An Implicit Membrane Generalized Born Theory for the Study of Structure, Stability, and Interactions of Membrane Proteins

This is the published version. Copyright 2003 by Elsevier.Exploiting recent developments in generalized Born (GB) electrostatics theory, we have reformulated the calculation of the self-electrostatic solvation energy to account for the influence of biological membranes. Consistent with continuum Poisson-Boltzmann (PB) electrostatics, the membrane is approximated as an solvent-inaccessible infinite planar low-dielectric slab. The present membrane GB model closely reproduces the PB electrostatic solvation energy profile across the membrane. The nonpolar contribution to the solvation energy is taken to be proportional to the solvent-exposed surface area (SA) with a phenomenological surface tension coefficient. The proposed membrane GB/SA model requires minor modifications of the pre-existing GB model and appears to be quite efficient. By combining this implicit model for the solvent/bilayer environment with advanced computational sampling methods, like replica-exchange molecular dynamics, we are able to fold and assemble helical membrane peptides. We examine the reliability of this model and approach by applications to three membrane peptides: melittin from bee venom, the transmembrane domain of the M2 protein from Influenza A (M2-TMP), and the transmembrane domain of glycophorin A (GpA). In the context of these proteins, we explore the role of biological membranes (represented as a low-dielectric medium) in affecting the conformational changes in melittin, the tilt of transmembrane peptides with respect to the membrane normal (M2-TMP), helix-to-helix interactions in membranes (GpA), and the prediction of the configuration of transmembrane helical bundles (GpA). The present method is found to perform well in each of these cases and is anticipated to be useful in the study of folding and assembly of membrane proteins as well as in structure refinement and modeling of membrane proteins where a limited number of experimental observables are available

Membrane models for molecular simulations of peripheral membrane proteins

Peripheral membrane proteins (PMPs) bind temporarily to the surface of biological membranes. They also exist in a soluble form and their tertiary structure is often known. Yet, their membrane-bound form and their interfacial-binding site with membrane lipids remain difficult to observe directly. Their binding and unbinding mechanism, the conformational changes of the PMPs and their influence on the membrane structure are notoriously challenging to study experimentally. Molecular dynamics simulations are particularly useful to fill some knowledge-gaps and provide hypothesis that can be experimentally challenged to further our understanding of PMP-membrane recognition. Because of the time-scales of PMP-membrane binding events and the computational costs associated with molecular dynamics simulations, membrane models at different levels of resolution are used and often combined in multiscale simulation strategies. We here review membrane models belonging to three classes: atomistic, coarse-grained and implicit. Differences between models are rooted in the underlying theories and the reference data they are parameterized against. The choice of membrane model should therefore not only be guided by its computational efficiency. The range of applications of each model is discussed and illustrated using examples from the literature.publishedVersio

The power of coarse graining in biomolecular simulations

Computational modeling of biological systems is challenging because of the multitude of spatial and temporal scales involved. Replacing atomistic detail with lower resolution, coarse grained (CG), beads has opened the way to simulate large-scale biomolecular processes on time scales inaccessible to all-atom models. We provide an overview of some of the more popular CG models used in biomolecular applications to date, focusing on models that retain chemical specificity. A few state-of-the-art examples of protein folding, membrane protein gating and self-assembly, DNA hybridization, and modeling of carbohydrate fibers are used to illustrate the power and diversity of current CG modeling

All-Atom Modeling of Protein Folding and Aggregation

Theoretical investigations of biorelevant processes in the life-science research require highly optimized simulation methods. Therefore, massively parallel Monte Carlo algorithms, namely MTM, were successfully developed and applied to the field of reversible protein folding allowing the thermodynamic characterization of proteins on an atomistic level. Further, the formation process of trans-membrane pores in the TatA system could be elucidated and the structure of the complex could be predicted

Toward biologically realistic computational membrane protein structure prediction and design

Membrane proteins function as gates and checkpoints that control the transit of molecules and information across the lipid bilayer. Understanding their structures will provide mechanistic insights in how to keep cells healthy and defend against disease. However, experimental difficulties have slowed the progress of structure determination. Previous work has demonstrated the promise of computational modeling for elucidating membrane protein structures. A remaining challenge is to model proteins coupled with the heterogeneous cell membrane environment. In the first half of this dissertation, I detail the development, testing and integration of a biologically realistic implicit lipid bilayer model in Rosetta. First, I describe the initial iteration of the implicit model that captures the anisotropic structure, shape of water-filled pores, and nanoscale dimensions of membranes with different lipid compositions. Second, I explain my approach to energy function benchmarking and optimization given the challenge of sparse and low-quality experimental data. Third, I outline the second generation that incorporates a new electrostatics and pH model. All of these developments have advanced the accuracy of Rosetta membrane protein structure prediction and design. In the second half of this dissertation, I investigate three challenging biological and engineering applications involving membrane proteins. In the first application, I examine mutation-induced stability changes in the integral membrane zinc metalloprotease ZMPSTE24: a protein with a large voluminous chamber that is not captured by current implicit models. In the second application, I model interactions between the SERCA2a calcium pump and the regulatory transmembrane protein phospholamban: a key membrane protein-protein interaction implicated in the heart’s response to adrenaline. Finally, I explore the challenge of membrane protein design to engineer a self-assembling transmembrane protein pore for nanotechnology applications. These applications highlight the next steps required to improve computational membrane protein modeling tools. Taken together, my work in both methods development and applications has advanced our understanding and ability to model and design membrane protein structures

Molecular modeling of the transmembrane domain of envelope glycoproteins from flaviviridae viruses

The putative transmembrane (TM) domains of the envelope glycoproteins from the family Flaviviridae consist of a highly polar segment in between two hydrophobic stretches. This type of sequence pattern does not yet exist in the database of high resolution structures of membrane proteins. Mutagenesis studies have shown that the TM domains act as membrane and signal anchors, and are responsible for heterodimerization. In hepatitis C virus (HCV), the TM domains of the envelope glycoproteins E1 and E2 were hypothesized to heterodimerize via an ion pair of Lys-Asp. Our MD simulations showed that the E1-E2 heterodimer formed by the charged residues located in the core of the lipid bilayer stabilized the helical conformation of E2. We compared the effect of other types of ion pair interactions using engineered peptides and obtained similar results. We found that an Asp amino acid had the strongest kink-inducing effect on the helix when it was located in the middle of a single-pass TM helix. The extended analyses on dengue, Japanese encephalitis, West Nile and bovine viral diarrhea viruses again showed that their putative TM domains behave similarly. All the TM domains of the E1/prM tended to tilt and remain helical in membrane bilayer. In contrast, the TM domains of the E2/E that contain a central Asp residue were severely kinked. Altogether, these TM domains illustrated a similar structural behavior in the lipid bilayer milieu.Die mutmaßlichen Transmembran (TM)-Domänen der Hüllglykoproteine der Familie Flaviviridae bestehen aus einem hochpolaren Segment zwischen zwei hydrophilen Abschnitten. Diess Sequenzmuster sind noch nicht in der Datenbank hochaufgelöster Strukturen von Membranproteinen enthalten. Gemäß Mutagenesestudien agieren die TM-Domänen als Membran- und Signalanker und sind für die Heterodimerisierung verantwortlich. Im Hepatitis C-Virus (HCV) heterodimerisieren die TM-Domänen der Hüllglykoproteine E1 und E2 möglicherweise über ein Ionenpaar zwischen Lys-Asp. Unsere MD-Simulationen zeigten, dass das E1-E2-Dimer, das durch die geladenen Residuen im Kern der Lipiddoppelschicht gebildet wird, die helikale Konformation von E2 stabilisiert. Der Effekt anderer Ionenpaarinteraktionen in künstlichen Peptiden führte zu ähnlichen Ergebnissen. Asp in der Mitte einer TM-Helix verursachte den stärksten Krümmungseffekt. Weitere Analysen mit anderen Flaviviridae (Dengue, Japanese encephalitis, West Nile und bovine viral diarrhea virus) zeigten ebenfalls ein ähnliches Verhalten ihrer mutmaßlichen TM-Domänen. Alle TM-Domänen von E1/prM tendierten zur Krümmung und blieben in der Membrandoppelschicht helikal. Hingegen waren die TM-Domänen von E2/E, die ein zentrales Asp enthalten, stark gekrümmt. Insgesamt zeigten diese mutmaßlichen TM-Domänen ein ähnliches strukturelles Verhalten in der Membran

Computational Investigation of the Pore Formation Mechanism of Beta-Hairpin Antimicrobial Peptides

β-hairpin antimicrobial peptides (AMPs) are small, usually cationic peptides that provide innate biological defenses against multiple agents. They have been proposed as the basis for novel antibiotics, but their pore formation has not been directly observed on a molecular level. We review previous computational studies of peptide-induced membrane pore formation and report several new molecular dynamics simulations of β-hairpin AMPs to elucidate their pore formation mechanism. We simulated β-barrels of various AMPs in anionic implicit membranes, finding that most of the AMPs’ β-barrels were not as stable as those of protegrin. We also performed an optimization study of protegrin β-barrels in implicit membranes, finding that nonamers were the most stable, but that multiplicities 7–13 were almost equally favorable. This indicated the possibility of a diversity of pore states consisting of various numbers of protegrin peptides. Finally, we used the Anton 2 supercomputer to perform multimicrosecond, all-atom molecular dynamics simulations of various protegrin-1 oligomers on the membrane surface and in transmembrane topologies. We also considered an octamer of the β-hairpin AMP tachyplesin. The simulations on the membrane surface indicated that protegrin dimers are stable, while trimers and tetramers break down because they assume a bent, twisted β-sheet shape. Tetrameric arcs remained stably inserted, but the pore water was displaced by lipid molecules. Unsheared protegrin β-barrels opened into long, twisted β-sheets that surrounded stable aqueous pores, whereas tilted barrels with sheared hydrogen bonding patterns were stable in most topologies. A third type of observed pore consisted of multiple small oligomers surrounding a small, partially lipidic pore. The octameric tachyplesin bundle resulted in small pores surrounded by 6 peptides as monomers and dimers. The results imply that multiple protegrin configurations may produce aqueous pores and illustrate the relationship between topology and pore formation steps. However, these structures’ long-term stability requires further investigation

Computational studies of biomembrane systems: Theoretical considerations, simulation models, and applications

This chapter summarizes several approaches combining theory, simulation and experiment that aim for a better understanding of phenomena in lipid bilayers and membrane protein systems, covering topics such as lipid rafts, membrane mediated interactions, attraction between transmembrane proteins, and aggregation in biomembranes leading to large superstructures such as the light harvesting complex of green plants. After a general overview of theoretical considerations and continuum theory of lipid membranes we introduce different options for simulations of biomembrane systems, addressing questions such as: What can be learned from generic models? When is it expedient to go beyond them? And what are the merits and challenges for systematic coarse graining and quasi-atomistic coarse grained models that ensure a certain chemical specificity

Computational Studies of Protein Structure, Dynamics, and Function in Native-like Environments

Proteins are among the four unique organic constituents of cells. They are responsible for a variety of important cell functions ranging from providing structural support to catalyzing biological reactions. They vary in shape, dynamic behavior, and localization. All of these together determine the specificity in their functions, but the question is how. The ultimate goal of the research conducted in this thesis is to answer this question. Two types of proteins are of particular interest. They include transmembrane proteins and protein assemblies. Using computer simulations with available experimental data to validate the simulation results, the research described here aims to reveal the structure and dynamics of proteins in their native-like environment and the indication on the mechanism of their functions. The first part of the thesis focuses on studying the structure and functions of transmembrane proteins. These proteins are consisted of transmembrane α-helices or β-strands, and each of the secondary structure elements adopts a unique orientation in the membrane following its local interactions. The structure of the entire protein is a collection of the orientations of these elements and their relative positions with respect to one another. These two basic aspects of membrane protein structure are studied in Chapter II and III. In Chapter II, efforts are given to determine the favorable orientation of a β-hairpin peptide, protegrin-1, in different lipid bilayers. The orientational preference results from the interplay between the protein and the surrounding lipid molecules. Chapter III is centered on revealing the structure and dynamics of caveolin-1 in DMPC bilayers. Caveolin-1 forms a re-entrant helix-turn-helix structure with two α-helices embedded in the membrane bilayer. The study shows that caveolin-1 monomer is rather dynamic and maintains its inserted conformation via both specific and non-specific protein-lipid interactions. To investigate the structural and dynamic impact on the function of a membrane protein, molecular dynamics simulations of the voltage-dependent anion channel are performed and the results are presented in Chapter IV. It is found in this chapter that the electrostatic interactions between charged residues on the channel wall facing the lumen are responsible for retarding the cation current, therefore giving the channel its anion selectivity. The second category of protein that is of interest in this thesis is the assembled protein complex, especially the ones that are highly symmetric. Actually, many membrane proteins belong to this category as well, but the study presented here in Chapter V involves simulations performed on a soluble protein complex, bacterioferritin B from Pseudomonas Aeruginosa. It is revealed by the simulations that the dynamic behavior of the protein is magnified by the symmetry and is tightly associated to its function