Binding of the motor protein SecA to the bacterial protein translocation channel SecYEG in the absence of ATP

Abstract

Translocation of many secretory proteins through the bacterial plasma membrane is facilitated by a complex of the protein translocase SecYEG with the motor protein SecA. The ATP-free complex is unstable in detergent, raising the question how SecA may perform several rounds of ATP hydrolysis without being released. Here we show that dual recognition of (i) SecYEG and (ii) acidic lipids in its immediate vicinity confers an apparent nanomolar affinity. As a result the complex between SecA and reconstituted SecYEG may be stable for tens of seconds as visualized by high-speed atomic force microscopy. These long lasting events and complementary shorter ones both give rise to single ion channel openings of equal duration. The existence of two different conformations of the complex was confirmed by luminescence resonance energy transfer. The deep protrusion of SecA’s two-helix finger into the aqueous channel is in line with the power stroke mechanism of translocation.