Three-dimensional structural analysis of tetanus toxin by electron crystallography

Ordered Arrays of Type B Botulinum Toxin On Phospholipid Vesicles

Botulinum neurotoxins comprise a family of potent toxins produced by the bacterium Clostridium botulinum. There are seven serologically distinct types of botulinum neurotoxins and they act near the neuromuscular junction, producing a flaccid paralysis. These neurotoxins are synthesized as single polypeptide chains which may be cleaved artificially or by endogenous proteases to more active dichain forms which are covalently held by disulfide linkage. The carboxy terminal 2/3 of the molecule is designated the heavy (H) chain, the rest is referred to as the light (L) chain. The neurotoxins have a molecular weight of approximately 150kD (for general review, Simpson, 1986)

Simpson (1986) has proposed, by analogy to other bacterial toxins and viral infection models that the mode of action of these toxins proceed in three stages: binding to the surface of a cell, internalization and an enzymatic action. These functions can be separated biochemically; for example, the chains bind to the the cell surface and internalize. The enzymatic activity probably involves the L chain. This enzymatic poisoning step in neural cells is not known.

The structure of the tetanus toxin reveals pH-mediated domain dynamics

The tetanus neurotoxin (TeNT) is a highly potent toxin produced by Clostridium tetani that inhibits neurotransmission of inhibitory interneurons, causing spastic paralysis in the tetanus disease. TeNT differs from the other clostridial neurotoxins by its unique ability to target the central nervous system by retrograde axonal transport. The crystal structure of the tetanus toxin reveals a "closed" domain arrangement stabilised by two disulphide bridges, and the molecular details of the toxin's interaction with its polysaccharide receptor. An integrative analysis combining X-ray crystallography, solution scattering and single particle electron cryo-microscopy reveals pH-mediated domain rearrangements that may give TeNT the ability to adapt to the multiple environments encountered during intoxication, and facilitate binding to distinct receptors.

Domain organization in Clostridium botulinum neurotoxin type E is unique: its implication in faster translocation

Clostridium botulinum produces seven antigenically distinct neurotoxins [C. botulinum neurotoxins (BoNTs) A-G] sharing a significant sequence homology. Based on sequence and functional similarity, it was believed that their three-dimensional structures will also be similar. Indeed, the crystal structures of BoNTs A and B exhibit similar fold and domain association where the translocation domain is flanked on either side by binding and catalytic domains. Here, we report the crystal structure of BoNT E holotoxin and show that the domain association is different and unique, although the individual domains are similar to those of BoNTs A and B. In BoNT E, both the binding domain and the catalytic domain are on the same side of the translocation domain, and all three have mutual interfaces. This unique association may have an effect on the rate of translocation, with the molecule strategically positioned in the vesicle for quick entry into cytosol. Botulism, the disease caused by BoNT E, sets in faster than any other serotype because of its speedy internalization and translocation, and the present structure offers a credible explanation. We propose that the translocation domain in other BoNTs follows a two-step process to attain translocation-competent conformation as in BoNT E. We also suggest that this translocation-competent conformation in BoNT E is a probable reason for its faster toxic rate compared to BoNT A. However, this needs further experimental elucidation.

Retrovirus capsid protein assembly arrangements

During retrovirus particle assembly and morphogenesis, the retrovirus structural (Gag) proteins organize into two different arrangements: an immature form assembled by precursor Gag (PrGag) proteins; and a mature form, composed of proteins processed from PrGag. Central to both Gag protein arrangements is the capsid (CA) protein, a domain of PrGag, which is cleaved from the precursor to yield a mature Gag protein composed of an N-terminal domain (NTD), a flexible linker region, and a C-terminal domain (CTD). Because Gag interactions have proven difficult to examine in virions, a number of investigations have focused on the analysis of structures assembled in vitro. We have used electron microscope (EM) image reconstruction techniques to examine assembly products formed by two different CA variants of both human immunodeficiency virus type 1 (HIV-1) and the Moloney murine leukemia virus (M-MuLV). Interestingly, two types of hexameric protein arrangements were observed for each virus type. One organizational scheme featured hexamers composed of putative NTD dimer subunits, with sharing of subunits between neighbor hexamers. The second arrangement used apparent NTD monomers to coordinate hexamers, involved no subunit sharing, and employed putative CTD interactions to connect hexamers. Conversion between the two assembly forms may be achieved by making or breaking the proposed symmetric NTD dimer contacts in a process that appears to mimic viral morphogenesis.

Two-dimensional structures of the Shiga toxin B-subunit and of a chimera bound to the glycolipid receptor Gb3

The B-subunit of Shiga toxin has been demonstrated as a powerful vector for carrying attached peptides into cells for intracellular transport studies and for medical research. We have investigated the structure of the B-subunit and of a chimera bearing a peptide extension, bound to the membranous lipidic receptor, the globotriaosylceramide (Gb3). Two-dimensional crystals of both B-subunits have been obtained by the lipid layer method and projection maps have been calculated at 8.5A resolution from ice-embedded samples. The B-subunits as the chimera are organized in a pentameric form similar to the X-ray structure of the B-subunit not bound to Gb3. A difference map of both proteins has been calculated in which no density could be attributed to the peptide extension. Cross-correlations with projections of the B-subunit X-ray structure revealed that pentamers in the 2D crystals were oriented with their binding sites pointing to the lipid layer. Thus, it is likely that the peptide extension was disordered and confined to the surface of the pentamer opposite to the Gb3 binding sites. This location confirms the hypothesis that addition of peptide extension to the C-terminus conserves the ability of the modified B-subunit to bind the membranous receptor Gb3.

Analysis of Rous sarcoma virus capsid protein variants assembled on lipid monolayers

During assembly and morphogenesis of Rous sarcoma virus (RSV), proteolytic processing of the structural precursor (Pr76Gag) protein generates three capsid (CA) protein variants, CA476, CA479, and CA488. The proteins share identical N-terminal domains (NTDs), but are truncated at residues corresponding to gag codons 476, 479, and 488 in their CA C-terminal domains (CTDs). To characterize oligomeric forms of the RSV CA variants, we examined 2D crystals of the capsid proteins, assembled on lipid monolayers. Using electron microscopy and image analysis approaches, the CA proteins were observed to organize in hexagonal (p6) arrangements, where rings of membrane-proximal NTD hexamers were spaced at 95 A intervals. Differences between the oligomeric structures of the CA variants were most evident in membrane-distal regions, where apparent CTDs interconnect hexamer rings. In this region, CA488 connections were observed readily, while CA476 and CA479 contacts were resolved poorly, suggesting that in vivo processing of CA488 to the shorter forms may permit virions to adopt a dissembly-competent conformation. In addition to crystalline arrays, the CA479 and CA488 proteins formed small spherical particles with diameters of 165-175 A. The spheres appear to be arranged from hexamer or hexamer plus pentamer ring subunits that are related to the 2D crystal forms. Our results implicate RSV CA hexamer rings as basic elements in the assembly of RSV virus cores.

Structure analysis of soluble proteins using electron crystallography

Electron crystallography as a structural determination technique has grown dramatically in use over recent years. Improvements in microscopes, equipment, practical techniques, computation facilities and image processing methods are reflected in the increasing number of near-atomic resolution structures that have been published. In this review we shall summarize the techniques involved in structure determination of soluble proteins using electron crystallography. Many soluble protein structures have been investigated in this manner over the past two decades. Here we present several examples where a variety of approaches have been used to gradually increase the information obtained.

Three-dimensional organization of retroviral capsid proteins on a lipid monolayer

We have used a method for the two-dimensional crystallization of retroviral structural proteins to obtain a three-dimensional structure of negatively stained, membrane-bound, histidine-tagged Moloney murine leukemia virus (M-MuLV) capsid protein (his-MoCA) arrays. Tilted and untilted micrographs from crystals formed by purified his-MoCA proteins incubated beneath lipid monolayers containing nickel-chelating lipids were used in 3D reconstructions. The 2D crystals had unit cell dimensions of a=72.6 A, b=72.5 A and gamma=119.5 degrees, but appeared to have no intrinsic symmetry (p1) in 3D, in contrast to the trigonal or hexagonal appearance of their 2D projections. Membrane-bound his-MoCA proteins showed a strand-like organization, apparently with dimer building blocks. Membrane-proximal regions, or putative N-terminal domains (NTDs), dimerized with different partners than the membrane-distal putative C-terminal domains (CTDs). Evidence also suggests that CTDs can adopt alternate orientations relative to their NTDs, forming interstrand connections. Our results are consistent with helical-spiral models for retrovirus particle assembly, but are not easily reconcilable with icosahedral models.

Assembly of retrovirus capsid-nucleocapsid proteins in the presence of membranes or RNA

Retrovirus Gag precursor (PrGag) proteins direct the assembly of roughly spherical immature virus particles, while after proteolytic processing events, the Gag capsid (CA) and nucleocapsid (NC) domains condense on viral RNAs to form mature retrovirus core structures. To investigate the process of retroviral morphogenesis, we examined the properties of histidine-tagged (His-tagged) Moloney murine leukemia (M-MuLV) capsid plus nucleocapsid (CANC) (His-MoCANC) proteins in vitro. The His-MoCANC proteins bound RNA, possessed nucleic acid-annealing activities, and assembled into strand, circle (or sphere), and tube forms in the presence of RNA. Image analysis of electron micrographs revealed that tubes were formed by cage-like lattices of CANC proteins surrounding at least two different types of protein-free cage holes. By virtue of a His tag association with nickel-chelating lipids, His-MoCANC proteins also assembled into planar sheets on lipid monolayers, mimicking the membrane-associated immature PrGag protein forms. Membrane-bound His-MoCANC proteins organized into two-dimensional (2D) cage-like lattices that were closely related to the tube forms, and in the presence of both nickel-chelating lipids and RNAs, 2D lattice forms appeared similar to lattices assembled in the absence of RNA. Our observations are consistent with a M-MuLV morphogenesis model in which proteolytic processing of membrane-bound Gag proteins permits CA and NC domains to rearrange from an immature spherical structure to a condensed mature form while maintaining local protein-protein contacts.

Is formation of visible channels in a phospholipid bilayer by botulinum neurotoxin type B sensitive to its disulfide?

Botulinum neurotoxin, produced by Clostridium botulinum as a approximately 150-kDa single-chain protein, is nicked proteolytically either endogenously or exogenously. The approximately 50- and approximately 100-kDa chains of the dichain molecule remain held together by an interchain disulfide bridge and noncovalent interactions. The neurotoxin binds to receptors of the target cell and is internalized by endocytosis. Thereafter, a portion of the neurotoxin, the approximately 50-kDa chain, escapes to the cytosol, where it blocks neurotransmitter release. Botulinum neurotoxin serotype B is released by the bacteria primarily as an unnicked single chain. We reduced this unnicked protein and used its binding to ganglioside in a lipid layer to produce helical tubular crystals of unnicked botulinum neurotoxin type B in its disulfide-reduced state. The helical arrangement of the neurotoxin allowed determination of the structure of the molecule using cryo-electron microscopy and image processing. The resulting model reveals that neurotoxin molecules formed loops extending out from the surface of the bilayer and bending toward a neighboring loop. Although channels have been seen with disulfide-linked neurotoxin (Schmid, Robinson, and DasGupta (1993) Direct visualization of botulinum neurotoxin-induced channels in phospholipid vesicles, Nature 364, 827-830), no channels were seen here, a finding which suggests that the reduced, unnicked neurotoxin is incapable of forming a visible channel.

Electron density projection map of the botulinum neurotoxin 900-kilodalton complex by electron crystallography

The 900-kDa botulinum neurotoxin complex serotype A has been crystallized by the lipid-layer two-dimensional crystallization technique. Based on the binding characteristics of the hemagglutinating portion of the complex, a number of ganglioside/ lipid mixtures were tested but only lactosyl ceramide/1-palmityl-2-oleoyl-sn-glycero-3-phosphocholine was found to crystallize the complex. The optimum lipid mixture contained 75 mass % lactosyl ceramide and 25 mass % 1-palmityl-2-oleoyl-sn-glycero-3-phosphocholine. Using protein concentrations from 5 to 500 micrograms/ml and pH and 5 acetate buffer, we have obtained crystals that diffract to better than 15 A when prepared in negative stain. A projection map with a resolution of 30 A was calculated with unit cell dimensions of a = b = 157 A and P3 symmetry. The complex is triangular in shape with six distinct lobes observed. Additionally, six smaller structures protrude from the triangular core.

Localization of the regions on the C-terminal domain of the heavy chain of botulinum A recognized by T lymphocytes and by antibodies after immunization of mice with pentavalent toxoid

We have mapped the regions recognized by T and/or B cells (Abs) on the C-terminal domain (Hc) of the heavy chain of botulinum neurotoxin serotype A (BoNT/A) after immunization of two inbred mouse strains with pentavalent toxoid (BoNTs A, B, C, D and E). Using a set of synthetic overlapping peptides, encompassing the entire Hc domain (residues 855-1296), we demonstrated that T cells of Balb/c (H-2d) mice, primed with one injection of toxoid, recognized two major regions within residues 897-915 and 939-957. After multiple inoculations with toxoid, T cells of Balb/c expanded their recognition ability and responded very well to challenge with peptide 1261-1279 and moderately to stimulation with peptide 1149-1167. Unlike Balb/c T cells, those of toxoid-primed SJL (H-2s) mice exhibited a more complex profile and responded to challenge with a large number of overlapping peptides. After one toxoid injection, however, three peptides, 897-915, 939-957/953-971 overlap and 1051-1069, were the most potent T cells stimulators. After three toxoid injections, peptides 897-915 and 1051-1069 remained immunodominant while the third region was shifted upstream to 925-943/939-957 overlap. The immunodominant epitope within peptide 897-915 was recognized exclusively by T cells, since no Abs were detected against this region. The Ab binding profiles of the two mouse strains were quite similar, showing only small quantitative differences. Both, Balb/c and SJL anti-toxoid Abs displayed strong binding mainly to peptide 1177-1195, followed by peptides 869-887/883-901 overlap and 1275-1296. In addition, a significant amount of Balb/c anti-toxoid Abs was bound to peptide 1135-1153. Unlike Balb/c Abs, that interacted weakly with peptides 995-1013 and 1051-1069, the anti-toxoid Abs of SJL mice exhibited strong binding toward both peptides. The results showed that, in a given strain, the regions recognized by anti-toxoid Abs and T cells may coincide or may be uniquely B or T cell determinants.

Electron crystallography of macromolecular periodic arrays on phospholipid monolayers

Electron crystallography has the potential of yielding structural information equivalent to x-ray diffraction. The major difficulty has been preparing specimens with the required structural order and size for diffraction and imaging in the electron microscope. 2D crystallization on phospholipid monolayers is capable of fulfilling both of these requirements. Crystals can form as a result of specific interactions with a protein's ligand or an analog, suitably linked to a lipid tail; or on a surface of complementary head-group charge. With such choices, the availability of a suitable lipid is limited only by synthetic chemistry. Ultimately, it is the quality and regularity of the protein-protein interactions that determine the crystalline order, as it is with any protein crystal. In the case of streptavidin, the monolayer crystal diffracts beyond 2.5 A. A 3 A projection map reconstructed from electron diffraction amplitudes and phases from images shows density which can be interpreted as beta-sheets and clusters of side chains. It remains to be shown that the monolayer crystals are flat and diffract as well at high tilt angle as untilted. Technological issues such as charging must be resolved. With parallel advances in data collection and processing, electron crystallography of monolayer macromolecular crystals will eventually take its place beside x-ray crystallography and NMR as a routine and efficient structural technique.

Two-dimensional crystallization of rabbit C-reactive protein on lipid monolayers

Two-dimensional (2D) crystals of rabbit C-reactive protein (CRP) have been obtained by protein binding on lipid monolayers at the air/water interface. Two different types of crystalline arrays of CRP were obtained, by specific binding and non-specific adsorption to the lipids. Electron crystallographic analysis of the negatively stained specimens showed that the unit cell parameters of the CRP 2D crystals formed by specific binding were a=81 angstroms, b=78 angstroms, gamma=118.35 degrees, and those formed by nonspecific adsorption were a=74 angstroms, b=67 angstroms, gamma=95.5 degrees, both with the layer group p1. Projection maps were obtained at a resolution of 26 angstroms and 22 angstroms respectively. They showed that only the monomers of the CRP were packed in the 2D arrays and the orientations of the monomers on the lipid monolayers were different in the two types of crystals. By comparing the two projection maps, a preliminary shape of the CRP monomer has been derived. A model of the pentameric structure of the oligomeric CRP has been proposed.

Gangliosides in phospholipid bilayer membranes: interaction with tetanus toxin

The interaction between tetanus toxin and its fragments with gangliosides and negatively charged phosphatidylglycerols has been studied in phosphatidylcholine host membranes by protein circular dichroism measurement, calorimetry to determine lipid phase transitions, and by fluorescence spectroscopy to follow the toxin-induced pore formation by measuring the release of intravesicular entrapped dye. CD-spectroscopic secondary structure analysis showed conformational change of the toxin only in the presence of GT1b clearly demonstrating the involvement of the ganglioside headgroups for this lipid-protein-interaction. In a dot-blot analysis we showed that fragment C binds to GT1b in reconstituted vesicles and that this fragment is then accessible to a fragment C specific antibody which is only possible if fragment C is exposed at least partially on the surface of the vesicle. Our calorimetric study demonstrates the preferential binding of tetanus toxin to ganglioside GT1b. However, this protein is also able to bind to other gangliosides and also to negatively charged phospholipids causing phase separation due to electrostatic interaction. Since tetanus toxin preferentially binds short chain phosphatidylglycerol, we conclude that the protein adopts lipids with respect to charge, head group structure and chain length from the bulk phase. One consequence of this lipid-protein interaction is the ability of tetanus toxin to permeabilize lipid vesicles. Pore formation is favoured in the presence of GT1b in phosphatidylcholine membranes but only at a sufficiently high enough ganglioside content. Gangliosides others than GT1b are less effective in pore formation. In the presence of negatively charged phosphatidylglycerol tetanus toxin causes a dye release which in contrast to GT1b-containing vesicles is not saturable. We conclude that tetanus toxin acts in combination with a given number of GT1b molecules. Twenty ganglioside molecules are found to be necessary to form the stable pore. Other negatively charged lipids also cause the toxin to intercalate into the membrane but in this case the release velocity is determined by the formation of membrane defects.

Structure and function of tetanus and botulinum neurotoxins

Tetanus and botulinum neurotoxins are produced by Clostridia and cause the neuroparalytic syndromes of tetanus and botulism. Tetanus neurotoxin acts mainly at the CNS synapse, while the seven botulinum neurotoxins act peripherally. Clostridial neurotoxins share a similar mechanism of cell intoxication: they block the release of neurotransmitters. They are composed of two disulfide-linked polypeptide chains. The larger subunit is responsible for neurospecific binding and cell penetration. Reduction releases the smaller chain in the neuronal cytosol, where it displays its zinc-endopeptidase activity specific for protein components of the neuroexocytosis apparatus. Tetanus neurotoxin and botulinum neurotoxins B, D, F and G recognize specifically VAMP/ synaptobrevin. This integral protein of the synaptic vesicle membrane is cleaved at single peptide bonds, which differ for each neurotoxin. Botulinum A, and E neurotoxins recognize and cleave specifically SNAP-25, a protein of the presynaptic membrane, at two different sites within the carboxyl-terminus. Botulinum neurotoxin type C cleaves syntaxin, another protein of the nerve plasmalemma. These results indicate that VAMP, SNAP-25 and syntaxin play a central role in neuroexocytosis. These three proteins are conserved from yeast to humans and are essential in a variety of docking and fusion events in every cell. Tetanus and botulinum neurotoxins form a new group of zinc-endopeptidases with characteristic sequence, mode of zinc coordination, mechanism of activation and target recognition. They will be of great value in the unravelling of the mechanisms of exocytosis and endocytosis, as they are in the clinical treatment of dystonias.

Formation of two-dimensional complexes of F-actin and crosslinking proteins on lipid monolayers: demonstration of unipolar alpha-actinin-F-actin crosslinking

A method is described for forming two-dimensional (2-D) paracrystalline complexes of F-actin and bundling/gelation proteins on positively charged lipid monolayers. These arrays facilitate detailed structural studies of protein interactions with F-actin by eliminating superposition effects present in 3-D bundles. Bundles of F-actin have been produced using the glycolytic enzymes aldolase and glyceraldehyde-3-phosphate dehydrogenase, the cytoskeletal protein erythrocyte adducin as well as smooth muscle alpha-actinin from chicken gizzard. All of the 2-D bundles formed contain F-actin with a 13/6 helical structure. F-actin-aldolase bundles have an interfilament spacing of 12.6 nm and a superlattice arrangement of actin filaments that can be explained by expression of a local twofold axis in the neighborhood of the aldolase. Well ordered F-actin-alpha-actinin 2-D bundles have an interfilament spacing of 36 nm and contain crosslinks 33 nm in length angled approximately 25-35 degrees to the filament axis. Images and optical diffraction patterns of these bundles suggest that they consist of parallel, unipolar arrays of actin filaments. This observation is consistent with an actin crosslinking function at adhesion plaques where actin filaments are bound to the cell membrane with uniform polarity.

Structure of membrane-bound human factor Va

Coagulation factor Va is an essential cofactor which combines with the serine protease factor Xa on a phospholipid surface to form the prothrombinase complex. In the present study, the structure of factor Va interacting with lipid surfaces containing phosphatidylserine was studied by electron microscopy. Two-dimensional crystals of factor Va were obtained on planar lipid films under quasi-physiological conditions. The two-dimensional projected structure of factor Va was calculated at a resolution of 2 nm, revealing dimers of factor Va arranged on the surface lattice with the symmetry of the plane group p2. Average unit cell dimensions are a = 14.4 nm, b = 8.8 nm, gamma = 107 degrees. Each factor Va molecule presents two distinct domains of protein density consisting of one small domain, of 3 nm in diameter, connected to a larger domain of about 6 nm x 4.5 nm. The projected structure of factor Va covers an area equivalent to about fifty phospholipid molecules. In addition, edge-on views of factor Va molecules bound to liposomes reveal a globular structure connected through a thin stem to the liposome surface. A three-dimensional model of membrane-bound factor Va is proposed.

Mechanism of action of tetanus and botulinum neurotoxins

The clostridial neurotoxins responsible for tetanus and botulism are metallo-proteases that enter nerve cells and block neurotransmitter release via zinc-dependent cleavage of protein components of the neuroexocytosis apparatus. Tetanus neurotoxin (TeNT) binds to the presynaptic membrane of the neuromuscular junction and is internalized and transported retroaxonally to the spinal cord. Whilst TeNT causes spastic paralysis by acting on the spinal inhibitory interneurons, the seven serotypes of botulinum neurotoxins (BoNT) induce a flaccid paralysis because they intoxicate the neuromuscular junction. TeNT and BoNT serotypes B, D, F and G specifically cleave VAMP/synaptobrevin, a membrane protein of small synaptic vesicles, at different single peptide bonds. Proteins of the presynaptic membrane are specifically attacked by the other BoNTs: serotypes A and E cleave SNAP-25 at two different sites located within the carboxyl terminus, whereas the specific target of serotype C is syntaxin.

Tetanus and botulism neurotoxins: a new group of zinc proteases

The active forms of tetanus and botulinum neurotoxins, released from the precursor molecule by specific proteolysis and reduction, block the release of neurotransmitters via a Zn(2+)-dependent protease activity. VAMP/synaptobrevin, an integral membrane protein of the synaptic vesicles, is cleaved at a single site by tetanus and botulinum B, D and F neurotoxins. The unique sequence, mechanism of activation and site of activity of clostridial neurotoxins mark them out as an independent group of Zn(2+)-endopeptidases.

Direct visualization of botulinum neurotoxin-induced channels in phospholipid vesicles

The seven botulinum neurotoxin (NT) serotypes produced by strains of Clostridium botulinum inhibit neurotransmitter release from synaptic vesicles. Neurotoxin is synthesized as a roughly 150K single-chain protein. Proteolysis produces two fragments, the 50K L-chain and 100K H-chain, that remain linked by a disulphide bond. Intoxication involves membrane attachment by the C-terminal half of the H-chain, endocytotic/lysosomal internalization, vesicle channel formation mediated by the 50K N-terminal half of the H-chain at low pH, and finally blockade of synaptic vesicle fusion after the L-chain reaches the cytosol. We report here the visualization of the neurotoxin-membrane complex by electron cryomicroscopy and image processing. Three-dimensional reconstructions show the neurotoxin bound to the exterior of ganglioside/PC lipid vesicles and show channels entirely perforating the vesicle wall. Each channel appears to arise from the interaction of four neurotoxin molecules.

Novel targets and catalytic activities of bacterial protein toxins

Among bacterial protein toxins with intracellular targets, tetanus and botulinum toxins form a group with unique properties. They are absolutely neurospecific and act in the cytosol of neurons. Recent evidence indicates that they are zinc proteases specific for proteins of the neuroexocytosis apparatus.

Teaching electron diffraction and imaging of macromolecules

Electron microscopic analysis can be used to determine the three-dimensional structures of macromolecules at resolutions ranging between 3 and 30 A. It differs from nuclear magnetic resonance spectroscopy or x-ray crystallography in that it allows an object's Coulomb potential functions to be determined directly from images and can be used to study relatively complex macromolecular assemblies in a crystalline or noncrystalline state. Electron imaging already has provided valuable structural information about various biological systems, including membrane proteins, protein-nucleic acid complexes, contractile and motile protein assemblies, viruses, and transport complexes for ions or macromolecules. This article, organized as a series of lectures, presents the biophysical principles of three-dimensional analysis of objects possessing different symmetries.

Two-dimensional crystallization of catalase on a monolayer film of poly(1-benzyl-L-histidine) spread at the air/water interface

Two-dimensional (2D) crystals of beef liver catalase were prepared by adsorption to a film of synthetic polypeptide, poly(1-benzyl-L-histidine) (PBLH), spread at the air/water interface. The crystallization experiments were carried out in the pH range of 4.8-6.4 for catalase solutions at low concentration (10 micrograms/ml). The pH-dependence suggested an electrostatic interaction in the binding of catalase to the PBLH film. At lower pH, small crystals were formed at a low binding rate, and at higher pH the binding was rapid and densely-packed 2D arrays with poor crystallinity were formed. To stimulate crystal growth, a thermal treatment was applied. One-shot heating of the interfacial catalase-PBLH film to 35-40 degrees C was remarkably effective to form larger 2D crystals. The structure of catalase 2D crystals has been analyzed by Fourier filtering of the transmission electron micrographs. The crystal form is a new one, containing four catalase molecules in the unit cell with lattice parameters of alpha = 187 A, b = 225 A and gamma = 92.8 degrees.

2D crystallization: from art to science

The techniques as well as the principles of the 2D crystallization of membrane and water-soluble proteins for electron crystallography are reviewed. First, the biophysics of the interactions between proteins, lipids and detergents is surveyed. Second, crystallization of membrane proteins in situ and by reconstitution methods is discussed, and the various factors involved are addressed. Third, we elaborate on the 2D crystallization of water-soluble proteins, both in solution and at interfaces, such as lipid monolayers, mica, carbon film or mercury surfaces. Finally, techniques and instrumentations that are required for 2D crystallization are described.

The Staphylococcus aureus alpha-toxin channel complex and the effect of Ca2+ ions on its interaction with lipid layers

Using the techniques of two-dimensional crystallization on supported lipid bilayers together with computer image processing, two distinct two-dimensional crystal types of staphylococcal alpha-toxin complex are formed depending on the presence or absence of Ca2+ ions. Without Ca2+, these are hexagonally packed (in A, a = b = 89.5 +/- 2.5 A; theta = 119.7 degrees) With Ca2+ present, rectangular crystal packing is seen (in A, a = 114.8 +/- 1.6 A, b = 140.2 +/- 0.7 A; theta = 89.1 degrees). A third, banded crystal type is also seen which is interpreted as a side-to-side packing of regular tubules. We use these tubular crystals for cross-correlation searches with top and side-on views of the complex from single particle reconstructions, and with the repeating units from the two-dimensional crystal types. The results lead us to propose a model in which the different two-dimensional crystal types are formed as a result of alpha-toxin hexamers packing in different orientations. In the hexagonal crystals the hexamers lie end-on with a 6-fold axis in projection. On the addition of Ca2+, the hexamers reorient to lie tilted with respect to the support, thus giving rise to a rectangular projection.

Rational design and synthesis of phospholipids for the two-dimensional crystallization of DNA gyrase, a key element in chromosome organization

Properties required of lipids for two-dimensional crystallization of proteins on lipid layers at the air/water interface are discussed in terms of molecular structure. These properties are related to essential features of the overall system such as (i) the fluidity and stability of the lipid film, (ii) the affinity of the protein to be crystallized for the lipids and (iii) the accessibility of the protein to the ligand in the lipid layer as well as (iv) technical constraints of the crystallization technique. The resulting ideas were tested through the rational design and synthesis of original phospholipid structures linked to novobiocin subsequently used in the production of two-dimensional crystals of DNA gyrase (B subunit), a prokaryotic type II DNA topoisomerase.

Surface topography of histidine residues of tetanus toxin probed by immobilized-metal-ion affinity chromatography

Tetanus toxin contains 14 histidine residues: six of them are localized in the light chain (L), one is present in the N-terminal half of the heavy chain (HN) and the remaining seven histidines are localized in the C-terminal half of the heavy chain (Hc). Using immobilized-metal-ion affinity chromatography with Chelating Superose-Zn(II), we show that histidines of Hc are exposed to the protein surface and are responsible for the binding of tetanus toxin and of Hc to the immobilized metal. The histidines of the L chain are not available for co-ordination of matrix-bound Zn2+; however, two of them and three of the histidines of fragment Hc are accessible to diethyl pyrocarbonate. Chromatography on Superose-Zn(II) is also shown to be a simple and efficient method for the rapid isolation of tetanus toxin and of its Hc fragment, which can be extended to the botulinum neurotoxins.

Patch averaging of electron images of GP3*I crystals with variable thickness

The combination of Fourier and correlation averaging techniques with multivariate statistical analysis and classification, a method known as patch averaging, is used to analyze untilted and tilted images of negatively stained GP32*I crystals, which exhibit variable thicknesses in a single crystal. Within a single image, coherent areas of the same apparent thickness can be distinguished from areas of differing thicknesses. Analysis using the phase relationships among symmetry-related reflections from reconstituted images obtained from untilted micrographs confirms the ability of the method to classify these variable thicknesses properly. Furthermore, the phases from some of the reconstituted images obtained from both untilted and tilted micrographs were found to match well with the phases in a previously determined three-dimensional data set of this crystal with pg symmetry along the crystallographic b axis. These results indicate the utility of the patch averaging procedures in the structural determination of protein crystals with different thicknesses.

Formation of 2-D paracrystals of F-actin on phospholipid layers mixed with quaternary ammonium surfactants

Two-dimensional paracrystalline arrays of F-actin have been formed on positively charged lipid layers composed of phosphatidylcholine (PC) and quaternary ammonium surfactants. These quaternary ammonium surfactants were found to be better promoters of two-dimensional order than PC lipid layers mixed with stearylamine. In addition, the length of the hydrocarbon chain was found to influence the achievement of 2-D order. Lipid layers composed of dilauryl-PC and didodecyldimethylammonium bromide, which are saturated C12 lipids, promoted 2-D crystallization better than mixtures of dipalmitoyl-PC, a saturated C16 lipid, and dioctadecyldimethylammonium bromide, a saturated C18 lipid. Thus, the hydrocarbon chain length, which influences lipid layer fluidity, had a significant effect on paracrystal formation. We suggest that quaternary ammonium surfactants may have advantages in some cases for forming ordered arrays on lipid layers. In addition to investigating the effect of lipid layer composition on paracrystal formation, we found that the injection of G-actin rather than F-actin under a fluid lipid layer into a polymerizing solution produced better ordered paracrystals.

Properties and use of botulinum toxin and other microbial neurotoxins in medicine

Crystalline botulinum toxin type A was licensed in December 1989 by the Food and Drug Administration for treatment of certain spasmodic muscle disorders following 10 or more years of experimental treatment on human volunteers. Botulinum toxin exerts its action on a muscle indirectly by blocking the release of the neurotransmitter acetylcholine at the nerve ending, resulting in reduced muscle activity or paralysis. The injection of only nanogram quantities (1 ng = 30 mouse 50% lethal doses [U]) of the toxin into a spastic muscle is required to bring about the desired muscle control. The type A toxin produced in anaerobic culture and purified in crystalline form has a specific toxicity in mice of 3 x 10(7) U/mg. The crystalline toxin is a high-molecular-weight protein of 900,000 Mr and is composed of two molecules of neurotoxin (ca. 150,000 Mr) noncovalently bound to nontoxic proteins that play an important role in the stability of the toxic unit and its effective toxicity. Because the toxin is administered by injection directly into neuromuscular tissue, the methods of culturing and purification are vital. Its chemical, physical, and biological properties as applied to its use in medicine are described. Dilution and drying of the toxin for dispensing causes some detoxification, and the mouse assay is the only means of evaluation for human treatment. Other microbial neurotoxins may have uses in medicine; these include serotypes of botulinum toxins and tetanus toxin. Certain neurotoxins produced by dinoflagellates, including saxitoxin and tetrodotoxin, cause muscle paralysis through their effect on the action potential at the voltage-gated sodium channel. Saxitoxin used with anaesthetics lengthens the effect of the anaesthetic and may enhance the effectiveness of other medical drugs. Combining toxins with drugs could increase their effectiveness in treatment of human disease.

Effects of radiation damage with 400-kV electrons on frozen, hydrated actin bundles

Electron images can be used to provide amplitudes and phases for the structural determination of biological specimens. Radiation damage limits the amount of structural information retrievable by computer processing. A 400-kV electron microscope was used to investigate radiation damage effects on frozen, hydrated actin bundles kept at -168 degrees C. The quality of phases within and among images in a damage series was evaluated quantitatively out to 16 A resolution. It was found that the phases of structure factors with good signal-to-noise ratio (IQ less than or equal to 4) can be reliably retrieved from images taken at a cumulative dose of at least 25 electrons/A2.

Crystallographic analysis of acrosomal bundle from Limulus sperm

The acrosomal process of Limulus sperm contains a bundle of filaments composed of actin and a 102 kDa protein in a 1:1 molar ratio. The structure of the bundle in true discharge was investigated by electron cryomicroscopy, X-ray scattering and crystallographic image analysis. A bundle can be characterized as a quasi-crystal with continuously varying views along the bundle axis. Each segment of the bundle is found to obey the symmetry of space group P1, with a = b = 147 A, c = 762 A, alpha = 90 degrees, beta = 90.6 degrees, gamma = 120 degrees. A unit cell contains a helical repeat of the filament with a selection rule following that of an actin filament. A 24 A projection map based on the h0l view was reconstructed after averaging 5300 unit cells from six electron images. Filaments in this projection are well separated and clearly display a 21 screw symmetry. This screw symmetry results from the helical parameters of the bundle filament and is found to be a non-crystallographic symmetry element present in the unit cell. Our structural analysis has led to the proposal that the assembly of a stable bundle with a defined maximum diameter can be controlled by the crystallographic packing of the twisted filaments.

Two-dimensional and epitaxial crystallization of a mutant form of yeast RNA polymerase II

A mutant form of yeast RNA polymerase II that lacks the fourth and seventh largest subunits, referred to as pol II delta 4/7, crystallized on positively charged lipid layers. Both single-layered (two-dimensional) crystals and several multi-layered crystal forms were obtained. The two-dimensional crystals, preserved in negative stain, diffracted strongly to about 1/20 A-1 and more weakly to 1/13 A-1 resolution. A projection map computed from averaged Fourier transforms revealed four pol II delta 4/7 complexes per unit cell and further revealed a cleft on the surface of the complex similar to that previously observed in the structure of Escherichia coli RNA polymerase. One of the multi-layered crystal forms, preserved in negative stain, diffracted strongly beyond 1/15 A-1 resolution. Coherent diffraction from the multi-layered crystal is indicative of protein-protein interactions between layers and ordering in the third dimension.

Structural analysis of two-dimensional arrays of cholera toxin B-subunit

Two-dimensional arrays of cholera toxin B-subunit (CTB) have been obtained by specific interaction with lipid films, as described by Ludwig et al. (1986). The relationship between two types of array, of either rectangular or hexagonal geometry, was analyzed using crystallographic methods of electron image analysis. Our results showed that the type of array obtained was highly dependent on the negative stain used and that both arrays presented related lattice parameters, indicating that they originated from a common unstained structure. Image analysis of hexagonal arrays at 17 A resolution revealed variable CTB projected structures, ranging from annularly symmetric particles to highly asymmetric particles, very distinct from the pentameric structure resolved from rectangular crystals. The present data suggest that hexagonal arrays result from an imperfect staining of CTB rectangular crystals. The staining distortion is such that the stain layer does not match faithfully the pentameric protein distribution whereas the regular organization of the specimen is maintained.

Analysis of symmetry and three-dimensional reconstruction of thin gp32*I crystals

Thin, multilayered crystals of gp32*I were analyzed by negative stain electron microscopy and image processing. Images of untilted crystals exhibited different projection symmetries and structural motifs. Systematic analysis of these images categorized the projections into four types. Areas producing the type 1 projection were reconstructed in three-dimensions from four tilt series containing 111 images. The three-dimensional data has excellent p121 plane group symmetry and reveals that the gp32*I molecule contains two large domains linked together by a small domain. Computer simulations utilizing projection data suggested that the type 2 and 3 projections arise from superposition of type 1 projections related by a 21 screw axis along the projection axis. The three-dimensional reconstruction was utilized in a final simulation that explained the occurrence of the fourth type of projection. This work provides a firm foundation for future high-resolution analysis of the crystal by electron cryomicroscopy.

Contrast analysis of cryo-images of n-paraffin recorded at 400 kV out to 2.1 A resolution

n-Paraffin was used as a test specimen for evaluating the relative merits of 400-kV versus 100-kV electron microscopy in recording data for electron crystallographic analysis of beam-sensitive materials. The parameter used for comparison, the relative contrast R, is the ratio of amplitudes from the computed Fourier transform of images and amplitudes from an electron diffraction pattern from the same crystal. R will thus be a measure of the contrast from an experimental image relative to that of a perfect image. Electron diffraction patterns and bright-field images were recorded at 400 kV at a specimen temperature of -167 degrees C. Using the flood-beam imaging technique the best R-value is 0.08 for all reflections in the resolution zone from 4 to 3 A. This value is equivalent to that found at 100 kV. In the resolution zone from 3 to 2A we have found R = 0.02. Using the spot-scan imaging technique, on the other hand, R was measured to be 0.42 for the reflections between 4- and 3-A resolution. This amount of relative contrast is 1.7 times that observed at 100 kV. Reflections at 3-2 A displayed an R-value of 0.05. Besides obtaining higher R-values when applying the spot-scan imaging technique at 400 kV, we observe a higher yield of images with isotropic diffraction and/or higher resolution reflections. Various contrast-attenuating factors, including the modulation transfer function of the photographic film and the cryo-holder, envelope functions for spatial and temporal coherence and lens and high-tension instabilities, the contrast transfer function and lastly the radiation damage effects, have been considered in interpreting the observed image contrast. Overall, use of 400 kV in combination with spot-scan does offer important improvements in contrast levels, which can be very useful in determining the three-dimensional structure from protein crystals.

Two-dimensional crystallization of proteins on lipid monolayers

This review includes details of recent macromolecular crystallizations using lipid monolayers. Crystallization conditions are discussed together with suggestions for improving resolution.

Antibody organization on lipid films. Influence of pH and interchain disulphide reduction

Two distinct lattice structures are observed for two-dimensional (2-D) antibody organization on phospholipid films. A low-order, small-unit-cell, square lattice is obtained at pH 7 and below for mouse IgE, mouse IgG2a and IgG2b and rabbit IgG. At pH 7.5 and above, the observed lattice structure switches to a large-unit-cell, hexagonal type for rabbit IgG and mouse IgE. Interchain disulphide reduction by exposure to 2 mM-dithiothreitol results in the formation of the compact 2-D lattice for all cases and for all pH conditions.

Method for forming two-dimensional paracrystals of biological filaments on lipid monolayers

A method is described for electron microscopic observation of two-dimensional paracrystals on unsupported lipid monolayers. The method uses a hydrophobic holey C-coated grid placed on a monolayer made positively charged by the inclusion of stearylamine (SA) and has been used to align scallop thin filaments and reconstituted actin/tropomyosin filaments to form paracrystals. The use of unsupported monolayers allows the paracrystals to be viewed in either negative stain or with cryoelectron microscopy. Those paracrystals in frozen hydrated specimens have better order than those with negative stain. It was found that varying the lipid composition between the less fluid distearolyphosphotidylcholine/SA and the more fluid egg yolk phosphotidylcholine/SA alters the size and order of the paracrystals, the more fluid system having smaller, more ordered paracrystalline domains. The advantage of the technique for studying actin/thin filaments is the ability to form large two-dimensional paracrystals under physiological conditions of [Mg2+] and pH.

Purification and lipid-layer crystallization of yeast RNA polymerase II

Yeast RNA polymerase II was purified to homogeneity by a rapid procedure involving immunoaffinity chromatography. The purified enzyme contained 10 subunits, as reported for conventional preparations, but with no detectable proteolysis of the largest subunit. In assays of initiation of transcription at the yeast CYC1 promoter, the enzyme complemented the deficiency of an extract from a strain that produces a temperature-sensitive polymerase II. Mammalian RNA polymerase II was inactive in this initiation assay. The purified yeast enzyme formed two-dimensional crystals on positively charged lipid layers, as previously found for Escherichia coli RNA polymerase holoenzyme. Image analysis of electron micrographs of crystals in negative stain, which diffracted to about 30-A resolution, showed protein densities of dimensions consistent with those of single polymerase molecules.

Three-dimensional structure of the nicotinic acetylcholine receptor and location of the major associated 43-kD cytoskeletal protein, determined at 22 A by low dose electron microscopy and x-ray diffraction to 12.5 A

The three-dimensional structure of the nicotinic acetylcholine receptor (AChR) from Torpedo californica, crystallized both before and after removal of associated proteins, most notably the main 43-kD cytoskeletal protein that interacts both with AChR and actin, is determined to a resolution of 22 A. This is the first structural analysis where the 43-kD protein has been removed from the sample before crystallization. Thus, it provides the most reliable assessment of what constitutes the structure of the minimal five subunit AChR complex, and, by comparison with the native membrane, of the location of the 43-kD cytoskeletal protein. Image reconstruction of two-dimensional crystals includes information from electron images of up to +/- 52 degrees tilted specimens of latticed AChR. Hybrid density maps that include x-ray diffraction perpendicular to the membrane to 12.5 A resolution were used and eliminate some of the distortions introduced in maps based only on electron microscopic analyses. Comparison of the difference Fourier density maps between AChR with its normal complement of associated proteins, and without them shows that the main density, assigned to the actin-binding 43-kD component is closely associated with the lipid bilayer as well as with the cytoplasmic domain of the AChR. It binds beside the AChR, not beneath it as suggested by others (C. Toyoshima and N. Unwin 1988. Nature [Lond.]. 336:237-240). There is good agreement between the volumes of density for structural components and expected volumes based on their molecular weight. Acetylcholine receptors aggregate in the absence of any cytoskeletal proteins, suggesting that the AChR alone is sufficient to encode and stabilize clustering, and perhaps to do so during synaptogenesis. The main 43-kD component may play a role in location and rate of association of AChR. We show that the disulfide bond that cross-links delta-delta chains of adjacent pentamers in about 80% of AChR, is not required to stabilize the lattice of AChR. Latticed tube structures are stable indefinitely. The lattices described here have 20% less volume of lipid than those originally obtained and characterized by J. Kistler and R. M. Stroud (1981. Proc. Natl. Acad. Sci. USA. 78:3678-3682), or those subsequently characterized by A. Brisson and P. N. T. Unwin (1984. J. Cell Biol. 99:1202-1211) and A. Brisson and P. N. T. Unwin (1985. Nature (Lond.). 315:474-477).