Distinct Requirements for Tail-Anchored Membrane Protein Biogenesis in Escherichia coli

Tail-anchored membrane proteins (TAMPs) are a distinct subset of inner membrane proteins (IMPs) characterized by a single C-terminal transmembrane domain (TMD) that is responsible for both targeting and anchoring. Little is known about the routing of TAMPs in bacteria. Here, we have investigated the role of TMD hydrophobicity in tail-anchor function in Escherichia coli and its influence on the choice of targeting/insertion pathway. We created a set of synthetic, fluorescent TAMPs that vary in the hydrophobicity of their TMDs and corresponding control polypeptides that are extended at their C terminus to create regular type II IMPs. Surprisingly, we observed that TAMPs have a much lower TMD hydrophobicity threshold for efficient targeting and membrane insertion than their type II counterparts. Using strains conditional for the expression of known membrane-targeting and insertion factors, we show that TAMPs with strongly hydrophobic TMDs require the signal recognition particle (SRP) for targeting. Neither the SecYEG translocon nor YidC appears to be essential for the membrane insertion of any of the TAMPs studied. In contrast, corresponding type II IMPs with a TMD of sufficient hydrophobicity to promote membrane insertion followed an SRP- and SecYEG translocon-dependent pathway. Together, these data indicate that the capacity of a TMD to promote the biogenesis of E. coli IMPs is strongly dependent upon the polypeptide context in which it is presented.