The human homolog of the rat inositol phosphate multikinase is an inositol 1,3,4,6-tetrakisphosphate 5-kinase

We have demonstrated that the human homolog of the rat inositol phosphate multikinase is an inositol 1,3,4,6-tetrakisphosphate 5-kinase (InSP4 5-kinase). The cDNA of the human gene contained a putative open reading frame of 1251 bp encoding 416 amino acids with 83.6% identity compared with the rat protein. The substrate specificity of the recombinant human protein demonstrated preference for Ins(1,3,4,6)P-4 with a catalytic efficiency (V-max/K-m) 43-fold greater than that of Ins(1,3,4,5)P-4 and 2-fold greater than that of Ins(1,4,5)P-3. The apparent V-max was 114 nmol of Ins(1,3,4,5,6)P-5 formed/min/mg of protein, and the apparent K-m was 0.3 mum Ins(1,3,4,6)P-4. The functional homolog in yeast is Ipk2p, and ipk2-null yeast strains do not synthesize Ins(1,3,4,5,6)P-5 or InSP6. Synthesis of these compounds was restored by transformation with wild-type yeast IPK2 but not with human InSP4 5-kinase. Thus the human gene does not complement for the loss of the yeast gene because yeast cells do not contain the substrate Ins(1,3,4,6)P-4, and the reaction of the human protein with Ins(1,3,4,5)P-4 is insufficient to effect rescue or synthesis of InSP5 and InsP,. Therefore the major activity of human InSP4 5-kinase is phosphorylation at the D-5 position, and the pathways for synthesis of Ins(1,3,4,5,6)P-5 in yeast versus humans are different.