Metabolic and signaling properties of an Itpk gene family in Glycine max

We have cloned and characterized four Itpk genes from soybean. All four recombinant Itpk proteins showed canonical Ins(1,3,4)P 3 5/6-kinase activity, but a kinetic analysis raised questions about its biological significance. Instead, we provide evidence that one alternative biological role for soybean Itpks is to interconvert the Cl − channel inhibitor, Ins(3,4,5,6)P 4, and its metabolic precursor, Ins(1,3,4,5,6)P 5, within a substrate cycle. The soybean Itpks also phosphorylated Ins(3,4,6)P 3 to Ins(1,3,4,6)P 4 which was further phosphorylated to Ins(1,3,4,5,6)P 5 by soybean Ipk2. Thus, soybean Itpks may participate in an inositol lipid-independent pathway of InsP 6 synthesis.