Structure of actin paracrystals induced by nerve growth factor

When nerve growth factor is added to F-actin, well-ordered bundles of filaments are formed. These bundles are observed even at low concentrations of NGF²¹, but when N-bromosuccinimide-treated NGF, a biologically inactive form of the protein is used, a much higher concentration is required to produce aggregation. Moreover, the bundles induced by the modified NGF are not very well ordered and show amorphous aggregates attached at various points.Electron microscopy of paracrystals induced by native NGF shows that, although they resemble pure actin paracrystals induced by Mg²⁺, the interfilament spacing is larger and bridges connect the filaments. Optical diffraction patterns show, in addition to the off-meridional reflections characteristic of the actin helix, meridional reflections on the first and fourth layer-lines, at axial spacings of 37 and 9 nm. Measurements of the axial positions of the layer-lines show that the actin helical symmetry is not significantly different from that in pure actin paracrystals. The presence of the meridional reflections indicates that groups of two or three bridges with spacing 9 nm or nearly 9 nm are arranged along the bundles at a repeating interval of 37 nm.Actin filament bundles have been observed in several non-muscle cells, and specific actin-binding proteins have been identified as responsible for this aggregation. Our in vitro observations show that the biologically active form of NGF interacts with actin and organizes it into well-ordered paracrystalline arrays. The in vitro formation of NGF-actin complexes may be related to the in vivo mechanism of action of this growth factor.     

Structure of actin filament bundles from microvilli of sea urchin eggs

The detailed substructure of actin filament bundles in microvilli of fertilized sea urchin eggs has been studied by analysing electron microscope images of negatively stained specimens. Transverse stripes which repeat about every 130 Å along the axis of a bundle, as previously observed by Burgess & Schroeder (1977), reflect the positions of cross-bridges that connect the filaments into a bundle. Analysis of optical transforms of the micrographs reveals that there are approximately 14 actin monomers between cross-overs of the two long-pitch helical strands of the actin filaments, with three cross-bridges in this interval. The structure is basically similar to that of the hexagonally packed bundles prepared in vitro from high speed supernatants of sea urchin eggs by Kane (1975) and analyzed by DeRosier et al. (1977). One clear difference, however, is that the in vivo microvillar filament bundles are supercoiled, giving rise to long axial repeats of 1500 to 2000 Å.Computationally filtered images of regions that were only slightly supercoiled reveal the relative alignment of filaments within the bundles and show that crossbridges appear to interact with four actin monomers, apparently linking two actin monomers on one strand of one filament to the nearest two monomers on a neighbouring filament. However, the cross-bridges are not spaced at equal intervals corresponding to four actin subunits, presumably because of the lack of hexagonal symmetry in the individual filaments, which have about 14 actin monomers between cross-overs. Instead, the cross-bridges are arranged quasiequivalently along the longitudinal axis of the bundles, in steps of four or five actin subunit spacings (28 Å each).     

Structural changes in thin filaments of crab striated muscle.

Low angle X-ray diffraction patterns were recorded from crab leg muscle in living resting state and in rigor (glycerol-extracted). Both resting and rigor patterns showed a series of layer-lines arising from a helical arrangement of actin subunits in the thin filaments. In the resting state, the crossover repeat of the long-pitch actin helices was 36.6 nm, and the symmetry of the genetic actin helix was an intermediate between $\text{26}\text{12}$ and $\text{28}\text{13}$. When the muscle went into rigor, the crossover repeat changed to 38.3 nm and the helical symmetry to $\text{28}\text{13}$.In the living resting pattern, six other reflections were observed on the meridian and in the near-meridional region. These were indexed as orders of 2 × 38.2 nm and could be assigned to troponin molecules; the spacings and the intensity distributions of these reflections could be explained by the model proposed by Ohtsuki (1974) for the arrangement of troponin molecules in the thin filaments.The muscle in rigor gave meridional and near-meridional reflections at orders of 2 × 38.3 nm. These were identified as the same series of reflections as was assigned to troponin in the living resting pattern, but were more intense and could be seen up to higher orders. We consider that the myosin heads attached to the thin filament at regular intervals along its axis also contribute to these reflections in the rigor pattern.     

Stepping between Membrane Microdomains

In isolated thick filaments from many types of muscle, the two head domains of each myosin molecule are folded back against the filament backbone in a conformation called the interacting heads motif (IHM) in which actin interaction is inhibited. This conformation is present in resting skeletal muscle, but it is not known how exit from the IHM state is achieved during muscle activation. Here, we investigated this by measuring the in situ conformation of the light chain domain of the myosin heads in relaxed demembranated fibers from rabbit psoas muscle using fluorescence polarization from bifunctional rhodamine probes at four sites on the C-terminal lobe of the myosin regulatory light chain (RLC). The order parameter 〈P₂〉 describing probe orientation with respect to the filament axis had a roughly sigmoidal dependence on temperature in relaxing conditions, with a half-maximal change at ∼19°C. Either lattice compression by 5% dextran T500 or addition of 25 μM blebbistatin decreased the transition temperature to ∼14°C. Maximum entropy analysis revealed three preferred orientations of the myosin RLC region at 25°C and above, two with its long axis roughly parallel to the filament axis and one roughly perpendicular. The parallel orientations are similar to those of the so-called blocked and free heads in the IHM and are stabilized by either lattice compression or blebbistatin. In relaxed skeletal muscle at near-physiological temperature and myofilament lattice spacing, the majority of the myosin heads have their light chain domains in IHM-like conformations, with a minority in a distinct conformation with their RLC regions roughly perpendicular to the filament axis. None of these three orientation populations were present during active contraction. These results are consistent with a regulatory transition of the thick filament in skeletal muscle associated with a conformational equilibrium of the myosin heads.     

SOME ASPECTS OF THE STRUCTURAL ORGANIZATION OF THE MYOFIBRIL AS REVEALED BY ANTIBODY-STAINING METHODS

From observations of fluorescent antibody staining and antibody staining in electron microscopy, evidence is presented for the following: (a) Direct contact of the actin and myosin filaments occurs at all stages of contraction. This results in inhibition of antibody staining of the H-meromyosin portion of the myosin molecule in the region of overlap of the thin and thick filaments. (b) Small structural changes occur in the thick filaments during contraction. This leads to exposure of antigenic sites of the L-meromyosin portion of the myosin molecule. The accessibility of these antigenic sites is dependent upon the sarcomere length. (c) The M line is composed of a protein which is weakly bound to the center of the thick filament and is not actin, myosin, or tropomyosin. (d) Tropomyosin as well as actin is present in the I band. (e) If actin or tropomyosin is present in the Z line, it is masked and unavailable for staining with antibody.     

A model for actin-filament length distribution in a lamellipod

A mathematical model is derived to describe the distributions of lengths of cytoskeletal actin filaments, along a 1 D transect of the lamellipod (or along the axis of a filopod) in an animal cell. We use the facts that actin filament barbed ends are aligned towards the cell membrane and that these ends grow rapidly in the presence of actin monomer as long as they are uncapped. Once a barbed end is capped, its filament tends to be degraded by fragmentation or depolymerization. Both the growth (by polymerization) and the fragmentation by actin-cutting agents are depicted in the model, which takes into account the dependence of cutting probability on the position along a filament. It is assumed that barbed ends are capped rapidly away from the cell membrane. The model consists of a system of discrete-integro-PDE's that describe the densities of barbed filament ends as a function of spatial position and length of their actin filament “tails”. The population of capped barbed ends and their trailing filaments is similarly represented. This formulation allows us to investigate hypotheses about the fragmentation and polymerization of filaments in a caricature of the lamellipod and compare theoretical and observed actin density profiles. Received: 19 May 2000 / Revised version: 12 March 2001 / Published online: 19 September 2001     

Structure of molluscan thick filaments: a common origin for diverse appearances.

Thick filaments from the smooth adductor muscles of the oysters Ostrea edulis and Crassostrea angulata have been examined in the electron microscope after negative staining. The two well-known patterns of stain (whose origin and relation have been uncertain), one a series of transverse narrow lines at intervals of 144 Å along the filament axis and the other a regular two-dimensional arrangement of stained spots (Bear &; Selby, 1956), are found to be mutually interconvertible by rotating the grid around the filament axis. This is interpreted to mean that the spots are the projections of stained regions running through the filament in a common direction. Only when looking along this direction will the net pattern be seen with maximum clarity and sharpness. On rotation of the filament round its axis, the spots broaden transversely to the axis, overlapping and ultimately only the axial periodicity will remain. The structure is therefore not helical, but resembles a crystal lattice, although no period can be discerned normal to the net plane.The addition of 10 mm-EDTA to all solutions used in the filament preparation (except the stain), especially when ammonium molybdate is the stain employed, removes many puzzling appearances (probably caused by positive staining) which render the interpretation difficult. The appearance of the negatively stained filament can be related to the stain patterns in negatively stained paramyosin paracrystals (Cohen et al., 1971).     

An X-Ray Diffraction Study of Contracting Molluscan Smooth Muscle

The living anterior byssus retractor muscle of Mytilus (ABRM), a smooth, “catch” muscle, has been studied by X-ray diffraction while relaxed and while tonically contracted. X-ray reflections were observed from the actin and paramyosin filaments and from the α-helical substructure of the paramyosin filaments. No differences in spacings or relative intensities were observed when the relaxed and contracting muscle patterns were compared. This result is consistent with a sliding filament mechanism involving an interaction between actin and paramyosin filaments.     

Twirling of actin by myosins II and V observed via polarized TIRF in a modified gliding assay

The force generated between actin and myosin acts predominantly along the direction of the actin filament, resulting in relative sliding of the thick and thin filaments in muscle or transport of myosin cargos along actin tracks. Previous studies have also detected lateral forces or torques that are generated between actin and myosin, but the origin and biological role of these sideways forces is not known. Here we adapt an actin gliding filament assay to measure the rotation of an actin filament about its axis (“twirling”) as it is translocated by myosin. We quantify the rotation by determining the orientation of sparsely incorporated rhodamine-labeled actin monomers, using polarized total internal reflection microscopy. To determine the handedness of the filament rotation, linear incident polarizations in between the standard s- and p-polarizations were generated, decreasing the ambiguity of our probe orientation measurement fourfold. We found that whole myosin II and myosin V both twirl actin with a relatively long (∼1 μm), left-handed pitch that is insensitive to myosin concentration, filament length, and filament velocity.     

The crystalline structure of collagen fibrils in tendon.

New data have been collected on the crystalline structure of collagen fibrils in tendon. The unit cell in decrimped tendon has been determined by measurements of the Bragg reflections in the X-ray diffraction pattern. The results are consistent with a triclinic cell with $\text{b = 75.5}\text{A}\text{?}$, β = 93 °, $\text{a =}\text{b}\text{sin}\text{β}$, a = 90 °, $\text{c = n × 668}\text{A}\text{?}$, where n is probably 4 and γ = 90 °. A selection rule observed for prominent reflections is explicable either in terms of a specific orientation of the microfibrils on the lattice, or by a helical distortion of the microfibril axis. The cell parameter β can be varied by changing the ionic envirionment.     

Communicative interaction of myosins along an actin filament in the presence of ATP

Myosin molecules contacting an actin filament in the presence of ATP were found to regulate the filamental fluctuations due to ATP hydrolysis in a communicative manner along the filament. As an evidence of the occurrence of the communication, ATP-activated fluctuating displacements of the filament in the direction perpendicular to its longitudinal axis were identified to propagate at a finite velocity not less than about 0.2 μm/s unidirectionally along the filament.     

Orientational information of troponin C within the thin filaments obtained by neutron fiber diffraction

In striated muscles contraction is regulated by the thin filament-based proteins, troponin consisting of three subunits (TnC, TnI, and TnT), and tropomyosin. Knowledge of in situ structures of these proteins is indispensable for elucidating this Ca(2+)-sensitive regulatory mechanism. We employed neutron scattering to investigate the structure of TnC within the thin filament, and found that TnC assumes extended dumbbell-like structures and moves toward the filament axis by binding of Ca(2+). Here, in order to obtain more detailed in situ structural information of TnC, neutron fiber diffraction measurements were performed. Sols of native thin filaments and the thin filaments containing deuterated TnC were prepared in (2)H(2)O. The oriented samples were obtained by placing these sols sealed in quartz capillaries with a diameter of 3 mm in a magnetic field of 18 Tesla. Neutron fiber diffraction patterns were obtained from these oriented samples in the absence and presence of Ca(2+). The patterns obtained showed strong equatorial diffraction due to the thin filaments, 59 A and 51 A layer-lines due to actin, and meridional reflections due to Tn-complex. Analysis of the meridional reflections due to Tn-complex with aid of model calculation showed that the angle between the thin filament axis and the long axis of TnC was estimated to be 67(+/-7) degrees and 49(+/-17) degrees , in the absence and presence of Ca(2+), respectively, suggesting that TnC, which assumes orientations rather perpendicular to the filament axis in the absence of Ca(2+), tilts toward the filament axis and the orientational and positional disorder increases by binding Ca(2+). It also showed that the relative position of the TnC moved by about 22 A by binding Ca(2+), and this apparent movement was concomitant with the movements of other Tn-subunits. This implies that by binding Ca(2+), significant structural rearrangements of Tn-subunits occur.     

Reactivities of Actin as a Contractile Protein

The molecular basis for the mechanism of contraction in striated muscle, with primary emphasis on the interaction between the thick and thin filaments and the role of the thin (actin) filaments, is the theme presented. Recent information relating to actin-myosin interaction points up the fact that definitive statements cannot be made regarding the molecular interaction(s) that lead to contraction. Nevertheless, the properties of actin indicate that (a) actin in the monomeric state has properties differing markedly from actin in the polymer (filament) state; (b) these property differences may be significant in the contractile process, for they include changes in the reactivity of the bound nucleotide and actin-myosin complex formation; (c) the bound nucleotide seems to be required in the contraction process. For these, and other, reasons discussed, the tentative hypothesis is advanced that the contraction reaction involves local changes in the actin filament providing local monomer or monomer-like actin units in the reaction with myosin.     

Structural analysis of muscle thin filament

Thin sheets of Ac-Tm-Tn paracrystals were prepared in the presence of high concentration of Ca²⁺ ion and three-dimensional image analysis was performed. The optical diffraction pattern of an electron micrograph showed spots up to 1/1.6 nm⁻¹ in the radial direction and up to 1/2.5 nm⁻¹ in the axial direction, the best resolution ever obtained so far. The translationally filtered image showed clear polarity of filament which looked like a “spearhead” per each crossover repeat of actin helix.The three-dimensionally reconstructed model looked very similar to the inner regions (A+B domains) of the Ac-Tm-S1 complex obtained by Toyoshima and Wakabayashi (14, 15) when they were placed so that the “spearhead” pattern of the Tc-Tm-Tn complex and the “arrowhead” pattern of the Ac-Tm-S1 complex pointed in the same direction. The myosin-binding site of actin was identified by comparison of two structures.The model of actin molecule cut out from the thin filament model had a low density region within itself, which was located about 2.5 nm from the helix axis. That low density region divided actin molecule into two domains, a large and a small domain. A dense “pillar” was detected which connected two neighboring actin molecules along a left-handed generic helix 1 nm from the helix axis. Two actin-actin binding sites which were responsible for the connection through the “pillar” were located on the inner surface of actin molecule.To obtain better crystalline arrays of actin, we tried a method utilizing adsorption to lipid. A positively-charged monolayer of lipids was formed on the surface of a small volume of buffer solution which was put in a microwell. Solution of negatively-charged F-actin was then injected into the buffer solution and was allowed to be joined to the lipid monolayer by electrostatic attraction. Fluidity of the lipid monolayer enabled the two-dimensional crystallization of actin. Electron microscopy revealed that larger paracrystalline arrays were formed more rapidly (< 1 hr) than those formed within solution, which demonstrated the advantage of this adsorption method.     

Quantitative Analysis of Filament Branch Orientation in Listeria Actin Comet Tails

Several bacterial and viral pathogens hijack the host actin cytoskeleton machinery to facilitate spread and infection. In particular, Listeria uses Arp2/3-mediated actin filament nucleation at the bacterial surface to generate a branched network that will help propel the bacteria. However, the mechanism of force generation remains elusive due to the lack of high-resolution three-dimensional structural data on the spatial organization of the actin mother and daughter (i.e., branch) filaments within this network. Here, we have explored the three-dimensional structure of Listeria actin tails in Xenopus laevis egg extracts using cryo-electron tomography. We found that the architecture of Listeria actin tails is shared between those formed in cells and in cell extracts. Both contained nanoscopic bundles along the plane of the substrate, where the bacterium lies, and upright filaments (also called Z filaments), both oriented tangentially to the bacterial cell wall. Here, we were able to identify actin filament intersections, which likely correspond to branches, within the tails. A quantitative analysis of putative Arp2/3-mediated branches in the actin network showed that mother filaments lie on the plane of the substrate, whereas daughter filaments have random deviations out of this plane. Moreover, the analysis revealed that branches are randomly oriented with respect to the bacterial surface. Therefore, the actin filament network does not push directly toward the surface but rather accumulates, building up stress around the Listeria surface. Our results favor a mechanism of force generation for Listeria movement where the stress is released into propulsive motion.     

Drosophila Fascin is a novel downstream target of prostaglandin signaling during actin remodeling

Although prostaglandins (PGs)—lipid signals produced downstream of cyclooxygenase (COX) enzymes—regulate actin cytoskeletal dynamics, their mechanisms of action are unknown. We previously established Drosophila oogenesis, in particular nurse cell dumping, as a new model to determine how PGs regulate actin remodeling. PGs, and thus the Drosophila COX-like enzyme Pxt, are required for both the parallel actin filament bundle formation and the cortical actin strengthening required for dumping. Here we provide the first link between Fascin (Drosophila Singed, Sn), an actin-bundling protein, and PGs. Loss of either pxt or fascin results in similar actin defects. Fascin interacts, both pharmacologically and genetically, with PGs, as reduced Fascin levels enhance the effects of COX inhibition and synergize with reduced Pxt levels to cause both parallel bundle and cortical actin defects. Conversely, overexpression of Fascin in the germline suppresses the effects of COX inhibition and genetic loss of Pxt. These data lead to the conclusion that PGs regulate Fascin to control actin remodeling. This novel interaction has implications beyond Drosophila, as both PGs and Fascin-1, in mammalian systems, contribute to cancer cell migration and invasion.     

Preparation and crystal structure of bis(acetato)(trans-1,2-diaminocyclohexane)platinum(II)

The structure of the complex [Pt(trans-1,2-di- aminocyclohexane) (acetate)₂]·H₂O has been determined by X-ray diffraction. This racemic compound is orthorhombic, space group Aba2, a = 20.813(9), b = 7.926(5), c = 17.296(8) Å, Z = 8. The structure was refined on 1214 nonzero Cu Kα reflections to R = 0.028. The square planar environment of Pt includes the amino groups of the diamine in cis positions and oxygens from two monodentate acetates. The PtN and PtO distances average 2.00(3) and 2.02(3) Å, respectively. The bite of the diamine ligand imposes a NPtN angle of 85(1)°, whereas the small OPtO angle of 85(1)° probably results from packing effects. The average plane through the puckered cyclohexyl ring makes an angle of 19° with the PtN₂O₂ plane. The molecules are stacked by pairs along the b axis. The two molecules of each pair are 180° apart about the stacking axis, and form altogether four NH···O hydrogen bonds.     

A Microscopic Formulation for the Actin-Driven Motion of Listeria in Curved Paths

Using a generalized Brownian ratchet model that accounts for the interactions of actin filaments with the surface of Listeria mediated by proteins like ActA and Arp2/3, we have developed a microscopic model for the movement of Listeria. Specifically, we show that a net torque can be generated within the comet tail, causing the bacteria to spin about its long axis, which in conjunction with spatially varying polymerization at the surface leads to motions of bacteria in curved paths that include circles, sinusoidal-like curves, translating figure eights, and serpentine shapes, as observed in recent experiments. A key ingredient in our formulation is the coupling between the motion of Listeria and the force-dependent rate of filament growth. For this reason, a numerical scheme was developed to determine the kinematic parameters of motion and stress distribution among filaments in a self-consistent manner. We find that a 5-15% variation in polymerization rates can lead to radii of curvatures of the order of 4-20 μm, measured in experiments. In a similar way, our results also show that most of the observed trajectories can be produced by a very low degree of correlation, <10%, among filament orientations. Since small fluctuations in polymerization rate, as well as filament orientation, can easily be induced by various factors, our findings here provide a reasonable explanation for why Listeria can travel along totally different paths under seemingly identical experimental conditions. Besides trajectories, stress distributions corresponding to different polymerization profiles are also presented. We have found that although some actin filaments generate propelling forces that push the bacteria forward, others can exert forces opposing the movement of Listeria, consistent with recent experimental observations.