Inter-membrane association of the Sec and BAM translocons for bacterial outer-membrane biogenesis

Abstract

The outer-membrane of Gram-negative bacteria is critical for surface adhesion, pathogenicity, antibiotic resistance and survival. The major constituent – hydrophobic b-barrel Outer-Membrane Proteins (OMPs) – are first secreted across the inner-membrane through the Sec-translocon for delivery to periplasmic chaperones e.g. SurA, which prevent aggregation. OMPs are then offloaded to the b-Barrel Assembly Machinery (BAM) in the outer-membrane for insertion and folding. We show the Holo-TransLocon (HTL) – an assembly of the protein-channel core-complex SecYEG, the ancillary sub-complex SecDF, and the membrane 'insertase' YidC – contacts BAM through periplasmic domains of SecDF and YidC, ensuring efficient OMP maturation. Furthermore, the proton-motive-force (PMF) across the inner-membrane acts at distinct stages of protein secretion: (1) SecA-driven translocation through SecYEG; and (2) communication of conformational changes via SecDF across the periplasm to BAM. The latter presumably drives efficient passage of OMPs. These interactions provide insights of inter-membrane organisation and communication, the importance of which is becoming increasingly apparent.

The Dynamic SecYEG Translocon

“…The best-studied example is of course the SecYEG-YidC interaction ( Scotti et al, 2000 ), where YidC likely helps substrates to exit the SecY channel ( Beck et al, 2001 ; Houben et al, 2002 ), although YidC can also act as SecYEG-independent insertase ( Samuelson et al, 2000 ; Serek et al, 2004 ). But there are more examples, like the SecYEG-Tat interaction ( Keller et al, 2012 ) or the SecYEG-Bam interaction ( Alvira et al, 2020 ), and the collaboration between different transport systems needs to be further explored. Finally, it is largely unknown how the protein transport machinery responds to environmental changes or to stress conditions.…”

“…The SecDF complex likely stabilizes the SecYEG-YidC interaction ( Nouwen and Driessen, 2002 ; Tsukazaki, 2018 ), although the SecYEG-YidC interaction is also observed in the absence of the SecDFYajC complex ( Boy and Koch, 2009 ; Sachelaru et al, 2015 ). Finally, SecDF might also play a role in efficient maturation and folding of OMPs ( Alvira et al, 2020 ) and it was proposed that SecDF is part of an inter-membrane trafficking machinery that connects transport processes across the inner membrane with those at the outer membrane ( Alvira et al, 2020 ) (see below).…”

“…This was recently verified by showing the interaction of the HTL with the BAM complex. This transient contact was shown to be conferred by the periplasmic loops of SecDF, YidC, and the BAM complex ( Alvira et al, 2020 ). The periplasmic domain of SecD has multiple contact sites with BamBCD, while YidC interacts with BamABCD.…”

“…The interaction between the HTL and BAM could facilitate the energetic coupling of inner membrane with outer membrane transport. Once OMP precursors are translocated across the SecYEG complex and the signal sequence is cleaved, the mature but yet unfolded protein is bound by periplasmic chaperons, such as PpiD ( Antonoaea et al, 2008 ) and is then recognized by the BAM complex, forming a trans-periplasmic supercomplex with SecDF as potential energy supplier ( Carlson et al, 2019 ; Alvira et al, 2020 ).…”

“…For being active in protein transport, the SecYEG translocon depends on the coordinated interaction with multiple partner proteins that select potential SecYEG substrates ( Lill et al, 1990 ; van der Does et al, 1996 ; Angelini et al, 2005 ), provide the driving force for protein transport ( Tsukazaki et al, 2011 ; Knyazev et al, 2018 ), coordinate substrate release from the SecYEG channel ( Beck et al, 2001 ; Houben et al, 2004 ; Sachelaru et al, 2017 ) and communicate with components of the proteostasis network ( Kihara et al, 1996 ; Schäfer et al, 1999 ; Jauss et al, 2019 ). The SecYEG translocon also cooperates with additional protein transport systems ( Figure 1 ), like the YidC insertase ( Scotti et al, 2000 ; Sachelaru et al, 2015 , 2017 ; Dalbey et al, 2017 ; Petriman et al, 2018 ), the Tat transport machinery ( Keller et al, 2012 ; Kudva et al, 2013 ; Tooke et al, 2017 ) and the Bam complex ( Wang et al, 2016 ; Alvira et al, 2020 ), which inserts β-barrel proteins into the outer membrane. Additional partner proteins of the SecYEG translocon have been recently identified by proteomic approaches ( Chorev et al, 2018 ; Carlson et al, 2019 ; Jauss et al, 2019 ), further highlighting the dynamic nature of the SecYEG translocon, which is probably the basis for its ability to transport a large variety of highly different substrates.…”