Dataset Information

Roles of Interleaflet Coupling and Hydrophobic Mismatch in Lipid Membrane Phase-Separation Kinetics.

ABSTRACT: Characterizing the nanoscale dynamic organization within lipid bilayer membranes is central to our understanding of cell membranes at a molecular level. We investigate phase separation and communication across leaflets in ternary lipid bilayers, including saturated lipids with between 12 and 20 carbons per tail. Coarse-grained molecular dynamics simulations reveal a novel two-step kinetics due to hydrophobic mismatch, in which the initial response of the apposed leaflets upon quenching is to increase local asymmetry (antiregistration), followed by dominance of symmetry (registration) as the bilayer equilibrates. Antiregistration can become thermodynamically preferred if domain size is restricted below ?20 nm, with implications for the symmetry of rafts and nanoclusters in cell membranes, which have similar reported sizes. We relate our findings to theory derived from a semimicroscopic model in which the leaflets experience a "direct" area-dependent coupling, and an "indirect" coupling that arises from hydrophobic mismatch and is most important at domain boundaries. Registered phases differ in composition from antiregistered phases, consistent with a direct coupling between the leaflets. Increased hydrophobic mismatch purifies the phases, suggesting that it contributes to the molecule-level lipid immiscibility. Our results demonstrate an interplay of competing interleaflet couplings that affect phase compositions and kinetics, and lead to a length scale that can influence lateral and transverse bilayer organization within cells.

SUBMITTER: Fowler PW

PROVIDER: S-EPMC5025830 | biostudies-literature | 2016 Sep

REPOSITORIES: biostudies-literature

ACCESS DATA

Publications

Roles of Interleaflet Coupling and Hydrophobic Mismatch in Lipid Membrane Phase-Separation Kinetics.

Fowler Philip W PW Williamson John J JJ Sansom Mark S P MS Olmsted Peter D PD

Journal of the American Chemical Society 20160830 36

Characterizing the nanoscale dynamic organization within lipid bilayer membranes is central to our understanding of cell membranes at a molecular level. We investigate phase separation and communication across leaflets in ternary lipid bilayers, including saturated lipids with between 12 and 20 carbons per tail. Coarse-grained molecular dynamics simulations reveal a novel two-step kinetics due to hydrophobic mismatch, in which the initial response of the apposed leaflets upon quenching is to inc ...[more]

PMID: 27574865

Similar Datasets

Project description:Using Förster resonance energy transfer, raft/liquid-ordered-domain formation was assessed in asymmetric vesicles containing outer leaflets composed of high-Tm (melting temperature) saturated phosphatidylcholines (diC18:0PC, diC16:0PC, diC15:0PC, or diC14:0PC), low-Tm unsaturated dioleoylphosphatidylcholine (DOPC) and cholesterol, and inner leaflets composed of lipids that by themselves would not form ordered domains (DOPC and cholesterol). Ordered-domain formation in the outer leaflet was compared to that in symmetric vesicles with the same lipid composition as the asymmetric vesicle outer leaflets. The difference between ordered-domain thermal stability in asymmetric and symmetric vesicles was highly dependent on high-Tm PC acyl-chain length. At one extreme, in diC14:0PC-containing asymmetric vesicles, the outer leaflet did not segregate to form ordered domains over the entire experimental temperature range even though ordered domains formed in the symmetric vesicles, indicating the inner leaflet dominated outer-leaflet physical behavior in the asymmetric vesicles. At the other extreme, in diC18:0PC-containing asymmetric vesicles, ordered domains formed over the entire temperature range at which they were present in symmetric vesicles, indicating the inner leaflet did not dominate outer-leaflet physical behavior. DiC15:0PC- and diC16:0PC-containing vesicles exhibited intermediate behaviors. A different set of vesicles was prepared with high-Tm lipid sphingomyelin (SM) in place of saturated phosphatidylcholine, and the % SM was varied. The thermal stability of outer-leaflet ordered domains in asymmetric vesicles was found to decrease more than in symmetric vesicles as SM levels decreased, indicating that the inner leaflet increasingly dominated outer leaflet physical state as SM levels decreased. Overall, inhibition of outer-leaflet ordered-domain formation in asymmetric vesicles by inner-leaflet lipids decreased as the ability of outer-leaflet lipids to form an ordered state by themselves increased, i.e., when outer-leaflet high-Tm lipid content or acyl-chain length increased. This has implications for how ordered-domain formation may be controlled in vivo.

Project description:Cell membranes possess a complex three-dimensional architecture, including nonrandom lipid lateral organization within the plane of a bilayer leaflet, and compositional asymmetry between the two leaflets. As a result, delineating the membrane structure-function relationship has been a highly challenging task. Even in simplified model systems, the interactions between bilayer leaflets are poorly understood, due in part to the difficulty of preparing asymmetric model membranes that are free from the effects of residual organic solvent or osmotic stress. To address these problems, we have modified a technique for preparing asymmetric large unilamellar vesicles (aLUVs) via cyclodextrin-mediated lipid exchange in order to produce tensionless, solvent-free aLUVs suitable for a range of biophysical studies. Leaflet composition and structure were characterized using isotopic labeling strategies, which allowed us to avoid the use of bulky labels. NMR and gas chromatography provided precise quantification of the extent of lipid exchange and bilayer asymmetry, while small-angle neutron scattering (SANS) was used to resolve bilayer structural features with subnanometer resolution. Isotopically asymmetric POPC vesicles were found to have the same bilayer thickness and area per lipid as symmetric POPC vesicles, demonstrating that the modified exchange protocol preserves native bilayer structure. Partial exchange of DPPC into the outer leaflet of POPC vesicles produced chemically asymmetric vesicles with a gel/fluid phase-separated outer leaflet and a uniform, POPC-rich inner leaflet. SANS was able to separately resolve the thicknesses and areas per lipid of coexisting domains, revealing reduced lipid packing density of the outer leaflet DPPC-rich phase compared to typical gel phases. Our finding that a disordered inner leaflet can partially fluidize ordered outer leaflet domains indicates some degree of interleaflet coupling, and invites speculation on a role for bilayer asymmetry in modulating membrane lateral organization.

Project description:Lipid rafts are thought to be key organizers of membrane-protein complexes in cells. Many proteins that interact with rafts have bulky polymeric components such as intrinsically disordered protein domains and polysaccharide chains. Therefore, understanding the interaction between membrane domains and membrane-bound polymers provides insights into the roles rafts play in cells. Multiple studies have demonstrated that high concentrations of membrane-bound polymeric domains create significant lateral steric pressure at membrane surfaces. Furthermore, our recent work has shown that lateral steric pressure at membrane surfaces opposes the assembly of membrane domains. Building on these findings, here we report that membrane-bound polymers are potent suppressors of membrane phase separation, which can destabilize lipid domains with substantially greater efficiency than globular domains such as membrane-bound proteins. Specifically, we created giant vesicles with a ternary lipid composition, which separated into coexisting liquid ordered and disordered phases. Lipids with saturated tails and poly(ethylene glycol) (PEG) chains conjugated to their head groups were included at increasing molar concentrations. When these lipids were sparse on the membrane surface they partitioned to the liquid ordered phase. However, as they became more concentrated, the fraction of GUVs that were phase-separated decreased dramatically, ultimately yielding a population of homogeneous membrane vesicles. Experiments and physical modeling using compositions of increasing PEG molecular weight and lipid miscibility phase transition temperature demonstrate that longer polymers are the most efficient suppressors of membrane phase separation when the energetic barrier to lipid mixing is low. In contrast, as the miscibility transition temperature increases, longer polymers are more readily driven out of domains by the increased steric pressure. Therefore, the concentration of shorter polymers required to suppress phase separation decreases relative to longer polymers. Collectively, our results demonstrate that crowded, membrane-bound polymers are highly efficient suppressors of phase separation and suggest that the ability of lipid domains to resist steric pressure depends on both their lipid composition and the size and concentration of the membrane-bound polymers they incorporate.

Dataset Information

Roles of Interleaflet Coupling and Hydrophobic Mismatch in Lipid Membrane Phase-Separation Kinetics.

Publications

Roles of Interleaflet Coupling and Hydrophobic Mismatch in Lipid Membrane Phase-Separation Kinetics.

Similar Datasets

OmicsDI is part of the ELIXIR infrastructure

Tweets