Abstract
We have cloned and characterized four Itpk genes from soybean. All four recombinant Itpk proteins showed canonical Ins(1,3,4)P3 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)P4, and its metabolic precursor, Ins(1,3,4,5,6)P5, within a substrate cycle. The soybean Itpks also phosphorylated Ins(3,4,6)P3 to Ins(1,3,4,6)P4 which was further phosphorylated to Ins(1,3,4,5,6)P5 by soybean Ipk2. Thus, soybean Itpks may participate in an inositol lipid-independent pathway of InsP6 synthesis.