Characterization of a Bacillus subtilis SecA mutant protein deficient in translocation ATPase and release from the membrane

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Abstract
SecA is the precursor protein binding subunit of the bacterial precursor protein translocase, which consists of the SecY/E protein as integral membrane domain. SecA is an ATPase, and couples the hydrolysis of ATP to the release of bound precursor proteins to allow their proton‐motive‐force‐driven translocation across the cytoplasmic membrane. A putative ATP‐binding motif can be predicted from the amino acid sequence of SecA with homology to the consensus Walker A‐type motif. The role of this domain is not known. A lysine residue at position 106 at the end of the glycine‐rich loop in the A motif of the Bacillus subtilis SecA was replaced by an asparagine through site‐directed mutagenesis (K106N SecA). A similar replacement was introduced at an adjacent lysine residue at position 101 (K101N SecA). Wild‐type and mutant SecA proteins were expressed to a high level and purified to homogeneity. The catalytic efficacy (kcat/km) of the K106N SecA for lipid‐stimulated ATP hydrolysis was only 1% of that of the wild‐type and K101N SecA. K106N SecA retained the ability to bind ATP, but its ATPase activity was not stimulated by precursor proteins. Mutant and wild‐type SecA bind with similar affinity to Escherichia coli inner membrane vesicles and insert into a phospholipid mono‐layer, in contrast to the wild type, membrane insertion of the K106N SecA was not prevented by ATP. K106N SecA blocks the ATP and proton‐motive‐force‐dependent chase of a translocation intermediate to fully translocated proOmpA. It is concluded that the GKT motif in the amino‐terminal domain of SecA is part of the catalytic ATP‐binding site. This site may be involved in the ATP‐driven protein recycling function of SecA which allows the release of SecA from its association with precursor proteins, and the phospholipid bilayer.