Hydrophobic Mismatch Controls the Mode of Membrane-Mediated Interactions of Transmembrane Peptides

Curvature Footprints of Transmembrane Proteins in Simulations with the Martini Force Field

Membranes play essential roles in biological systems and are tremendously diverse in the topologies and chemical and elastic properties that define their functions. In many cases, a given membrane may display considerable heterogeneity, with localized clusters of lipids and proteins exhibiting distinct characteristics compared to adjoining regions. These lipid-protein assemblies can span nanometers to micrometers and are associated with cellular processes such as transport and signaling. While lipid-protein assemblages are dynamic, they can be stabilized by coupling between local membrane composition and shape. Due to the inherent difficulty in resolving atomistic details of membrane proteins in their native lipid environments, these complexes are notoriously challenging to study experimentally; however, molecular dynamics (MD) simulations might be a viable alternative. Here, we aim to assess the utility of coarse-grained (CG) MD simulations with the Martini force field for studying membrane curvature induced by transmembrane (TM) proteins that are reported to generate local curvature. The direction and magnitude of curvature induced by five different TM proteins, as well as certain lipid-protein and protein-protein interactions, were found to be in good agreement with available reference data.

Alteration of Average Thickness of Lipid Bilayer by Membrane-Deforming Inclusions

Thickness of lipid bilayer membranes is a key physical parameter determining membrane permeability and stability with respect to formation of through pores. Most membrane inclusions or impurities like amphipathic peptides, transmembrane peptides, lipid inclusions of a different molecular shape, lipid domains, and protein-lipid domains, locally deform the membrane. The detailed structure of the locally deformed region of the membrane is a kind of "fingerprint" for the inclusion type. However, most experimental methods allow determining only averaged parameters of membranes with incorporated inclusions, thus preventing the direct obtaining of the characteristics of the inclusion. Here we developed a model that allows the obtaining of characteristic parameters of three types of membrane inclusions (amphipathic peptides, transmembrane peptides, monolayer lipid patches) from experimentally observable dependencies of the average thickness of lipid bilayer on the surface concentration of the inclusions. In the case of amphipathic peptides, the model provided the peptide parameters that were in qualitative agreement with the available experimental data.

Hypo-Osmotic Stress and Pore-Forming Toxins Adjust the Lipid Order in Sheep Red Blood Cell Membranes

Lipid ordering in cell membranes has been increasingly recognized as an important factor in establishing and regulating a large variety of biological functions. Multiple investigations into lipid organization focused on assessing ordering from temperature-induced phase transitions, which are often well outside the physiological range. However, particular stresses elicited by environmental factors, such as hypo-osmotic stress or protein insertion into membranes, with respect to changes in lipid status and ordering at constant temperature are insufficiently described. To fill these gaps in our knowledge, we exploited the well-established ability of environmentally sensitive membrane probes to detect intramembrane changes at the molecular level. Our steady state fluorescence spectroscopy experiments focused on assessing changes in optical responses of Laurdan and diphenylhexatriene upon exposure of red blood cells to hypo-osmotic stress and pore-forming toxins at room temperature. We verified our utilized experimental systems by a direct comparison of the results with prior reports on artificial membranes and cholesterol-depleted membranes undergoing temperature changes. The significant changes observed in the lipid order after exposure to hypo-osmotic stress or pore-forming toxins resembled phase transitions of lipids in membranes, which we explained by considering the short-range interactions between membrane components and the hydrophobic mismatch between membrane thickness and inserted proteins. Our results suggest that measurements of optical responses from the membrane probes constitute an appropriate method for assessing the status of lipids and phase transitions in target membranes exposed to mechanical stresses or upon the insertion of transmembrane proteins.

Simulation study of domain formation in a model bacterial membrane

Recent experimental studies revealed that functional membrane microdomains (FMMs) are formed in prokaryotic cells which are structurally and functionally similar to the lipid rafts formed in eukaryotic cells. In this study, we employ coarse-grained molecular dynamics simulations to investigate the mechanism of domain formation and its physiochemical properties in a model methicillin-resistant staphylococcus aureus (MRSA) cell membrane. We find that domains are formed through lateral segregation of staphyloxanthin (STX), a carotenoid which shields the bacteria from the host's immune because of its antioxidant nature. Simulation results suggest that membrane integrity increases with the size of the domain, which is assessed by computing bond order parameter of the lipid tails, membrane expansion modulus and water permeability across the membrane. Various membrane domain proteins such as flotillin-like protein floA and penicillin binding protein (PBP2a) preferentially bind with the STX and accumulate in the membrane domain which is consistent with the recent experimental results.

Exploring the trimerization process of a transmembrane helix with an ionizable residue by molecular dynamics simulations: a case study of transmembrane domain 5 of LMP-1

The oligomerization of membrane proteins is an important biological process that plays a critical role in the initialization of membrane protein receptor signaling. Unveiling how transmembrane segments oligomerize is critical for understanding the mechanism of membrane receptor signaling activation. Owing to the complicated membrane environment and the extraordinary dynamic properties of the ionizable residues in the transmembrane segment, it is extremely challenging to thoroughly understand the oligomerization process of the transmembrane domain. In this study, transmembrane domain 5 (TMD5) of latent membrane protein-1 from Epstein-Barr virus was used as a prototype model to investigate the trimerization process of the transmembrane segment with ionizable residues. The trimerization process of TMD5 was rebuilt and investigated via conventional molecular dynamics simulations and constant-pH molecular dynamics simulations. When TMD5s approached each other, the tilting angles of the TMD5 monomer decreased. TMD5s formed stable trimers until two interacting sites (D150s and Q139s) along each transmembrane helix were created to lock the TMD5s. The pK_a values of D150 shifted toward neutral states in the membrane environment. When TMD5s were monomers, the pK_a shift of D150 was mainly influenced by its microenvironment in the lipid bilayer. When TMD5s were moving close to each other, protein-protein interactions became the main contributing factor for the pK_a shift of D150s. Overall, this work elucidates the behavior of the TMD5 helix and the pK_a shift of ionizable residue D150 in the process of TMD5 oligomerization. This study may provide insight into the development of agents for targeting the oligomerization of membrane proteins.