Cholesterol Promotes the Peripheral Binding of Retroviral Proteins to Lipid Bilayers

Abstract

Rous sarcoma virus (RSV) serves as a model retrovirus to study interactions of the structural protein, Gag, of enveloped viruses such as HIV, with the plasma membrane (PM). Gag-PM interactions are a critical step in the viral lifecycle and understanding their mechanisms is important for facilitating the development of retroviral therapies. Both in vitro and in vivo experiments have provided important insights into Gag's binding behavior but a much needed molecular picture of the protein-bilayer interactions is missing. It is known that the protein binds peripherally to the membrane via its 104 amino-acids-long membrane-binding domain (MBD). This interaction is primarily driven by electrostatics but multiple experiments have reported an additional enhancement of binding in the presence of cholesterol. This effect has been observed in both the myristoylated MBD of HIV, and the non-myristoylated MBD of RSV. To learn the mechanism of this effect we have examined the interactions of RSV MBD with cholesterol-rich and cholesterol-depleted bilayers composed of the charged and neutral lipids POPC and POPS with all-atom MD simulations. The results are consistent with the reported experimental findings, showing that the structure of the protein remains mostly intact, but its position with respect to the membrane changes depending on the lipid composition. Thus, residue-based analysis of protein-lipid interactions identifies up to 20% increase in Lys-POPS contacts in the cholesterol containing bilayer, whose area per lipid is more than 25% lower than that of the POPC/POPS bilayer without cholesterol. We have analyzed the effect of MBD on the lipids interacting directly with it in the two systems, and report on potential cholesterol-mediated bilayer properties (including lipid headgroup tilt, acyl chain order and surface charge density) that contribute to the enhanced binding.