IP6K structure and the molecular determinants of catalytic specificity in an inositol phosphate kinase family

Abstract

Inositol trisphosphate kinases (IP3Ks) and inositol hexakisphosphate kinases (IP6Ks) each regulate specialized signalling activities by phosphorylating either InsP 3 or InsP 6 respectively. The molecular basis for these different kinase activities can be illuminated by a structural description of IP6K. Here we describe the crystal structure of an Entamoeba histolytica hybrid IP6K/IP3K, an enzymatic parallel to a living fossil. Through molecular modelling and mutagenesis, we extrapolated our findings to human IP6K2, which retains vestigial IP3K activity. Two structural elements, an α-helical pair and a rare, two-turn 3 10 helix, together forge a substrate-binding pocket with an open clamshell geometry. InsP 6 forms substantial contacts with both structural elements. Relative to InsP 6, enzyme-bound InsP 3 rotates 55° closer to the α-helices, which provide most of the protein's interactions with InsP 3. These data reveal the molecular determinants of IP6K activity, and suggest an unusual evolutionary trajectory for a primordial kinase that could have favored efficient bifunctionality, before propagation of separate IP3Ks and IP6Ks. © 2014 Macmillan Publishers Limited. All rights reserved.