Specific labeling of connexin43 in NRK cells using tyramide-based signal amplification and fluorescence photooxidation.

Imaging of gap junction proteins, the connexins, has been performed in tissue culture cells both by labeling of connexins with immunocytochemical tags and by cloning and expressing chimeras of connexins and fluorescent proteins such as Green Fluorescent Protein. These two approaches have been used to gain information about protein localization or trafficking at light microscopic resolution. Electron microscopy provides higher resolution; however, analysis of electron micrographs of unlabeled connexins has been generally limited to recognition of gap junction structures. Immunolabeling of gap junction proteins in whole cells at the electron microscopic level has been difficult to achieve because of the fixation sensitivity of most gap junction antibodies. To obtain reasonable sensitivity, immunoperoxidase procedures are typically employed, and these suffer from relatively poor resolution. Here we describe the combination of tyramide signal amplification techniques and fluorescence photooxidation for higher resolution immunolocalization studies for correlative light and electron microscopic imaging. By using correlative microscopy, we can not only localize connexin pools or structures, but also discover what other cellular substructures interact with gap junction proteins. The use of tyramide signal amplification techniques is necessary to increase fluorescence levels that have decreased due to increased specimen fixation required to maintain cell ultrastructure. The fluorescence photooxidation technique provides a high-resolution method for staining of proteins in cells. Unlike colloidal gold-based methods, fluorescence photooxidation allows for three-dimensional localization using high-voltage electron microscopy.

Electron crystallographic methods for investigating gap junction structure.

Gap junctions are clusters of closely packed intercellular membrane channels embedded in the plasma membranes of two adjoining cells. The central pore of the membrane channels serves as a conduit between cell cytoplasms for molecules less than 1000 Da in size. Advances in the purification of gap junctions and electron cryocrystallography and computer reconstruction techniques have produced new insights into the intercellular channel structure. Methods are described here for the purification of gap junction membranes, biochemical treatments to produce hemichannel layers ("split junctions"), assessment of the purity of gap junction preparations, electron cryomicroscopy, image processing and reconstruction, three-dimensional visualization, and interpretation. The critical step in electron crystallographic structure determination remains the isolation of crystalline material in sufficient and pure quantities for recording of electron microscope images. Along with sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting, the quality of gap junction purification is assessed using electron microscopy of negatively stained preparations. Electron microscopy is also used to assess the crystallinity of the purified gap junctions and split junctions. Electron cryocrystallography is a powerful technique for high-resolution structural characterization. Image processing is used to combine and enhance two-dimensional images. Electron crystallographic analysis is used to generate a three-dimensional structure from a set of electron micrographs. This three-dimensional information is extracted from a set of images recorded after tilting the specimen in the electron microscope stage and recombined using Fourier analysis techniques analogous to those used in X-ray crystallography. Computer modeling of the three-dimensional gap junction structures is a useful tool for analyzing hemichannel docking.

The Electron Microscopy Outreach Program: A Web-based resource for research and education.

We have developed a centralized World Wide Web (WWW)-based environment that serves as a resource of software tools and expertise for biological electron microscopy. A major focus is molecular electron microscopy, but the site also includes information and links on structural biology at all levels of resolution. This site serves to help integrate or link structural biology techniques in accordance with user needs. The WWW site, called the Electron Microscopy (EM) Outreach Program (URL: http://emoutreach.sdsc.edu), provides scientists with computational and educational tools for their research and edification. In particular, we have set up a centralized resource containing course notes, references, and links to image analysis and three-dimensional reconstruction software for investigators wanting to learn about EM techniques either within or outside of their fields of expertise.

Formation of the gap junction intercellular channel requires a 30 degree rotation for interdigitating two apposing connexons.

Intercellular communication via gap junction membrane channels cannot occur until two apposing hemichannels (connexons) meet and dock to form a sealed cell-cell conduit. In particular, an important question is how does the structure at the extracellular surface influence the molecular recognition of the two connexons. In this study, cryoelectron microscopy and computer modeling provide evidence that the formation of the gap junction intercellular channel requires a 30 degree rotation between hemichannels for proper docking. With this amount of rotation, the peaks (protrusions) on one connexon fit into the valleys of the apposed connexon in the 3-D model, which would make for an ionically tight interface necessary for a functional cell-cell channel. Docking appears to be governed by a "lock and key" mechanism via a simple interdigitation of the six protrusions from each connexon. This interdigitation increases significantly the contact surface area and potential number of hydrogen bonds or hydrophobic interactions and/or other attractive interactions. Having a larger surface area than if the surfaces were flat would explain the biochemical requirements for conditions characterized previously for splitting of channels into hemichannels. The docked connexons were computationally fitted into two gap junction structures, which further confirmed the interdigitated manner of docking.

Molecular organization of gap junction membrane channels.

Gap junctions regulate a variety of cell functions by creating a conduit between two apposing tissue cells. Gap junctions are unique among membrane channels. Not only do the constituent membrane channels span two cell membranes, but the intercellular channels pack into discrete cell-cell contact areas forming in vivo closely packed arrays. Gap junction membrane channels can be isolated either as two-dimensional crystals, individual intercellular channels, or individual hemichannels. The family of gap junction proteins, the connexins, create a family of gap junctions channels and structures. Each channel has distinct physiological properties but a similar overall structure. This review focuses on three aspects of gap junction structure: (1) the molecular structure of the gap junction membrane channel and hemichannel, (2) the packing of the intercellular channels into arrays, and (3) the ways that different connexins can combine into gap junction channel structures with distinct physiological properties. The physiological implications of the different structural forms are discussed.

Preparation, characterization, and structure of half gap junctional layers split with urea and EGTA.

Gap junctions, collections of membrane channels responsible for intercellular communication, contain two paired hemichannels (also called connexons). We have investigated conditions for splitting the membrane pair using urea. We have developed a protocol which consistently splits the gap junction samples with 60-90% efficiency. Our results indicate that hydrophobic forces are important in holding the two connexons together but that Ca2+ ions are also important in the assembly of the membrane pair. Greater yields and better structural integrity of split junctions were obtained with a starting preparation of gap junctions which had been detergent treated. Image analysis of edge views of single connexon layers reveal an asymmetry in the appearance of the cytoplasmic and extracellular surface. Cryo-electron microscopy and image analysis of split junctions show that the packing and structural detail of membranes containing arrays of single connexons are the same as for intact junctions, and that the urea treatment causes no gross structural changes in the connexon assembly.

Orientation of cholera toxin bound to model membranes.

The orientation of cholera toxin bound to its cell-surface receptor, ganglioside GM1, in a supporting lipid membrane was determined by electron microscopy of negatively stained toxin-lipid samples. Image analysis of two dimensional crystalline arrays has shown previously that the B-subunits of cholera toxin orient at the membrane surface as a pentameric ring with a central channel (Reed, R. A., J. Mattai, and G.G. Shipley. 1987. Biochemistry. 26:824-832; Ribi, H. O., D. S. Ludwig, K. L. Mercer, G. K. Schoolnik, and R. D. Kornberg. 1988. Science (Wash, DC). 239:1272-1276). We recorded images of negatively stained cholera toxin and isolated B-pentamers oriented perpendicular to the lipid surface so that the pentamer ring is viewed from the side. The pentamer dimensions, estimated from the average of 100 molecules, are approximately 60 by 30 A. Images of side views of whole cholera toxin clearly show density above the pentamer ring away from the lipid layer. On the basis of difference maps between averages of side views of whole toxin and B-pentamers, this density above the pentamer has been identified as a portion of the A-subunit. The A-subunit may also extend into the pore of the pentamer. In addition, Fab fragments from a monoclonal antibody to the A-subunit were mixed with the toxin prior to binding to GM1. Density from the Fab was localized to the region of toxin above the pentamer ring confirming the location of the A-subunit. The structure determined for the homologous heat-labile enterotoxin from Escherichia coli shows that the A-subunit lies mostly on one face of this pentamer with a small region penetrating the pentamer pore (Sixma, T. K., S. E. Pronk, K. H. Kalk, E. S. Wartna, B. A. M. van Zanten, B. Witholt,and W. G. J. Hol. 1991. Nature (Lond.). 351:371-377). The putative GM1 binding sites are located on the opposite face of the B-pentamer. Cholera toxin, therefore appears to bind to a model membrane with its GM1 binding surface adjacent to the membrane. Low resolution density maps were constructed from the x-ray coordinates of the E. coli toxin and compared with the electron microscopy-derived maps.

Isolation, characterization and structure of bacterial flagellar motors containing the switch complex.

A putative complex of the three switch proteins, FliG, FliM and FliN appears to be directly involved in torque generation and control of direction of rotation. We have developed a preparative procedure for flagellar motors that retains these proteins as evidenced by Western blots using anti-FliG, anti-FliM and anti-FliN antibodies. Immunogold labeling with these three antibodies shows that the three switch proteins are localized to the motor. Electron micrographs of frozen-hydrated preparations reveal a large, new component we have termed the "C ring complex" attached to the cytoplasmic face of the M ring. In a three-dimensional reconstruction of the cylindrically averaged structure, the M-S ring complex appears thicker and wider by the addition of extra material to the cytoplasmic surface of the M ring. In addition, extending into the cytoplasm from the thickened M ring is the C ring complex, a thin-walled cylinder having a length of 170 A and an outer diameter of 450 A compared to the 290 A diameter of the M ring. We provide evidence that the thickened M ring contains FliG and that the C ring complex may contain FliM and FliN. The large diameter of the C ring complex may permit interaction with the M ring and with the circlet of studs thought to be the MotA/MotB complex.

Structure of the extracellular surface of the gap junction by atomic force microscopy.

The extracellular surface of the gap junction cell-to-cell channels was imaged in phosphate-buffered saline with an atomic force microscope. The fully hydrated isolated gap junction membranes adsorbed to mica were irregular sheets approximately 1-2 microns across and 13.2 (+/- 1.3) nm thick. The top bilayer of the gap junction was dissected by increasing the force applied to the tip or sometimes by increasing the scan rate at moderate forces. The exposed extracellular surface revealed a hexagonal array with a center-to-center spacing of 9.4 (+/- 0.9) nm between individual channels (connexons). Images of individual connexons with a lateral resolution of < 3.5 nm, and in the best case approximately 2.5 nm, were reliably and reproducibly obtained with high-quality tips. These membrane channels protruded 1.4 (+/- 0.4) nm from the extracellular surface of the lipid membrane, and the atomic force microscope tip reached up to 0.7 nm into the pore, which opened up to a diameter of 3.8 (+/- 0.6) nm on the extracellular side.

Mass determination and estimation of subunit stoichiometry of the bacterial hook-basal body flagellar complex of Salmonella typhimurium by scanning transmission electron microscopy.

The basal body, a part of the rotary motor of the bacterial flagellum, is a multiprotein assembly that consists of four rings (denoted M, S, P, and L) and an axial rod (denoted R). From analysis of scanning transmission electron microscopy images of hook-basal body preparations isolated from Salmonella typhimurium, we have determined the masses of the basal body and three of its subcomplexes. The mass of the basal body (i.e., the four rings and rod) is 4400 +/- 490 kDa (mean +/- SD; n = 54). The mass of the LPR subcomplex (i.e., L and P rings and the whole rod) is 2600 +/- 380 kDa (n = 55), that of the L and P rings and the distal part of the rod is 2100 +/- 320 kDa (n = 25), and the mass of the L and P ring subcomplex is 1700 +/- 260 kDa (n = 514). These results, together with the masses of the component proteins, indicate that the rings contain approximately 26 subunits each and that the mass of the rod is consistent with a composition of approximately 6 copies each of three of the rod proteins FlgB, FlgC, and FlgF and approximately 26 copies of FlgG as determined by Jones et al. [Jones, C. J., Macnab, R. M., Okino, H. & Aizawa, S.-I. (1990) J. Mol. Biol. 212, 377-387] using quantitative gel electrophoresis. The results of Jones et al., together with ours, account for all proteins in the basal body to within approximately 5% (or 200 kDa).

Image analysis of gap junction structures.

Isolated gap junction plaques contain hexagonal crystalline arrays of membrane channels called connexons which are a suitable specimen for electron crystallography. Image analysis of gap junction lattices has shown that while there is sufficient lattice order for structural analysis to approximately 25 A, there is enough disorder in both the lattice and the connexon to create a family of related images. This review is focused on how these images can be interpreted in terms of what is known about both the connexon and its constituent protein, connexin.

Substructure of the flagellar basal body of Salmonella typhimurium.

The Salmonella typhimurium basal body, a part of the flagellar rotary motor, consists of four rings (denoted M, S, P and L) and a coaxial rod. Using low-dose electron microscopy and image averaging methods on negatively stained and frozen-hydrated preparations, we examined whole basal body complexes and subcomplexes obtained by dissociation in acid. Dissociation occurs in steps, allowing us to obtain images of substructures lacking the M ring, lacking the M and S rings, and lacking the M and S rings and the proximal portion of the rod. We obtained images of the L and P ring subcomplex. The existence of a subcomplex missing only the M ring suggests either that the S and M rings derive from two different proteins, or that the M ring is a labile domain of a single protein, which makes up both rings. At the 25 to 30 A resolution of our averaged images, the L, P and S rings appear cylindrically symmetric. Images of the M ring show variability that may be due to differences in angular orientation of the grid, but equally could be due to structural variations. Three-dimensional reconstructions of these structures from the averaged images reveal the internal structure and spatial organization of these components.

Structural studies of tropomyosin by cryoelectron microscopy and x-ray diffraction.

A comparison has been made between cryoelectron microscope images and the x-ray structure of one projection of the Bailey tropomyosin crystal. The computed transforms of the electron micrographs extend to a resolution of approximately 18 A compared with the reflections from x-ray crystallography which extend to 15 A. After correction of the images for lattice distortions and the contrast transfer function, the structure factors were constrained to the plane group (pmg) symmetry of this projection. Amplitude and phase data for five images were compared with the corresponding view from the three-dimensional x-ray diffraction data (Phillips, G.N., Jr., J.P. Fillers, and C. Cohen. 1986. J. Mol. Biol. 192: 111-131). The average R factor between the electron microscopy and x-ray amplitudes was 15%, with an amplitude-weighted mean phase difference of 4.8 degrees. The density maps derived from cryoelectron microscopy contain structural features similar to those from x-ray diffraction: these include the width and run of the filaments and their woven appearance at the crossover regions. Preliminary images obtained from frozen-hydrated tropomyosin/troponin cocrystals suggest that this approach may provide structural details not readily obtainable from x-ray diffraction studies.

Correlation analysis of gap junction lattice images.

Fourier averages of connexon images computed from low-irradiation electron micrographs of isolated negatively stained gap junction domains exhibited differences in stain distribution and connexon orientation. To analyze these polymorphic structures, correlation averaging methods were applied to images from negatively stained and frozen-hydrated specimens. For the negatively stained specimens, separate averages over two subsets of connexons with differing degrees of stain accumulation in the axial channel were obtained. Two populations of connexons with opposite skew orientations were distinguishable within a single junctional domain of a frozen-hydrated specimen. Correlation maps calculated using the left- and right-skewed references showed that the selected connexons tend to locally cluster. Using correlation methods to analyze packing disorder in a typical connexon lattice, we estimated the root-mean-square variation in the nearest neighbor pair separation to be approximately 11% of the lattice constant. Displacements of the connexons relative to each other increased with increasing pair separation in the lattice, rather like a liquid, although long-range orientation order was conserved as in a crystal. These results support the hypothesis that the hexagonal ordering of the connexons results from short-range repulsive forces.

Image reconstruction of the flagellar basal body of Caulobacter crescentus.

The bacterium Caulobacter crescentus has a single polar flagellum, which is present for only a portion of its cell cycle. The flagellum is ejected from the swarmer cell and then synthesized de novo later in the cell cycle. The flagellum is composed of a transmembrane basal body, a hook and a filament. Single-particle averaging and image reconstruction methods were applied to the electron micrographs of negatively stained basal bodies from C. crescentus. These basal bodies have five rings threaded on a rod. The L and P rings are connected by a bridge of material at their outer radii. The E ring is a thin, flat disk. The S ring has a triangular cross section, the sides of the triangle abutting the E ring, the rod and the M ring. The M ring, which is at the inner membrane of the cell, has a different structure depending on the method of preparation. With one method, the M ring makes a snug contact with the S ring and is often capped by an axial button, a new component apparently distinct from the M ring. With the other method, the M ring is similar to that of S. typhimurium; that is, it contacts the S ring only at an outer radius and lacks the button. Averages of the rod-hook-filament subassembly ejected by swarmer cells reveal that the rod consists of two parts with the E ring marking the approximate position of the break. The structures of basal bodies from two mutants defective in the hook assembly were found to be indistinguishable from wild-type basal bodies, suggesting that the assembly of the basal body is independent of the hook or filament assembly.

Gap junction structures. VIII. Membrane cross-sections.

Profiles of negatively stained gap junctions have been measured by grid sectioning. After normal levels of electron irradiation, the membrane thickness shrinks to about half that of unirradiated controls, but no shrinkage occurs in the hexagonal lattice plane. Even under low irradiation conditions, there is significant thinning of the membranes. Edge views, in which rows of connexons are aligned parallel to the beam, were obtained from grid sections, folds in normal negatively stained specimens, and sections of a positively stained specimen. Averaging these micrographs with the translational and mirror symmetry of the projected lattice image displays conserved and variable features in the stain distribution of different specimens. Variations in the relative amount of negative stain in the gap at the surfaces and in the channel are uncorrelated with the irradiation but appear to depend on the local staining conditions and the integrity of the connexons. The dimensions measured from previously unirradiated grid sections, folds, and positively stained sections are in accord with x-ray diffraction measurements. Radiation-induced shrinkage can be accounted for by mass loss principally from the membrane bilayer. Disordering of the surface structure appears to be correlated with the radiation sensitivity of the bilayer; in contrast, the gap structure is well preserved under a variety of conditions.

Gap junction structures. VII. Analysis of connexon images obtained with cationic and anionic negative stains.

Micrographs of isolated gap junction specimens, negatively stained with one molybdate, three tungstate and three uranyl stains, were recorded at low and high irradiation. Fourier-averaged images of the negatively stained gap junctions have been self-consistently scaled to identify conserved and variable features. Intrinsic features in the hexagonally averaged images have been distinguished from residual noise by statistical comparisons among similarly prepared specimens. The cationic uranyl stains can penetrate the axial connexon channel, whereas the anionic stains are largely excluded; these observations indicate that the channel is negatively charged. Variability in the extent of the axial stain penetration, and enhancement of this staining by radiation damage and heating may be accounted for by a leaky, labile channel gate. The peripheral stain concentrations marking the perimeter of the skewed, six-lobed connexon image and the stain-excluding region at the 3-fold axis of the lattice, which are seen only under conditions of low irradiation with both anionic and cationic stains, are identified as intrinsic features of the isolated gap junction structure. The stain concentrations located approximately 30 A from the connexon center appear to be symmetrically related on opposite sides of the junction by non-crystallographic 2-fold axes oriented approximately 8 degrees to the lattice axes at the plane of the gap. The radiation-sensitive hexagonal features seen in the negatively stained images may correspond to substructure on the cytoplasmic surfaces of the paired gap junction membranes.