Computational and Experimental Studies of Lipid-Protein Interactions in Biomemrane Function

Abstract

Integral membrane proteins constitute more than 30% of the human genome. Substantial lipid-protein interactions contribute to the structure and function of membrane proteins such as GPCRs and viral membrane ion channels. We address how physical properties of the membrane as well as positive charge on the lipid head group modulate protein activation. The flexible surface model (FSM) describes physical properties of the lipid environment that effect membrane protein activation [3]. According to the FSM, elastic coupling of the membrane lipids to integral membrane proteins define lipid-protein interactions through a balance of curvature and hydrophobic forces. Using UV-visible spectroscopy, FTIR spectroscopy [2], and molecular dynamics (MD) simulations, we discovered that lipid bilayer properties like head group and chain length modulate membrane protein functioning by influencing the conformational states. Using rhodopsin, a canonical G-protein-coupled receptor (GPCR) as a prototype, we show the effect of positively charged membrane lipid head group on rhodopsin activation using UV-visible spectroscopy and MD simulations. We discovered rhodopsin reconstituted in membrane lipids with a positively charged head group like DOTAP shifts the equilibrium to active Meta-II state. We propose a mechanism where membrane lipids with positively charged head group interact with Glu134 present on lipid-protein interface breaking the salt bridge with Arg135 in the conserved ERY motif. The interaction between Glu134 and the positively charged head group stabilizes the active Meta-II state. Our study gives insight on the role of membrane lipid-protein interactions in rhodopsin activation mechanism which can ultimately be extended to influenza virus membrane ion channels and other integral proteins. [1] A.V. Struts et al. (2015) Meth. Mol. Biol. 1271, 133-158. [2] E. Zaitseva et al. (2010) JACS 132, 4815-4821. [3] M.F. Brown (2012) Biochemistry 51, 9782-9795.