A comparative study of the cryo-EM structures of S. cerevisiae and human anaphase-promoting complex/cyclosome (APC/C)

The anaphase-promoting complex/cyclosome (APC/C) is a large multi-subunit E3 ubiquitin ligase that controls progression through the cell cycle by orchestrating the timely proteolysis of mitotic cyclins and other cell cycle regulatory proteins. Although structures of multiple human APC/C complexes have been extensively studied over the past decade, the S. cerevisiae APC/C has been less extensively investigated. Here, we describe medium resolution structures of three S. cerevisiae APC/C complexes: unphosphorylated apo-APC/C and the ternary APC/CCDH1-substrate complex, and phosphorylated apo-APC/C. Whereas the overall architectures of human and S. cerevisiae APC/C are conserved, as well as the mechanism of CDH1 inhibition by CDK-phosphorylation, specific variations exist, including striking differences in the mechanism of coactivator-mediated stimulation of E2 binding, and the activation of APC/CCDC20 by phosphorylation. In contrast to human APC/C in which coactivator induces a conformational change of the catalytic module APC2:APC11 to allow E2 binding, in S. cerevisiae apo-APC/C the catalytic module is already positioned to bind E2. Furthermore, we find no evidence of a phospho-regulatable auto-inhibitory segment of APC1, that in the unphosphorylated human APC/C, sterically blocks the CDC20C-box binding site of APC8. Thus, although the functions of APC/C are conserved from S. cerevisiae to humans, molecular details relating to their regulatory mechanisms differ.


Introduction
The anaphase-promoting complex/cyclosome (APC/C), through targeting specific proteins for proteolysis via the ubiquitin-proteosome system, is a key regulator of cell cycle transitions.APC/C activity and substrate selection are controlled at various levels to ensure that specific cell cycle events occur in the correct order and time, reviewed in (Alfieri, Zhang et al. 2017, Watson, Brown et al. 2019, Barford 2020, Bodrug, Welsh et al. 2021).These regulatory mechanisms include the binding of cell cycle-specific coactivator subunits (CDC20 and CDH1), reversible APC/C and coactivator phosphorylation, and inhibitory complexes and proteins.
Research on S. cerevisiae APC/C over 25 years provided insights into APC/C structure and mechanism.Analysis of the sequences of proteins encoded by the CDC16, CDC23 and CDC27 genes led to the characterisation of the 34-residue tetratricopeptide repeat (TPR) motif that is usually arranged in multiple copies in contiguous arrays (Sikorski, Michaud et al. 1991, Lamb, Michaud et al. 1994, King, Peters et al. 1995).Similar arrangements of TPR motifs, that serve as protein-protein interaction sites, are conserved across a range of proteins with widely varying functions.The isolation of endogenous APC/C from S. cerevisiae confirmed the composition of large APC/C subunits that form a MDa-sized complex, and also the discovery of four small subunits (APC9, APC12/CDC26, APC13/SWM1 and APC15/MND2) (Zachariae, Shin et al. 1996, Zachariae, Shevchenko et al. 1998, Hall, Torres et al. 2003, Passmore, McCormack et al. 2003), that are also conserved with metazoan APC/C.S. cerevisiae APC/C was one of the first large multi-subunit complexes to be reconstituted in vitro using the baculovirus/insect cell over-expression system (Schreiber, Stengel et al. 2011).Early electron microscopy studies of endogenous and recombinant S. cerevisiae APC/C revealed that a TPR module and a platform module assemble to create a triangular-shaped complex defining a central cavity which accommodates a combined substrate recognition module of the coactivator and APC10, and the catalytic module of APC2:APC11 (Ohi, Feoktistova et al. 2007, da Fonseca, Kong et al. 2011, Schreiber, Stengel et al. 2011).Coactivator and APC10 cooperate to generate a co-receptor for D-box degron recognition (Carroll and Morgan 2002, Carroll, Enquist-Newman et al. 2005, Buschhorn, Petzold et al. 2011, da Fonseca, Kong et al. 2011, Chao, Kulkarni et al. 2012, Chang, Zhang et al. 2014, Brown, VanderLinden et al. 2015, Hartooni, Sung et al. 2022), whereas coactivator alone binds the KEN box and ABBA motif degrons (Chao, Kulkarni et al. 2012, Tian, Li et al. 2012, He, Chao et al. 2013).
For our earlier research on the structure and mechanism of the APC/C we investigated the S. cerevisiae system due to the ease of purifying TAP-tagged APC/C from yeast cultures (Passmore, McCormack et al. 2003, Passmore, Barford et al. 2005), and at the time the benefits of yeast genetics had provided considerable insights into its function.By 2011 our capacity to over-express recombinant APC/C using the baculovirus insect cell system (Schreiber, Stengel et al. 2011, Zhang, Yang et al. 2013, Zhang, Yang et al. 2016) allowed us and others to investigate human APC/C.These studies, together with those of others, provided insights into the overall architecture of the APC/C, mechanisms of substrate recognition and ubiquitylation, and APC/C regulation through mechanisms including reversible phosphorylation and the binding of the mitotic checkpoint complex and EMI1 (Frye, Brown et al. 2013, Brown, Watson et al. 2014, Chang, Zhang et al. 2014, Brown, VanderLinden et al. 2015, Chang, Zhang et al. 2015, Alfieri, Chang et al. 2016, Brown, VanderLinden et al. 2016, Yamaguchi, VanderLinden et al. 2016, Zhang, Chang et al. 2016, Watson, Grace et al. 2019, Höfler, Yu et al. 2024).
Recently, we returned to S. cerevisiae APC/C to extend the 10 Å-resolution structure published in 2011 (da Fonseca, Kong et al. 2011, Schreiber, Stengel et al. 2011) to atomic resolution.We completed building a 4.0 Å structure of the S. cerevisiae APC/C CDH1 -substrate complex (APC/C CDH1:Hsl1 ), allowing a comparative study of the S. cerevisiae and human systems.While human and S. cerevisiae APC/C architectures are in general very similar, there are significant differences, including the additional TPR subunit APC7 in human APC/C, and the structures of the smaller, less well conserved subunits.Aspects of regulation also differ.For example, in contrast to human APC/C, the catalytic module APC2:APC11 of S. cerevisiae adopts an active conformation in the absence of coactivator, whereas in human APC/C coactivators induce a conformational change of APC2:APC11 from a downwards state, in which the E2-binding site is blocked, to an upwards state competent to bind E2 (Chang, Zhang et al. 2014).Also unknown is the degree of conservation of the mechanism of APC/C CDC20 activation by APC/C phosphorylation.In human APC/C, phosphorylation of an autoinhibitory (AI) segment incorporated within a long loop of the APC1 subunit removes a steric blockade to the binding of the CDC20 C-box to its binding site on APC8 (Fujimitsu, Grimaldi et al. 2016, Qiao, Weissmann et al. 2016, Zhang, Chang et al. 2016).In our S. cerevisiae apo-APC/C cryo-EM maps we observe no evidence of an auto-inhibitory segment bound to the coactivator C boxbinding site in APC8.Thus it seems likely that mechanisms of activation of APC/C CDC20 by phosphorylation are not conserved from S. cerevisiae to human.Here, we present a comparative study of human and S. cerevisiae APC/C.

Results
Overall structure of S. cerevisiae APC/C complexes We reconstituted S. cerevisiae APC/C using the baculovirus/insect cell system (Zhang, Yang et al. 2016) (Supplementary Fig. 1a), similar to methods described previously (Schreiber, Stengel et al. 2011).When in complex with the CDH1 coactivator, the reconstituted APC/C ubiquitylated the highaffinity S. cerevisiae substrate Hsl1 (Supplementary Fig. 1b, c).Phosphorylated APC/C was generated using CDK2-cyclin A, and mass spectrometry revealed the majority of phosphosites mapped to the APC1, APC3 and APC6 subunits (Supplementary Table 1).We prepared cryo-EM grids for three APC/C complexes; (i) a ternary APC/C CDH1:Hsl1 complex, (ii) apo-APC/C and (iii) phosphorylated apo-APC/C (Supplementary Figs 2, 3).A cryo-EM reconstruction of the ternary APC/C CDH1:Hsl1 complex was determined at 4 Å, whereas the structures of unphosphorylated and phosphorylated apo-APC/C were determined at 4.9 Å and 4.5 Å resolution, respectively (Figs 1-3, Supplementary Figs 2-6 and Supplementary Table 2).We built the atomic coordinates based on prior structures of human (Chang, Zhang et al. 2015, Höfler, Yu et al. 2024), E. cuniculi (Zhang, Roe et al. 2010), S. cerevisiae (Au, Leng et al. 2002) and S. pombe (Zhang, Kulkarni et al. 2010, Zhang, Chang et al. 2013) APC/C subunits, and used AlphaFold2 predictions (Jumper, Evans et al. 2021, Tunyasuvunakool, Adler et al. 2021) to guide de novo model building (Supplementary Table 3).The overall structure of the recombinant APC/C CDH1:Hsl1 ternary complex is similar to the 10 Å resolution cryo-EM structure of APC/C CDH1:Hsl1 reconstituted using native APC/C purified from endogenous sources (da Fonseca, Kong et al. 2011), and our earlier negative stain EM reconstruction (Schreiber, Stengel et al. 2011).The complex adopts a triangular structure delineated by a lattice-like shell that generates a central cavity.The APC/C is comprised of two major modules; the TPR lobe composed of the canonical TPR subunits APC3/CDC27, APC6/CDC16 and APC8/CDC23, and the platform module composed of the large APC1 subunit, together with APC4 and APC5.The TPR lobe adopts a quasisymmetric structure through the sequential stacking of the three structurally-related TPR homodimers, APC8/CDC23, APC6/CDC16 and APC3/CDC27.Each TPR protein is composed of 12-13 copies of the TPR motif, a 34-residue repeat that forms a pair of anti-parallel a-helices.Consecutive arrays of TPR motifs fold into a TPR superhelical structure with a pitch of seven TPR motifs (Das, Cohen et al. 1998).The TPR helix defines an inner TPR groove suitable for protein binding.Each of the three TPR proteins of the TPR lobe homo-dimerise through their N-terminal TPR helix.The two C-terminal TPR helices of the APC3 homo-dimer provide the IR-tail binding site for coactivators and APC10 (Matyskiela and Morgan 2009, Chang, Zhang et al. 2014, Chang, Zhang et al. 2015), whereas structurally equivalent sites on APC8 interacts with the C-box of coactivator N-terminal domains (NTDs) (Chang, Zhang et al. 2015), and also with the IR tail of the the CDC20 subunit of the mitotic checkpoint complex (MCC CDC20 ) of the APC/C MCC complex (Alfieri, Chang et al. 2016, Yamaguchi, VanderLinden et al. 2016).Assembly of the complete APC/C complex is augmented by four small non-globular intrinsically disordered proteins (IDPs) that by interacting simultaneously with multiple large globular subunits stabilise the APC/C.Together, the platform and TPR modules create a scaffold for the juxtaposition of the catalytic and substrate recognition modules, with the catalytic module of the cullin subunit APC2 and the RINGdomain subunit APC11 binding to the platform module.Two subunits mediate substrate recognition through a substrate-recognition module: the exchangeable coactivator subunits (CDC20, CDH1 and Ama1) and the core APC/C subunit APC10.Coactivators and APC10 share in common a conserved C-terminal IR (Ile-Arg) tail that each interacts with the symmetry-related subunits of the APC3 homodimer.Additionally, APC10 interacts with APC1, and the NTDs of coactivators interact with both the TPR and platform modules.Cryo-EM density bridging the CDH1 WD40 domain (CDH1 WD40 ) and APC10 corresponds to the D-box degron of Hsl1 (da Fonseca, Kong et al. 2011).In all three states, the APC2:APC11 catalytic module adopts an upward conformation that positions the APC11 RING domain (APC11 RING ) proximal (~30 Å) to the substrate (Figs 1-3), similar to that observed in the active human APC/C CDH1:EMI1 complex (Fig. 4a, b) (Chang, Zhang et al. 2015, Höfler, Yu et al. 2024).

APC2
A recent analysis of human APC/C CDH1:EMI1 determined at 2.9 Å resolution revealed a zinc-binding module (APC2 ZBM ) inserted within cullin repeat 2 (CR2) of APC2 (Höfler, Yu et al. 2024).This segment constitutes a 45-residue insert between the A and B a-helices of CR2 (Fig. 5a), and is most similar in structure to treble-clef/GATA-like zinc fingers.Four Cys residues coordinate the zinc ion in human APC2.A multiple sequence alignment showed that APC2 ZBM is conserved within metazoan APC2 sequences (from humans to C. elegans), but not in yeast APC2 sequences (Chang, Zhang et al. 2015, Höfler, Yu et al. 2024).Consistent with this analysis, there is no structural equivalent of human APC2 ZBM in the cryo-EM structure of S. cerevisiae APC/C CDH1:Hsl1 (Fig. 5b, c).APC2 ZBM substantially increases the thermal stability of human APC2.Whether it performs additional roles is unknown.

APC4
The APC4 subunit comprises a WD40 domain with a long four-helix bundle insert between blades 3 and 4, augmented by an insert within blade 3 that is an a-helix in human APC4 and a b-hairpin in S. cerevisiae APC4 (Fig. 5d, e).Strikingly, in contrast to human APC4 which incorporates a typical seven-b-bladed propeller, in S. cerevisiae the APC4 WD40 domain (APC4 WD40 ) is constructed from six blades (blades 1-6) (Fig. 5d).The absence of cryo-EM density for a blade7 in S. cerevisiae APC4 is supported by an AlphaFold2 prediction (Tunyasuvunakool, Adler et al. 2021) (Fig. 5f, g).The predicted APC4 model includes an a-helix instead of blade7.However, this helix is not supported by corresponding cryo-EM density, and is a low confidence prediction based on a low pLDDT score (Fig. 5f, g).The unusual APC4 WD40 domain architecture might be specific to S. cerevisiae.The AlphaFold2 prediction of S. pombe APC4 proposes a seven-bladed WD40 domain (Tunyasuvunakool, Adler et al. 2021).

Structure of intrinsically disordered regions and intrinsically disordered proteins
Our structure indicates how the four smaller non-globular subunits (APC9, CDC26/APC12, SWM1/APC13, MND2) (Zachariae, Shin et al. 1996, Zachariae, Shevchenko et al. 1998, Hall, Torres et al. 2003, Passmore, McCormack et al. 2003), with a high content of intrinsically disordered regions (IDRs), mediate inter-subunit interactions between the larger globular subunits (Fig. 4).CDC26/APC12 and SWM1/APC13 share sequence conservation with their metazoan counterparts (Schwickart, Havlis et al. 2004) (Supplementary Table 3), consistent with the proposal that SWM1 is the ortholog of metazoan APC13 (Hall, Torres et al. 2003, Passmore, McCormack et al. 2003).MND2 and APC9 share no clear sequence similarities with metazoan APC/C subunits, although previous structure-based mapping of MND2 and APC9 to the S. cerevisiae APC/C EM-reconstruction suggested counterparts to APC15 and APC16, respectively (Schreiber, Stengel et al. 2011), consistent with the proposal that MND2 is the S. cerevisiae ortholog of human APC15 (Mansfeld, Collin et al. 2011), and that MND2 and APC15 share related functions in promoting the autoubiquitylation of CDC20 in the context of the S. cerevisiae and human APC/C:MCC complexes (Mansfeld, Collin et al. 2011, Foster and Morgan 2012, Uzunova, Dye et al. 2012, Alfieri, Chang et al. 2016).Analysis of the subunit composition of APC/C complexes purified from S. cerevisiae strains harbouring specific gene deletions revealed that APC9 is required for the efficient incorporation of CDC27/APC3 into the assembled APC/C (Zachariae, Shevchenko et al. 1998, Passmore, McCormack et al. 2003), whereas SWM1/APC13 and CDC26/APC12 are required for the stoichiometric assembly of APC3, APC6 and APC9 into the complex (Zachariae, Shevchenko et al. 1998, Schwickart, Havlis et al. 2004).MND2 deletion did not affect the association of other subunits (data not shown from (Schwickart, Havlis et al. 2004)).

CDC26/APC12
The N-terminal 13 residues of CDC26/APC12 insert into the TPR groove of APC6, adopting an extended conformation and mode of binding highly conserved with previous cryo-EM and crystallographic structures of human and S. pombe orthologs (Wang, Dye et al. 2009, Zhang, Kulkarni et al. 2010, Chang, Zhang et al. 2014, Chang, Zhang et al. 2015) (Fig. 4), although at the resolution of our APC/C CDH1:Hsl1 cryo-EM map we cannot define an N-terminal acetyl group visualised in the crystal structure of the S. pombe APC6-APC12 complex (Zhang, Kulkarni et al. 2010).CDC26/APC12 then folds into an a-helix that packs against a-helices of the inner groove of the APC6 TPR superhelix, similar to the human and S. pombe APC6:APC12 complexes, as is the disordered C-terminus of CDC26/APC12.In neither the human and S. cerevisiae APC/C complexes, apart from APC6, do we observe interactions of CDC26/APC12 with other APC/C subunits.Thus, the role of CDC26/APC12 in stabilising the association of APC3, APC6 and APC9 with the APC/C complex (Zachariae, Shevchenko et al. 1998, Schwickart, Havlis et al. 2004) is likely through its specific role in stabilising the APC6 TPR super-helix (Wang, Dye et al. 2009) which bridges APC3 with APC8 (Fig. 4e, f)

SWM1/APC13
In human APC/C, the APC13 N-terminus inserts into the TPR groove formed by the N-terminal TPR helix of APC8A (Chang, Zhang et al. 2014, Brown, VanderLinden et al. 2015, Chang, Zhang et al. 2015).The extended chain of APC13 then interacts with structurally equivalent symmetry-related shallow grooves present on the outer TPR-helical ridges of APC8B, APC6A, APC6B and APC3A (sites 4-7, Fig. 4f).These shallow grooves are lined with side chains of aromatic residues that engage hydrophobic residues of APC13 (Chang, Zhang et al. 2015).This pattern of TPR subunit engagement by APC13 is continued by APC16 interactions with equivalent sites on APC3B, APC7A and APC7B (sites 1-3, Fig. 4f).An earlier electron microscopy analysis of S. cerevisiae APC/C using an APC13green fluorescent protein fusion positioned APC13 close to the APC6-APC3 interface (Schreiber, Stengel et al. 2011), correlating with the position of human APC13.In our structure of S. cerevisiae APC/C CDH1:Hsl1 , we built APC13 into discontinuous cryo-EM density that spans APC8 to APC3.Similar to human APC13, S. cerevisiae APC13 engages the structurally equivalent sites on APC8B, APC6A, APC6B and APC3B (sites 4-7, Fig. 4e), with its N-terminus also inserting into the TPR groove of APC8A.
Human APC13 and S. cerevisiae SWM1/APC13 perform comparable structural roles, contacting the successive TPR subunits of the TPR lobe.We proposed previously that these contacts function to order the stacking of TPR homo-dimers of the TPR groove by breaking the symmetry of inter-TPR dimer interfaces.Although human and S. cerevisiae APC13 share only low sequence similarity (Supplementary Table 3), their conserved structural roles presumably explains the ability of human APC13 to substitute for the function of S. cerevisiae SWM1/APC13 in vivo (Schwickart, Havlis et al. 2004, Penkner, Prinz et al. 2005).

APC9 (human APC16 ortholog)
In human APC/C, APC16 is visible as a single 40-residue extended a-helix that lies along the shallow grooves on the outer TPR-helical ridges of APC3B, APC7A and APC7B, forming equivalent interactions as observed for APC13 contacts with APC3, APC6 and APC8 (Chang, Zhang et al. 2014, Brown, VanderLinden et al. 2015, Chang, Zhang et al. 2015) (Fig. 4f).S. cerevisiae APC9 is over twice the size of APC16 with the two proteins sharing no clear sequence similarity.Based on a subunit deletion approach, we had previously assigned a region of APC9 to the C-terminal TPR super-helix of APC3A (Schreiber, Stengel et al. 2011).This clearly differs from the association of APC16 with human APC/C.Guided by an AlphaFold2 prediction of APC9 (Tunyasuvunakool, Adler et al. 2021) and a complex of APC9 and APC3 (Jumper, Evans et al. 2021), we built 131 of 265 residues into the S. cerevisiae APC/C CDH1:Hsl1 cryo-EM map as three discontinuous segments (Fig. 4c and Supplementary Table 3).The C-terminal segment (segment 3) is a short three-turn a-helix that is structurally homologous to the mode of human APC16 binding to APC3B (Fig. 4e, f).Segment 2, composed of three separated a-helices, docks onto the outer surface of APC3A, bridging the two turns of the TPR helix (Fig. 4e).Finally, segment 1 bridges APC3A and APC6A (Fig. 4c insert), likely explaining why APC9 deletion causes loss of both APC3 and APC9 from purified APC/C complexes (Zachariae, Shevchenko et al. 1998, Passmore, McCormack et al. 2003).

MND2/APC15
In human APC/C, the visible regions of APC15 adopt a relatively simple elongated structure, with the N-terminus of the protein inserting as an extended chain into the groove formed by the N-terminal TPR helix of APC5, then forming an a-helix (APC15 NTH ) that contacts the N-terminal a-helical domain of APC5, before engaging a groove at the interface of APC6A and APC8A.APC15 interacts with APC8A at a site structurally equivalent to the sites on APC8B, APC6 and APC3 engaged by the other small subunits APC9/APC16, APC12, and APC13 (site 8, Fig. 4b, d and f). S. cerevisiae MND2/APC15 also contacts the N-terminal a-helical domain of APC5, and APC6A and APC8A, as for human APC15, but has an overall more irregular structure, with its N-terminus docking into a groove formed between the APC5 TPR helix and the four-helical bundle of APC4 (Fig. 4a, c and e).In contrast to human APC5, the APC5 TPR groove is occupied by a loop emanating from the APC5 Nterminal a-helical domain.An insert of S. cerevisiae APC15, not conserved in human, protrudes into the APC/C cavity, engaging the outer TPR helix of APC8B that forms the C-box binding site (Fig. 4a).A stretch of ~40 acidic residues, common to both human and S. cerevisiae APC15, is C-terminal to the ordered regions of APC15 in both complexes.
In the human APC/C MCC structure, the mitotic checkpoint complex (MCC) was observed to adopt both closed and open conformations (APC/C MCC -closed and APC/C MCC -open), with APC/C MCC -closed being associated with disorder of APC15 NTH and concomitant conformational changes of APC4 and APC5, whereas an ordered APC15 NTH is associated with APC/C MCC -open (Alfieri, Chang et al. 2016, Yamaguchi, VanderLinden et al. 2016).In APC/C MCC -closed, the UbcH10-binding site on the APC2:APC11 catalytic module is sterically occluded by the MCC.These structural observations explained why APC15 deletion in human APC/C abolished CDC20 MCC auto-ubiquitylation in the context of APC/C MCC (Uzunova, Dye et al. 2012, Alfieri, Chang et al. 2016, Yamaguchi, VanderLinden et al. 2016).Our observation that APC15 NTH is structurally conserved in S. cerevisiae APC/C is consistent with a defect in CDC20 MCC ubiquitylation caused by deleting APC15 (Foster and Morgan 2012).In contrast to MND2/APC15 promoting CDC20 MCC ubiquitylation during the SAC, in S. cerevisiae, MND2 specifically suppresses securin/Pds1 ubiquitylation by the Ama1 coactivator-APC/C complex that prevents premature sister chromatid segregation during meiosis (Oelschlaegel, Schwickart et al. 2005).The 14 sites of MND2 phosphorylation required for efficient APC/C Ama1regulated progression through meiosis I (Torres and Borchers 2007), are located within, and Cterminal to, the acidic stretch in MND2, and are therefore not visible in our structure.
A common feature of all the small APC/C subunits is that they form extended, mainly irregular structures, that simultaneously contact multiple large globular APC/C subunits.CDC26/APC12, SWM1/APC13 and human APC15 are anchored at their N-termini by inserting into the TPR grooves of APC6, APC8A and APC5, respectively.These interactions serve to stabilise the TPR groove through formation of protein-protein interactions (Wang, Dye et al. 2009).In human APC/C, a 40residue loop of APC1 WD40 inserts into the TPR groove of APC8B, although this structural feature is not conserved in S. cerevisiae APC/C.

Interactions of CDH1 with APC/C and conserved regulation by phosphorylation
Coactivators interact with the APC/C through an N-terminal C-box motif (Schwab, Neutzner et al. 2001) and a C-terminal IR tail (Passmore, McCormack et al. 2003, Vodermaier, Gieffers et al. 2003).Both motifs are highly evolutionarily conserved.Interestingly, although the seven-residue C-box and two-residue IR tail lack obvious sequence similarity, the C-box binding site within the TPR helix of APC8B is structurally conserved with the IR-tail binding site on APC3, with an Arg residue, common to both, anchoring the motifs to their respective binding sites (Chang, Zhang et al. 2015).The docking of the coactivator C-box motif to this site on APC8B/CDC23 rationalises the temperature-sensitive cell cycle mutations of S. cerevisiae CDC23 (Sikorski, Boguski et al. 1990), and prior mutagenesis data implicating Asn405 of APC8/CDC23 in CDH1 binding (Matyskiela and Morgan 2009).Additionally, other segments within CDH1 NTD mediate APC/C-coactivator interactions.When in complex with the APC/C, CDH1 NTD folds into five a-helices (labelled a2-a6) (Fig. 6).Four of these a-helices mediate APC/C -CDH1 interactions, three of which; a2, a3 and a6, that form anti-parallel interactions with APC1 PC a-helices, being conserved with human CDH1 (Fig. 6b, e).Unique to S. cerevisiae CDH1 is a three-turn a-helix (a5) that docks onto the outer TPR ridge of APC6B, running antiparallel to exposed a-helices, and bridging APC8B (Fig. 6a, c).CDK phosphorylation of CDH1 NTD inhibits CDH1 binding to the APC/C, and substituting alanines for all eleven predicted CDK sites in S. cerevisiae CDH1 almost completely eliminated CDH1 phosphorylation in mitosis (Zachariae, Schwab et al. 1998).Mimicking six CDK phosphosites with aspartates abolished binding of CDH1 to the APC/C in vivo, whereas a mutant with non-phosphorylatable sites bound APC/C more tightly than wild type CDH1 (Hockner, Neumann-Arnold et al. 2016).Five of these CDK sites, with additional non-consensus CDK sites, were reported to be phosphorylated in vivo (Hall, Warren et al. 2004).Mapping these phosphosites onto the S. cerevisiae APC/C CDH1:Hsl1 structure suggested a rationale for how CDH1 NTD phosphorylation inhibits binding of CDH1 to the APC/C (Fig. 6a).Two CDK sites (Ser227, Ser239) are conserved in human CDH1 (Fig. 6b) (Chang, Zhang et al. 2015), and map on or close to the a6 helix that docks onto APC1 PC (Fig. 6a).Phosphorylation of either site in human CDH1 contributed significantly to CDH1 inactivation (Chang, Zhang et al. 2015), likely by disrupting CDH1 NTD -APC1 PC interactions.Thus, this analysis reveals an evolutionarily conserved molecular mechanism for inhibiting CDH1 binding to the APC/C through CDK phosphorylation.Additionally, three reported non-consensus CDK-phosphosites within a5 (Ser216, Ser220, Ser225) (Hall, Warren et al. 2004) would likely interfere with a5 interactions with APC6B and APC8B (Fig. 6a).
The recent 2.9 Å resolution cryo-EM structure of APC/C CDH1:EMI1 revealed an N-terminal amphipathic a-helix of CDH1 (CDH1 a1 ) interacting through non-polar interactions with a hydrophobic groove in the APC1 WD40 domain (Höfler, Yu et al. 2024).This a-helix is conserved in metazoan coactivators, but consistent with MSA analysis, is not present in S. cerevisiae CDH1.
Finally, the mode of binding of CDH1 IR to the IR-tail binding site on APC3A is conserved with that of human APC/C CDH1:EMI1 (Matyskiela and Morgan 2009, Brown, VanderLinden et al. 2015, Chang, Zhang et al. 2015, Höfler, Yu et al. 2024) (Fig. 7a-d).In S. cerevisiae APC/C, the interaction of APC9 with APC3A (segment 2 of APC9) and APC3B (segment 3 of APC9) partially mimics the respective APC7 and APC16 interactions with human APC3 (Fig. 7e, f).APC9 segment 2 interactions with the Cterminal TPR helix of APC3A may stabilise the CDH1 IR tail-binding site of APC3A.
Comparison of apo-APC/C with APC/C CDH1:Hsl1 Biochemical studies on both metazoan (Xenopus) and S. cerevisiae APC/C revealed that coactivators function as substrate adaptors (Burton and Solomon 2001, Hilioti, Chung et al. 2001, Schwab, Neutzner et al. 2001, Kraft, Vodermaier et al. 2005), and also enhance APC/C catalytic activity (Kimata, Baxter et al. 2008, Van Voorhis andMorgan 2014).For S. cerevisiae APC/C, catalytic enhancement results from a substantially more efficient interaction of the APC/C with its E2s Ubc4 and Ubc1 (Van Voorhis and Morgan 2014).Similarly, the affinity of the human APC/C for UbcH10 is increased three-fold when in complex with CDH1 (Chang, Zhang et al. 2014).In human APC/C, the association of CDH1 induces a conformational change of the APC2:APC11 catalytic module from a downwards state, in which the UbcH10-binding site is blocked, to an upwards state competent to bind UbcH10 (Chang, Zhang et al. 2014).Comparing unphosphorylated S. cerevisiae apo-APC/C and APC/C CDH1:Hsl1 showed that in both states the APC2:APC11 catalytic module adopts the same raised position (Fig. 8a, b), similar to human APC/C CDH1:EMI1 .Docking Ubc4 onto the RING domain of APC11 (based on the S. cerevisiae Not4:Ubc4 coordinates; PDB 5AIE (Bhaskar, Basquin et al. 2015)) indicated that the conformation of the APC2:APC11 catalytic module in apo-APC/C does not sterically occlude Ubc4 binding (Fig. 8c, d).Thus, the mechanism by which coactivator stimulates S. cerevisiae APC/C catalytic activity through enhancing E2 binding differs from human APC/C, although the structural mechanism is not defined in our study.
Why isn't S. cerevisiae apo-APC/C in an inactive conformation?The interaction of CDH1 NTD with APC1 and APC8B of human APC/C explains how CDH1 association promotes a conformational change of the APC/C that results in the upward movement of the APC2:APC11 catalytic module (Chang, Zhang et al. 2014, Höfler, Yu et al. 2024).In the human ternary complex, CDH1 NTD disrupts the interaction between APC1 PC and APC8B by binding to a site on APC1 PC that overlaps the site in contact with APC8B.This wedges APC1 PC and APC8B apart.The resultant downwards-displacement of APC8B tilts the platform to translate the APC2 CTD :APC11 module upwards.In contrast to human APC/C, in the more open structure of S. cerevisiae apo-APC/C, the TPR lobe does not directly contact APC1 PC , and thus the position of APC8B relative to the APC8B -CDH1 NTD1 -binding site does not impede CDH1 association.Instead, on binding to S. cerevisiae APC/C, of CDH1 NTD interconnects the TPR lobe with APC1 PC to reinforce TPR and platform lobe interactions (Fig. 8a, b; lower panels).To accommodate closure of the APC/C cavity, conformational changes are distributed over the subunits of the TPR and platform lobes, including a tilt of APC1 PC relative to the combined domains of APC1 Mid -APC1 WD40 (Supplementary Figure 7 and Supplementary Video 1).

Mechanism of APC/C activation by phosphorylation
Hyperphosphorylation of APC/C subunits APC1 and APC3 is required for CDC20 to activate the APC/C (Lahav-Baratz, Sudakin et al. 1995, Shteinberg, Protopopov et al. 1999, Kramer, Scheuringer et al. 2000, Rudner and Murray 2000, Golan, Yudkovsky et al. 2002, Kraft, Vodermaier et al. 2005, Fujimitsu, Grimaldi et al. 2016, Qiao, Weissmann et al. 2016, Zhang, Chang et al. 2016).In metazoan unphosphorylated apo-APC/C, an auto-inhibitory (AI) segment incorporated into the 300s loop inserted within the WD40 domain of APC1 (APC1 300L ) sterically occludes the C-box binding site on APC8B (APC8 C-box ) thereby competing for CDC20 binding (Fujimitsu, Grimaldi et al. 2016, Qiao, Weissmann et al. 2016, Zhang, Chang et al. 2016).Phosphorylation of residues within (APC1 300L ), including the AI segment, displaces the AI segment from APC8 C-box allowing CDC20 binding.Multiple lines of data support this model, including the observation of cryo-EM density for the AI segment bound to APC8 C-box (Zhang, Chang et al. 2016, Höfler, Yu et al. 2024), the constitutive activation of APC/C CDC20 on deleting APC1 300L , and the location of multiple CDK and PLK1 sites within APC1 300L (Fujimitsu, Grimaldi et al. 2016, Li, Chang et al. 2016, Qiao, Weissmann et al. 2016, Zhang, Chang et al. 2016, Höfler, Yu et al. 2024).Replacing these phosphosites with glutamates activated APC/C CDC20 , whereas in contrast, mutations to Ala prevented CDK activation of APC/C CDC20 (Fujimitsu, Grimaldi et al. 2016, Qiao, Weissmann et al. 2016, Zhang, Chang et al. 2016, Höfler, Yu et al. 2024).Our cryo-EM structure suggests that activation of S. cerevisiae APC/C CDC20 by phosphorylation operates by a different mechanism.We observe no cryo-EM density occupying the C-box binding site of APC8B (Fig. 9a, b).Arguably, the low resolution of the unphosphorylated apo-APC/C structure means that we cannot definitively state that APC8 C-box is not occluded in the unphosphorylated apo-state.However, the Morgan lab recently reported that deleting the APC1 WD40 loop of S. cerevisiae (residues 225-365), equivalent to human APC1 300L , caused only a minor growth defect, but importantly no evidence for APC/C hyperactivation (Ng, Adly et al. 2024), a finding that is not consistent with a model in which S. cerevisiae and human APC/C share a common phosphoregulatory mechanism.
Superimposing both apo-APC/C states (phosphorylated and unphosphorylated) reveals that the only obvious difference between the two is a small relative shift of the TPR lobe (Fig. 9c-e).Since APC/C phosphorylation stimulates CDC20 association, and not CDH1, we would expect specific and perhaps localised conformational changes in the APC/C.It is possible that in contrast to the activation of human APC/C CDC20 through release of a negative auto-inhibitory segment, phosphorylated regions of S. cerevisiae APC/C might contact CDC20 directly or indirectly to enhance its binding.We note that 'unphosphorylated' APC/C isolated from insect cells is partially phosphorylated prior to its treatment with CDK2 (Supplementary Table 1).Although CDK2-dependent phosphorylation causes the expected mobility shift in APC3/CDC27 (Supplementary Fig. 1a), and an increase in phosphorylation sites (Supplementary Table 1), it is possible that 'unphosphorylated' APC/C may already containctivating phosphorylation sites, thus explaining the absence of conformational change on in vitro CDK2-mediated phosphorylation.Our phosphoproteomic study did not assess phospho-site stoichiometry.

Discussion
We present a comparative study of human and S. cerevisiae APC/C.This revealed a similar overall architecture, with the largest difference being the absence of an APC7 homolog in S. cerevisiae.A recent study discovered a specific role for APC7 in mammalian brain development (Ferguson, Urso et al. 2022).Homozygous ANAPC7 mutations in humans, leading to loss of APC7 protein, underlies an inherited intellectual disability syndrome, caused by defective APC/C DAPC7 -mediated degradation of the chromosome-associated protein Ki-67 (Ferguson, Urso et al. 2022).Thus in mammals, APC7 confers specific post-mitotic functions in the developing brain through selective protein-recognition.The mode of binding of the CDH1 coactivator subunit is also very similar, except for the presence in human APC/C of the N-terminal a-helix (CDH1 a1 ), not conserved in S. cerevisiae APC/C, that mediates contacts to APC1 WD40 .Also conserved is the mechanism of inactivation of CDH1 by CDK phosphorylation of two conserved serine residues on CDH1 at the CDH1 NTD -APC1 PC interface.Major differences between human and S. cerevisiae APC/C are how coactivators enhance APC/C affinity for E2s (UbcH10 in human (Chang, Zhang et al. 2014), Ubc1 and Ubc4 in S. cerevisiae (Van Voorhis and Morgan 2014)), and how APC/C CDC20 is activated by CDK-mediated APC/C phosphorylation.What is clear from our structural analysis is that unlike human APC/C, in the apostate of S. cerevisiae APC/C, the E2-binding site on APC11 RING is accessible, and that CDH1 does not promote a substantial conformational change of the APC2:APC11 catalytic module.However, how CDH1 enhances E2 affinity for APC/C isn't clear from a comparison of the apo-APC/C and APC/C CDH1:Hsl1 ternary structures.It is likely that the structures of APC/C CDH1:Hsl1 in complex with E2s would reveal this mechanism.Finally, the mechanism of stimulating CDC20-binding to the APC/C through phosphorylation-dependent relief of an auto-inhibitory segment of APC1, as in human APC/C (Fujimitsu, Grimaldi et al. 2016, Qiao, Weissmann et al. 2016, Zhang, Chang et al. 2016), is not conserved in S. cerevisiae APC/C, a structural finding that is consistent with in vivo data (Ng, Adly et al. 2024).Since comparison of all three reported structures did not reveal a likely mechanism, one explanation could be that a phosphorylated region of the APC/C either directly or indirectly contacts CDC20 to enhance its affinity for the APC/C.The column was washed with APC/C buffer [50 mM Tris-HCl (pH 8.0), 200 mM NaCl, 5 % (v/v) glycerol, 2 mM DTT, 1 mM EDTA and 2 mM benzamidine].The recombinant APC/C complex was eluted with the wash buffer supplemented with 2.5 mM desthiobiotin (Sigma-Aldrich).Before loading the elution fractions onto a 6 mL Resource Q anion-exchange column (Cytiva), they were diluted 1.6fold into buffer A without NaCl [buffer A: 20 mM HEPES (pH 8.0), 120 mM NaCl, 5 % (v/v) glycerol and 2 mM DTT].The Resource Q column was washed with buffer A and the APC/C was eluted with a gradient of buffer B [20 mM HEPES (pH 8.0), 1 M NaCl, 5 % (v/v) glycerol and 2 mM DTT].The resulting elution was concentrated to around 3 mg/mL and ultracentrifuged for 10 min at 40,000 g.The soluble supernatant was loaded onto a Superose 6 Increase 3.2/300 column (Cytiva) equilibrated in APC/C gel-filtration buffer [20 mM HEPES (pH 8.0), 150 mM NaCl and 0.5 mM TCEP].The gel filtration was run on a ÄKTAmicro (Cytiva) with a flow rate of 50 µL/min.

Electron microscopy
Freshly purified APC/C samples were visualised by negative-staining EM to assess the quality and homogeneity of the sample.Micrographs were recorded on a CryoSpirit electron microscope (ThermoFisher Scientific) at an accelerating voltage of 120 keV and at a defocus of approximately -1.5 µm.For cryo-EM, Quantifoil R3.5/1 grids coated with a layer of continuous carbon film (approximately 50 Å thick) were glow-discharged for 30 s before deposition of 3 µL fresh sample (~0.12 mg/mL), blotted for 8 s, and vitrified by plunging into liquid ethane with a custom-made manual plunger at 4 °C.Micrographs were collected on a Titan Krios microscope (ThermoFisher Scientific) at an acceleration voltage of 300 keV and Falcon III direct electron detector.Micrographs were taken using EPU software (ThermoFisher Scientific) at a nominal magnification of 59,000x, with a calibrated pixel size of 1.38 Å. Fifty nine movie frames with an average electron dose of 59 e -/Å 2 were recorded in integration mode for 1.49 s.The defocus range was set at -2.0 to -4.0 µm.

Image processing
All movie frames were aligned and averaged with MotionCor2 (Zheng, Palovcak et al. 2017) or RELION's own implementation of the MotionCor2 algorithm (Zivanov, Nakane et al. 2018).Contrast transfer function parameters were calculated with Gctf (Zhang 2016).Particles were automatically selected by template-free particle picking in Gautomatch (developed by Kai Zhang, http://www.mrclmb.cam.ac.uk/kzhang/Gautomatch/).All subsequent steps were performed in RELION-2/3 (Zivanov, Nakane et al. 2018).Sorting of particle images was performed by 2D and 3D classification, using 60 Å low-pass filtered human unphosphorylated apo-APC/C EM map (EMD-3386) (Zhang, Chang et al. 2016) as an initial 3D reference.The reconstruction generated from all the corresponding S. cerevisiae particles, low-pass filtered at 40 Å, was used as a subsequent reference.Finally, the data set was subject to either particle polishing or Bayesian particle polishing (Zivanov, Nakane et al. 2018), CTF refinement and an extra 3D classification step to discard remaining bad particles.All resolution estimations were based on the gold-standard Fourier shell correlation (FSC) calculations using the FSC = 0.143 criterion.Supplementary Data Table 2 summarises EM reconstructions obtained in this work.

Model building of APC/C
Model building of the apo-APC/C, APC/C CDH1:Hsl1 structures and phosphorylated apo-APC/C were performed in COOT (Emsley and Cowtan 2004).Initially, available atomic structure of human unphosphorylated apo-APC/C (5G05) (Zhang, Chang et al. 2016) and human APC/C CDH1:EMI1 (4UI9) (Chang, Zhang et al. 2015) and prior structures of human (Chang, Zhang et al. 2015, Höfler, Yu et al. 2024), E. cuniculi (Zhang, Roe et al. 2010), S. cerevisiae (Au, Leng et al. 2002) and S. pombe (Zhang, Kulkarni et al. 2010, Zhang, Chang et al. 2013) APC/C subunits were rigid-body fitted as individual subunits into the cryo-EM maps in Chimera (Yang, Lasker et al. 2012).Subsequently, the different human and S. cerevisiae APC/C protein subunit sequences were aligned using MUSCLE program (Edgar 2004) and Jalview (Waterhouse, Procter et al. 2009).Based on the multiple alignment results, the amino acids from the human fitted atomic structure were substituted for the corresponding S. cerevisiae amino acids using COOT.Finally, all fitted structures were rebuilt according to the cryo-EM map and guided by AlphaFold2 predictions of these subunits (Jumper, Evans et al. 2021, Tunyasuvunakool, Adler et al. 2021).APC9, APC13, APC15, the CDH1 NTD and several loop regions not seen in previous structures were built ab initio, also guided by AlphaFold2 predictions of these subunits, including a prediction of the APC3:APC9 complex.Real-space refinement was performed in PHENIX (Adams, Afonine et al. 2010) and the refined model was validated using the MolProbity tool (Williams, Headd et al. 2018).Maps and models were visualised with COOT (Emsley and Cowtan 2004), and figures generated using Chimera X (Goddard, Huang et al. 2018).The refinement statistics are summarised in Supplementary Data Table 2.

Mass spectrometry
Purified proteins were prepared for mass spectrometric analysis by in solution enzymatic digestion, without prior reduction and alkylation.Protein samples were digested with trypsin or elastase (Promega), both at an enzyme to protein ratio of 1:20.The resulting peptides were analysed by nano-scale capillary LC-MS/MS using an Ultimate U3000 HPLC (ThermoScientific Dionex) to deliver a flow of approximately 300 nL/min.A C18 Acclaim PepMap100 5 μm, 100 μm Å~ 20 mM nanoViper column (ThermoFisherScientific Dionex), trapped the peptides before separation on a C18 Acclaim PepMap100 3 μm, 75 μm Å~ 250 mM nanoViper column (ThermoFischerScientific Dionex).Peptides were eluted with a 90 min gradient of acetonitrile (2% to 50%).The analytical column outlet was directly interfaced via a nano-flow electrospray ionization source, with a hybrid quadrupole orbitrap mass spectrometer (Q-Exactive Plus Orbitrap, ThermoFischerScientific).LC-MS/MS data were then searched against an in-house LMB database using the Mascot search engine (Matrix Science) (Perkins, Pappin et al. 1999), and the peptide identifications validated using the Scaffold program (Proteome Software Inc.) (Keller, Nesvizhskii et al. 2002).All data were additionally interrogated manually.The phosphorylation sites in red are present in both complexes: phosphorylated apo-APC/C and the control unphosphorylated apo-APC/C.The sequence coverage is ≥77% for all the subunits.* Consensus CDK site (S/T)P ** Consensus CDK site (S/T)Px(K/R)

Figure 1 .
Figure 1.Overall structure of the APC/C CDH1:Hsl1 complex.a, Two views of the APC/C CDH1:Hsl1 ternary complex fitted into the 4.0 Å cryo-EM map.b, Two views of the APC/C CDH1:Hsl1 ternary complex shown as ribbon representations.

Figure 2 .
Figure 2. Overall structure of unphosphorylated apo-APC/C.a, Two views of apo-APC/C fitted into the 4.9 Å cryo-EM map.b, Two views of apo-APC/C shown as ribbon representations.

Figure 3 .
Figure 3. Overall structure of phosphorylated apo-APC/C.a, Two views of phosphorylated apo-APC/C fitted into the 4.5 Å cryo-EM map.b, Two views of apo-APC/C shown as ribbon representations.

Figure 4 .
Figure 4. Comparison of S. cerevisiae APC/C CDH1:Hsl1 with human APC/C CDH1:EMI1 .a, S. cerevisiae and b, human complexes subunits colour-coded.c, S. cerevisiae and d, human complexes with large subunits coloured in grey and small IDP subunits (APC9, APC12, APC13, APC15, APC16) colour-coded.The structure of human APC/C CDH1:EMI1 from (Höfler, Yu et al. 2024) (PDB 7QE7).e, and f, Four small IDP-subunits (APC9, APC13, APC15, APC16) contribute to interacting with equivalent, quasi-symmetrical sites on the outer surfaces of the TPR subunits of the TPR lobe.TPR lobes of S. cerevisiae (e) and human APC/C (f) are depicted as a surface representations.The contact sites with the three small subunits that contact the S. cerevisiae TPR lobe are numbered 3 to 8 after human APC/C(Chang, Zhang et al. 2015) that has sites 8 sites sue to APC7.

Figure 5 .
Figure 5.Comparison of S. cerevisiae and human APC2 and APC4 subunits.a, Human APC2 with APC2 ZBM of CRL2 coloured red.b, S. cerevisiae APC2.The position of human APC2 ZBM is indicated.c, Superimposition S. cerevisiae APC2 (purple) with human APC2 (grey) on CRL2.Human APC2 from (Höfler, Yu et al. 2024) (PDB 7QE7).d, S. cerevisiae APC4, colour-coded from N-to Ctermini with a blue to red ramp.e, Human APC4, colour-coded from N-to C-termini with a blue to red ramp.S. cerevisiae APC4 WD40 is a six-bladed b-propeller in contrast to the seven bladed b-propeller of human APC4 WD40 .f, AlphaFold2 prediction of S. cerevisiae APC4 colour-coded according to pLDDT score.No seventh bade is predicted.The a-helix in place of the seventh blade is predicted with low confidence.g, Fit of S. cerevisiae APC4 WD40 into the APC/C CDH1:Hsl1 cryo-EM map showing lack of density for a seventh blade.Human APC4 from (Höfler, Yu et al. 2024) (PDB 7QE7).

Figure 7 .
Figure 7.Comparison of CDH1 IR binding to S. cerevisiae and human APC/C CDH1 complexes.a, Overall view of the CDH1 IR -binding site on S. cerevisiae APC/C CDH1:Hsl1 .b, Zoomed view of this complex.c, Overall view of the CDH1 IR -binding site on human APC/C CDH1:EMI1 .d, Zoomed view of this complex.The structure of human APC/C CDH1:EMI1 from (Höfler, Yu et al. 2024) (PDB 7QE7).e, f, APC9 of S. cerevisiae APC/C partially mimics APC7 and APC16 of human APC/C.(e) In S. cerevisiae APC/C, the interaction of APC9 with APC3A (segment 2 of APC9) and APC3B (segment 3 of APC9) partially mimics the respective APC7 and APC16 interactions with human APC3.APC9 segment 2 interactions with the C-terminal TPR helix of APC3A may stabilise the CDH1 IR -binding site of APC3A.f, Similarly, in human APC/C, the interface of APC7B with APC3B might function to stabilise the APC10 IR -binding site on APC3B.Human APC/C CDH1:EMI1 from (Höfler, Yu et al. 2024) (PDB 7QE7).

Figure 8 .
Figure 8. Comparing apo-APC/C and the APC/C CDH1:Hsl1 complex shows both apo-APC/C and the APC/C CDH1:Hsl1 complexes are competent to bind the Ubc4.a, Two views of apo-APC/C.b, Two views of APC/C CDH1:Hsl1 .The coordinates were superimposed on APC1 PC .In the apo-APC/C complex, there are no contacts between APC1 PC of the platform module and APC6B and APC8B of the TPR lobe.In the APC/C CDH1:Hsl1 , CDH1 NTD bridges APC1 PC with APC6B and APC8B.The overall conformations are similar, specifically the APC2:APC11 catalytic module adopts a raised conformation in both states.c, Model of APC/C CDH1:Hsl1 in complex with Ubc4.d, Model of apo-APC/C in complex with Ubc4.The position of the APC2:APC11 catalytic module in apo-APC/C does not exclude Ubc4

Figure 9 .
Figure 9. Regulation of APC/C by phosphorylation.The C-box binding site is un-obstructed in unphosphorylated apo-APC/C.a, Cryo-EM density from the unphosphorylated apo-APC/C map corresponding to the C-box binding site of APC8B with the fitted CDH1 C-box modelled from APC/C CDH1:Hsl1 .We observe no density at the APC8B C-box binding site.b, Cryo-EM map density for the CDH1 C-box from the APC/C CDH1:Hsl1 reconstruction with fitted coordinates.c-e, Comparison of unphosphorylated apo-APC/C and phosphorylated apo-APC/C: (c) Unphosphorylated apo-APC/C.(d) Phosphorylated apo-APC/C.(e) Superimposition of unphosphorylated apo-APC/C (coloured by subunit assignment) and phosphorylated apo-APC/C (grey).There is a small relative shift of the TPR lobe.The coordinates were superimposed on APC1 PC .