The Pathway for the Production of Inositol Hexakisphosphate in Human Cells

John W Verbsky,Shao-Chun Chang,Monita P Wilson,Yasuhiro Mochizuki,Philip W Majerus

doi:10.1074/jbc.m411528200

Abstract

The yeast and Drosophila pathways leading to the production of inositol hexakisphosphate (InsP(6)) have been elucidated recently. The in vivo pathway in humans has been assumed to be similar. Here we show that overexpression of Ins(1,3,4)P(3) 5/6-kinase in human cell lines results in an increase of inositol tetrakisphosphate (InsP(4)) isomers, inositol pentakisphosphate (InsP(5)) and InsP(6), whereas its depletion by RNA interference decreases the amounts of these inositol phosphates. Expression of Ins(1,3,4,6)P(4) 5-kinase does not increase the amount of InsP(5) and InsP(6), although its depletion does block InsP(5) and InsP(6) production, showing that it is necessary for production of InsP(5) and InsP(6). Expression of Ins(1,3,4,5,6)P(5) 2-kinase increases the amount of InsP(6) by depleting the InsP(5) in the cell, and depletion of 2-kinase decreases the amount of InsP(6) and causes an increase in InsP(5). These results are consistent with a pathway that produces InsP(6) through the sequential action of Ins(1,3,4)P(3) 5/6-kinase, Ins(1,3,4,6)P(4) 5-kinase, and Ins(1,3,4,5,6)P5 2-kinase to convert Ins(1,3,4)P(3) to InsP(6). Furthermore, the evidence implicates 5/6-kinase as the rate-limiting enzyme in this pathway.

Highlights

The yeast and Drosophila pathways leading to the production of inositol hexakisphosphate (InsP6) have been elucidated recently
Deletion of IPK2 causes an increase in Ins(1,4,5)P3, whereas loss of IPK1 causes a loss of InsP6 and an accumulation of InsP5, proving that there is no other pathway to go from InsP3 to InsP6 in yeast
When the authors were determining the activities of the Drosophila and Arabidopsis homologs of Ipk[2], they found a significant 5-kinase activity on Ins(1,3,4,6)P4

Summary

Introduction

The yeast and Drosophila pathways leading to the production of inositol hexakisphosphate (InsP6) have been elucidated recently. Ins(1,4,5)P3 is converted directly to InsP6 by the sequential action of two proteins: Ipk[2], which produces InsP5 in a two-step phosphorylation of Ins(1,4,5)P3, first to Ins(1,4,5,6)P4 and to InsP5; and Ipk[1], which produces InsP6 (1, 15) This pathway differs from that proposed by Menniti et al (12) in that no isomerization of Ins(1,4,5)P3 to Ins(1,3,4)P3 is required and in that the intermediate InsP4 isomer is not Ins(1,3,4,6)P4 but Ins(1,4,5,6)P4 (15). When the authors were determining the activities of the Drosophila and Arabidopsis homologs of Ipk[2], they found a significant 5-kinase activity on Ins(1,3,4,6)P4 This activity is necessary for the pathway proposed in human cells. Because the yeast pathway works through Ins(1,4,5,6)P4, which is already phosphorylated at the D-5 position, they suggest that this activity is not relevant for the production of InsP6 in these organisms

Methods

Results

Conclusion