Nucleotide and phospholipid-dependent control of PPXD and C-domain association for SecA ATPase.

The SecA ATPase motor is a central component of the eubacterial protein translocation machinery. It is comprised of N- and C-domain substructures, where the N-domain is comprised of two nucleotide-binding domains that flank a preprotein-binding domain (PPXD), while the C-domain binds phospholipids as well as SecB chaperone. Our recent crystal structure of Bacillus subtilis SecA protomer [Hunt, J. F., Weinkauf, S., Henry, L., Fak, J. J., McNicholas, P., Oliver, D. B., and Deisenhofer, J. (2002) Science 297, 2018-2026] along with experimental support for the correct dimer structure [Ding, H., Hunt, J. F., Mukerji, I., and Oliver, D. (2003) Biochemistry 42, 8729-8738] have now allowed us to study SecA structural dynamics during interaction with various translocation ligands and to relate these findings to current models of SecA-dependent protein translocation. In this paper, we utilized fluorescence resonance energy transfer methodology with genetically engineered SecA proteins containing unique pairs of tryptophan and fluorophore-labeled cysteine residues within the PPXD and C-domains of SecA to investigate the interaction of these two domains and their response to temperature, model membranes, and nucleotide. Consistent with the crystal structure of SecA, we found that the PPXD and C-domains are proximal to one another in the ground state. Increasing temperature or binding to model membranes promoted a loosening of PPXD and C-domain association, while ADP binding promoted a tighter association. A similar pattern of PPXD and C-domain association was obtained also for Escherichia coli SecA protein. Furthermore, a hyperactive Azi-PrlD SecA protein of E. coli had increased PPXD and C-domain separation, consistent with its activation in the ground state. Interestingly, PPXD and C-domain separation occurred prior to the onset of major temperature-induced conformational changes in both the PPXD and C-domains of SecA. Our results support a model in which PPXD and C-domain proximity is important for regulating the initial stages of SecA activation, and they serve also as a template for future structural studies aimed at elucidation of the chemomechanical cycle of SecA-dependent protein translocation.