Membrane Protein Insertion in Bacteria from a Structural Perspective

Abstract

M embrane proteins are inserted into the lipid bilayer in bacteria by two pathways. The Sec machinery is responsible for the insertion of the majority of the membrane proteins after targeting by the SRP/FtsY components. However, there is also a class of membrane proteins that insert independent of the Sec machinery. These proteins require a novel protein called YidC. Recently, the structural details of the Sec machinery have come to light via X-crystallographic analysis. There are now structures of the membrane-embedded Sec protein-conducting channel, the SecA ATPase motor, and the targeting components. The structures give clues to how a polypeptide is translocated across the membrane and how the transmembrane segments of a membrane protein are released from the Sec complex. Additionally, the structure of the targeting components sheds light on how the membrane substrates are selected for transport and delivered to the membrane.

Folding and self-association of atTic20 in lipid membranes: implications for understanding protein transport across the inner envelope membrane of chloroplasts

“…Although we did not attempt to characterize the nature of the targeting, it is known that the signal recognition particle (SRP) targets multi-spanning transmembrane (TM) domain proteins to the membrane in E. coli. SRP is able to recognize either cleavable N-terminal signal sequences or internal TM domains (signal anchor sequences) for targeting proteins to bacterial membranes, and insertion can occur via the SecYEG or YidC translocon [ 38 , 39 ]. Furthermore, SRP is known to be recruited by the presence of tryptophan-enriched transmembrane helices, such as is found in the first predicted TM domain of atTic20 [ 38 ].…”

“…SRP is able to recognize either cleavable N-terminal signal sequences or internal TM domains (signal anchor sequences) for targeting proteins to bacterial membranes, and insertion can occur via the SecYEG or YidC translocon [ 38 , 39 ]. Furthermore, SRP is known to be recruited by the presence of tryptophan-enriched transmembrane helices, such as is found in the first predicted TM domain of atTic20 [ 38 ]. It therefore seems likely that atTic20 was targeted to the bacterial membrane in an SRP-dependent manner, with the first TM domain serving as a signal anchor sequence.…”