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G protein-coupled receptors (GPCRs) relay diverse extracellular signals into cells by catalyzing nucleotide release from heterotrimeric G proteins, but the mechanism underlying this quintessential molecular signaling event has remained unclear. Here we use atomic-level simulations to elucidate the nucleotide-release mechanism. We find that the G protein a subunit Ras and helical domains—previously observed to separate widely upon receptor binding to expose the nucleotide-binding site—separate spontaneously and frequently even in the absence of a receptor. Domain separation is necessary but not sufficient for rapid nucleotide release. Rather, receptors catalyze nucleotide release by favoring an internal structural rearrangement of the Ras domain that weakens its nucleotide affinity. We use double electron-electron resonance spectroscopy and protein engineering to confirm predictions of our computationally determined mechanism.

ClyA is a pore‐forming toxin from virulent Escherichia coli and Salmonella enterica strains. Here, we show that the intrinsic hemolytic activity of ClyA is independent of its redox state, and that the assembly of both reduced and oxidized ClyA to the ring‐shaped oligomer is triggered by contact with lipid or detergent. A rate‐limiting conformational transition in membrane‐bound ClyA monomers precedes their assembly to the functional pore. We obtained a three‐dimensional model of the detergent‐induced oligomeric complex at 12 Å resolution by combining cryo‐ and negative stain electron microscopy with mass measurements by scanning transmission electron microscopy. The model reveals that 13 ClyA monomers assemble into a cylinder with a hydrophobic cap region, which may be critical for membrane insertion.

The atomic force microscope (AFM) was used to image native OmpF porin and to detect the electrostatic potential generated by the protein. To this end the OmpF porin trimers from Escherichia coli was reproducibly imaged at a lateral resolution of ∼0.5 nm and a vertical resolution of ∼0.1 nm at variable electrolyte concentrations of the buffer solution. At low electrolyte concentrations the charged AFM probe not only contoured structural details of the membrane protein surface but also interacted with local electrostatic potentials. Differences measured between topographs recorded at variable ionic strength allowed mapping of the electrostatic potential of OmpF porin. The potential map acquired by AFM showed qualitative agreement with continuum electrostatic calculations based on the atomic OmpF porin embedded in a lipid bilayer at the same electrolyte concentrations. Numerical simulations of the experimental conditions showed the measurements to be reproduced quantitatively when the AFM probe was included in the calculations. This method opens a novel avenue to determine the electrostatic potential of native protein surfaces at a lateral resolution better than 1 nm and a vertical resolution of ∼0.1 nm.

G protein-coupled receptors (GPCRs) relay diverse extracellular signals into cells by catalyzing nucleotide release from heterotrimeric G proteins, but the mechanism underlying this quintessential molecular signaling event has remained unclear. Here we use atomic-level simulations to elucidate the nucleotide-release mechanism. We find that the G protein α subunit Ras and helical domains-previously observed to separate widely upon receptor binding to expose the nucleotide-binding site-separate spontaneously and frequently even in the absence of a receptor. Domain separation is necessary but not sufficient for rapid nucleotide release. Rather, receptors catalyze nucleotide release by favoring an internal structural rearrangement of the Ras domain that weakens its nucleotide affinity. We use double electron-electron resonance spectroscopy and protein engineering to confirm predictions of our computationally determined mechanism.

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 ∼80 Å 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 Å resolution. Cryoelectron microscopy provided a 3.7 Å 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.

The rotor stoichiometry of F-ATPases has been revealed by the combined approaches of X-ray diffraction (XRD), electron crystallography, and atomic force microscopy (AFM). XRD showed the rotor from the yeast mitochondrial F-ATPase to contain 10 subunits. AFM was used to visualize the tetradecameric chloroplast rotors, and electron crystallography and AFM together revealed the rotors from Ilyobacter tartaricus to be composed of 11 subunits. While biochemical methods had determined an approximate stoichiometric value, precise measurements and new insights into a species-dependent rotor stoichiometry became available by applying the three structural tools together. The structures of AQP1, a water channel, and GlpF, a glycerol channel, were determined by electron crystallography and XRD. The combination of both of these structural tools with molecular dynamics simulations gave a differentiated description of the mechanisms determining the selectivity of water and glycerol channels. This illustrates that the combination of different methods in structural biology reveals more than each method alone.Key words: AQP1, GlpF, F-ATPase, XRD, electron crystallography, AFM.