A molecular switch in SecA protein couples ATP hydrolysis to protein translocation

SecA, the dimeric ATPase subunit of bacterial protein translocase, catalyse s translocation during ATP-driven membrane cycling at SecYEG. We now show t hat the SecA protomer comprises two structural modules: the ATPase N-domain , containing the nucleotide binding sites NBD1 and NBD2, and the regulatory C-domain. The C-domain binds to the N-domain in each protomer and to the C -domain of another protomer to form SecA dimers. NBD1 is sufficient for sin gle rounds of SecA ATP hydrolysis. Multiple ATP turnovers at NBD1 require b oth the NBD2 site acting in cis and a conserved C-domain sequence operating in trans. This intramolecular regulator of ATP hydrolysis (IRA) mediates N -/C-domain binding and acts as a molecular switch: it suppresses ATP hydrol ysis in cytoplasmic SecA while it releases hydrolysis in SecY-bound SecA du ring translocation. We propose that the IRA switch couples ATP binding and hydrolysis to SecA membrane insertion/deinsertion and substrate translocati on by controlling nucleotide-regulated relative motions between the N-domai n and the C-domain. The IRA switch is a novel essential component of the pr otein translocation catalytic pathway.