Membrane Binding of the Osh4 Curvature-Sensing Peptide

Abstract

This work presents the study of the binding mechanism and function of peripheral membrane protein Osh4 curvature sensing peptide (ALPS-like motif). Osh4 is a lipid transport protein, member of a family of seven homologue oxysterol binding proteins in yeast. It was previously shown, using molecular dynamics simulations, that Osh4 has six membrane binding regions (JMB 2012, 423:847-862). Non-specific interactions with anionic lipids are an important driving force for the Osh4 attraction to yeast membranes. The ALPS-like motif of Osh4, a 29 amino acid peptide which also forms the lid to protect sterols, has also been identified as a membrane curvature sensor (Nature SMB 2007, 14(2):138-146). This work examines the binding mechanism of the peptide with bilayers containing different lipid types. A previous study showed ALPS peptides bind to membranes with surface-packing defects (BJ 2013, 104:575-584). Unsaturated lipids and increasing values of surface tension were implemented to increase the surface packing defects of our membranes. The simplest model had only phosphatidylcholine (PC) lipids; phosphatidylserine (PS) and ergosterol (ERG) were added to model yeast membranes more closely. Simulations using pure 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC) bilayers, a lipid with one double bond per tail, were carried out at different values of surface tension (γ). Additional systems exploring the role of charged lipids and ERG in binding of the peptide were DOPC-DOPS (60:40) and DOPC-ERG (50:50) mixtures. Molecular dynamics were carried out for 200ns using the CHARMM36 force field and the NPT ensemble. Binding events were characterized through hydrogen bonding and binding free energy calculations. Since binding events are stochastic, replicate simulations were carried out for each system. Time in the Anton machine allowed us to run longer simulations for the DOPC-DOPS (60-40) bilayers to study trajectories of 2µs.