The Sec-independent function of Escherichia coli YidC is evolutionary-conserved and essential

YidC plays a role in the integration and assembly of many (if not all) Escherichia coli inner membrane proteins. Strikingly, YidC operates in two distinct pathways: one associated with the Sec translocon that also mediates protein translocation across the inner membrane and one independent from the Sec translocon. YidC is homologous to Alb3 and Oxa1 that function in the integration of proteins into the thylakoid membrane of chloroplasts and inner membrane of mitochondria, respectively. Here, we have expressed the conserved region of yeast Oxa1 in a conditional E. coli yidC mutant. We find that Oxa1 restores growth upon depletion of YidC. Data obtained from in vivo protease protection assays and in vitro cross-linking and folding assays suggest that Oxa1 complements the insertion of Sec-independent proteins but is unable to take over the Sec-associated function of YidC. Together, our data indicate that the Sec-independent function of YidC is conserved and essential for cell growth.