Crystal structure of YidC in complex with YibN.

YidC is a widely conserved membrane protein that facilitates membrane protein insertion and folding. Escherichia coli YidC contains a conserved core region with five transmembrane helices and an additional N-terminal transmembrane helix, TM1. TM1 has not been resolved in previously reported E. coli YidC structures, leaving its position and structural role unclear. YibN is a single-pass inner membrane protein known to interact with YidC, but the structural basis of their association remains unknown. Here, we determined the crystal structure of the E. coli YidC-YibN complex. The structure reveals that TM1 is positioned away from the YidC core and forms the major intramembrane interface with the N-terminal amphipathic helix and transmembrane helix of YibN. YibN also makes additional contacts with the YidC core. Comparison with previously reported YidC structures further shows that the paddle domain is positioned closer to TM5 while retaining its overall fold. These findings define a previously unresolved role of YidC TM1 in YibN association and suggest that YibN binding may be coupled to repositioning of the YidC paddle domain.