Part of the Sec protein translocase complex. Interacts with the SecYEG preprotein conducting channel. Has a central role in coupling the hydrolysis of ATP to the transfer of proteins into and across the cell membrane, serving as an ATP-driven molecular motor driving the stepwise translocation of polypeptide chains across the membrane. ATP + H2O + cellular proteinSide 1 = ADP + phosphate + cellular proteinSide 2. Monomer and homodimer. Part of the essential Sec protein translocation apparatus which comprises SecA, SecYEG and auxiliary proteins SecDF. Other proteins may also be involved. Distribution is 50-50. No mRNA is detectable in stationary phase cells; the protein is subject to protein degradation in stationary phase. Does not complement E.coli temperature-sensitive secA mutants. Belongs to the SecA family.