Identification of residues essential for catalysis and binding of calmodulin in rat brain inositol 1,4,5-trisphosphate 3-kinase.

In order to identify the amino acid residues involved in calmodulin (CaM) binding and catalytic activity, rat brain inositol 1,4,5-trisphosphate (InsP3) 3-kinase was expressed in Escherichia coli as a beta-galactosidase fusion protein [clone C5; Takazawa, Vandekerckhove, Dumont & Erneux (1990) Biochem. J. 272, 107-112]. Three deletion mutants in the plasmid of clone C5 were generated using convenient restriction enzymes. The results show that the removal of 34 amino acids from the C-terminal end of InsP3 3-kinase resulted in an inactive protein which still interacted with CaM-Sepharose in a Ca2(+)-dependent way. The catalytic domain is thus located at the C-terminal end of the protein. A series of 5' deletion mutants was prepared and used to produce proteins with the same C-terminal end but shortened N-termini, varying in length by over 80 amino acids. Assay of InsP3 3-kinase activity in bacterial extracts indicated that a maximum of 275 amino acids in the C-terminal region may be sufficient for the construction of a catalytically active domain. Affinity chromatography on CaM-Sepharose of 5' and 3' deletion mutants revealed that the sequence stretching from Ser-156 to Leu-189 is involved in CaM binding and enzyme stimulation.