YidC, the Escherichia coli homologue of mitochondrial Oxa1p, is a component of the Sec translocase

In Escherichia coli, both secretory and inner membrane proteins initially a re targeted to the core SecYEG inner membrane translocase, Previous work ha s also identified the peripherally associated SecA protein as well as the S ecD, SecF and YajC inner membrane proteins as components of the translocase . Here, we use a cross-linking approach to show that hydrophilic portions o f a co-translationally targeted inner membrane protein (FtsQ) are close to SecA and SecY, suggesting that insertion takes place at the SecA/Y interfac e. The hydrophobic FtsQ signal anchor sequence contacts both lipids and a n ovel 60 kDa translocase-associated component that we identify as YidC, YidC is homologous to Saccharomyces cerevisiae Oxa1p, which has been shown to f unction in a novel export pathway at the mitochondrial inner membrane. We p ropose that YidC is involved in the insertion of hydrophobic sequences into the lipid bilayer after initial recognition by the SecAYEG translocase.