Bacterial YidC, an evolutionally conserved membrane protein, functions as a membrane protein chaperone in cooperation with the Sec translocon and as an independent insertase for membrane proteins. In Gram-negative bacteria, the transmembrane and periplasmic regions of YidC interact with the Sec proteins, forming a multi-protein complex for Sec-dependent membrane protein integration. Here, we report the crystal structure of full-length Escherichia coli YidC. The structure reveals that a hydrophilic groove, formed by five transmembrane helices, is a conserved structural feature of YidC, as compared to the previous YidC structure from Bacillus halodurans, which lacks a periplasmic domain. Structural mapping of the substrate- or Sec protein-contact sites suggested the importance of the groove for the YidC functions as a chaperone and an insertase, and provided structural insight into the multi-protein complex.