Human copper transporter ATP7B (Wilson disease protein) forms stable dimers in vitro and in cells

ATP7B is a copper-transporting P_1B-type ATPase (Cu-ATPase) with an essential role in human physiology. Mutations in ATP7B cause the potentially fatal Wilson disease, and changes in ATP7B expression are observed in several cancers. Despite its physiologic importance, the biochemical information about ATP7B remains limited because of a complex multidomain organization of the protein. By analogy with the better characterized prokaryotic Cu-ATPases, ATP7B is assumed to be a single-chain monomer. We show that in eukaryotic cells, human ATP7B forms dimers that can be purified following solubilization. Deletion of the four N-terminal metal-binding domains, characteristic for human ATP7B, does not disrupt dimerization, i.e. the dimer interface is formed by the domains that are conserved among Cu-ATPases. Unlike the full-length ATP7B, which is targeted to the trans-Golgi network, 1-4ΔMBD-7B is targeted primarily to vesicles. This result and the analysis of differentially tagged ATP7B variants indicate that the dimeric structure is retained during ATP7B trafficking between the intracellular compartments. Purified dimeric species of 1-4ΔMBD-7B were characterized by a negative stain electron microscopy in the presence of ADP/MgCl₂ Single-particle analysis yielded a low-resolution 3D model that provides the first insight into an overall architecture of a human Cu-ATPase, positions of the main domains, and a dimer interface.

Atox1 contains positive residues that mediate membrane association and aid subsequent copper loading

Copper chaperones bind intracellular copper and ensure proper trafficking to downstream targets via protein-protein interactions. In contrast to the mechanisms of copper binding and transfer to downstream targets, the mechanisms of initial copper loading of the chaperones are largely unknown. Here, we demonstrate that antioxidant protein 1 (Atox1 in human cells), the principal cellular copper chaperone responsible for delivery of copper to the secretory pathway, possesses the ability to interact with negatively charged lipid headgroups via distinct surface lysine residues. Moreover, loss of these residues lowers the efficiency of copper loading of Atox1 in vivo, suggesting that the membrane may play a scaffolding role in copper distribution to Atox1. These findings complement the recent discovery that the membrane also facilitates copper loading of the copper chaperone for superoxide dismutase 1 and provide further support for the emerging paradigm that the membrane bilayer plays a central role in cellular copper acquisition and distribution.

Understanding the role of amphipathic helices in N-BAR domain driven membrane remodeling

Endophilin N-BAR (N-terminal helix and Bin/amphiphysin/Rvs) domain tubulates and vesiculates lipid membranes in vitro via its crescent-shaped dimer and four amphipathic helices that penetrate into membranes as wedges. Like F-BAR domains, endophilin N-BAR also forms a scaffold on membrane tubes. Unlike F-BARs, endophilin N-BARs have N-terminal H0 amphipathic helices that are proposed to interact with other N-BARs in oligomer lattices. Recent cryo-electron microscopy reconstructions shed light on the organization of the N-BAR lattice coats on a nanometer scale. However, because of the resolution of the reconstructions, the precise positioning of the amphipathic helices is still ambiguous. In this work, we applied a coarse-grained model to study various membrane remodeling scenarios induced by endophilin N-BARs. We found that H0 helices of N-BARs prefer to align in an antiparallel manner at two ends of the protein to form a stable lattice. The deletion of H0 helices causes disruption of the lattice. In addition, we analyzed the persistence lengths of the protein-coated tubes and found that the stiffness of endophilin N-BAR-coated tubules qualitatively agrees with previous experimental work studying N-BAR-coated tubules. Large-scale simulations on membrane liposomes revealed a systematic relation between H0 helix density and local membrane curvature fluctuations. The data also suggest that the H0 helix is required for BARs to form organized structures on the liposome, further illustrating its important function.

Evaluation of electron crystallographic data from images of two-dimensional crystals

In the past, various formats have been used to present data fidelity of electron crystallographic data from images of two-dimensional crystals. While each scheme provides some information about the data, not all schemes are equally useful for presenting data reliability. This brief chapter gives guidelines on how to look at datasets that are solely derived from images, and on how to summarize the data structure through graphical outputs that provide unbiased and detailed information about data quality. By the time of publication, the actual procedures described here will have been integrated into the 2dx package and therefore, these evaluation tools will be available at a "push of a button." Because of this, the chapter focuses on brief explanations for why certain ways of presenting data make more sense than others.

Structural basis of membrane bending by the N-BAR protein endophilin

Functioning as key players in cellular regulation of membrane curvature, BAR domain proteins bend bilayers and recruit interaction partners through poorly understood mechanisms. Using electron cryomicroscopy, we present reconstructions of full-length endophilin and its N-terminal N-BAR domain in their membrane-bound state. Endophilin lattices expose large areas of membrane surface and are held together by promiscuous interactions between endophilin's amphipathic N-terminal helices. Coarse-grained molecular dynamics simulations reveal that endophilin lattices are highly dynamic and that the N-terminal helices are required for formation of a stable and regular scaffold. Furthermore, endophilin accommodates different curvatures through a quantized addition or removal of endophilin dimers, which in some cases causes dimerization of endophilin's SH3 domains, suggesting that the spatial presentation of SH3 domains, rather than affinity, governs the recruitment of downstream interaction partners.

Electron crystallography--the waking beauty of structural biology

Since its debut in the mid 1970s, electron crystallography has been a valuable alternative in the structure determination of biological macromolecules. Its reliance on single-layered or double-layered two-dimensionally ordered arrays and the ability to obtain structural information from small and disordered crystals make this approach particularly useful for the study of membrane proteins in a lipid bilayer environment. Despite its unique advantages, technological hurdles have kept electron crystallography from reaching its full potential. Addressing the issues, recent initiatives developed high-throughput pipelines for crystallization and screening. Adding progress in automating data collection, image analysis and phase extension methods, electron crystallography is poised to raise its profile and may lead the way in exploring the structural biology of macromolecular complexes.

Structure and function of copper uptake transporters

Owing to their redox and coordination chemistry copper ions play essential roles in cellular function. Research over the past 20 years has shed much light on the biochemistry of copper homeostasis, and the emergence of high-resolution crystal structures for many of the proteins that partake in cellular copper biology have began to provide insight into the molecular mechanisms by which cells handle this important metal. A notable gap in our understanding is related to the process by which cells acquire copper ions. This chapter describes recent progress in the structure determination of cellular copper uptake transporters and how the emerging structural information aids understanding of the molecular mechanisms that govern cellular copper acquisition and distribution.

Two modes of interaction between the membrane-embedded TARP stargazin's C-terminal domain and the bilayer visualized by electron crystallography

Glutamate-mediated neurotransmission through ligand-gated, ionotropic glutamate receptors is the main form of excitatory neurotransmission in the vertebrate central nervous system where it plays central roles in learning, memory and a variety of disorders. Acting as auxiliary subunits, transmembrane α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) regulatory proteins (TARPs) are essential regulators for glutamate-mediated neurotransmission in the central nervous system. Here, we report the first electron crystallographic reconstructions of full-length mouse stargazin (γ-2) at ∼20Å resolution in a membrane bilayer environment. Formation of ordered arrays required anionic lipids and was modulated by cholesterol and monovalent cations. Projection structures revealed that the C-termini of stargazin monomers closely interacted with the bilayer surface in an extended conformation that placed the C-terminal PDZ-binding motif ∼100Å away from the transmembrane domain and in close proximity to a membrane re-entrant region. The C-termini interaction with the bilayer was modulated by the ionic strength of the solution and overall protein secondary structure increased when membrane-bound. Our data suggest that stargazin interactions with and within the membrane play significant roles in TARP structure and directly visualize TARP functional mechanisms essential for AMPAR trafficking and clustering.

Structural fold, conservation and Fe(II) binding of the intracellular domain of prokaryote FeoB

FeoB is a G-protein coupled membrane protein essential for Fe(II) uptake in prokaryotes. Here, we report the crystal structures of the intracellular domain of FeoB (NFeoB) from Klebsiella pneumoniae (KpNFeoB) and Pyrococcus furiosus (PfNFeoB) with and without bound ligands. In the structures, a canonical G-protein domain (G domain) is followed by a helical bundle domain (S-domain), which despite its lack of sequence similarity between species is structurally conserved. In the nucleotide-free state, the G-domain's two switch regions point away from the binding site. This gives rise to an open binding pocket whose shallowness is likely to be responsible for the low nucleotide-binding affinity. Nucleotide binding induced significant conformational changes in the G5 motif which in the case of GMPPNP binding was accompanied by destabilization of the switch I region. In addition to the structural data, we demonstrate that Fe(II)-induced foot printing cleaves the protein close to a putative Fe(II)-binding site at the tip of switch I, and we identify functionally important regions within the S-domain. Moreover, we show that NFeoB exists as a monomer in solution, and that its two constituent domains can undergo large conformational changes. The data show that the S-domain plays important roles in FeoB function.

Tryptophan scanning analysis of the membrane domain of CTR-copper transporters

Membrane proteins of the CTR family mediate cellular copper uptake in all eukaryotic cells and have been shown to participate in uptake of platinum-based anticancer drugs. Despite their importance for life and the clinical treatment of malignancies, directed biochemical studies of CTR proteins have been difficult because high-resolution structural information is missing. Building on our recent 7A structure of the human copper transporter hCTR1, we present the results of an extensive tryptophan-scanning analysis of hCTR1 and its distant relative, yeast CTR3. The comparative analysis supports our previous assignment of the transmembrane helices and shows that most functionally and structurally important residues are clustered around the threefold axis of CTR trimers or engage in helix packing interactions. The scan also identified residues that may play roles in interactions between CTR trimers and suggested that the first transmembrane helix serves as an adaptor that allows evolutionarily diverse CTRs to adopt the same overall structure. Together with previous biochemical and biophysical data, the results of the tryptophan scan are consistent with a mechanistic model in which copper transport occurs along the center of the trimer.

A dimeric structure for archaeal box C/D small ribonucleoproteins

Methylation of ribosomal RNA (rRNA) is required for optimal protein synthesis. Multiple 2'-O-ribose methylations are carried out by box C/D guide ribonucleoproteins [small ribonucleoproteins (sRNPs) and small nucleolar ribonucleoproteins (snoRNPs)], which are conserved from archaea to eukaryotes. Methylation is dictated by base pairing between the specific guide RNA component of the sRNP or snoRNP and the target rRNA. We determined the structure of a reconstituted and catalytically active box C/D sRNP from the archaeon Methanocaldococcus jannaschii by single-particle electron microscopy. We found that archaeal box C/D sRNPs unexpectedly formed a dimeric structure with an alternative organization of their RNA and protein components that challenges the conventional view of their architecture. Mutational analysis demonstrated that this di-sRNP structure was relevant for the enzymatic function of archaeal box C/D sRNPs.

Structural basis of membrane invagination by F-BAR domains

BAR superfamily domains shape membranes through poorly understood mechanisms. We solved structures of F-BAR modules bound to flat and curved bilayers using electron (cryo)microscopy. We show that membrane tubules form when F-BARs polymerize into helical coats that are held together by lateral and tip-to-tip interactions. On gel-state membranes or after mutation of residues along the lateral interaction surface, F-BARs adsorb onto bilayers via surfaces other than their concave face. We conclude that membrane binding is separable from membrane bending, and that imposition of the module's concave surface forces fluid-phase bilayers to bend locally. Furthermore, exposure of the domain's lateral interaction surface through a change in orientation serves as the crucial trigger for assembly of the helical coat and propagation of bilayer bending. The geometric constraints and sequential assembly of the helical lattice explain how F-BAR and classical BAR domains segregate into distinct microdomains, and provide insight into the spatial regulation of membrane invagination.

Characterization of a novel prokaryotic GDP dissociation inhibitor domain from the G protein coupled membrane protein FeoB

The FeoB family of membrane embedded G proteins are involved with high affinity Fe(II) uptake in prokaryotes. Here, we report that FeoB harbors a novel GDP dissociation inhibitor-like domain that specifically stabilizes GDP-binding through an interaction with the switch I region of the G protein. We show that the stabilization of GDP binding is conserved between species despite a high degree of sequence variability in their guanine nucleotide dissociation inhibitor (GDI)-like domains, and demonstrate that the presence of the membrane embedded domain increases GDP-binding affinity roughly 150-fold over the level accomplished by action of the GDI-like domain alone. To our knowledge, this is the first example for a prokaryotic GDI, targeting a bacterial G protein-coupled membrane process. Our findings suggest that Fe(II) uptake in bacteria involves a G protein regulatory pathway reminiscent of signaling mechanisms found in higher-order organisms.

Two conformational states of the membrane-associated Bacillus thuringiensis Cry4Ba delta-endotoxin complex revealed by electron crystallography: implications for toxin-pore formation

The insecticidal nature of Cry delta-endotoxins produced by Bacillus thuringiensis is generally believed to be caused by their ability to form lytic pores in the midgut cell membrane of susceptible insect larvae. Here we have analyzed membrane-associated structures of the 65-kDa dipteran-active Cry4Ba toxin by electron crystallography. The membrane-associated toxin complex was crystallized in the presence of DMPC via detergent dialysis. Depending upon the charge of the adsorbed surface, 2D crystals of the oligomeric toxin complex have been captured in two distinct conformations. The projection maps of those crystals have been generated at 17A resolution. Both complexes appeared to be trimeric; as in one crystal form, its projection structure revealed a symmetrical pinwheel-like shape with virtually no depression in the middle of the complex. The other form revealed a propeller-like conformation displaying an obvious hole in the center region, presumably representing the toxin-induced pore. These crystallographic data thus demonstrate for the first time that the 65-kDa activated Cry4Ba toxin in association with lipid membranes could exist in at least two different trimeric conformations, conceivably implying the closed and open states of the pore.

F-BAR Proteins Join the BAR Family Fold

Expanding the range of curvature generating and curvature stabilizing protein modules, the first F-BAR domain structures support their assignment to the BAR domain superfamily and emphasize how modifications to a basic structural frame can generate a broad spectrum of properties.

A structural perspective on copper uptake in eukaryotes

Over a decade ago, genetic studies identified a family of small integral membrane proteins, commonly referred to as copper transporters (CTRs) that are both required and sufficient for cellular copper uptake in a yeast genetic complementation assay. We recently used electron crystallography to determine a projection density map of the human high affinity transporter hCTR1 embedded into a lipid bilayer. At 6 A resolution, this first glimpse of the structure revealed that hCTR1 is trimeric and possesses the type of radial symmetry that traditionally has been associated with the structure of certain ion channels such as potassium or gap junction channels. Representative for this particular type of architecture, a region of low protein density at the center of the trimer is consistent with the existence of a copper permeable pore along the center three-fold axis of the trimer. In this contribution, we will briefly discuss how recent structure-function studies correlate with the projection density map, and provide a perspective with respect to the cellular uptake of other transition metals.

Assembly of the inner rod determines needle length in the type III secretion injectisome

Assembly of multi-component supramolecular machines is fundamental to biology, yet in most cases, assembly pathways and their control are poorly understood. An example is the type III secretion machine, which mediates the transfer of bacterial virulence proteins into host cells. A central component of this nanomachine is the needle complex or injectisome, an organelle associated with the bacterial envelope that is composed of a multi-ring base, an inner rod, and a protruding needle. Assembly of this organelle proceeds in sequential steps that require the reprogramming of the secretion machine. Here we provide evidence that, in Salmonella typhimurium, completion of the assembly of the inner rod determines the size of the needle substructure. Assembly of the inner rod, which is regulated by the InvJ protein, triggers conformational changes on the cytoplasmic side of the injectisome, reprogramming the secretion apparatus to stop secretion of the needle protein.

Projection structure of the human copper transporter CTR1 at 6-A resolution reveals a compact trimer with a novel channel-like architecture

Human CTR1 is a high-affinity copper transporter that also mediates the uptake of the anticancer drug cisplatin by largely unknown transport mechanisms. Here we report the 6-A projection structure obtained for human CTR1 by using electron crystallography of 2D protein crystals in a native phospholipid bilayer. The projection of CTR1 reveals a symmetrical trimer that is <40 A wide. Notably, the center threefold axis of each trimer forms a region of very low electron density likely to be involved in copper translocation. The formation of a putative pore for metal ions at the interface of three identical subunits deviates from the structural design of typical primary and secondary active transporters and reveals that copper uptake transporters have a novel architecture that is structurally more closely related to channel proteins.

Transmembrane protein structures without X-rays

Transmembrane (TM) proteins constitute 15-30% of the genome, but <1% of the structures in the Protein Data Bank. This discrepancy is disturbing, and emphasizes that structure determination of TM proteins remains challenging. The challenge is greatest for proteins from eukaryotes, the structures of which remain intractable despite tremendous advances that have been made towards structure determination of bacterial TM proteins. Notably, >50% of the membrane protein families in eukaryotes lack bacterial homologs. Therefore, it is conceivable that many more years will elapse before high-resolution structures of eukaryotic TM proteins emerge. Until then, integrated approaches that combine biochemical and computational analyses with low-resolution structures are likely to have increasingly important roles in providing frameworks for the mechanistic understanding of membrane-protein structure and function.

Structural insights into the assembly of the type III secretion needle complex

Type III secretion systems (TTSSs) mediate translocation of virulence factors into host cells. We report the 17-angstrom resolution structures of a central component of Salmonella typhimurium TTSS, the needle complex, and its assembly precursor, the bacterial envelope-anchored base. Both the base and the fully assembled needle complex adopted multiple oligomeric states in vivo, and needle assembly was accompanied by recruitment of the protein PrgJ as a structural component of the base. Moreover, conformational changes during needle assembly created scaffolds for anchoring both PrgJ and the needle substructure and may provide the basis for substrate-specificity switching during type III secretion.

A Calpha model for the transmembrane alpha helices of gap junction intercellular channels

Gap junction channels connect the cytoplasms of apposed cells via an intercellular conduit formed by the end-to-end docking of two hexameric hemichannels called connexons. We used electron cryomicroscopy to derive a three-dimensional density map at 5.7 angstroms in-plane and 19.8 angstroms vertical resolution, allowing us to identify the positions and tilt angles for the 24 alpha helices within each hemichannel. The four hydrophobic segments in connexin sequences were assigned to the alpha helices in the map based on biochemical and phylogenetic data. Analyses of evolutionary conservation and compensatory mutations in connexin evolution identified the packing interfaces between the helices. The final model, which specifies the coordinates of Calpha atoms in the transmembrane domain, provides a structural basis for understanding the different physiological effects of almost 30 mutations and polymorphisms in terms of structural deformations at the interfaces between helices, revealing an intimate connection between molecular structure and disease.

Eukaryotic CTR copper uptake transporters require two faces of the third transmembrane domain for helix packing, oligomerization, and function

Members of the copper uptake transporter (CTR) family from yeast, plants, and mammals including human are required for cellular uptake of the essential metal copper. Based on biochemical data, CTRs have three transmembrane domains and have been shown to oligomerize in the membrane. Among individual members of the family, there is little amino acid sequence identity, raising questions as to how these proteins adopt a common fold, oligomerize, and participate in copper transport. Using site-directed mutagenesis, tryptophan scanning, genetic complementation, subcellular localization, chemical cross-linking, and the yeast unfolded protein response, we demonstrated that at least half of the third transmembrane domain (TM3) plays a vital role in CTR structure and function. The results of our analysis showed that TM3 contains two functionally distinct faces. One face bears a highly conserved Gly-X-X-X-Gly (GG4) motif, which we showed to be essential for CTR oligomerization. Moreover, we showed that steric constraints reach past the GG4-motif itself including amino acid residues that are not conserved throughout the CTR family. A second face of TM3 contains three amino acid positions that, when mutated to tryptophan, cause predominantly abnormal localization but are still partially functional in growth complementation experiments. These mutations cluster on the face opposite to the GG4-bearing face of TM3 where they may mediate interactions with the remaining two transmembrane domains. Taken together, our data support TM3 as being buried within trimeric CTR where it plays an essential role in CTR assembly.

Human meiotic recombinase Dmc1 promotes ATP-dependent homologous DNA strand exchange

Homologous recombination is crucial for the repair of DNA breaks and maintenance of genome stability. In Escherichia coli, homologous recombination is dependent on the RecA protein. In the presence of ATP, RecA mediates the homologous DNA pairing and strand exchange reaction that links recombining DNA molecules. DNA joint formation is initiated through the nucleation of RecA onto single-stranded DNA (ssDNA) to form helical nucleoprotein filaments. Two RecA-like recombinases, Rad51 and Dmc1, exist in eukaryotes. Whereas Rad51 is needed for both mitotic and meiotic recombination events, the function of Dmc1 is restricted to meiosis. Here we examine human Dmc1 protein (hDmc1) for the ability to promote DNA strand exchange, and show that hDmc1 mediates strand exchange between paired DNA substrates over at least several thousand base pairs. DNA strand exchange requires ATP and is strongly dependent on the heterotrimeric ssDNA-binding molecule replication factor A (RPA). We present evidence that hDmc1-mediated DNA recombination initiates through the nucleation of hDmc1 onto ssDNA to form a helical nucleoprotein filament. The DNA strand exchange activity of hDmc1 is probably indispensable for repair of DNA double-strand breaks during meiosis and for maintaining the ploidy of meiotic chromosomes.

Expression, solubilization, and biochemical characterization of the tight junction transmembrane protein claudin-4

The tight junction tetraspan protein claudin-4 creates a charge-selective pore in the paracellular pathway across epithelia. The structure of the pore is unknown, but is presumed to result from transcellular adhesive contacts between claudin's extracellular loops. Here we report the expression of claudin-4 by baculovirus infection of Sf9 cells and describe the biochemical analysis suggesting it has a hexameric quaternary configuration. We show the detergent perfluoro-octanoic acid is able to maintain oligomeric claudin species. Sucrose velocity centrifugation and laser light scattering are also used to investigate the oligomeric state of claudin-4. In contrast to proteins of similar topology, such as gap junction family connexins, the oligomeric state of claudins appears more dynamic. These data suggest the structural organization of claudins in tight junction pores is unique.

The membrane protein FeoB contains an intramolecular G protein essential for Fe(II) uptake in bacteria

G proteins are critical for the regulation of membrane protein function and signal transduction. Nevertheless, coupling between G proteins and membrane proteins with multiple membrane-spanning domains has so far been observed only in higher organisms. Here we show that the polytopic membrane protein FeoB, which is essential for Fe(II) uptake in bacteria, contains a guanine-nucleotide-specific nucleotide binding site. We identify the G4-motif, NXXD, responsible for guanine nucleotide specificity, and show that GTP hydrolysis occurs very slowly. In contrast to typical G proteins, the association and dissociation of GDP were found to be faster than for GTP, suggesting that in the absence of additional factors, FeoB's G protein domain may exist mostly in the GTP-bound form. Furthermore, the binding of GTP is required for efficient Fe(II) uptake through the FeoB-dependent system. Notably, even in bacteria, this covalent linkage between a G protein and a polytopic membrane protein appears, to our knowledge, to be unique. These findings raise the intriguing question whether FeoB represents a primordial archetype of G protein-regulated membrane proteins.

Electron cryomicroscopy methods

Electron cryomicroscopy methods comprise a rapidly expanding field providing insights into the structure and function of biological macromolecules and their supramolecular assemblies. The 3.8 A resolution structure of the membrane protein aquaporin, a view of the herpesvirus capsid at 8.5 A and the 10 A resolution structure of the spliceosomal U1 small nuclear ribonucleoprotein complex are three outstanding examples emphasizing the versatility of this technique.

Assessment of electron crystallographic data obtained from two-dimensional crystals of biological specimens

Over the past few years, an increasing number of electron crystallographic studies using two-dimensional crystals have shed light on the structure of biologically important macromolecules. Steady progress in the development of specimen-preparation techniques and image-processing tools enable researchers to achieve resolutions in the range of 5-10 A almost routinely. However, reaching near-atomic resolution remains a formidable task that is likely to require several years. Without doubt, this process will become far less time-consuming as methods are improved further. However, the immediate future is more likely to be dominated by structures solved to an intermediate level of resolution. Since the reliability of such structures is more difficult to assess than that of density maps at near-atomic resolution and as the popularity of electron cryo-microscopy increases it becomes more important to define standardized criteria for the evaluation of electron crystallographic data. This article discusses some of the relevant issues with the aim of stimulating further discussion about the assessment and presentation of electron crystallographic data.

Expression, two-dimensional crystallization, and electron cryo-crystallography of recombinant gap junction membrane channels

We used electron cryo-microscopy and image analysis to examine frozen-hydrated, two-dimensional (2D) crystals of a recombinant, 30-kDa C-terminal truncation mutant of the cardiac gap junction channel formed by 43-kDa alpha(1) connexin. To our knowledge this is the first example of a structural analysis of a membrane protein that has been accomplished using microgram amounts of starting material. The recombinant alpha(1) connexin was expressed in a stably transfected line of baby hamster kidney cells and spontaneously assembled gap junction plaques. Detergent treatment with Tween 20 and 1,2-diheptanoyl-sn-phosphocholine resulted in well-ordered 2D crystals. A three-dimensional density (3D) map with an in-plane resolution of approximately 7.5 A revealed that each hexameric connexon was formed by 24 closely packed rods of density, consistent with an alpha-helical conformation for the four transmembrane domains of each connexin subunit. In the extracellular gap the aqueous channel was bounded by a continuous wall of protein that formed a tight electrical and chemical seal to exclude exchange of substances with the extracellular milieu.

Electron cryo-crystallography of a recombinant cardiac gap junction channel

Gap junctions in the heart play an important functional role by electrically coupling cells, thereby organizing the pattern of current flow to allow co-ordinated muscle contraction. Cardiac gap junctions are therefore intimately involved in normal conduction as well as the genesis of potentially lethal arrhythmias. We recently utilized electron cryo-microscopy and image analysis to examine frozen-hydrated 2D crystals of a recombinant, C-terminal truncated form of connexin 43 (Cx43; alpha 1), the principal cardiac gap junction protein. The projection map at 7 A resolution revealed that each 30 kDa connexin subunit has a transmembrane alpha-helix that lines the aqueous pore and a second alpha-helix in close contact with the membrane lipids. The distribution of densities allowed us to propose a model in which the two apposing connexons that form the channel are staggered by approximately 30 degrees. We are now recording images of tilted, frozen-hydrated 2D crystals, and a preliminary 3D map has been computed at an in-plane resolution of approximately 7.5 A and a vertical resolution of approximately 25 A. As predicted by our model, the two apposing connexons that form the channel are staggered with respect to each other for certain connexin molecular boundaries within the hexamer. Within the membrane interior each connexin subunit displays four rods of density, which are consistent with an alpha-helical conformation for the four transmembrane domains. Preliminary studies of BHK hamster cells that express the truncated Cx43 designated alpha 1 Cx263T demonstrate that oleamide, a sleep inducing lipid, blocks in vivo dye transfer, suggesting that oleamide causes closure of alpha 1 Cx263T channels. The comparison of the 3D structures in the presence and absence of oleamide may provide an opportunity to explore the conformational changes that are associated with oleamide-induced blockage of dye transfer. The structural details revealed by our analysis will be essential for delineating the molecular basis for intercellular current flow in the heart, as well as the general molecular design and functional properties of this important class of channel proteins.

Three-dimensional structure of a recombinant gap junction membrane channel

Gap junction membrane channels mediate electrical and metabolic coupling between adjacent cells. The structure of a recombinant cardiac gap junction channel was determined by electron crystallography at resolutions of 7.5 angstroms in the membrane plane and 21 angstroms in the vertical direction. The dodecameric channel was formed by the end-to-end docking of two hexamers, each of which displayed 24 rods of density in the membrane interior, which is consistent with an alpha-helical conformation for the four transmembrane domains of each connexin subunit. The transmembrane alpha-helical rods contrasted with the double-layered appearance of the extracellular domains. Although not indicative for a particular type of secondary structure, the protein density that formed the extracellular vestibule provided a tight seal to exclude the exchange of substances with the extracellular milieu.

Synthesis, assembly and structure of gap junction intercellular channels

Gap junction membrane channels assemble as dodecameric complexes, in which a hexameric hemichannel (connexon) in one plasma membrane docks end to end with a connexon in the membrane of a closely apposed cell. Steps in the synthesis, assembly and turnover of gap junction channels appear to follow the general secretory pathway for membrane proteins. In addition to homo-oligomeric connexons, different connexin polypeptide subunits can also assemble as hetero-oligomers. The ability to form homotypic and heterotypic channels that consist of two identical or two different connexons, respectively, adds even greater versatility to the functional modulation of gap junction channels. Electron cryocrystallography of recombinant gap junction channels has recently provided direct evidence for alpha-helical folding of at least two of the transmembrane domains within each connexin subunit. The potential to correlate the structure and biochemistry of gap junction channels with recently identified human diseases involving connexin mutations makes this a particularly exciting area of research.

An alpha-carbon template for the transmembrane helices in the rhodopsin family of G-protein-coupled receptors

A model for the alpha-carbon positions in the seven transmembrane helices in the rhodopsin family of G-protein-coupled receptors is presented. The model incorporates structural information derived from the analysis of approximately 500 sequences in this family. The location, relative to the centre of the lipid bilayer, of each of the seven helical sequence segments and their probable lengths are deduced from sequence analysis, along with the orientation, relative to the centre of the helix bundle, of each helical segment around its axis. The packing of the helices in the model is guided by the density in a three-dimensional map of frog rhodopsin determined by electron cryo-microscopy. The model suggests which of the residues that are highly conserved in this family of receptors interact with each other. Helices III, V and VI are predicted to protrude more than the others from the central lipid core towards the aqueous phase on the intracellular side of the membrane. This feature could be a property of the receptor structure in some but not all of the conformations that it adopts, since recent studies suggest that relative movement occurs between these helices on photoactivation of rhodopsin. Results from other techniques, including the creation of metal-binding sites and disulphide bridges, site-directed spin-labelling studies, the substituted-cysteine accessibility method and other site-directed mutagenesis studies, are discussed in terms of the model.

Arrangement of rhodopsin transmembrane alpha-helices

Rhodopsins, the photoreceptors in rod cells, are G-protein-coupled receptors with seven hydrophobic segments containing characteristic conserved sequence patterns that define a large family. Members of the family are expected to share a conserved transmembrane structure. Direct evidence for the arrangement of seven alpha-helices was obtained from a 9A projection map of bovine rhodopsin. Structural constraints inferred from a comparison of G-protein-coupled receptor sequences were used to assign the seven hydrophobic stretches in the sequence to features in the projection map. A low-resolution three-dimensional structure of bovine rhodopsin and two projection structures of frog rhodopsin confirmed the position of the three least tilted helices, 4, 6 and 7. A more elongated peak of density for helix 5 indicated that it is tilted or bent, but helices 1, 2 and 3 were not resolved. Here we have used electron micrographs of frozen-hydrated two-dimensional frog rhodopsin crystals to determine the structure of frog rhodopsin. Seven rods of density in the map are used to estimate tilt angles for the seven helices. Density visible on the extracellular side of the membrane suggests a folded domain. Density extends from helix 6 on the intracellular side, and a short connection between helices 1 and 2, and possibly a part of the carboxy terminus, are visible.

Projection structure of a gap junction membrane channel at 7 A resolution

Electron cryo-microscopy and image analysis of frozen-hydrated, two-dimensional crystals of gap junction membrane channels formed by recombinant alpha 1 connexin (Cx43) reveal a ring of transmembrane alpha-helices that lines the aqueous pore and a second ring of alpha-helices in close contact with the membrane lipids.

Low resolution structure of bovine rhodopsin determined by electron cryo-microscopy

The visual pigment rhodopsin is a member of the G protein-coupled receptor family. Electron cryo-microscopy was used to determine the three-dimensional structure of bovine rhodopsin from tilted two-dimensional crystals embedded in vitrified water. The effective resolution in a map obtained from the 23 best crystals was about 9.5 A horizontally and approximately 47 A normal to the plane of the membrane. Four clearly resolved tracks of density in the map correspond to four alpha-helices oriented nearly perpendicular to the plane of the membrane. One of these helices appears to be more tilted than anticipated from the projection structure published previously. The remaining three helices are presumably more highly tilted, given that they form a continuous "arc-shaped" feature and could not be resolved to the same extent. The overall density distribution in the low resolution map shows an arrangement of the helices in which the "arc-shaped" feature is extended by a fourth, less tilted helix. The band of these four tilted helices is flanked by a straight helix on the outer side and a pair of straight helices on its inner side.