Abstract
The structure of the first protein-conducting channel was determined more than a decade ago. Today, we are still puzzled by the outstanding problem of protein translocation—the dynamic mechanism underlying the consignment of proteins across and into membranes. This review is an attempt to summarize and understand the energy transducing capabilities of protein-translocating machines, with emphasis on bacterial systems: how polypeptides make headway against the lipid bilayer and how the process is coupled to the free energy associated with ATP hydrolysis and the transmembrane protein motive force. In order to explore how cargo is driven across the membrane, the known structures of the protein-translocation machines are set out against the background of the historic literature, and in the light of experiments conducted in their wake. The paper will focus on the bacterial general secretory (Sec) pathway (SecY-complex), and its eukaryotic counterpart (Sec61-complex), which ferry proteins across the membrane in an unfolded state, as well as the unrelated Tat system that assembles bespoke channels for the export of folded proteins.
Highlights
Energy conservation through chemiosmosis and oxidative phosphorylation provides the means to control the chemistry and dynamics of the cell
The central motif by which substrates are recognized is a stretch of hydrophobic amino acids preceded by one or more positively charged residues [10]: for nascent membrane proteins, the hydrophobic region is usually the first transmembrane helix (TMH)—the signal anchor (SA), while secreted substrates possess an N-terminal signal sequence (SS) which is cleaved by signal peptidase following translocation
Based on the biochemical cross-linking sites [44], we have modelled and energy minimized a probable course for the peptide through SecA
Summary
Energy conservation through chemiosmosis and oxidative phosphorylation provides the means to control the chemistry and dynamics of the cell. Protein secretion and membrane protein insertion occurs via the general protein secretion, or Sec, machinery. This is the case at the plasma membranes of bacteria and archaea, the chloroplast thylakoid membrane and the endoplasmic reticulum (ER) of eukaryotes. The post-translational pathway, is used mainly by prokaryotes for protein secretion [7], an increasing number of eukaryotic proteins have been shown to follow this pathway [8,9] Both types of secretion are initiated when the N-terminal region of a preprotein is targeted to the SecY/Sec complex at the plasma ( prokaryotes) or. B 370: 20150025 post-translational translocation (eukaryotes) post-translational translocation (bacteria) outside membrane inside
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