PPIP5K1 Modulates Ligand Competition Between Diphosphoinositol Polyphosphates and PtdIns(3,4,5)P3 for Polyphosphoinositide-Binding Domains.

We describe new signalling consequences for PPIP5K1-mediated phosphorylation of InsP6 and 5-InsP7 to 1-InsP7 and InsP8. In NIH-3T3 cells, either hyperosmotic stress or receptor-activation by PDGF promoted translocation of PPIP5K1 from the cytoplasm to the plasma membrane. The PtdIns(3,4,5)P3-binding domain (PBD) in PPIP5K1 recapitulated that translocation. Mutagenesis of PBD to reduce affinity for PtdIns(3,4,5)P3 prevented translocation. Using surface plasmon resonance, we found that PBD association with vesicular PtdIns(3,4,5)P3 was inhibited by InsP6 and diphosphoinositol polyphosphates. However, the inhibition by PPIP5K1 substrates (IC50 values: 5-InsP7 = 5 μM; InsP6 = 7 μM) was substantially more potent than that of the PPIP5K1 products (IC50 values: InsP8 = 32 μM, 1-InsP7 = 43 μM). This rank order of ligand competition with PtdIns(3,4,5)P3 was also exhibited by the pleckstrin-homology (PH) domains of AKT, GRP1 and SIN1. We propose that, in vivo, PH domain binding of InsP6 and 5-InsP7 suppresses inappropriate signalling from stochastic increases in PtdIns(3,4,5)P3 ("noise"). That restraint may be relieved by localized depletion of InsP6 and 5-InsP7 at the plasma membrane following PPIP5K1 recruitment. We tested this hypothesis in insulin-stimulated L6 myoblasts, using mTOR-mediated phosphorylation of AKT on Ser-473 as a readout for SIN1-mediated translocation of mTORC2 to the plasma membrane (Zoncu,R., Efeyan,A, Sabatini,D.M. (2011) Nat.Rev.Mol...
Source: BJ Signal - Category: Biochemistry Authors: Tags: BJ Signal Source Type: research