Molecular Models of Nanodiscs

Nanodiscs are discoloidal protein–lipid particles that self-assemble from a mixture of lipids and membrane scaffold proteins. They form a highly soluble membrane mimetic that closely resembles a native-like lipid environment, unlike micelles. Nanodiscs are widely used for experimental studies of membrane proteins. In this work, we present a new method for building arbitrary nanodiscs using a combination of the Martini coarse-grained and all-atom force fields. We model the basic membrane scaffold protein MSP1 and its extended versions, such as MSP1E1 and MSP1E2, using a crystal structure of human apolipoprotein Apo-I. We test our method by generating nanodiscs of different sizes and compositions, including nanodiscs with embedded membrane proteins, such as bacteriorhodopsin, outer membrane protein X, and the glucose transporter. We show that properties of our nanodiscs are in general agreement with experimental data and previous computational studies

Computational Insights into the Role of Cholesterol in Inverted Hexagonal Phase Stabilization and Endosomal Drug Release

Cholesterol is a major component of many lipid-based drug delivery systems, including cationic lipid nanoparticles. Despite its critical role in the drug release stage, the underlying molecular mechanism by which cholesterol assists in endosomal escape remains unclear. An efficient drug release from the endosome requires endosomal disruption. This disruption is believed to involve a lamellar-to-inverted hexagonal (Lα–HII) phase transition upon fusion of the lipid nanoparticle with the endosomal membrane. We used molecular dynamics simulations to study the structural properties of HII systems composed of an anionic lipid distearoyl phosphatidylserine (DSPS), an ionizable cationic lipid (KC2H), and cholesterol for several hydration levels and molar ratios. This system corresponds to the lipid mixtures in the hypothesized HII structure formed upon fusion and is of interest for the rational design of ionizable cationic lipids, including KC2, for an optimal drug release. Simulations suggest a geometry- and symmetry-driven lipid sorting and cholesterol–DSPS co-location around the water cores. Cholesterol preferentially co-locates with negatively charged saturated DSPS lipids at interstitial angles. The observed cholesterol–DSPS co-location results in an overall increase in the DSPS acyl chains’ order parameters, which we propose to assist in stabilizing the HII phase by stretching the DSPS acyl chains for filling the voids formed by three adjacent lipid tubules. Furthermore, a systematic increase in the cholesterol concentration increased the lattice plane spacing and the water core radius but decreased the undulations along the lipid tubule axis. We propose that cholesterol and the degree of saturation/polyunsaturation of the lipid acyl chains, and not the lipid charge, are the main contributors in facilitating the Lα–HII phase transition and stabilizing/destabilizing the formed HII phase, whereas the positive charge of the ionizable cationic lipid promotes the LNP–endosomal membrane adhesion and assists in initiating the fusion process at the local contact area. We also propose that the effect of cholesterol on the HII structure and curvature is the main underlying reason for the well-documented HII stabilization and destabilization at low and high molar concentrations of cholesterol, respectively

Water Defect and Pore Formation in Atomistic and Coarse-Grained Lipid Membranes: Pushing the Limits of Coarse Graining

Defects in lipid bilayers are important in a range of biological processes, including interactions between antimicrobial peptides and membranes, transport of polar molecules (including drugs) across membranes, and lipid flip–flop from one monolayer to the other. Passive lipid flip–flop and the translocation of polar molecules across lipid membranes occur on a slow time scale because of high-energy intermediates involving water defects and pores in the membrane. Such defects are an interesting test case for coarse-grained models because of their relatively small characteristic size at the level of water molecules and the complex environment of water and polar head groups in a low-dielectric membrane interior. Here we compare coarse-grained simulations with the MARTINI model with the standard MARTINI water and two recently developed coarse-grained polarizable water models to atomistic simulations. Although in several cases the MARTINI model reproduces the correct free energies, there are structural differences between the atomistic and coarse-grained models. The polarizable water model improves the free energies but only moderately improves the structures. Atomistic test simulations in which water molecules are artificially tethered to each other in groups of four, the resolution of MARTINI, suggest that the limiting factor is not the size of the coarse-grained particles but rather the simple interaction potential and/or the entropy lost in coarse graining the system. By increasing the attractive interaction between the lipids’ headgroup and water, we did observe pore formation but at the expense of the correct equilibrium properties of the bilayers

Supramolecular Organization of SARS-CoV and SARS-CoV‑2 Virions Revealed by Coarse-Grained Models of Intact Virus Envelopes

The coronavirus disease 19 (COVID-19) pandemic is causing a global health crisis and has already caused a devastating societal and economic burden. The pathogen, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has a high sequence and architecture identity with SARS-CoV, but far more people have been infected by SARS-CoV-2. Here, combining the structural data from cryo-electron microscopy and structure prediction, we constructed bottom-up Martini coarse-grained models of intact SARS-CoV and SARS-CoV-2 envelopes. Microsecond molecular dynamics simulations were performed, allowing us to explore their dynamics and supramolecular organization. Both SARS-CoV and SARS-CoV-2 envelopes present a spherical morphology, with structural proteins forming multiple string-like islands in the membrane and clusters between the heads of spike proteins. Critical differences between the SARS-CoV and SARS-CoV-2 envelopes are the interaction pattern between the spike proteins and the flexibility of the spike proteins. Our models provide structural and dynamic insights into the SARS virus envelopes and could be used for further investigation, such as drug design and membrane fusion and fission processes

Phase Separation in Atomistic Simulations of Model Membranes

Understanding the lateral organization in plasma membranes remains an open problem despite a large body of research. Model membranes with coexisting micrometer-size domains are routinely employed as simplified models of plasma membranes. Many molecular dynamics simulations have investigated phase separation in model membranes at the coarse-grained level, but atomistic simulations remain computationally challenging. We simulate DPPC:DOPC and DPPC:DOPC:cholesterol lipid bilayers to investigate phase transitions at temperatures from 310 to 270 K. In this temperature range, the binary mixture forms a liquid phase (Lα) and a coexistence of Lα and either gel or ripple phases. The ternary mixture forms a liquid disordered (Ld) phase and a coexistence of liquid ordered (Lo) and either Ld or gel phases. We quantify the coexisting phases and discuss their properties against the background of experimental results. We observe partial registration of growing domains in both mixtures. We characterize specific cholesterol–cholesterol and cholesterol–phospholipid interaction geometries underlying its increased partitioning and the smoothed phase transition in the ternary mixture compared to the binary mixture. By comparing coexisting domains with homogeneous bilayers of the same composition, we demonstrate how domain coexistence affects their properties. Our simulations provide important insights into the lipid–lipid interactions in model lipid bilayers and improve our understanding of the lateral organization in plasma membranes with higher compositional complexity

Microsecond Molecular Dynamics Simulations of Lipid Mixing

Molecular dynamics (MD) simulations of membranes are often hindered by the slow lateral diffusion of lipids and the limited time scale of MD. In order to study the dynamics of mixing and characterize the lateral distribution of lipids in converged mixtures, we report microsecond-long all-atom MD simulations performed on the special-purpose machine Anton. Two types of mixed bilayers, POPE:POPG (3:1) and POPC:cholesterol (2:1), as well as a pure POPC bilayer, were each simulated for up to 2 μs. These simulations show that POPE:POPG and POPC:cholesterol are each fully miscible at the simulated conditions, with the final states of the mixed bilayers similar to a random mixture. By simulating three POPE:POPG bilayers at different NaCl concentrations (0, 0.15, and 1 M), we also examined the effect of salt concentration on lipid mixing. While an increase in NaCl concentration is shown to affect the area per lipid, tail order, and lipid lateral diffusion, the final states of mixing remain unaltered, which is explained by the largely uniform increase in Na⁺ ions around POPE and POPG. Direct measurement of water permeation reveals that the POPE:POPG bilayer with 1 M NaCl has reduced water permeability compared with those at zero or low salt concentration. Our calculations provide a benchmark to estimate the convergence time scale of all-atom MD simulations of lipid mixing. Additionally, equilibrated structures of POPE:POPG and POPC:cholesterol, which are frequently used to mimic bacterial and mammalian membranes, respectively, can be used as starting points of simulations involving these membranes

Supramolecular Organization of SARS-CoV and SARS-CoV‑2 Virions Revealed by Coarse-Grained Models of Intact Virus Envelopes

The coronavirus disease 19 (COVID-19) pandemic is causing a global health crisis and has already caused a devastating societal and economic burden. The pathogen, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has a high sequence and architecture identity with SARS-CoV, but far more people have been infected by SARS-CoV-2. Here, combining the structural data from cryo-electron microscopy and structure prediction, we constructed bottom-up Martini coarse-grained models of intact SARS-CoV and SARS-CoV-2 envelopes. Microsecond molecular dynamics simulations were performed, allowing us to explore their dynamics and supramolecular organization. Both SARS-CoV and SARS-CoV-2 envelopes present a spherical morphology, with structural proteins forming multiple string-like islands in the membrane and clusters between the heads of spike proteins. Critical differences between the SARS-CoV and SARS-CoV-2 envelopes are the interaction pattern between the spike proteins and the flexibility of the spike proteins. Our models provide structural and dynamic insights into the SARS virus envelopes and could be used for further investigation, such as drug design and membrane fusion and fission processes

Microsecond Molecular Dynamics Simulations of Lipid Mixing

Molecular dynamics (MD) simulations of membranes are often hindered by the slow lateral diffusion of lipids and the limited time scale of MD. In order to study the dynamics of mixing and characterize the lateral distribution of lipids in converged mixtures, we report microsecond-long all-atom MD simulations performed on the special-purpose machine Anton. Two types of mixed bilayers, POPE:POPG (3:1) and POPC:cholesterol (2:1), as well as a pure POPC bilayer, were each simulated for up to 2 μs. These simulations show that POPE:POPG and POPC:cholesterol are each fully miscible at the simulated conditions, with the final states of the mixed bilayers similar to a random mixture. By simulating three POPE:POPG bilayers at different NaCl concentrations (0, 0.15, and 1 M), we also examined the effect of salt concentration on lipid mixing. While an increase in NaCl concentration is shown to affect the area per lipid, tail order, and lipid lateral diffusion, the final states of mixing remain unaltered, which is explained by the largely uniform increase in Na⁺ ions around POPE and POPG. Direct measurement of water permeation reveals that the POPE:POPG bilayer with 1 M NaCl has reduced water permeability compared with those at zero or low salt concentration. Our calculations provide a benchmark to estimate the convergence time scale of all-atom MD simulations of lipid mixing. Additionally, equilibrated structures of POPE:POPG and POPC:cholesterol, which are frequently used to mimic bacterial and mammalian membranes, respectively, can be used as starting points of simulations involving these membranes

Phase Separation in Atomistic Simulations of Model Membranes

Understanding the lateral organization in plasma membranes remains an open problem despite a large body of research. Model membranes with coexisting micrometer-size domains are routinely employed as simplified models of plasma membranes. Many molecular dynamics simulations have investigated phase separation in model membranes at the coarse-grained level, but atomistic simulations remain computationally challenging. We simulate DPPC:DOPC and DPPC:DOPC:cholesterol lipid bilayers to investigate phase transitions at temperatures from 310 to 270 K. In this temperature range, the binary mixture forms a liquid phase (Lα) and a coexistence of Lα and either gel or ripple phases. The ternary mixture forms a liquid disordered (Ld) phase and a coexistence of liquid ordered (Lo) and either Ld or gel phases. We quantify the coexisting phases and discuss their properties against the background of experimental results. We observe partial registration of growing domains in both mixtures. We characterize specific cholesterol–cholesterol and cholesterol–phospholipid interaction geometries underlying its increased partitioning and the smoothed phase transition in the ternary mixture compared to the binary mixture. By comparing coexisting domains with homogeneous bilayers of the same composition, we demonstrate how domain coexistence affects their properties. Our simulations provide important insights into the lipid–lipid interactions in model lipid bilayers and improve our understanding of the lateral organization in plasma membranes with higher compositional complexity

Phase Separation in Atomistic Simulations of Model Membranes

Understanding the lateral organization in plasma membranes remains an open problem despite a large body of research. Model membranes with coexisting micrometer-size domains are routinely employed as simplified models of plasma membranes. Many molecular dynamics simulations have investigated phase separation in model membranes at the coarse-grained level, but atomistic simulations remain computationally challenging. We simulate DPPC:DOPC and DPPC:DOPC:cholesterol lipid bilayers to investigate phase transitions at temperatures from 310 to 270 K. In this temperature range, the binary mixture forms a liquid phase (Lα) and a coexistence of Lα and either gel or ripple phases. The ternary mixture forms a liquid disordered (Ld) phase and a coexistence of liquid ordered (Lo) and either Ld or gel phases. We quantify the coexisting phases and discuss their properties against the background of experimental results. We observe partial registration of growing domains in both mixtures. We characterize specific cholesterol–cholesterol and cholesterol–phospholipid interaction geometries underlying its increased partitioning and the smoothed phase transition in the ternary mixture compared to the binary mixture. By comparing coexisting domains with homogeneous bilayers of the same composition, we demonstrate how domain coexistence affects their properties. Our simulations provide important insights into the lipid–lipid interactions in model lipid bilayers and improve our understanding of the lateral organization in plasma membranes with higher compositional complexity