BPS2026 – An atomistic view of lipid exchange by Sec14-like phosphatidylinositol transfer proteins

Lipid trafficking between intracellular compartments is a fundamental process that underlies membrane dynamics, lipid signaling, and the intracellular communication networks. Yet, the biophysical processes that govern the complex protein-lipid interactions associated with the lipid exchange activities of transfer proteins is not at all understood. Here, we report the molecular mechanism of how Sec14, a phosphatidylinositol (PtdIns)/phosphatidylcholine (PtdCho) transfer protein (PITP) executes its lipid exchange cycle. The heterotypic PtdCho/PtdIns exchange is the essential engine by which Sec14-like PITPs potentiate phosphatidylinositol-4-phosphate (PtdIns4P) signaling in eukaryotic cells. Using an integrated structural biology approach that combines X-ray crystallography, HDX-MS, 19 F-detected NMR spectroscopy, and molecular dynamics simulations, we describe in atomistic detail a heterotypic PtdCho/PtdIns exchange reaction for Sec14-like PITPs. We dissect the mechanism into discrete steps and identify the protein elements that are responsible for membrane encounter, clustering of PtdIns, eviction of bound PtdCho from the binding cavity into an “acceptor” membrane, reciprocal extraction of a PtdIns molecule from that membrane bilayer, and its incorporation into the PITP lipid binding cavity. We also capture the protein conformational dynamics that facilitates a concerted process where the invading PtdIns and exiting PtdCho molecules interact during the exchange. Together, these results enabled us to assign specific functions to highly conserved but previously uncharacterized elements of Sec14-like proteins, and identify molecular concepts that provide a framework for interpreting functional mechanisms of other lipid transfer proteins.