Assembly of bacterial inner membrane proteins.

Numerous membrane proteins form multisubunit protein complexes, which contain both integral and peripheral subunits, in addition to prosthetic groups. Bacterial membrane proteins are inserted into the inner membrane by the Sec translocase and YidC insertase. Their folding can be facilitated by YidC and the phospholipid phosphatidylethanolamine (PE). Glycine zippers and other motifs promote transmembrane-transmembrane (TM-TM) helix interactions that may lead to the formation of α-helical bundles of membrane proteins. During or after membrane insertion, the subunits of oligomeric membrane proteins must find each other to build the homo-oligomeric and the hetero-oligomeric membrane complexes. Although chaperones may function as assembly factors in the formation of the oligomer, many protein oligomers appear to fold and oligomerize spontaneously. Current studies show that most subunits of hetero-oligomers follow a sequential and ordered pathway to form the membrane protein complex. If the inserted protein is misfolded or the membrane protein is misassembled, quality control mechanisms exist that can degrade the proteins.