YidC, an assembly site for polytopic Escherichia coli membrane proteins located in immediate proximity to the SecYE translocon and lipids

Like its mitochondrial homolog Oxa1p, the inner membrane protein YidC of Es cherichia coli is involved in the integration of membrane proteins. We have analyzed individual insertion steps of the polytopic E coli membrane prote in MtIA targeted as ribosome-nascent chain complexes to inner membrane vesi cles. YidC can accommodate at least the first two transmembrane segments of MtIA at the protein lipid interface and retain them even though the length of the nascent chain would amply allow insertion into membrane lipids. An even longer insertion intermediate of MtIA is described that still has the first transmembrane helix bound to YidC while the third contacts SecE and Y idC during integration. Our findings suggest that YidC forms a contiguous i ntegration unit with the SecYE translocon and functions as an assembly site for polytopic membrane proteins mediating the formation of helix bundles p rior to their release into the membrane lipids.