SecA and FtsY compete for access to the SecYEG translocon. (A) SecYEG(pBpa)-PL (10 nM SecYEG final concentration) were incubated with 1.0 µM SecA and UV exposed, when indicated. After Na₂CO₃ extraction the sample was separated by SDS-PAGE and, after Western transfer, decorated with polyclonal α-SecA antibodies. Indicated are SecA and the 170 kD SecY–SecA cross-link product. (B) SecYEG(pBpa)-PL were incubated with FtsY or SecA or with both proteins and UV exposed. When both FtsY and SecA were present, FtsY was added before SecA. After SDS-PAGE and Western transfer, the membrane was decorated with polyclonal α-FtsY antibodies (top). After stripping of the membrane by treatment with SDS/DTT, the membrane was decorated with polyclonal α-SecA antibodies (bottom). (C) As in B, but when both FtsY and SecA were present, SecA was added before FtsY. The blot was decorated with polyclonal α-SecA antibodies (top) or polyclonal α-FtsY antibodies (bottom).