RT Journal Article
SR Electronic
T1 Signal peptide hydrophobicity modulates interaction with the twin-arginine translocase
JF bioRxiv
FD Cold Spring Harbor Laboratory
SP 135103
DO 10.1101/135103
A1 Qi Huang
A1 Tracy Palmer
YR 2017
UL http://biorxiv.org/content/early/2017/05/07/135103.abstract
AB The general secretory pathway (Sec) and twin-arginine translocase (Tat) operate in parallel to export proteins across the cytoplasmic membrane of prokaryotes and the thylakoid membrane of plant chloroplasts. Substrates are targeted to their respective machineries by N-terminal signal peptides that share a common tripartite organization, however Tat signal peptides harbor a conserved and almost invariant arginine pair that are critical for efficient targeting to the Tat machinery. Tat signal peptides interact with a membrane-bound receptor complex comprised of TatB and TatC components, with TatC containing the twin-arginine recognition site. Here we isolated suppressors in the signal peptide of the Tat substrate, SufI, that restored Tat transport in the presence of inactivating substitutions in the TatC twin-arginine binding site. These suppressors increased signal peptide hydrophobicity, and co-purification experiments indicated that they restored binding to the variant TatBC complex. The hydrophobic suppressors could also act in cis to suppress substitutions at the signal peptide twin-arginine motif that normally prevent targeting to the Tat pathway. Highly hydrophobic variants of the SufI signal peptide containing four leucine substitutions retained the ability to interact with the Tat system. The hydrophobic signal peptides of two Sec substrates, DsbA and OmpA, containing twin lysine residues, were shown to mediate export by the Tat pathway and to co-purify with TatBC. These findings indicate that there is unprecedented overlap between Sec and Tat signal peptides and that neither the signal peptide twin-arginine motif nor the TatC twin-arginine recognition site are essential mechanistic features for operation of the Tat pathway.Importance Protein export is an essential process in all prokaryotes, The Sec and Tat export pathways operate in parallel, with the Sec machinery transporting unstructured precursors and the Tat pathway transporting folded proteins. Proteins are targeted to the Tat pathway by N-terminal signal peptides that contain an almost invariant twin-arginine motif. Here we make the surprising discovery that the twin-arginines are not essential for recognition of substrates by the Tat machinery, and that this requirement can be bypassed by increasing the signal peptide hydrophobicity. We further show that signal peptides of bona fide Sec substrates can also mediate transport by the Tat pathway. Our findings suggest that key features of the Tat targeting mechanism have evolved to prevent mis-targeting of substrates to the Sec pathway rather than being a critical requirement for function of the Tat pathway.