OpenStructure: an integrated software framework for computational structural biology.

Research projects in structural biology increasingly rely on combinations of heterogeneous sources of information, e.g. evolutionary information from multiple sequence alignments, experimental evidence in the form of density maps and proximity constraints from proteomics experiments. The OpenStructure software framework, which allows the seamless integration of information of different origin, has previously been introduced. The software consists of C++ libraries which are fully accessible from the Python programming language. Additionally, the framework provides a sophisticated graphics module that interactively displays molecular structures and density maps in three dimensions. In this work, the latest developments in the OpenStructure framework are outlined. The extensive capabilities of the framework will be illustrated using short code examples that show how information from molecular-structure coordinates can be combined with sequence data and/or density maps. The framework has been released under the LGPL version 3 license and is available for download from http://www.openstructure.org.

Progress in the analysis of membrane protein structure and function.

Structural information on membrane proteins is sparse, yet they represent an important class of proteins that is encoded by about 30% of all genes. Progress has primarily been achieved with bacterial proteins, but efforts to solve the structure of eukaryotic membrane proteins are also increasing. Most of the structures currently available have been obtained by exploiting the power of X-ray crystallography. Recent results, however, have demonstrated the accuracy of electron crystallography and the imaging power of the atomic force microscope. These instruments allow membrane proteins to be studied while embedded in the bi-layer, and thus in a functional state. The low signal-to-noise ratio of cryo-electron microscopy is overcome by crystallizing membrane proteins in a two-dimensional protein-lipid membrane, allowing its atomic structure to be determined. In contrast, the high signal-to-noise ratio of atomic force microscopy allows individual protein surfaces to be imaged at sub-nanometer resolution, and their conformational states to be sampled. This review summarizes the steps in membrane protein structure determination and illuminates recent progress.

Differential dimer activities of the transcription factor Oct-1 by DNA-induced interface swapping.

Two crystal structures of Oct-1 POU domain bound to DNA provide a rationale for differential, conformation-dependent recruitment of transcription cofactors. The POU-homeo and POU-specific subdomains of Oct-1 contain two different nonoverlapping pairs of surface patches that are capable of forming unrelated protein-protein interfaces. Members of the POU factor family contain one or two conserved sequence motifs in the interface that are known to be phosphorylated, as noted for Oct-1 and Pit-1. Modeling of Oct-4 reveals the unique case where the same conserved sequence is located in both interfaces. Our studies provide the basis for two distinct dimeric POU factor arrangements that are dictated by the architecture of each DNA response element. We suggest interface swapping in dimers could be a general mechanism of modulating the activity of transcription factors.

Role of charged residues at the OmpF porin channel constriction probed by mutagenesis and simulation.

The channel constriction of OmpF porin, a pore protein in the bacterial outer membrane, is highly charged due to the presence of three arginines (R42, R82, and R132) and two acidic residues (D113 and E117). The influence of these charges on ion conductance, ion selectivity, and voltage gating has been studied with mutants D113N/E117Q, R42A/R82A/R132A/D113N/E117Q, and V18K/G131K, which were designed to remove or add protein charge at the channel constriction. The crystal structures revealed no or only local changes compared to wild-type OmpF, thus allowing a comparative study. The single-channel conductance of the isosteric D113N/E117Q variant was found to be 2-fold reduced, and that of the pentuple mutant was 70% of the wild-type value, despite a considerably larger pore cross section. Ion selectivity was drastically altered by the mutations with cation/anion permeability ratios ranging from 1 to 12. Ion flow through these and eight other mutants, which have been characterized previously, was simulated by Brownian dynamics based on the detailed crystal structures. The calculated ion selectivity and relative channel conductance values agree well with the experimental data. This demonstrates that ion translocation through porin is mainly governed by pore geometry and charge, the two factors that are properly represented in the simulations.

The 6.9-A structure of GlpF: a basis for homology modeling of the glycerol channel from Escherichia coli.

The three-dimensional structure of GlpF, the glycerol facilitator of Escherichia coli, was determined by cryo-electron microscopy. The 6.9-A density map calculated from images of two-dimensional crystals shows the GlpF helices to be similar to those of AQP1, the erythrocyte water channel. While the helix arrangement of GlpF does not reflect the larger pore diameter as seen in the projection map, additional peripheral densities observed in GlpF are compatible with the 31 additional residues in loops C and E, which accordingly do not interfere with the inner channel construction. Therefore, the atomic structure of AQP1 was used as a basis for homology modeling of the GlpF channel, which is predicted to be free of bends, wider, and more vertically oriented than the AQP1 channel. Furthermore, the residues facing the GlpF channel exhibit an amphiphilic nature, being hydrophobic on one side and hydrophilic on the other side. This property may partially explain the contradiction of glycerol diffusion but limited water permeation capacity.

The 3.7 A projection map of the glycerol facilitator GlpF: a variant of the aquaporin tetramer.

GlpF, the glycerol facilitator protein of Escherichia coli, is an archetypal member of the aquaporin superfamily. To assess its structure, recombinant histidine-tagged protein was overexpressed, solubilized in octylglucoside and purified to homogeneity. Negative stain electron microscopy of solubilized GlpF protein revealed a tetrameric structure of approximately 80 A side length. Scanning transmission electron microscopy yielded a mass of 170 kDa, corroborating the tetrameric nature of GlpF. Reconstitution of GlpF in the presence of lipids produced highly ordered two-dimensional crystals, which diffracted electrons to 3.6 A resolution. Cryoelectron microscopy provided a 3.7 A projection map exhibiting a unit cell comprised of two tetramers. In projection, GlpF is similar to AQP1, the erythrocyte water channel. However, the major density minimum within each monomer is distinctly larger in GlpF than in AQP1.

Stability of trimeric OmpF porin: the contributions of the latching loop L2.

The channel-forming protein OmpF porin from Escherichia coli spans the bacterial outer membrane. Each of the three monomers comprises a hollow, 16-stranded beta-barrel. These are associated to homotrimers which are unusually stable, due mostly to hydrophobic interactions between the beta-barrels. In addition, a loop, L2 connects one subunit to its neighbor by latching into its channel. Residue E71 on loop 2 is integrated into an ionic network and forms salt bridges and hydrogen bonds with R100 and R132 on the channel wall in the adjacent subunit. To examine these contributions quantitatively, six single-site, two double, and one deletion mutant were constructed on the basis of the atomic coordinates of the protein. Differential scanning calorimetric analysis showed that the salt-bridge, E71-R100, contributes significantly to trimer stability: the substitution E71Q causes a decrease of the transition temperature from 72 to 48 degreesC, with DeltaHcal diminishing from 430 to 201 kcal mol-1. A nearby substitution in the loop, D74N, has lesser effects on thermal stability, while the deletion in L2 (Delta69-77) has an effect comparable to that of E71Q. X-ray structure analysis to 3.0 A resolution revealed only local structural differences in the mutants except for the substitution R100A, where another residue, R132, is found to fill the gap left by the truncated side chain of A100. Functional assays in planar lipid bilayers show significantly increased cation selectivities if the charge distribution was affected.