Electron microscopy of negatively stained scallop myosin molecules. Effect of regulatory light chain removal on head structure

Invertebrate muscles: thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

This is the second in a series of canonical reviews on invertebrate muscle. We cover here thin and thick filament structure, the molecular basis of force generation and its regulation, and two special properties of some invertebrate muscle, catch and asynchronous muscle. Invertebrate thin filaments resemble vertebrate thin filaments, although helix structure and tropomyosin arrangement show small differences. Invertebrate thick filaments, alternatively, are very different from vertebrate striated thick filaments and show great variation within invertebrates. Part of this diversity stems from variation in paramyosin content, which is greatly increased in very large diameter invertebrate thick filaments. Other of it arises from relatively small changes in filament backbone structure, which results in filaments with grossly similar myosin head placements (rotating crowns of heads every 14.5 nm) but large changes in detail (distances between heads in azimuthal registration varying from three to thousands of crowns). The lever arm basis of force generation is common to both vertebrates and invertebrates, and in some invertebrates this process is understood on the near atomic level. Invertebrate actomyosin is both thin (tropomyosin:troponin) and thick (primarily via direct Ca(++) binding to myosin) filament regulated, and most invertebrate muscles are dually regulated. These mechanisms are well understood on the molecular level, but the behavioral utility of dual regulation is less so. The phosphorylation state of the thick filament associated giant protein, twitchin, has been recently shown to be the molecular basis of catch. The molecular basis of the stretch activation underlying asynchronous muscle activity, however, remains unresolved.

Invertebrate Muscles: Muscle Specific Genes and Proteins

This is the first of a projected series of canonic reviews covering all invertebrate muscle literature prior to 2005 and covers muscle genes and proteins except those involved in excitation-contraction coupling (e.g., the ryanodine receptor) and those forming ligand- and voltage-dependent channels. Two themes are of primary importance. The first is the evolutionary antiquity of muscle proteins. Actin, myosin, and tropomyosin (at least, the presence of other muscle proteins in these organisms has not been examined) exist in muscle-like cells in Radiata, and almost all muscle proteins are present across Bilateria, implying that the first Bilaterian had a complete, or near-complete, complement of present-day muscle proteins. The second is the extraordinary diversity of protein isoforms and genetic mechanisms for producing them. This rich diversity suggests that studying invertebrate muscle proteins and genes can be usefully applied to resolve phylogenetic relationships and to understand protein assembly coevolution. Fully achieving these goals, however, will require examination of a much broader range of species than has been heretofore performed.

Calcium-dependent structural changes in scallop heavy meromyosin

The mechanism of calcium regulation of scallop myosin is not understood, although it is known that both myosin heads are required. We have explored possible interactions between the heads of heavy meromyosin (HMM) in the presence and absence of calcium and nucleotides by sedimentation and electron microscope studies. The ATPase activity of the HMM preparation was activated over tenfold by calcium, indicating that the preparation contained mostly regulated molecules. In the presence of ADP or ATP analogs, calcium increased the asymmetry of the HMM molecule as judged by its slower sedimentation velocity compared with that in EGTA. In the absence of nucleotide the asymmetry was high even in EGTA. The shift in sedimentation occurred with a sharp midpoint at a calcium level of about 0.5 microM. Sedimentation of subfragment 1 was not dependent on calcium or on nucleotides. Modeling accounted for the observed sedimentation behavior by assuming that both HMM heads bent toward the tail in the absence of calcium, while in its presence the heads had random positions. The sedimentation pattern showed a single peak at all calcium concentrations, indicating equilibration between the two forms with a t(1/2) less than 70 seconds. Electron micrographs of crosslinked, rotary shadowed specimens indicated that 81 % of HMM molecules in the presence of nucleotide had both heads pointing back towards the tail in the absence of calcium, as compared with 41 % in its presence. This is consistent with the sedimentation data. We conclude that in the "off" state, scallop myosin heads interact with each other, forming a rigid structure with low ATPase activity. When molecules are switched "on" by binding of calcium, communication between the heads is lost, allowing them to flex randomly about the junction with the tail; this could facilitate their interaction with actin in contracting muscle.

Calcium-induced quenching of intrinsic fluorescence in brain myosin V is linked to dissociation of calmodulin light chains

Myosin V isolated from chick brain (BM V) is a multimeric protein of about 640 kDa consisting of two intertwined heavy chains of 212 kDa and multiple light chains of 10 to 20 kDa. A distinctive feature of the heavy chain is an extended neck region with six consensus IQ sites for the binding of calmodulin (CaM) and myosin light chains. The actin-activated MgATPase has been shown to require >/=1 microM Ca2+ for full activity, and evidence points to a myosin-linked regulatory system where the CaM light chains participate as modulators for the Ca2+ signal. Still, the precise mechanism of Ca2+ regulation remains unknown. In the present study we have used the intrinsic tryptophan fluorescence of native BM V to monitor conformational changes of BM V induced by Ca2+, and we relate these changes to CaM dissociation from the BM V molecule. The fluorescence intensity decreases approximately 17% upon addition of sub-micromolar concentrations of Ca2+ (K0.5 = 0.038 microM). This decrease in fluorescence, which is dominated by a conformational change in the heavy chain, can be reversed by addition of 1, 2-di(2-aminoethoxy)ethane-N,N,N',N'tetraacetic acid (EGTA) followed by an excess of CaM, but not by addition of EGTA alone. Gel filtration of native BM V using HPLC shows that CaM is partially dissociated from the heavy chain in EGTA and dissociates further upon addition of sub-micromolar concentrations of Ca2+. These observations suggest that the affinity of CaM for at least one of the IQ sites on the BM V heavy chain decreases with Ca2+ and that the Ca2+ concentration required for this effect is lower than that needed to activate acto-BM V. Using a cosedimentation assay in the presence of actin, we also observe partial dissociation of CaM when Ca2+ is absent, but now the addition of Ca2+ has a biphasic effect: sub-micromolar Ca2+ concentrations lead to reassociation of CaM with the heavy chain, followed by dissociation when Ca2+ exceeds 5-10 microM. Thus, the binding of CaM to BM V is affected by both actin and Ca2+.

Electron cryomicroscopy of acto-myosin-S1 during steady-state ATP hydrolysis

The structure of the complex of actin and myosin subfragment-1 (S1) during steady-state ATP hydrolysis has been examined by electron microscopy. This complex is normally dissociated by ATP in vitro but was stabilized here by low ionic strength. Optimal conditions for attachment were established by light-scattering experiments that showed that approximately 70% of S1 could be bound in the presence of ATP. Micrographs of the unstained complex in vitreous water suggest that S1 attaches to actin in a variety of configurations in ATP; this contrasts with the single attached configuration seen in the presence of ADP. The data are therefore compatible with the idea that a change in attached configuration of the myosin cross-bridge is the origin of muscle force. In control experiments where ATP was allowed to hydrolyze completely the binding of the S1 seemed cooperative.

Fluorescence resonance energy transfer within the regulatory light chain of myosin

Rabbit skeletal muscle myosin regulatory light chain-2 (LC2) contains two reactive cysteine residues, Cys125 and Cys154, and one tryptophan at position 137. Using wild-type rabbit LC2 or its genetically engineered mutant with Cys125-->Arg (C125R), these residues can be selectively modified with fluorescent or chromophoric probes for spectroscopic studies. We have bound suitable donor/acceptor probe pairs to the two cysteine residues and Trp137 in LC2 or C125R, and measured the distance in solution between the probes by fluorescence resonance energy transfer spectroscopy. C125R was made to facilitate specific labelling of the less reactive Cys154, thus allowing the distance between Cys154 and Trp137 to be measured. Our measurements show that these residues are in close proximity to each other, the distance between them ranging from 1.7 nm (between Cys125 and Trp137) to 2.7 nm (Cys125 and Cys154). These results suggest that Cys125, Trp137 and Cys154, spanning up to 29 residues in the sequence of LC2, are spatially close, consistent with these residues residing within a C-terminal globular domain. The distances we obtained are in agreement with previous crosslinking studies [Huber, P. A., Brunner, U.T. & Schaub, M. C. (1989) Biochemistry 28, 9116-9123; Saraswat, L. & Lowey, S. (1991) J. Biol. Chem. 266, 19777-19785] and structure predictions of LC2. LC2 is located at the head-rod junction of the myosin crossbridge, and provides the primary regulatory mechanism in molluscan and smooth muscle. In skeletal muscle, its functional role is unclear, although it has been implicated in modulating actomyosin interaction [Metzger, J. M. & Moss, R. L. (1992) Biophys. J. 63, 460-468]. The incorporation of spectroscopic probes onto the light chains of myosin in solution or in fibres has become a valuable tool for evaluating the dynamic properties of the crossbridge during force generation.

Structure of the myosin head in solution and the effect of light chain 2 removal

Structural properties of rabbit skeletal myosin head (S1) and the influence of the DTNB light chain (LC2) on the size and shape of myosin heads in solution were investigated by small angle x-ray scattering. The LC2 deficient myosin head, S1 (-LC2), and the S1 containing LC2 light chain, S1 (+LC2) were studied in parallel. The respective values of the radius of gyration were found to be (40.2 +/- 0.5) A and (46.7 +/- 1) A, while the maximum dimension was (190 +/- 15) A for both species. The large difference between the two Rg values suggest that LC2 is located close to one extremity of the myosin head, in agreement with most electron microscopy observations. All models derived from the x-ray scattering pattern of the native myosin head share a common overall morphology, showing two main regions, an asymmetric globular portion which tapers smoothly into a thinner domain of roughly equivalent length making an angle of approximately 60 degrees, with a contour length of approximately 210 A.

Shape and length of myosin heads

There is controversy concerning the shape and length of myosin heads. In the present paper we try to analyse the data and to draw clear conclusions in this field. When the myosin heads are isolated (S1) from the rest of the molecule, their length is approximately 12 nm and their shape is close to that of a prolate ellipsoid with an axial ratio approximately 2.3 (in solution) or close to that of a comma when attached to F-actin (with a length of 12-13 nm). When the myosin heads are observed on a whole molecule, their length is approximately 19 nm and they are pear-shaped. Here we suggest that all these observations are compatible. We believe that, for a whole myosin molecule, a large part of the head-rod joint (S1/S2 joint) is measured with the head, owing to a particularly heavy staining or shadowing of this joint. On the other hand, S1 is probably built up of a head part plus the S1/S2 joint, which is not revealed by the usual techniques (hydrodynamics, X-ray and neutron scattering). Finally, the comma shape would be related to a flexible part in the head region of S1, which is significantly bent when S1 is attached to F-actin, but which would be less bent for S1 in solution. A similar bending also occurs in crystalline S1.

Efficiency of muscle contraction. The chemimechanic equilibrium

Although muscle contraction is one of the principal themes of biological research, the exact mechanism whereby the chemical free energy of ATP hydrolysis is converted into mechanical work remains elusive. The high thermodynamic efficiency of the process, above all, is difficult to explain on the basis of present theories. A model of the elementary effect in muscle contraction is proposed which aims at high thermodynamic efficiency based on an approximate equilibrium between chemical and mechanical forces throughout the transfer of free energy. The experimental results described in the literature support the assumption that chemimechanic equilibrium is approximated by a free energy transfer system based on the binding of divalent metal ions to the myosin light chains. Muscle contraction demonstrated without light chains is expected to proceed with a considerably lower efficiency. Free energy transfer systems based on the binding of ions to proteins seem to be widespread in the cell. By establishing an approximate chemimechanic equilibrium, they could facilitate biological reactions considerably and save large amounts of free energy. The concept of chemimechanic equilibrium is seen as a supplementation to the concept of chemiosmotic equilibrium introduced for the membrane transport by P. Mitchell.

Three-dimensional structure of myosin subfragment-1 from electron microscopy of sectioned crystals

Image analysis of electron micrographs of thin-sectioned myosin subfragment-1 (S1) crystals has been used to determine the structure of the myosin head at approximately 25-A resolution. Previous work established that the unit cell of type I crystals of myosin S1 contains eight molecules arranged with orthorhombic space group symmetry P212121 and provided preliminary information on the size and shape of the myosin head (Winkelmann, D. A., H. Mekeel, and I. Rayment. 1985. J. Mol. Biol. 181:487-501). We have applied a systematic method of data collection by electron microscopy to reconstruct the three-dimensional (3D) structure of the S1 crystal lattice. Electron micrographs of thin sections were recorded at angles of up to 50 degrees by tilting the sections about the two orthogonal unit cell axes in sections cut perpendicular to the three major crystallographic axes. The data from six separate tilt series were merged to form a complete data set for 3D reconstruction. This approach has yielded an electron density map of the unit cell of the S1 crystals of sufficient detail. to delineate the molecular envelope of the myosin head. Myosin S1 has a tadpole-shaped molecular envelope that is very similar in appearance to the pear-shaped myosin heads observed by electron microscopy of rotary-shadowed and negatively stained myosin. The molecule is divided into essentially three morphological domains: a large domain on one end of the molecule corresponding to approximately 60% of the total molecular volume, a smaller central domain of approximately 30% of the volume that is separated from the larger domain by a cleft on one side of the molecule, and the smallest domain corresponding to a thin tail-like region containing approximately 10% of the volume. This molecular organization supports models of force generation by myosin which invoke conformational mobility at interdomain junctions within the head.