Single-headed scallop myosin and regulation

X-ray solution scattering of squid heavy meromyosin: strengthening the evidence for an ancient compact off state

The overall conformations of regulated myosins or heavy meromyosins from chicken/turkey, scallop, tarantula, limulus, and scorpion sources have been studied by a number of techniques, including electron microscopy, sedimentation, and pulsed electron paramagnetic resonance. These studies have indicated that the binding of regulatory ions changes the conformation of the molecule from a compact shape found in the "off" state of the muscle to extended relationships between the tail and independently mobile heads that predominate in the "on" state. Here we strengthen the argument for the generality of this conformational change by using small angle X-ray scattering on heavy meromyosin from squid. Small angle X-ray scattering allows the protein to be visualized in solution under mild and relatively physiological conditions, and squid differs from the other species studied by at least 500 million years of evolution. Analysis of the data indicates that upon addition of Ca(2+) the radius of gyration increases. Differences in the squid "on" and "off" states are clearly distinguishable as bimodal and unimodal pair distance distribution functions respectively. These observations are consistent with a Ca(2+)-free squid heavy meromyosin that is compact, but which becomes extended when Ca(2+) is bound. Further, the scattering profile derived from the current model of tarantula heavy meromyosin in the "off" state is in excellent agreement with the measured "off" state scattering profile for squid heavy meromyosin. The previous and current studies together provide significant evidence that regulated myosin's compact off-state conformation is an ancient trait, inherited from a common ancestor during divergent evolution.

Purification, crystallization and preliminary X-ray crystallographic analysis of squid heavy meromyosin

All muscle-based movement is dependent upon carefully choreographed interactions between the two major muscle components, myosin and actin. Regulation of vertebrate smooth and molluscan muscle contraction is myosin based (both are in the myosin II class), and requires the double-headed form of myosin. Removal of Ca2+ from these muscles promotes a relatively compact conformation of the myosin dimer, which inhibits its interaction with actin. Although atomic structures of single myosin heads are available, the structure of any double-headed portion of myosin, including the ∼375 kDa heavy meromyosin (HMM), has only been visualized at low (∼20 Å) resolution by electron microscopy. Here, the growth of three-dimensional crystals of HMM with near-atomic resolution (up to ∼5 Å) and their X-ray diffraction are reported for the first time. These crystals were grown in off-state conditions, that is in the absence of Ca2+ and the presence of nucleotide analogs, using HMM from the funnel retractor muscle of squid. In addition to the crystallization conditions, the techniques used to isolate and purify this HMM are also described. Efforts at phasing and improving the resolution of the data in order to determine the structure are ongoing.

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.

Calcium regulates scallop muscle by changing myosin flexibility

Muscle myosins are molecular motors that convert the chemical free energy available from ATP hydrolysis into mechanical displacement of actin filaments, bringing about muscle contraction. Myosin cross-bridges exert force on actin filaments during a cycle of attached and detached states that are coupled to each round of ATP hydrolysis. Contraction and ATPase activity of the striated adductor muscle of scallop is controlled by calcium ion binding to myosin. This mechanism of the so-called "thick filament regulation" is quite different to vertebrate striated muscle which is switched on and off via "thin filament regulation" whereby calcium ions bind to regulatory proteins associated with the actin filaments. We have used an optically based single molecule technique to measure the angular disposition adopted by the two myosin heads whilst bound to actin in the presence and absence of calcium ions. This has allowed us to directly observe the movement of individual myosin heads in aqueous solution at room temperature in real time. We address the issue of how scallop striated muscle myosin might be regulated by calcium and have interpreted our results in terms of the structures of smooth muscle myosin that also exhibit thick filament regulation.

Cardiac myosin isoforms from different species have unique enzymatic and mechanical properties

The mammalian heart contains two cardiac myosin isoforms: beta-myosin heavy chain (MHC) is found predominantly in the ventricles of large mammals, and alpha-MHC is expressed in the atria. The sequence identity between these isoforms is approximately 93%, with nonidentical residues clustered in discrete, functionally important domains associated with actin binding and ATPase activity. It is well-established that rabbit alpha-cardiac myosin has a 2-fold greater unloaded shortening velocity than beta-cardiac myosin but a 2-fold lower average isometric force. Here, we test the generality of these relationships for another large mammal, the pig, as well as for a small rodent, the mouse, which expresses alpha-MHC in its ventricles throughout adulthood. Hydrophobic interaction chromatography (HIC) was used to purify myosin from mouse, rabbit, and pig hearts. The superior resolving power of HIC made it possible to prepare highly homogeneous, enzymatically active myosin from small amounts of tissue. The movement of actin filaments by myosin was measured in an in vitro motility assay. The same assay could be used to determine average isometric force by loading the actin filaments with increasing concentrations of alpha-actinin to stop filament motion. We conclude that myosin from the mouse has significantly higher velocities for both alpha and beta isoforms than myosin from rabbits and pigs, even though the 2-fold difference in velocity between isoforms is maintained. Unlike the larger mammals, however, the small rodent generates the same high isometric force for both alpha and beta isoforms. Thus, nature has adapted the function of cardiac myosin isoforms to optimize power output for hearts of a given species.

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.

Primary structure of myosin from the striated adductor muscle of the Atlantic scallop, Pecten maximus, and expression of the regulatory domain

We have determined the complete cDNA and deduced amino acid sequences of the heavy chain, regulatory light chain and essential light chain which constitute the molecular structure of myosin from the striated adductor muscle of the scallop, Pecten maximus. The deduced amino acid sequences of P. maximus regulatory light chain, essential light chain and heavy chain comprise 156, 156 and 1940 amino acids, respectively. These myosin peptide sequences, obtained from the most common of the eastern Atlantic scallops, are compared with those from three other molluscan myosins: the striated adductor muscles of Argopecten irradians and Placopecten magellanicus, and myosin from the siphon retractor muscle of the squid, Loligo pealei. The Pecten heavy chain sequence resembles those of the other two scallop sequences to a much greater extent as compared with the squid sequence, amino acid identities being 97.5% (A. irradians), 95.6% (P. magellanicus) and 73.6% (L. pealei), respectively. Myosin heavy chain residues that are known to be important for regulation are conserved in Pecten maximus. Using these Pecten sequences, we have overexpressed the regulatory light chain, and a combination of essential light chain and myosin heavy chain fragment, separately, in E. coli BL21 (DE3) prior to recombination, thereby producing Pecten regulatory domains without recourse to proteolytic digestion. The expressed regulatory domain was shown to undergo a calcium-dependent increase (approximately 7%) in intrinsic tryptophan fluorescence with a mid-point at a pCa of 6.6.

Locking regulatory myosin in the off-state with trifluoperazine

Scallop striated adductor muscle myosin is a regulatory myosin, its activity being controlled directly through calcium binding. Here, we show that millimolar concentrations of trifluoperazine were effective at removal of all regulatory light chains from scallop myosin or myofibrils. More important, 200 microM trifluoperazine, a concentration 10-fold less than that required for light-chain removal, resulted in the reversible elimination of actin-activated and intrinsic ATPase activities. Unlike desensitization induced by metal ion chelation, which leads to an elevation of activity in the absence of calcium concurrent with regulatory light-chain removal, trifluoperazine caused a decline in actin-activated MgATPase activity both in the presence and absence of calcium. Procedures were equally effective with respect to scallop myosin, myofibrils, subfragment-1, or desensitized myofibrils. Increased alpha-helicity could be induced in the isolated essential light chain through addition of 100-200 microM trifluoperazine. We propose that micromolar concentrations of trifluoperazine disrupt regulation by binding to a single high-affinity site located in the C-terminal domain of the essential light chain, which locks scallop myosin in a conformation resembling the off-state. At millimolar trifluoperazine concentrations, additional binding sites on both light chains would be filled, leading to regulatory light-chain displacement.

Two heads of myosin are better than one for generating force and motion

Several classes of the myosin superfamily are distinguished by their "double-headed" structure, where each head is a molecular motor capable of hydrolyzing ATP and interacting with actin to generate force and motion. The functional significance of this dimeric structure, however, has eluded investigators since its discovery in the late 1960s. Using an optical-trap transducer, we have measured the unitary displacement and force produced by double-headed and single-headed smooth- and skeletal-muscle myosins. Single-headed myosin produces approximately half the displacement and force (approximately 6 nm; 0.7 pN) of double-headed myosin (approximately 10 nm; 1.4 pN) during a unitary interaction with actin. These data suggest that muscle myosins require both heads to generate maximal force and motion.

Fluorescence measurements detect changes in scallop myosin regulatory domain

Ca2+-induced conformational changes of scallop myosin regulatory domain (RD) were studied using intrinsic fluorescence. Both the intensity and anisotropy of tryptophan fluorescence decreased significantly upon removal of Ca2+. By making a mutant RD we found that the Ca2+-induced fluorescence change is due mainly to Trp21 of the essential light chain which is located at the unusual Ca2+-binding EF-hand motif of the first domain. This result suggests that Trp21 is in a less hydrophobic and more flexible environment in the Ca2+-free state, supporting a model for regulation based on the 2 A resolution structure of scallop RD with bound Ca2+ [Houdusse A. and Cohen C. (1996) Structure 4, 21-32]. Binding of the fluorescent probe, 8-anilinonaphthalene-1-sulphonate (ANS) to the RD senses the dissociation of the regulatory light chain (RLC) in the presence of EDTA, by energy transfer from a tryptophan cluster (Trp818, 824, 826, 827) on the heavy chain (HC). We identified a hydrophobic pentapeptide (Leu836-Ala840) at the head-rod junction which is required for the effective energy transfer and conceivably is part of the ANS-binding site. Extension of the HC component of RD towards the rod region results in a larger ANS response, presumably indicating changes in HC-RLC interactions, which might be crucial for the regulatory function of scallop myosin.

Measurement of ATPase activities of myosin at the level of tracks and single molecules

In order to determine the degree of mechanochemical coupling in actomyosin in vitro motility assays, it is desirable to measure the sliding velocity and the associated ATP turnover simultaneously at the single filament level. Actin sliding over tracks of immobilised heavy meromyosin (HMM) has been initiated by flash photolysis of caged ATP. Flash photolysis has also been used to displace fluorescent Cy3-EDA-nucleotides from HMM tracks to monitor the ATPase activity. These assays are now being combined using total internal reflectance fluorescence (TIRF) microscopy with a dual-view detection system for Cy3/Cy5 labels on ATP and actin respectively. In other experiments, we are exploring the use of the single molecule kinetic technique developed by Funatsu et al. (Nature 374, 555-559, 1995) to scale down ATPase assays of Dictyostelium myosin fragments and to elucidate the mechanism of regulation of the molluscan (scallop) myosin ATPase. Although fluctuations occur from the binding and release of Cy3-EDA-nucleotides during turnover and might provide a measure of the ATPase activity, other sources of fluctuations also need to be considered.

Regulation of scallop myosin by calcium. Cooperativity and the "off" state

Scallop subfragment 1 (S1) is an unregulated molecule; it differs from heavy meromyosin (HMM) and myosin in that it has no "off" state, although it contains the full complement of light chains and the triggering calcium binding site. S1 differs from myosin by lacking the head-rod junction and being single-headed. The contribution of the head-rod junction was evaluated by studying single-headed myosin. Isolated single-headed myosins show some regulation; their actin activated ATPase is stimulated about 3-fold by calcium. However, in contrast to HMM and myosin, the calcium dependence of ATPase activation of single-headed myosin is non-cooperative. The single ATP turnover rate of single-headed myosin in the absence of calcium is less than 30 seconds (our experimental resolution) compared to the approximately 5 minute turnover rate of myosin. HMM and myosin exhibit several cooperative features not shown by S1. Calcium binding becomes cooperative in the presence of nucleotide analogues in HMM and myosin, but not in S1. Nucleotide analogues are bound cooperatively by myosin and HMM in the absence of calcium; the introduction of calcium to the system reduces the affinity and abolishes the cooperative binding of nucleotide in the double headed molecules. Conversely, S1 shows normal binding curves for nucleotide analogues both in the presence and absence of calcium. Therefore, there is direct communication between the calcium binding sites and nucleotide binding sites in regulated molecules that is mediated by interaction between the two heads. .

Dimerization of the head-rod junction of scallop myosin

We have compared the dimerization properties and coiled-coil stability of various recombinant fragments of scallop myosin around the head-rod junction. The heavy-chain peptide of the regulatory domain and its various extensions toward the alpha-helical rod region were expressed in Escherichia coli, purified, and reconstituted with the light chains. Rod fragments of the same length but without the light-chain binding domain were also expressed. Electron micrographs show that the regulatory domain complex containing 340 residues of the rod forms dimers with two knobs (two regulatory domains) at one end attached to an approximately 50-nm coiled coil. These parallel dimers are in equilibrium with monomers (Kd = 10.6 microM). By contrast, complexes with shorter rod extensions remain predominantly monomeric. Dimers are present, accounting for ca. 5% of the molecules containing a rod fragment of 87 residues and ca. 30% of those with a 180-residue peptide. These dimers appear to be antiparallel coiled coils, as judged by their length and the knobs observed at the two ends. The rod fragments alone do not dimerize and form a coiled-coil structure unless covalently linked by disulfide bridges. Our results suggest that the N-terminal end of the coiled-coil rod is stabilized by interactions with the regulatory domain, most likely with residues of the regulatory light chain. This labile nature of the coiled coil at the head-rod junction might be a structural prerequisite for regulation of scallop myosin by Ca2+-ions.

Structural requirements for in vivo myosin I function in Aspergillus nidulans

We have investigated the minimal requirements of the tail region for myosin I function in vivo using the filamentous fungus Aspergillus nidulans. The CL3 strain (McGoldrick, C. A., Gruver, C., and May, G. S. (1995) J. Cell Biol. 128, 577-587) was transformed with a variety of myoA constructs containing mutations in the IQ, TH-1-like, SH3, and proline-rich domains by frameshift or in-frame deletions of the tail domains. The resulting strains contained wild type myoA driven by the alcA promoter and a mutant myoA driven by its endogenous promoter. This strategy allowed for selective expression of the wild type and/or mutant form of MYOA by the choice of growth medium. Proper septation and hyphal branching were found to be dependent on the interaction of the IQ motifs with calmodulin, as well as, the presence of its proline-rich domain. Additionally, a single proline-rich motif was sufficient for nearly wild type MYOA function. Most surprisingly, the SH3 domain was not essential for MYOA function. These studies expand our previous knowledge of the function of MYOA to include roles in hyphal morphogenesis, septal wall formation, and cell polarity, laying the groundwork for more detailed investigations on the function of the various tail domains in MYOA.

Cooperativity and regulation of scallop myosin and myosin fragments

Scallop heavy meromyosin (HMM) preparation obtained by a new improved method showed a Mg-ATPase activity that was activated 15-fold by calcium. The ATPase activity depended on ionic strength and reached maximum at 0.1 M without altering calcium sensitivity. The highly regulated HMM and myosin preparations showed cooperative properties not seen with unregulated subfragment 1 (S1). ATPase activity of myosin and HMM increased steeply with calcium concentration, yielding Hill coefficients about 3 and 4, respectively. Calcium binding by HMM and myosin became cooperative in the presence of ADP, AMP-PNP, or ADP.Vi yielding Hill coefficients of 1.8 and 2.8, respectively. Binding of calcium by HMM in the presence of ATP was also cooperative at physiological ionic strength, whereas at low ionic strength the data fit best to a simple binding curve. In contrast, calcium binding by unregulated S1 followed a normal binding curve and was not affected by the presence of nucleotide analogues. Calcium decreased the affinity of ADP and ADP-PNP to myosin and HMM, but had no effect on the nucleotide binding to S1. The results indicate that communication between the nucleotide and calcium binding sites requires the presence of two heads and exists only in the "off" state. We propose that in the presence of calcium, interaction between the two heads is disrupted and they act independently.