Inositol polyphosphate multikinase regulates inositol 1,4,5,6-tetrakisphosphate

Abstract

The human inositol phosphate multikinase (IPMK, 5-kinase) has a preferred 5-kinase activity over 3-kinase and 6-kinase activities and a substrate preference for inositol 1,3,4,6-tetrakisphosphate (Ins(1,3,4,6)P 4) over inositol 1,4,5-trisphosphate (Ins(1,4,5)P 3) and inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P 4). We now report that the recombinant human protein can catalyze the conversion of inositol 1,4,5,6-tetrakisphosphate (Ins(1,4,5,6)P 4) to Ins(1,3,4,5,6)P 5 in vitro; the reaction product was identified by HPLC to be Ins(1,3,4,5,6)P 5. The apparent V max was 42 nmol of Ins(1,3,4,5,6)P 5 formed/min/mg protein, and the apparent K m was 222 nM using Ins(1,3,4,6)P 4 as a substrate; the catalytic efficiency was similar to that for Ins(1,4,5)P 3. Stable over-expression of the human protein in HEK-293 cells abrogates the in vivo elevation of Ins(1,4,5,6)P 4 from the Salmonella dublin SopB protein. Hence, the human 5-kinase may also regulate the level of Ins(1,4,5,6)P 4 and have an effect on chloride channel regulation.