SECA MEMBRANE CYCLING AT SECYEG IS DRIVEN BY DISTINCT ATP BINDING ANDHYDROLYSIS EVENTS AND IS REGULATED BY SECD AND SECF

The SecA subunit of E. coli preprotein translocase promotes protein se cretion during cycles of membrane insertion and deinsertion at SecYEG. This process is regulated both by nucleotide binding and hydrolysis a nd by the SecD and SecF proteins. In the presence of associated prepro tein, the energy of ATP binding at nucleotide-binding domain 1 (NBD1) drives membrane insertion of a 30 kDa domain of SecA, while deinsertio n of SecA requires the hydrolysis of this ATP. SecD and SecF stabilize the inserted state of SecA. ATP binding at NBD2, though needed for pr eprotein translocation, is not needed for SecA insertion or deinsertio n.