skip to main content
US FlagAn official website of the United States government
dot gov icon
Official websites use .gov
A .gov website belongs to an official government organization in the United States.
https lock icon
Secure .gov websites use HTTPS
A lock ( lock ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.


Title: Building Asymmetric Lipid Bilayers for Molecular Dynamics Simulations: What Methods Exist and How to Choose One?
The compositional asymmetry of biological membranes has attracted significant attention over the last decade. Harboring more differences from symmetric membranes than previously appreciated, asymmetric bilayers have proven quite challenging to study with familiar concepts and techniques, leaving many unanswered questions about the reach of the asymmetry effects. One particular area of active research is the computational investigation of composition- and number-asymmetric lipid bilayers with molecular dynamics (MD) simulations. Offering a high level of detail into the organization and properties of the simulated systems, MD has emerged as an indispensable tool in the study of membrane asymmetry. However, the realization that results depend heavily on the protocol used for constructing the asymmetric bilayer models has sparked an ongoing debate about how to choose the most appropriate approach. Here we discuss the underlying source of the discrepant results and review the existing methods for creating asymmetric bilayers for MD simulations. Considering the available data, we argue that each method is well suited for specific applications and hence there is no single best approach. Instead, the choice of a construction protocol—and consequently, its perceived accuracy—must be based primarily on the scientific question that the simulations are designed to address.  more » « less
Award ID(s):
1817929
PAR ID:
10469786
Author(s) / Creator(s):
; ;
Publisher / Repository:
MDPI
Date Published:
Journal Name:
Membranes
Volume:
13
Issue:
7
ISSN:
2077-0375
Page Range / eLocation ID:
629
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. null (Ed.)
    The plasma membrane of eukaryotic cells is known to be compositionally asymmetric. Certain phospholipids, such as sphingomyelin and phosphatidylcholine species, are predominantly localized in the outer leaflet, while phosphatidylethanolamine and phosphatidylserine species primarily reside in the inner leaflet. While phospholipid asymmetry between the membrane leaflets is well established, there is no consensus about cholesterol distribution between the two leaflets. We have performed a systematic study, via molecular simulations, of how the spatial distribution of cholesterol molecules in different “asymmetric” lipid bilayers are affected by the lipids’ backbone, head-type, unsaturation, and chain-length by considering an asymmetric bilayer mimicking the plasma membrane lipids of red blood cells, as well as seventeen other asymmetric bilayers comprising of different lipid types. Our results reveal that the distribution of cholesterol in the leaflets is solely a function of the extent of ordering of the lipids within the leaflets. The ratio of the amount of cholesterol matches the ratio of lipid order in the two leaflets, thus providing a quantitative relationship between the two. These results are understood by the observation that asymmetric bilayers with equimolar amount of lipids in the two leaflets develop tensile and compressive stresses due to differences in the extent of lipid order. These stresses are alleviated by the transfer of cholesterol from the leaflet in compressive stress to the one in tensile stress. These findings are important in understanding the biology of the cell membrane, especially with regard to the composition of the membrane leaflets. 
    more » « less
  2. Membrane undulations play a vital role in many biological processes, including the regulation of membrane protein activity. The asymmetric lipid composition of most biological membranes complicates theoretical description of these bending fluctuations, yet experimental data that would inform any such a theory is scarce. Here, we used neutron spin-echo (NSE) spectroscopy to measure the bending fluctuations of large unilamellar vesicles (LUV) having an asymmetric transbilayer distribution of high- and low-melting lipids. The asymmetric vesicles were prepared using cyclodextrin-mediated lipid exchange, and were composed of an outer leaflet enriched in egg sphingomyelin (ESM) and an inner leaflet enriched in 1-palmitoyl-2-oleoyl-phosphoethanolamine (POPE), which have main transition temperatures of 37 °C and 25 °C, respectively. The overall membrane bending rigidity was measured at three temperatures: 15 °C, where both lipids are in a gel state; 45 °C, where both lipids are in a fluid state; and 30 °C, where there is gel-fluid co-existence. Remarkably, the dynamics for the fluid asymmetric LUVs (aLUVs) at 30 °C and 45 °C do not follow trends predicted by their symmetric counterparts. At 30 °C, compositional asymmetry suppressed the bending fluctuations, with the asymmetric bilayer exhibiting a larger bending modulus than that of symmetric bilayers corresponding to either the outer or inner leaflet. We conclude that the compositional asymmetry and leaflet coupling influence the internal dissipation within the bilayer and result in membrane properties that cannot be directly predicted from corresponding symmetric bilayers. 
    more » « less
  3. Many cellular lipid bilayers consist of leaflets that differ in their lipid composition — a non-equilibrium state actively maintained by cellular sorting processes that counter passive lipid flip-flop. While this lipidomic aspect of membrane asymmetry has been known for half a century, its elastic and thermodynamic ramifications have garnered attention only fairly recently. Notably, the torque arising when lipids of different spontaneous curvature reside in the two leaflets can be counterbalanced by a difference in lateral mechanical stress between them. Such membranes can be essentially flat in their relaxed state, despite being compositionally strongly asymmetric, but they harbor a surprisingly large but macroscopically invisible differential stress. This hidden stress can affect a wide range of other membrane properties, such as the resistance to bending, the nature of phase transitions in its leaflets, and the distribution of flippable species, most notably sterols. In this short note we offer a concise overview of our recently proposed basic framework for capturing the interplay between curvature, lateral stress, leaflet phase behavior, and cholesterol distribution in generally asymmetric membranes, and how its implied signatures might be used to learn more about the hidden but physically consequential differential stress. 
    more » « less
  4. KCNE3 is a potassium channel accessory transmembrane protein that regulates the function of various voltage-gated potassium channels such as KCNQ1. KCNE3 plays an important role in the recycling of potassium ion by binding with KCNQ1. KCNE3 can be found in the small intestine, colon, and in the human heart. Despite its biological significance, there is little information on the structural dynamics of KCNE3 in native-like membrane environments. Molecular dynamics (MD) simulations are a widely used as a tool to study the conformational dynamics and interactions of proteins with lipid membranes. In this study, we have utilized all-atom molecular dynamics simulations to characterize the molecular motions and the interactions of KCNE3 in a bilayer composed of: a mixture of POPC and POPG lipids (3:1), POPC alone, and DMPC alone. Our MD simulation results suggested that the transmembrane domain (TMD) of KCNE3 is less flexible and more stable when compared to the N- and C-termini of KCNE3 in all three membrane environments. The conformational flexibility of N- and C-termini varies across these three lipid environments. The MD simulation results further suggested that the TMD of KCNE3 spans the membrane width, having residue A69 close to the center of the lipid bilayers and residues S57 and S82 close to the lipid bilayer membrane surfaces. These results are consistent with previous biophysical studies of KCNE3. The outcomes of these MD simulations will help design biophysical experiments and complement the experimental data obtained on KCNE3 to obtain a more detailed understanding of its structural dynamics in the native membrane environment. 
    more » « less
  5. We describe a method to determine membrane bending rigidity from capacitance measurements on large area, free-standing, planar, biomembranes. The bending rigidity of lipid membranes is an important biological mechanical property that is commonly optically measured in vesicles, but difficult to quantify in a planar, unsupported system. To accomplish this, we simultaneously image and apply an electric potential to free-standing, millimeter area, planar lipid bilayers composed of DOPC and DOPG phospholipids to measure the membrane Young’s (elasticity) modulus. The bilayer is then modeled as two adjacent thin elastic films to calculate bending rigidity from the electromechanical response of the membrane to the applied field. Using DOPC, we show that bending rigidities determined by this approach are in good agreement with the existing work using neutron spin echo on vesicles, atomic force spectroscopy on supported lipid bilayers, and micropipette aspiration of giant unilamellar vesicles. We study the effect of asymmetric calcium concentration on symmetric DOPC and DOPG membranes and quantify the resulting changes in bending rigidity. This platform offers the ability to create planar bilayers of controlled lipid composition and aqueous ionic environment, with the ability to asymmetrically alter both. We aim to leverage this high degree of compositional and environmental control, along with the capacity to measure physical properties, in the study of various biological processes in the future. 
    more » « less