Biochemical and functional characterization of inositol 1,3,4,5,6-pentakisphosphate 2-kinases

Synthesis of inositol 1,2,3,4,5,6-hexakisphosphate (IP6), also known as phytate, is integral to cellular function in all eukaryotes. Production of IP6 predominately occurs through phosphorylation of inositol 1,3,4,5,6-pentakisphosphate (IP5) by a 2-kinase. Recent cloning of the gene encoding this kinase from Saccharomyces cerevisiae, designated scIpk1, has identified a cellular role for IP6 production in the regulation of mRNA export from the nucleus. In this report, we characterize the biochemical and functional parameters of recombinant scIpk1. Purified recombinant scIpk1 kinase activity is highly selective for IP5 substrate and exhibits apparent Ii, values of 644 nM and 62.8 muM for IP5 and ATP, respectively. The observed apparent catalytic efficiency (k(cat)/ K-m) of scIpk1 is 31,610 s(-1) M-1. A sequence similarity search was used to identify an IP5 2-kinase from the fission yeast Schizosaccharomyces pombe. Recombinant spIpk1 has similar substrate selectivity and catalytic efficiency to its budding yeast counterpart, despite sharing only 24% sequence identity. Cells lacking scIPk1 are deficient in IP6 production and exhibit lethality in combination with a gle1 mutant allele. Both of these phenotypes are complemented by expression of the spIPK1 gene in the sc-ipk1 cells. Analysis of several inactive mutants and multiple sequence alignment of scIpk1, spIpk1, and a putative Candida albicans Ipk1 have identified residues involved in catalysis. This includes two conserved motifs: E(i/l/m)KPKWL(t/y) and LXMTLRDV(t/g) (l/c)(f/y)I. Our data suggest that the mechanism for IP6 production is conserved across species.