| Abstract |
More than 30% of the bacterial proteome functions in the cell periphery or outside the cell it is produced. For that reason an array of sixteen systems that transport membrane and secretory proteins has been produced during evolution. Of these, only the Sec secretion pathway is ubiquitous and essential for life.
The majority of the secretory proteins destined for the periplasm or the outer bacterial membrane, by-pass the inner membrane through the Sec translocase. Translocase holoenzyme consists of a SecYEG preprotein conducting channel and an ATPase, SecA. The preprotein conducting channel transports the polypeptides to the periplasm or place them into the lipid bilayer. The ATPase pushes processively the polypeptide chains into the channel through multiple rounds of ATP hydrolysis. In this study we have focused on the way that preproteins are targeted to the Sec-translocase and eventually get secreted.
We have developed the proper substrates and established that secretory preproteins are targeted through SecA to the SecYEG conducting channel. The mature domain is recognized by SecA and is responsible for targeting secretory preproteins only if present in a “non-native” state. This way of targeting is independent of signal peptides and SecB chaperone and probably generic in all bacteria.
After the mature domain gets targeted to the translocase holoenzyme, the signal peptide “catalyzes” its translocation through two ordered events. Firstly the signal peptide lowers translocase activation energy by loosening interactions between the two sub-domains of the “DEAD” motor. After that, the physical presence of the signal peptide in its SecA groove causes detachment between the two sub-domains of the ATPase motor inducing multiple rounds of ATP hydrolysis and cycles of SecA membrane insertion and de-insertion, events that cause translocation of the polypeptide chain to the periplasm.
The results of this study clarify some key points of the molecular mechanism of the Sec secretion system and encourage a radical revision of the current dogma on bacterial preprotein recognition and the role of the signal peptides in preprotein translocation.
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Μοριακή ανάλυση της στόχευσης και μεταφοράς προπρωτεινών από την Sec Μεταθετάση
