RT Journal Article
SR Electronic
T1 Structural basis for the allosteric regulation of the SbtA bicarbonate transporter by the P<sub>II</sub>-like protein, SbtB, from <em>Cyanobium</em> sp. PCC7001
JF bioRxiv
FD Cold Spring Harbor Laboratory
SP 762807
DO 10.1101/762807
A1 Joe A. Kaczmarski
A1 Nan-Sook Hong
A1 Bratati Mukherjee
A1 Laura T. Wey
A1 Loraine Rourke
A1 Britta Förster
A1 Thomas S. Peat
A1 G. Dean Price
A1 Colin J. Jackson
YR 2019
UL http://biorxiv.org/content/early/2019/09/09/762807.abstract
AB Cyanobacteria have evolved a suite of enzymes and inorganic carbon (Ci) transporters that improve photosynthetic performance by increasing the localized concentration of CO2 around the primary CO2-fixating enzyme, Rubisco. This CO2-concentrating mechanism (CCM) is highly regulated, responds to illumination/darkness cycles and allows cyanobacteria to thrive under limiting Ci conditions. While the transcriptional control of CCM activity is well understood, less is known about how regulatory proteins might allosterically regulate Ci transporters in response to changing conditions. Cyanobacterial sodium-dependent bicarbonate transporters (SbtAs) are inhibited by PII-like regulatory proteins (SbtBs), with the inhibitory effect being modulated by adenylnucleotides. Here, we used isothermal titration calorimetry to show that SbtB from Cyanobium sp. PCC7001 (SbtB7001) binds AMP, ADP, cAMP and ATP with micromolar-range affinities. X-ray crystal structures of apo- and nucleotide-bound SbtB7001 revealed that while AMP, ADP and cAMP have little effect on the SbtB7001 structure, binding of ATP stabilizes the otherwise flexible T-loop and that the flexible C-terminal C-loop adopts several distinct conformations. We also show that ATP binding affinity is increased ten-fold in the presence of Ca2+ and we present an X-ray crystal structure of Ca2+ATP:SbtB7001 that shows how this metal ion facilitates additional stabilizing interactions with the apex of the T-loop. We propose that the Ca2+ATP-induced conformational change observed in SbtB7001 is important for allosteric regulation of SbtA activity by SbtB and is consistent with changing adenylnucleotide levels in illumination/darkness cycles.Ciinorganic carbonCCMCO2-concentrating mechanismSbtAsodium-dependent bicarbonate transporter ASbtBsodium-dependent bicarbonate transport protein B