Molecular sorting of lipids by bacteriorhodopsin in dilauroylphosphatidylcholine/distearoylphosphatidylcholine lipid bilayers

Abstract

A combined experimental and theoretical study is performed on binary dilauroylphosphatidylcholine/distearoylphosphatidylcholine (DLPC/DSPC) lipid bilayer membranes incorporating bacteriorhodopsin (BR). The system is designed to investigate the possibility that BR, via a hydrophobic matching principle related to the difference in lipid bilayer hydrophobic thickness and protein hydrophobic length, can perform molecular sorting of the lipids at the lipid-protein interface, leading to lipid specificity/selectivity that is controlled solely by physical factors. The study takes advantage of the strongly nonideal mixing behavior of the DLPC/DSPC mixture and the fact that the average lipid acyl-chain length is strongly dependent on temperature, particularly in the main phase transition region. The experiments are based on fluorescence energy transfer techniques using specifically designed lipid analogs that can probe the lipid-protein interface. The theoretical calculations exploit a microscopic molecular interaction model that embodies the hydrophobic matching as a key parameter. At low temperatures, in the gel-gel coexistence region, experimental and theoretical data consistently indicate that BR is associated with the short-chain lipid DLPC. At moderate temperatures, in the fluid-gel coexistence region, BR remains in the fluid phase, which is mainly composed of short-chain lipid DLPC, but is enriched at the interface between the fluid and gel domains. At high temperatures, in the fluid phase, BR stays in the mixed lipid phase, and the theoretical data suggest a preference of the protein for the long-chain DSPC molecules at the expense of the short-chain DLPC molecules. The combined results of the experiments and the calculations provide evidence that a molecular sorting principle is active because of hydrophobic matching and that BR exhibits physical lipid selectivity. The results are discussed in the general context of membrane organization and compartmentalization and in terms of nanometer-scale lipid-domain formation.

Visualizing Association of N-Ras in Lipid Microdomains: Influence of Domain Structure and Interfacial Adsorption

“…The AFM images locating protein particles at the boundary between both domains thus confirms the proposition from computer simulation work stating that incorporation can be favorable at the domain boundaries as to the possibility to decrease the unfavorable line tension energy. 6,8 A similar patchy localization has been observed for GM1 in DOPC-DPPC monolayers. Insertion of a glycosylphosphatidylinositol-(GPI-) anchored alkaline phosphatase (AP) into raft mixtures showed spontaneous insertion of AP through its GPI anchor into liquid-ordered domains.…”

“…Such an interfacial adsorption effect of inserting proteins can generally be expected in many-phase lipid systems that have no particular preferences for any particular phase 6,8 sfor example due to hydrophobic mismatchsso that the proteins are expelled to the boundary. It is clear that the localization and accumulation of proteins in the interfaces of a lipid bilayer with domains may provide particularly strong direct protein-protein interactions and hence may serve as a vehicle for protein association.…”

“…The importance of such effects has been raised by computer simulation studies. [6][7][8] The multicomponent character of biomembranes leads to strong membrane heterogeneity and phase segregation originating from compositional fluctuations. In the state of equilibrium, lipid phase separation leads to the formation of macroscopically large domains (phases), whereas out of equilibrium, the phase separation process may produce small-scale domains and extended compositional fluctuations may be observed.…”

Adverse effect of obesity on red cell membrane arachidonic and docosahexaenoic acids in gestational diabetes

“…Hydrophobic matching thickness of lipid bilayer (i.e. lipid acylchain length) and integral membrane proteins have been proposed as a determinator of the coupling between lipid and membrane proteins [40]. Protein prefers to be associated with the lipid species that is hydrophobically best matched.…”

What do diffusion measurements tell us about membrane compartmentalisation? Emergence of the role of interprotein interactions

“…Since all lipids and proteins do not present the same hydrophobic thicknesses, it has been demonstrated that some interactions will be favoured to form domains where their size can vary from a few nanometres to several microns. The formation of these domains can be induced either by lipid-lipid interactions [4,10,11], lipid-protein interactions [12,13] or proteinprotein interactions [14,15]. Interactions of membrane proteins with the cytoskeleton or with the glycocalyx can also affect the membrane organisation and are likely to play a confining role [16].…”