SecA proteins of Bacillus subtilis and Escherichia coli possess homologous amino-terminal ATP-binding domains regulating integration into the plasma membrane.

The Bacillus subtilis secA homolog, div, was cloned and expressed at a variety of different levels in wild-type and secA mutant strains of Escherichia coli. Analysis of Div function showed that it could not substitute for SecA despite being present at a wide range of concentrations at or above the physiological level. Location of regions of functional similarity between the two proteins using div-secA chimeras revealed that only the amino-terminal ATP-binding domain of Div could functionally substitute for the corresponding region of SecA. The role of this domain was revealed by subcellular localization experiments that demonstrated that in both B. subtilis and E. coli Div had cytoplasmic, peripheral, and integral membrane distributions similar to those of its SecA homolog and that an intact ATP-binding domain was essential for regulating integration of this protein into the plasma membrane. These results suggest strongly that the previously observed cycle of membrane binding, insertion, and deinsertion of SecA protein (A. Economou and W. Wickner, Cell 78:835-843, 1994) is common to these two bacteria, and they demonstrate the importance of the conserved ATP-binding domain in promoting this cycle.