campus seminar: Multiscale dynamics of the bacterial translocon

During membrane protein synthesis, transmembrane segments (TMs) of nascent proteins emerge from the ribosome and insert into the bacterial plasma membrane at the SecYEG translocon. TMs enter the pore of the translocon and then access the lipid bilayer by passing through a lateral gate formed by two alpha helices of SecY. We have applied single-molecule fluorescence resonance energy transfer (smFRET) to monitor the dynamics of lateral gate opening and closing during membrane protein insertion. By using two complementary smFRET techniques (TIRF and PIE FRET), we have measured the whole range of translocon dynamics from the sub-millisecond to the second range, and found that the lateral gate is highly dynamic and permits lipid insertion of TMs as soon as they have emerged from the ribosome. In addition, we have found that the membrane-protein chaperone YidC strong stabilizes the open conformation of the lateral gate during TM insertion and may facilitate folding of polytopic membrane proteins in the lipid bilayer. These results reveal how rapid fluctuations in the lateral gate of the translocon contribute to the biogenesis of membrane proteins.