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

Joanna Komar,Jelger A Lycklama A Nijeholt,Tim R Dafforn,Christiane Schaffitzel,Ryan J Schulze,Sarah C Lee,Sara Alvira,Remy Martin,Ian Collinson,Imre Berger,Gabriele Deckers-Hebestreit

doi:10.1042/bcj20160545

Abstract

Protein secretion and membrane insertion occur 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 transmembrane 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 supercomplex of SecYEG–SecDF–YajC–YidC — is a bona fide resident of the Escherichia coli inner membrane. Moreover, when compared with 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.

Highlights

The structure of the conserved heterotrimeric Sec complex lends itself to both protein secretion and membrane protein insertion [1]
Immunoprecipitation experiments were conducted in order to confirm the presence of endogenous intact HTL in the inner membranes of the BL21-derived E. coli C43 strain [44]
The molecular mechanism underlying membrane protein insertion, folding and assembly is poorly understood compared with what we know about the transport of proteins across the membrane during secretion [8,53]

Summary

Introduction

The structure of the conserved heterotrimeric Sec complex lends itself to both protein secretion and membrane protein insertion [1]. Minor rearrangements of the complex, such as the signal sequence-induced displacement of TMH (transmembrane helix)-2b, TMH-7 and a central plug (helix 2a) of SecY/Sec61α, initiate the transport of secretory proteins across the membrane [2,3]. The SecY complex (SecYEG) associates with the motor ATPase SecA, which drives posttranslational translocation of pre-proteins across the membrane [6,7,8]. The recently characterised holo-translocon (HTL) super-complex comprises the SecYEG core complex, the accessory subcomplex SecDF–YajC and the so-called membrane protein insertase YidC [9]; in principle, all the components necessary for protein secretion and the insertion of TMHs of translocating membrane proteins

Methods

Results

Conclusion