Abstract
Membrane protein function is regulated by the lipid bilayer composition. In many cases the changes in function correlate with changes in the lipid intrinsic curvature (c0), and c0 is considered a determinant of protein function. Yet, water-soluble amphiphiles that cause either negative or positive changes in curvature have similar effects on membrane protein function, showing that changes in lipid bilayer properties other than c0 are important—and may be dominant. To further investigate the mechanisms underlying the bilayer regulation of protein function, we examined how maneuvers that alter phospholipid head groups effective “size”—and thereby c0—alter gramicidin (gA) channel function. Using dioleoylphospholipids and planar bilayers, we varied the head groups’ physical volume and the electrostatic repulsion among head groups (and thus their effective size). When 1,2-dioleyol-sn-glycero-3-phosphocholine (DOPC), was replaced by 1,2-dioleyol-sn-glycero-3-phosphoethanolamine (DOPE) with a smaller head group (causing a more negative c0), the channel lifetime (τ) is decreased. When the pH of the solution bathing a 1,2-dioleyol-sn-glycero-3-phosphoserine (DOPS) bilayer is decreased from 7 to 3 (causing decreased head group repulsion and a more negative c0), τ is decreased. When some DOPS head groups are replaced by zwitterionic head groups, τ is similarly decreased. These effects do not depend on the sign of the change in surface charge. In DOPE:DOPC (3:1) bilayers, pH changes from 5→9 to 5→0 (both increasing head group electrostatic repulsion, thereby causing a less negative c0) both increase τ. Nor do the effects depend on the use of planar, hydrocarbon-containing bilayers, as similar changes were observed in hydrocarbon-free lipid vesicles. Altering the interactions among phospholipid head groups may alter also other bilayer properties such as thickness or elastic moduli. Such changes could be excluded using capacitance measurements and single channel measurements on gA channels of different lengths. We conclude that changes gA channel function caused by changes in head group effective size can be predicted from the expected changes in c0.
Highlights
Membrane proteins are coupled to their host bilayer through hydrophobic interactions, and the need for hydrophobic adaptation, or matching, between lipid bilayers and the embedded membrane proteins (Israelachvili, 1977; Mouritsen and Bloom, 1984) causes protein conformational transitions involving the protein-bilayer interface to perturb the surrounding bilayer
To further investigate the regulation of membrane protein function by the bilayer material properties, we examine how maneuvers that alter the effective size of phospholipid head groups affect gA channel function and compare the results to predictions based on changes in c0 or bilayer elastic moduli
Gramicidin channel function is modulated by experimental maneuvers that alter steric or electrostatic interactions among the head groups of the bilayer-forming lipids
Summary
Membrane proteins are coupled to their host bilayer through hydrophobic interactions, and the need for hydrophobic adaptation, or matching, between lipid bilayers and the embedded membrane proteins (Israelachvili, 1977; Mouritsen and Bloom, 1984) causes protein conformational transitions involving the protein-bilayer interface to perturb the surrounding bilayer. Such bilayer perturbations incur energetic costs (Huang, 1986; Helfrich and Jakobsson, 1990; Ring, 1996; Nielsen et al, 1998; May, 2000; Nielsen and Andersen, 2000) that contribute to the free energy difference of the protein conformational changes (Gruner, 1991; Lundbæk et al, 2010; Rusinova et al, 2011). In an isolated relaxed monolayer the intermolecular force profile determines the effective cross-sectional areas of head groups and acyl chains, which in turn define geometric packing constraints (Israelachvili et al, 1977) and a molecular “shape” (Cullis and de Kruijff, 1979, cf. Figures 1B–D)
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