title: Identification of the magnesium-binding domain of the high-affinity ATP-binding site of the Bacillus subtilis and Escherichia coli SecA protein.
abstract: The homodimeric SecA protein is the peripheral subunit of the translocase, and couples the hydrolysis of ATP to the translocation of precursor proteins across the bacterial cytoplasmic membrane. The high affinity ATP binding activity of SecA resides in the amino-terminal domain of SecA. This domain contains a tandem repeat of the "so-called" Walker B-motif, hXhhD (Walker, J.E., Saraste, M., Runswick, M.J., and Gay, N.J. (1982) EMBO J. 1, 945-951), that in combination with motif A is responsible for the Mg(2+)-phosphate protein interaction. Two aspartate residues at positions 207 and 215 of the Bacillus subtilis SecA, and Asp-217 in the Escherichia coli SecA, that could be Mg2+ ion ligands, were individually mutated to an asparagine. Mutant SecA proteins were unable to growth-complement an E. coli secA amber mutant strain, and the E. coli SecA mutant interfered with the translocation of precursor proteins in vivo. B. subtilis mutant SecA proteins were expressed to a high level and purified to homogeneity. The high affinity ATP and Mg(2+)-ion binding activity was reduced in the Asp-207 mutant, and completely lost in the Asp-215 mutant. Both SecA proteins were defective in lipid-stimulated ATPase activity. Proteolytic studies suggest that the two subunits of the mutated dimeric SecA proteins are present in different conformational states. These data suggest that Asp-207 and Asp-215 are involved in the binding of the Mg(2+)-ion when Mg(2+)-ATP is bound to SecA, while Asp-207 fulfills an additional catalytic role, possibly in accepting a proton during catalysis.
authors.affiliation: Department of Microbiolgy, University of Groningen, Haren, The Netherlands.
authors.forename: J P
authors.initials: JP
authors.lastname: van der Wolk
authors.forename: M
authors.initials: M
authors.lastname: Klose
authors.forename: J G
authors.initials: JG
authors.lastname: de Wit
authors.forename: T
authors.initials: T
authors.lastname: den Blaauwen
authors.forename: R
authors.initials: R
authors.lastname: Freudl
authors.forename: A J
authors.initials: AJ
authors.lastname: Driessen
mesh_terms: D000251:Adenosine Triphosphatases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism*; D000595:Amino Acid Sequence; D001412:Bacillus subtilis / Q000378:metabolism*; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D001483:Base Sequence; D001665:Binding Sites; D004926:Escherichia coli / Q000378:metabolism*; D029968:Escherichia coli Proteins; D008274:Magnesium / Q000378:metabolism*; D026901:Membrane Transport Proteins; D008969:Molecular Sequence Data; D009154:Mutation; D011487:Protein Conformation; D000069816:SEC Translocation Channels; D000081416:SecA Proteins; D013329:Structure-Activity Relationship
chemical_list: D001426:Bacterial Proteins; D029968:Escherichia coli Proteins; D026901:Membrane Transport Proteins; D000069816:SEC Translocation Channels; D000255:Adenosine Triphosphate; D000251:Adenosine Triphosphatases; D000081416:SecA Proteins; D008274:Magnesium