The second messenger binding site of inositol 1,4,5-trisphosphate 3-kinaseis centered in the catalytic domain and related to the inositol trisphosphate receptor site

A segment of inositol 1,4,5 trisphosphate 3-kinase responsible for inositol 1,4,5-trisphosphate (InsP(3)) binding was characterized and confirmed by t hree different approaches employing the fully active expressed catalytic do main of the enzyme. Part of this moiety was protected from limited tryptic proteolysis by InsP(3). Sequencing of two fragments of 16 and 21 kDa, gener ated in the absence or presence of InsP(3), respectively, identified segmen t Glu-271 to Arg-305 as being protected. 15 monoclonal antibodies, all bind ing to epitopes within this region, inhibited enzyme activity and interfere d with inositol phosphate binding. Detailed enzyme kinetic parameters of 32 site-directed mutants revealed residues Arg-276 and Lys-303 in this segmen t and Arg-322, located nearby, as directly involved in and five other close ly neighbored residues, all located within a segment of 73 amino acids, as also influencing InsP(3) binding. Part of this region is similar in sequenc e to an InsP(3) binding segment in InsP(3) receptors, Combined with the fin ding that mutants influencing only ATP binding all lie outside this region, these data indicate that an InsP(3) binding core domain is inserted betwee n two segments acting together in ATP binding and phosphate transfer.