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

Abstract

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 À 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.

Mutations in genes controlling the biosynthesis and accumulation of inositol phosphates in seeds

“…Mu (Mutator) transposon insertion mutants as well as EMS (ethyl methanesulfonate)-induced mutations in this gene gave a reduction of 30-50 % in seed PA content [6,26]. This suggests that other parts of the interwoven inositol phosphate pathways are also involved in the formation of seed PA. ITPKs are divided into three phylogenetic subgroups [13,24,25]. The three seedspecific monocotyledon ITPKs, ZmIPK, OsITP5/6K-4 and OsITP5/6K-6, group together, whereas the dicotyledon seedspecific GmITPK3 from soya bean is found in another subgroup, indicating that the subgrouping cannot be used as a guide to tissue specificity of gene expression.…”

“…Finally, at least some ITPKs show phosphatase activity towards several inositol phosphates [25]. Generally, however, the ITPKs phosphorylate InsP 3 s [Ins(1,3,4)P 3 , Ins(3,5,6)P 3 and/or Ins(3,4,6)P 3 ] and InsP 4 s [Ins(3,4,5,6)P 4 , Ins(1,3,4,5)P 4 and/or Ins(1,2,5,6)P 4 ] [21,[24][25][26]. HvIPK interconverted the two substrates Ins(1,3,4,5)P 4 and Ins(1,3,4,6)P 4 by isomerase activity, and, when Ins(1,3,4)P 3 is used as substrate, this and other enzymes often produce a mixture of Ins(1,3,4,5)P 4 and Ins(1,3,4,6)P 4 .…”

“…ITPKs have been isolated from several plants. Four different ITPKs are found in both Arabidopsis [21][22][23] and soya bean [24]. One of the four soya bean ITPKs, GmITPK3, showed higher expression in early stages in seed development.…”

Molecular basis for the integration of inositol phosphate signaling pathways via human ITPK1

“…Interestingly, plants contain multiple homologues of ITPK1 (Josefsen et al, 2007; Shi et al, 2003; Stiles et al, 2008; Sweetman et al, 2007; Wilson and Majerus, 1997). Rice has six such genes (Suzuki et al, 2007).…”

“…Rice has six such genes (Suzuki et al, 2007). The catalytic efficiencies of the different plant ITPK isoforms can differ by up to two orders of magnitude (Stiles et al, 2008). There is good evidence of an important and ubiquitous role for at least some of these ITPK isoforms in the synthesis of higher inositol phosphates.…”