An electrostatic switching mechanism controls the PS/PI4P exchange activity of Osh6p
In eukaryotic cells, phosphatidylserine (PS) is made in the endoplasmic reticulum (ER), yet it is enriched in the plasma membrane (PM), where it provides negative charges and is critical for recruiting various signaling proteins. In yeast Osh6p and Osh7p contribute to this accumulation by transporting PS from the ER to the PM via PS/phosphatidylinositol 4-phosphate (PI4P) exchange cycles. It is unclear how, at each cycle, these proteins escape from the huge electrostatic attraction of the PM to go back to the ER. We addressed this issue with Osh6p using in vitro assays and simulations, backed to cellular observations. We show that it dissociates from anionic membranes, like the PM, once it traps one of its lipid ligand. The N-terminal lid of Osh6p, when it folds to close the binding pocket with the lipid inside, changes the electrostatic surface of the protein, thereby reducing its binding to anionic membranes. Owing to this electrostatic switching mechanism, Osh6p keeps a fast exchange activity between almost neutral and negatively-charged membranes, i.e in a model system mimicking the ER/PM interface. Attenuation of the electronegativity of the lid weakens such a capacity. This study demonstrates how a lipid transfer protein self-limits its residency time on membranes to be efficient.
Institut de Pharmacologie Moléculaire et Cellulaire CNRS and Université de Nice Sophia Antipolis
Domain 4 - UMR 144 - Subcellular Structure and Cellular Dynamics