Phosphorylation of a distinct structural form of phosphatidylinositol transfer protein alpha at Ser¹⁶⁶ by protein kinase C disrupts receptor-mediated phospholipase C signaling by inhibiting delivery of phosphatidylinositol to membranes

Abstract

Phosphatidylinositol transfer protein α (PITPα) participates in the supply of phosphatidylinositol (PI) required for many cellular events including phospholipase C (PLC) β and γ signaling by G-protein-coupled receptors and receptor-tyrosine kinases, respectively. Protein kinase C has been known to modulate PLC signaling by G-protein-coupled receptors and receptor-tyrosine kinases, although the molecular target has not been identified in most instances. In each case phorbol myristate acetate pretreatment of HL60, HeLa, and COS-7 cells abrogated PLC stimulation by the agonists formyl-Met-Leu-Phe, ATP, and epidermal growth factor, respectively. Here we show that phosphorylation of PITPα at Ser 166 resulted in inhibition of receptor-stimulated PLC activity. Ser 166 is localized in a small pocket between the 165-172 loop and the rest of the protein and was not solvent-accessible in either the PI- or phosphatidylcholine-loaded structures of PITPα. To allow phosphorylation at Ser 166 , a distinct structural form is postulated, and mutation of Thr 59 to alanine shifted the equilibrium to this form, which could be resolved on native PAGE. The elution profile observed by size exclusion chromatography of phosphorylated PITPα from rat brain or in vitro phosphorylated PITPα demonstrated that phosphorylated PITPα is structurally distinct from the non-phosphorylated form. Phosphorylated PITPα was unable to deliver its PI cargo, although it could deliver phosphatidylcholine. We conclude that the PITPα structure has to relax to allow access to the Ser 166 site, and this may occur at the membrane surface where PI delivery is required for receptor-mediated PLC signaling