The Core of the Bacterial Translocase Harbors a Tilted Transmembrane Segment 3 of SecE

The bacterial translocase mediates the translocation and membrane integration of proteins. The integral membrane proteins SecY and SecE are conserved core subunits of the translocase. Previous cysteine-scanning studies showed that the transmembrane segment (TMS) 3 of SecE contacts TMS 2 and 7 of SecY, and TMS 3 of another SecE. We now demonstrate that SecE also contacts TMS 10 of SecY. Combining all available cysteine-scanning mutagenesis data, a three-dimensional model has been built in which the positions of the helices that form the central core of the bacterial translocase are mapped. Remarkably, this model reveals that TMS 3 of SecE is strongly tilted relative to SecY.