YidC mediates membrane protein insertion in bacteria

The basic machinery for the translocation of proteins into or across membra nes is remarkably conserved from Escherichia coli to humans. In eukaryotes, proteins are inserted into the endoplasmic reticulum using the signal reco gnition particle (SRP) and the SRP receptor, as well as the integral membra ne Sec61 trimeric complex (composed of alpha, beta and gamma subunits)(1). In bacteria, most proteins are inserted by a related pathway that includes the SRP homologue Ffh(2-5), the SRP receptor FtsY(6,7), and the SecYEG trim eric complex(8), where Y and E are related to the Sec61 alpha and gamma sub units, respectively. Proteins in bacteria that exhibit no dependence on the Sec translocase were previously thought to insert into the membrane direct ly without the aid of a protein machinery(9,10). Here we show that membrane insertion of two Sec-independent proteins requires YidC. YidC is essential for E. coli viability and homologues are present in mitochondria and chlor oplasts. Depletion of YidC also interferes with insertion of Sec-dependent membrane proteins, but it has only a minor effect on the export of secretor y proteins. These results provide evidence for an additional component of t he translocation machinery that is specialized for the integration of membr ane proteins.