Structure-Function Analysis of Phosphatidylinositol Transfer Protein Alpha Bound to Human Phosphatidylinositol

Abstract

Phosphatidylinositol transfer protein α (PITPα) selectively transports and promotes exchange of phosphatidylinositol (PI) and phosphatidylcholine (PC) between lipid bilayers. In higher eukaryotes PITPα is required for cellular functions such as phospholipase C-mediated signaling, regulated exocytosis, and secretory vesicle formation. We have determined the crystal structure of human PITPα bound to its physiological ligand, PI, at 2.95 Å resolution. The structure identifies the critical side chains within the lipid-headgroup binding pocket that define the exquisite specificity for PI. Mutational analysis of the PI binding pocket is in good agreement with the structural data and allows manipulation of functional properties of PITPα. Surprisingly, there are no major conformational differences between PI- and PC-loaded PITPα, despite previous predictions. In the crystal, PITPα-PI is dimeric, with two identical dimers in the asymmetric unit. The dimer interface masks precisely the sequence we identify as contributing to PITPα membrane interaction. Our structure represents a soluble, transport-competent form of PI-loaded PITPα.