Membrane protein insertion and assembly by the bacterial holo-translocon SecYEG-SecDF-YajC-YidC

Protein secretion and membrane insertion occurs through the ubiquitous Sec machinery. In this system, insertion involves the targeting of translating ribosomes via the signal recognition particle and its cognate receptor to the SecY (bacteria and archaea) / Sec61 (eukaryotes) translocon. A common mechanism then guides nascent trans-membrane helices (TMHs) through the Sec complex, mediated by associated membrane insertion factors. In bacteria, the membrane protein ‘insertase’ YidC ushers TMHs through a lateral gate of SecY to the bilayer. YidC is also thought to incorporate proteins into the membrane independently of SecYEG. Here, we show the bacterial holo-translocon (HTL) – a super-complex of SecYEG-SecDF-YajC-YidC – is a bona fide resident of the E. coli inner membrane. Moreover, when compared to SecYEG and YidC alone, the HTL is more effective at the insertion and assembly of a wide range of membrane protein substrates, including those hitherto thought to require only YidC.