The three isoenzymes of human inositol-1,4,5-trisphosphate 3-kinase show specific intracellular localization but comparable Ca2+ responses on transfection in COS-7 cells

Inositol 1,4,5-trisphosphate [Ins(1,4,5)P₃] 3-kinase catalyses the phosphorylation of InsP₃ to inositol 1,3,4,5-tetrakisphosphate. cDNAs encoding three human isoenzymes of InsP₃ 3-kinase (A, B and C) have been reported previously [Choi, Kim, Lee, Moon, Sim, Kim, Chung and Rhee (1990) Science 248, 64–66; Dewaste, Pouillon, Moreau, Shears, Takazawa and Erneux (2000) Biochem. J. 352, 343–351; Dewaste, Roymans, Moreau and Erneux (2002) Biochem. Biophys. Res. Commun. 291, 400–405; Takazawa, Perret, Dumont and Erneux (1991) Biochem. Biophys. Res. Commun. 174, 529–535]. The localization of InsP₃ 3-kinase isoenzymes fused at their N-terminus to the green fluorescent protein has been studied by confocal microscopy. The A isoform appeared to associate with the cytoskeleton, whereas the C isoform was totally cytoplasmic. The B isoform had a more complex localization: it appeared in the plasma membrane, cytoskeleton and in the endoplasmic reticulum. The three human isoenzymes of InsP₃ 3-kinase can thus be distinguished by their N-terminal sequence, sensitivity to Ca²⁺/calmodulin and localization on transfection in COS-7 cells. We have compared the cytosolic Ca²⁺ responses induced by ATP in COS-7 cells transfected with the three isoenzymes. Cells expressing high levels of any of the three isoforms no longer respond to ATP, whereas cells expressing low levels of each enzyme showed a reduced response consisting of one to three Ca²⁺ spikes in response to 100 μM ATP. These effects were seen only in wild-type InsP₃ 3-kinase-transfected cells. 3-Kinase-dead mutant cells behaved as vector-transfected cells. The results highlight the potential role of the three isoforms of InsP₃ 3-kinase as direct InsP₃ metabolizing enzymes and direct regulators of Ca²⁺ responses to extracellular signals.