LYS-197 AND ASP-414 ARE CRITICAL RESIDUES FOR BINDING OF ATP MG2+ BY RAT-BRAIN INOSITOL 1,4,5-TRISPHOSPHATE 3-KINASE/

Rat brain inositol 1,4,5-trisphosphate (InsP3) 3-kinase A was expresse d in Escherichia coli in order to identify the amino acid residues inv olved in substrate ATP/Mg2+ binding. Two amino acid regions that are c onserved in the catalytic domain of InsP3 3-kinase isoenzymes A and B had characteristics consistent with two ATP/Mg2+-binding motives. Site -directed mutagenesis was performed on residues Lys-197, Lys-207 and A sp-414 to generate three mutant enzymes, referred to as C5 K197I, C5 K 207I and C5 D414N. Comparison of the wild-type and mutant proteins wit h regard to enzymic activity revealed that C5 K197I exhibited 10% of c ontrol enzyme activity, C5 D414N was totally inactive and C5 K207I was fully active. The reduced levels of enzyme activity for C5 K197I and C5 D414N were correlated with an altered ability of the mutant enzymes to bind ATP/Mg2+, as determined by ATP-agarose affinity chromatograph y. Neither Ca2+/calmodulin binding nor InsP3 binding appeared to be af fected. Mutant C5 K207I showed the same characteristics as the wild-ty pe enzyme. Taken together, these results strongly indicated (i) that a mino acid residues Lys-197 and Asp-414 are necessary for InsP3 3-kinas e activity and form part of the ATP/Mg2+-binding domain, and (ii) that amino acid residues Lys-197, Lys-207 and Asp-414 are not involved in either InsP3 binding or enzyme stimulation by Ca2+/calmodulin.