No abstract

SignificanceMost proteins need to fold into a specific 3D structure to function. The mechanism by which isolated proteins fold has been thoroughly studied by experiment and theory. However, in the cell proteins do not fold in isolation but are synthesized as linear chains by the ribosome during translation. It is therefore natural to ask at which point during synthesis proteins fold, and whether this differs from the folding of isolated protein molecules. By studying folding of a well-characterized protein domain, titin I27, stalled at different points during translation, we show that it already folds in the mouth of the ribosome exit tunnel and that the mechanism is almost identical to that of the isolated protein.

Protein transport via the Sec translocon represents an evolutionary conserved mechanism for delivering cytosolically-synthesized proteins to extra-cytosolic compartments. The Sec translocon has a three-subunit core, termed Sec61 in Eukaryotes and SecYEG in Bacteria. It is located in the endoplasmic reticulum of Eukaryotes and in the cytoplasmic membrane of Bacteria where it constitutes a channel that can be activated by multiple partner proteins. These partner proteins determine the mechanism of polypeptide movement across the channel. During SRP-dependent co-translational targeting, the ribosome threads the nascent protein directly into the Sec channel. This pathway is in Bacteria mainly dedicated for membrane proteins but in Eukaryotes also employed by secretory proteins. The alternative pathway, leading to post-translational translocation across the Sec translocon engages an ATP-dependent pushing mechanism by the motor protein SecA in Bacteria and a ratcheting mechanism by the lumenal chaperone BiP in Eukaryotes. Protein transport and biogenesis is also assisted by additional proteins at the lateral gate of SecY/Sec61α and in the lumen of the endoplasmic reticulum or in the periplasm of bacterial cells. The modular assembly enables the Sec complex to transport a vast array of substrates. In this review we summarize recent biochemical and structural information on the prokaryotic and eukaryotic Sec translocons and we describe the remarkably complex interaction network of the Sec complexes.

Background: YidC interacts with SecY during membrane protein insertion but details on this interaction are missing. Results: YidC binds to the lateral gate of SecY and is detached by nascent membrane proteins but not by SecA. Conclusion: Nascent membrane-induced lateral gate movements directly influence the SecY-YidC interaction. Significance: This is the first detailed analysis of the SecY-YidC interaction.

The YidC insertase also integrates multispanning membrane proteins that had been considered to be exclusively SecYEG dependent. Only membrane proteins that require SecA can be inserted only via SecYEG. Targeting to YidC is SRP dependent, and the C-terminus of YidC cross-links to SRP, FtsY, and ribosomal subunits.