The phosphorylation of Ins(1,4,5)P3 (InsP3) to Ins(1,3,4,5)P4 (InsP4) is catalysed by InsP3 3-kinase. Molecular-biological data have shown the presence of two human isoenzymes of InsP3 3-kinase, namely InsP3 3-kinases A and B. We have isolated from a rat thymus cDNA library a 2235 bp cDNA (clone B15) encoding rat InsP3 3-kinase B. Northern-blot analysis of mRNA isolated from rat tissues (thymus, testis, brain, spleen, liver, kidney, heart, lung and intestine) revealed that a rat InsP3 3-kinase B probe hybridized to a 6 kb mRNA in lung, thymus, testis, brain and heart. In contrast, Northern-blot analysis of the same tissues probed under stringent conditions with a rat InsP3 3-kinase A probe hybridized to a 2 kb mRNA only in brain and a 1.8-2.0 kb mRNA species in testis. Northern-blot analysis of three human cell lines (HL-60, SH-SY5Y and HTB-138) probed with a human InsP3 3-kinase B probe showed the presence of a 6 kb mRNA in all cell lines, except in the human neuroblastoma cell line (SH-SY5Y), where two mRNA species of 5.7 and 6 kb were detected. Using the same blot, no hybridization signal could be seen with a human InsP3 3-kinase A probe. Altogether, our data are consistent with the notion that the two InsP3 3-kinase isoenzymes, A and B, are specifically expressed in different tissues and cells.

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