Rhodopsin/Lipid Hydrophobic Matching—Rhodopsin Oligomerization and Function

Abstract

Lipid composition of the membrane and rhodopsin packing density strongly modulate the early steps of the visual response of photoreceptor membranes. In this study, lipid-order and bovine rhodopsin function in proteoliposomes composed of the sn-1 chain perdeuterated lipids 14:0d27-14:1-PC, 16:0d31-16:1-PC, 18:0d35-18:1-PC, or 20:0d39-20:1-PC at rhodopsin/lipid molar ratios from 1:70 to 1:1000 (mol/mol) were investigated. Clear evidence for matching of hydrophobic regions on rhodopsin transmembrane helices and hydrophobic thickness of lipid bilayers was observed from (2)H nuclear magnetic resonance order parameter measurements at low rhodopsin concentrations. Thin bilayers stretched to match the length of transmembrane helices observed as increase of sn-1 chain order, while thicker bilayers were compressed near the protein. A quantitative analysis of lipid-order parameter changes suggested that the protein adjusts its conformation to bilayer hydrophobic thickness as well, which confirmed our earlier circular-dichroism measurements. Changes in lipid order parameters upon rhodopsin incorporation vanished for bilayers with a hydrophobic thickness of 27 ± 1 Å, suggesting that this is the bilayer thickness at which rhodopsin packs in bilayers at the lowest membrane perturbation. The lipid-order parameter studies also indicated that a hydrophobic mismatch between rhodopsin and lipids triggers rhodopsin oligomerization with increasing rhodopsin concentrations. Both hydrophobic mismatch and rhodopsin oligomerization result in substantial shifts of the equilibrium between the photointermediates metarhodopsin I and metarhodopsin II; increasing bilayer thickness favors formation of metarhodopsin II while oligomerization favors metarhodopsin I. The results highlight the importance of hydrophobic matching for rhodopsin structure, oligomerization, and function.

Soft Matter in Lipid–Protein Interactions

“…A molecular-level understanding of the forces acting on lipids and proteins in membranes rests on knowledge of pure bilayer dynamics and thermodynamics in the absence of complicating protein molecules. For example, nuclear magnetic resonance (NMR) spectroscopy provides knowledge of average structure (142) and lipid mobility (27,95), and has been applied to investigate the influences of ions (25,141), water (84), cholesterol (1,26,106,114,161), and proteins (15,152). Segmental order parameters of the lipids are directly accessible (95), as interpreted by Seelig (142) and others (44,128) in terms of local bilayer structure and dynamics (27).…”

“…Experimental support comes from studies of the effects of the lipid acyl length on protein activity for Ca 2+ -ATPase (92) and rhodopsin (19,20,153). In the case of rhodopsin (19,20,152), the MI-MII activation equilibrium depends on the lipid acyl-chain length, where a thicker bilayer (128) favors the active MII state (see Figure 1). That leads us to the idea that the equilibrium can be tipped toward one state or the other, depending on structural features such as the bilayer thickness matching.…”

“…Among the open questions are the properties and role of the boundary lipids in the first solvation shell about membrane proteins; whether hydrophobic mismatch yields distortion of either the lipid bilayer or the embedded proteins or peptides; and the role of the lipid phase state (e.g., gel, liquid-crystalline, reverse hexagonal) in the proteolipid coupling. Experimentally, solid-state 2 H NMR spectroscopy shows that under matched conditions, the distortion of the lipid bilayer about integral membrane proteins like rhodopsin tends to be relatively small (70,152). Apparently, the energetics of the deformation are rather high, and small structural changes can have a large energetic penalty.…”

Cholesterol-induced suppression of membrane elastic fluctuations at the atomistic level

“…Notably, the structural and dynamical properties of biomembranes are mediated by the lipid composition and interactions with the proteins, water, cholesterol, and surfactants (Brown and Chan, 2007; Coskun and Simons, 2011; Kaiser et al, 2009; Kaye et al, 2011; Kinnun et al, 2015; Leftin et al, 2014b; Mallikarjunaiah et al, 2011; Rheinstädter et al, 2004; Tyler et al, 2015). Membrane remodeling requires mesoscopic elastic deformations of the lipids (Kinnun et al, 2015) that can play a central role in biological functioning with regard to lipid-protein interactions, domain formation, and various nano- and microstructures implicated in key cellular functions (Brown, 1997; Brown, 2012; Liang et al, 2014; Soubias et al, 2014, 2015; Teague et al, 2013). …”

Chemical synthesis of transmembrane peptide and its application for research on the transmembrane–juxtamembrane region of membrane protein

“…The TM region must influence the functions of membrane proteins by rotation, association, or changing its orientation and even its secondary structure. Increasing evidence has suggested that the characteristics of the membrane, which is composed of numerous lipid molecules, considerably influences membrane protein function . Therefore, molecular characterization of the TM region within the lipid bilayer is essential to understand the molecular mechanisms that regulate the functions of membrane proteins.…”