Structural relationships of actin, myosin, and tropomyosin revealed by cryo-electron microscopy.

We have calculated three-dimensional maps from images of myosin subfragment-1 (S1)-decorated thin filaments and S1-decorated actin filaments preserved in frozen solution. By averaging many data sets we obtained highly reproducible maps that can be interpreted simply to provide a model for the native structure of decorated filaments. From our results we have made the following conclusions. The bulk of the actin monomer is approximately 65 X 40 X 40 A and is composed of two domains. In the filaments the monomers are strongly connected along the genetic helix with weaker connections following the long pitch helix. The long axis of the monomer lies roughly perpendicular to the filament axis. The myosin head (S1) approaches the actin filament tangentially and binds to a single actin, the major interaction being with the outermost domain of actin. In the map the longest chord of S1 is approximately 130 A. The region of S1 closest to actin is of high density, whereas the part furthest away is poorly defined and may be disordered. By comparing maps from decorated thin filaments with those from decorated actin, we demonstrate that tropomyosin is bound to the inner domain of actin just in front of the myosin binding site at a radius of approximately 40 A. A small change in the azimuthal position of tropomyosin, as has been suggested by others to occur during Ca2+-mediated regulation in vertebrate striated muscle, appears to be insufficient to eclipse totally the major site of interaction between actin and myosin.

Intermolecular versus intramolecular interactions of Dictyostelium myosin: possible regulation by heavy chain phosphorylation.

Dictyostelium myosin has been examined under conditions that reveal intramolecular and intermolecular interactions that may be important in the process of assembly and its regulation. Rotary shadowed myosin molecules exhibit primarily two configurations under these conditions: straight parallel dimers and folded monomers. All of the monomers bend in a specific region of the 1860-A-long tail that is 1200 A from the head-tail junction. Molecules in parallel dimers are staggered by 140 A, which is a periodicity in the packing of myosin molecules originally observed in native thick filaments of muscle. The most common region for interaction in the dimers is a segment of the tail about 200-A-long, extending from 900 to 1100 A from the head-tail junction. Parallel dimers form tetramers by way of antiparallel interactions in their tail regions with overlaps in multiples of 140 A. The folded configuration of the myosin molecules is promoted by phosphorylation of the heavy chain by Dictyostelium myosin heavy chain kinase. It appears that the bent monomers are excluded from filaments formed upon addition of salt while the dimeric molecules assemble. These results may provide the structural basis for primary steps in myosin filament assembly and its regulation by heavy chain phosphorylation.

Site-specific inhibition of myosin-mediated motility in vitro by monoclonal antibodies.

Monoclonal antibodies directed against seven different sites on Dictyostelium myosin (Peltz, G., J. A. Spudich, and P. Parham, 1985, J. Cell Biol., 100: 1016-1023) were tested for their ability to inhibit movement of myosin in vitro, using the Nitella-based myosin-mediated bead movement assay (Sheetz, M. P., R. Chasan, and J. A. Spudich, 1984, J. Cell Biol., 99: 1867-1871). To complement this functional assay, we located the binding sites of these antibodies by electron microscopy, using the rotary shadowing technique. One antibody bound to the 18,000-dalton light chain and inhibited movement completely. All of the remaining antibodies bound to various positions along the rod portion of the myosin molecule, which is approximately 1,800 A long. Antibodies that bound to the rod about 470, 680, and 1400 A from the head-tail junction did not alter myosin movement. One antibody appeared to bind very close to the head-tail junction and to inhibit movement 50%. Surprisingly, three antibodies that bound about 1,200 A from the head-tail junction inhibited movement completely. This inhibition did not depend on using intact IgG, since Fab' fragments had the same effect.

The C-protein tetramer binds 230 to 240 nucleotides of pre-mRNA and nucleates the assembly of 40S heterogeneous nuclear ribonucleoprotein particles.

A series of in vitro protein-RNA binding studies using purified native (C1)3C2 and (A2)3B1 tetramers, total soluble heterogeneous nuclear ribonucleoprotein (hnRNP), and pre-mRNA molecules differing in length and sequence have revealed that a single C-protein tetramer has an RNA site size of 230 to 240 nucleotides (nt). Two tetramers bind twice this RNA length, and three tetramers fold monoparticle lengths of RNA (700 nt) into a unique 19S triangular complex. In the absence of this unique structure, the basic A- and B-group proteins bind RNA to form several different artifactual structures which are not present in preparations of native hnRNP and which do not function in hnRNP assembly. Three (A2)3B1 tetramers bind the 19S complex to form a 35S assembly intermediate. Following UV irradiation to immobilize the C proteins on the packaged RNA, the 19S triangular complex is recovered as a remnant structure from both native and reconstituted hnRNP particles. C protein-RNA complexes composed of three, six, or nine tetramers (one, two, or three triangular complexes) nucleate the stoichiometric assembly of monomer, dimer, and trimer hnRNP particles. The binding of C-protein tetramers to RNAs longer than 230 nt is through a self-cooperative combinatorial mode. RNA packaged in the 19S complex and in 40S hnRNP particles is efficiently spliced in vitro. These findings demonstrate that formation of the triangular C protein-RNA complex is an obligate first event in the in vitro and probably the in vivo assembly the 40S hnRNP core particle, and they provide insight into the mechanism through which the core proteins package 700-nt increments of RNA. These findings also demonstrate that unless excluded by other factors, the C proteins are likely to be located along the length of nascent transcripts.

Electron microscopy of scallop myosin. Location of regulatory light chains.

The heads of the Ca2+-sensitive myosin molecules from scallop muscle, contrasted for electron microscopy by rotary shadowing, display two appearances depending on the presence or absence of the regulatory light chains. The heads of intact myosin appear "pear-shaped" as described for vertebrate myosin (Elliott & Offer, 1978): they are widest at the end remote from the tail and taper to a narrower neck near their junction with the tail. In contrast, myosin heads that lack the regulatory light chains appear more globular. The neck region is no longer visible: the rounded heads appear directly attached to the tail or there is an apparent gap between the head and the tail. Two preparations of myosin subfragment-1 that differ in light chain content show a similar difference in appearance. Fab fragments of antibodies specific for the light chains bind to the myosin heads and can also be visualized in the electron microscope using rotary shadowing. Both Fab fragments specific for the regulatory light chains and Fab fragments specific for the essential light chains bind preferentially to intact scallop myosin in the narrow region of the myosin head near its junction with the tail.

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.

Ultrastructural morphology of the hnRNP C protein tetramer.

The C protein tetramer is one of three heterotetramers which comprise the majority of the protein mass of mammalian 40S nuclear ribonucleoprotein particles (hnRNP particles). The events of RNA processing occur while the nascent transcripts are packaged in these structures. The C protein tetramer contains three monomers of C1 and one C2 monomer [i.e., (C1)3C2]. The tetramer's mass (129.2 kDa), approximate sedimentation coefficient (5.8S), and Stokes radius (6.2 nm) suggest that the tetramer may be either highly anisotropic or may possess an unusually large hydration shell in solution. The tetramer binds approximately 235 nucleotides of pre-mRNA. Electron microscopy of purified individual RNA-free C protein tetramers has revealed the overall morphology of this important pre-mRNA binding complex. In negatively stained preparations, the tetramer clearly displays a nonlinear, three- or four-lobed appearance with a diameter of 8.5 +/- 0.5 nm. A detailed comparison of the substructure seen in individual images suggests a tetrahedral arrangement of the four polypeptides. Rotary-shadowed images confirm the size of the tetramer observed in negatively stained preparations. This study provides the first demonstration of the overall arrangement of polypeptides in the C protein tetramer.

Cryo-electron microscopy of S1-decorated actin filaments.

We have applied techniques for cryo-electron microscopy, combined with image processing, to both S1-decorated native thin filaments and S1-decorated actin filaments. In our reconstruction the actin subunit has a prolate ellipsoid shape and is composed of two domains. The long axis of the monomer lies roughly perpendicular to the filament axis. The myosin head (S1) approaches the actin filament tangentially, the major interaction being with the outermost domain of actin. To distinguish the position of tropomyosin unambiguously in our map, we compared the maps from decorated thin filaments with those from decorated actin filaments. Our difference map clearly shows a peak corresponding to the position of tropomyosin; tropomyosin is bound to the inner domain of actin just in front of the myosin binding site at a radius of about 40 A. As a first step toward looking at the actomyosin structure in a state other than rigor, we examined S1 crosslinked to actin filaments by the zero-length crosslinker EDC in the presence of ATP and after pPDM bridging of the reactive thiols of S1. S1 molecules of the cross-linked complexes in the presence of ATP and after pPDM treatment appear dramatically different from those in rigor. The S1s appear more disordered and no longer assume the characteristic rigor 45 degrees angle with the actin filaments.

An ultrastructural characterization of in vitro-assembled hnRNP C protein-RNA complexes.

In mammalian cells approximately 700 nucleotide lengths of pre-mRNA are packaged during transcription by a unique group of abundant nuclear proteins to form a repeating array of regular ribonucleoprotein complexes termed 30-40S heterogeneous nuclear ribonucleoprotein particles (hnRNP particles). We have used electron microscopy to examine complexes that form when in vitro-transcribed RNA is bound by one of the purified native core-particle proteins which comprise the 40S monoparticle (the C protein tetramer). Negatively stained images of the C protein tetramer bound to particle-length RNA (700 nt) demonstrate that three tetramers bind each RNA molecule to form a stable closed triangular complex. The triangular complexes have an isosceles shape with a base of 18.0 nm and sides of 23.0 nm. When RNA molecules of 230 nt are used as substrates single tetramers bind to form complexes that appear as small rounded structures with an average diameter of 9.7 nm. Twice this length of RNA (456 nt) supports the assembly of mostly bilobed complexes that are 20.4 nm long and 11.8 nm wide. Images of the C protein-RNA complexes which assemble on 1400-nucleotide lengths of RNA (two particle lengths of RNA) clearly show complexes composed of two triangles while three-triangle complexes are seen when 2100-nt lengths of RNA are used as the assembly substrate. These ultrastructural results demonstrate that groups of three C protein tetramers combine with the length of RNA packaged in monoparticles to form a discreet triad structure.(ABSTRACT TRUNCATED AT 250 WORDS)

Streptococcal M protein: alpha-helical coiled-coil structure and arrangement on the cell surface.

The conformation and molecular dimensions of purified type 6 streptococcal M proteins establish the close structural relationship of these molecules to tropomyosin. Ultracentrifuge studies reveal that the M molecules exist as stable dimers; circular dichroism spectra indicate that the molecules contain about 70% alpha helix; and fiber x-ray diffraction diagrams show the characteristic reflections of the alpha-helical pattern. Electron microscopic images of M protein shadowed with platinum reveal rod-shaped molecules having the same width as tropomyosin. However, the lengths of the M molecules are about 30% shorter than lengths predicted by assuming a completely alpha-helical molecule. These findings indicate that the structure of the M6 protein is primarily alpha-helical coiled coil. Comparison of the lengths of the fibers on the surface of the streptococcus and the isolated M proteins suggests that each fiber on the cell wall consists of a single M-protein molecule approximately 500 A long. The structure determined for these fimbriae is the first alpha-helical coiled-coil conformation to be demonstrated for bacterial surface projections.

End-stabilized microtubules observed in vitro: stability, subunit, interchange, and breakage.

We report a reliable method to prepare, in vitro, microtubules that are stabilized at both ends by axonemal structures, and report studies of their properties. Such "end-stabilized" microtubules neither grow nor shorten over times of several hours when tubulin subunits are present in the surrounding solution. When subunits are removed, the microtubules eventually break. Breakage occurs within a sinuous and flexible region, a few microns in length, that begins at a single point on the microtubule and grows. When breakage does occur, the resulting two free ends shorten very rapidly until the flexible part has depolymerized and the region of straight microtubule is reached. The remainder of the microtubule then shortens at rates comparable to those ordinarily observed in dynamic instability. Formation of the flexible region can be reversed if subunits are added to the buffer prior to breakage. End-stabilized microtubules are a useful tool for studying interactions of molecules with the microtubular wall. They may be a good model for interpreting stabilizing events that happen in the cell. A preliminary study of the effects of microtubule poisons on the wall is presented.