PPIP5K1: A Kinase With A Cryptic Polyphosphoinositide‐Binding Domain (PBD1)

Abstract

The inositol pyrophosphates (InsP7, InsP8) are multifunctional signaling molecules that regulate cell and organismal energy homeostasis. Two mammalian kinases that synthesize InsP8, PPIP5K1 and PPIP5K2, were recently cloned. The kinase domain was mapped to the N‐terminal one‐third of these kinases, but unexpectedly, both enzymes were also found to contain a “phosphatase‐like” domain. In this study, we report that these “phosphatase‐like” domains are not catalytically active towards inositol phosphates. Instead, we now demonstrate using surface plasmon resonance, that these domains (we re‐designated them as polyphosphoinsoitide‐binding domains or PBDs) bind PtdIns(3,4,5)P3 with high affinity (PBD1 Kd = 96 nM; PBD2 Kd = 705 nM). PtdIns(3,4)P2 and PtdIns(4,5)P2 are 6‐fold weaker ligands and there is no significant binding of PtdIns(3,5)P2 or any of the monophosphorylated phosphoinositides. This ligand specificity is suggestive of the participation of a pleckstrin‐homology (PH) domain. Indeed, evidence from sequence alignments and site‐directed mutagenesis reveal that the PDBs consist of a partial PH domain fused to a catalytically‐inactive substrate‐binding motif from the acid‐phosphatase family. This novel domain mediates receptor‐dependent compartmentalization of PPIP5K1 at the plasma membrane where localized synthesis of InsP8 may modulate other signaling pathways.