Computational analysis of membrane proteins: the largest class of drug targets.

Given the key roles of integral membrane proteins as transporters and channels, it is necessary to understand their structures and, hence, mechanisms and regulation at the molecular level. Membrane proteins represent approximately 30% of all proteins of currently sequenced genomes. Paradoxically, however, only approximately 2% of crystal structures deposited in the protein data bank are of membrane proteins, and very few of these are at high resolution (better than 2A). The great disparity between our understanding of soluble proteins and our understanding of membrane proteins is because of the practical problems of working with membrane proteins - specifically, difficulties in expression, purification and crystallization. Thus, computational modeling has been utilized extensively to make crucial advances in understanding membrane protein structure and function.

Binding site flexibility: molecular simulation of partial and full agonists within a glutamate receptor.

Ionotropic glutamate receptors mediate fast synaptic transmission in the mammalian central nervous system and play an important role in many different functions, including memory and learning. They have also been implicated in a variety of neuropathologies and as such have generated widespread interest in their structure and function. Molecular Dynamics simulations (5 x 20 ns) of the ligand-binding core of the GluR2 glutamate receptor were performed. Through simulations of both wild type and the L650T mutant, we show that the degree of protein flexibility can be correlated with the extent to which the binding cleft is open. In agreement with recent experiments, the simulations of kainate with the wild-type construct show a slight increase in beta-sheet content that we are able to localize to two specific regions. During one simulation, the protein made a transition from an open-cleft conformation to a closed-cleft conformation. This closed cleft conformation closely resembles the closed-cleft crystal structure, thus indicating a potential pathway for conformational change associated with receptor activation. Analysis of the binding pocket suggests that partial agonists possess a greater degree of flexibility within the pocket that may help to explain why they are less efficient at opening the channel than full agonists. Examination of water molecules surrounding the ligands reveals that mobility in distinct subsites can be a discriminator between full and partial agonism and will be an important consideration in the design of drugs against these receptors.

Comparative molecular dynamics--similar folds and similar motions?

Proteins possessing the same fold may undergo similar motions, particularly if these motions involve large conformational transitions. The increasing amounts of structural data provide a useful starting point with which to test this hypothesis. We have performed a total of 0.29 micros of molecular dynamics across a series of proteins within the same fold family (periplasmic binding proteinlike) in order to address to what extent similarity of motion exists. Analysis of the local conformational space on these timescales (10-20 ns) revealed that the behavior of the proteins could be readily distinguished between an apo-state and a ligand-bound state. Moreover, analysis of the root-mean-square fluctuations reveals that the presence of the ligand exerts a stabilizing effect on the protein, with similar motions occurring, but with reduced magnitude. Furthermore, the conformational space in the presence of the ligand appears to be dictated by sequence but not by the type of ligand present. In contrast, apo-simulations showed considerable overlap of conformational space across the fold as a result of their ability to undergo larger fluctuations. Indeed, we observed several transitions from different simulations between states corresponding to the closed-cleft and open-cleft forms of the fold, with the predominant motions being conserved across the different proteins. Thus, large-scale conformational changes do indeed appear to be conserved across this fold architecture, but smaller conformational motions appear to reflect the differences in sequence and local fold.

Membrane protein structure quality in molecular dynamics simulation.

Our goal was to assess the relationship between membrane protein quality, output from protein quality checkers and output from molecular dynamics (MD) simulations. Membrane transport proteins are essential for a wide range of cellular processes. Structural features of integral membrane proteins are still under-explored due to experimental limitations in structure determination. Computational techniques can be used to exploit biochemical and medium resolution structural data, as well as sequence homology to known structures, and enable us to explore the structure-function relationships in several transmembrane proteins. The quality of the models produced is vitally important to obtain reliable predictions. An examination of the relationship between model stability in molecular dynamics (MD) simulations derived from RMSD (root mean squared deviation) and structure quality assessment from various protein quality checkers was undertaken. The results were compared to membrane protein structures, solved at various resolution, by either X-ray or electron diffraction techniques. The checking programs could predict the potential success of MD in making functional conclusions. MD stability was shown to be a good indicator for the quality of structures. The quality was also shown to be dependent on the resolution at which the structures were determined.

A prokaryotic glutamate receptor: homology modelling and molecular dynamics simulations of GluR0.

GluR0 is a prokaryotic homologue of mammalian glutamate receptors that forms glutamate-activated, potassium-selective ion channels. The topology of its transmembrane (TM) domain is similar to that of simple potassium channels such as KcsA. Two plausible alignments of the sequence of the TM domain of GluR0 with KcsA are possible, differing in the region of the P helix. We have constructed homology models based on both alignments and evaluated them using 6 ns duration molecular dynamics simulations in a membrane-mimetic environment. One model, in which an insertion in GluR0 relative to KcsA is located in the loop between the M1 and P helices, is preferred on the basis of lower structural drift and maintenance of the P helix conformation during simulation. This model also exhibits inter-subunit salt bridges that help to stabilise the TM domain tetramer. During the simulation, concerted K(+) ion-water movement along the selectivity filter is observed, as is the case in simulations of KcsA. K(+) ion exit from the central cavity is associated with opening of the hydrophobic gate formed by the C-termini of the M2 helices. In the intact receptor the opening of this gate will be controlled by interactions with the extramembranous ligand-binding domains.

Interdomain dynamics and ligand binding: molecular dynamics simulations of glutamine binding protein.

Periplasmic binding proteins from Gram-negative bacteria possess a common architecture, comprised of two domains linked by a hinge region, a fold which they share with the neurotransmitter-binding domains of ionotropic glutamate receptors (GluRs). Glutamine-binding protein (GlnBP) is one such protein, whose crystal structure has been solved in both open and closed forms. Multi-nanosecond molecular dynamics simulations have been used to explore motions about the hinge region and how they are altered by ligand binding. Glutamine binding is seen to significantly reduce inter-domain motions about the hinge region. Essential dynamics analysis of inter-domain motion revealed the presence of both hinge-bending and twisting motions, as has been reported for a related sugar-binding protein. Significantly, the influence of the ligand on GlnBP dynamics is similar to that previously observed in simulations of rat glutamate receptor (GluR2) ligand-binding domain. The essential dynamics analysis of GlnBP also revealed a third class of motion which suggests a mechanism for signal transmission in GluRs.

Ion channels: structural bioinformatics and modelling.

Ion channels are membrane proteins of key physiological and pharmacological importance. As is the case for many integral membrane proteins, X-ray structures are known for a few bacterial channels, yet structures of human homologues are required for analysis of channel-associated diseases and for drug design. Homology modelling can be used to help remedy this deficit. In combination with molecular dynamics simulations and associated calculations, modelling provides a powerful approach to understanding structure/function relationships in human ion channels. Modelling techniques have been applied to two classes of potassium channels: voltage-gated (Kv) and inward rectifier (Kir) channels. Kir channel models, based on the structure of the bacterial channel KcsA, have been used as a starting point for detailed simulation studies that have increased our understanding of ion permeation and selectivity mechanisms. The transmembrane domain of GluR0, a bacterial homologue of mammalian glutamate receptors, also may be modelled using the KcsA structure as a template. Models of the nicotinic acetylcholine receptor may be constructed in a modular fashion. The snail acetylcholine-binding protein provides a template for the extracellular ligand-binding domain. The transmembrane pore region can be modelled on the basis of NMR structures of the pore-lining M2 helix.

Molecular dynamics simulations of the ligand-binding domain of the ionotropic glutamate receptor GluR2.

Ionotropic glutamate receptors are essential for fast synaptic nerve transmission. Recent x-ray structures for the ligand-binding (S1S2) region of the GluR2 alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-sensitive receptor have suggested how differences in protein/ligand interactions may determine whether a ligand will behave as a full agonist. We have used multiple molecular dynamics simulations of 2-5 ns duration to explore the structural dynamics of GluR2 S1S2 in the presence and absence of glutamate and in a complex with kainate. Our studies indicate that not only is the degree of domain closure dependent upon interactions with the ligand, but also that protein/ligand interactions influence the motion of the S2 domain with respect to S1. Differences in domain mobility between the three states (apo-S1S2, glutamate-bound, and kainate-bound) are surprisingly clear-cut. We discuss how these changes in dynamics may provide an explanation relating the mechanism of transmission of the agonist-binding event to channel opening. We also show here how the glutamate may adopt an alternative mode of binding not seen in the x-ray structure, which involves a key threonine (T480) side chain flipping into a new conformation. This new conformation results in an altered pattern of hydrogen bonding at the agonist-binding site.