The C terminus of cardiac troponin I stabilizes the Ca2+-activated state of tropomyosin on actin filaments

RATIONALE

Ca(2+) control of troponin-tropomyosin position on actin regulates cardiac muscle contraction. The inhibitory subunit of troponin, cardiac troponin (cTn)I is primarily responsible for maintaining a tropomyosin conformation that prevents crossbridge cycling. Despite extensive characterization of cTnI, the precise role of its C-terminal domain (residues 193 to 210) is unclear. Mutations within this region are associated with restrictive cardiomyopathy, and C-terminal deletion of cTnI, in some species, has been associated with myocardial stunning.

OBJECTIVE

We sought to investigate the effect of a cTnI deletion-removal of 17 amino acids from the C terminus- on the structure of troponin-regulated tropomyosin bound to actin.

METHODS AND RESULTS

A truncated form of human cTnI (cTnI(1-192)) was expressed and reconstituted with troponin C and troponin T to form a mutant troponin. Using electron microscopy and 3D image reconstruction, we show that the mutant troponin perturbs the positional equilibrium dynamics of tropomyosin in the presence of Ca(2+). Specifically, it biases tropomyosin position toward an "enhanced C-state" that exposes more of the myosin-binding site on actin than found with wild-type troponin.

CONCLUSIONS

In addition to its well-established role of promoting the so-called "blocked-state" or "B-state," cTnI participates in proper stabilization of tropomyosin in the "Ca(2+)-activated state" or "C-state." The last 17 amino acids perform this stabilizing role. The data are consistent with a "fly-casting" model in which the mobile C terminus of cTnI ensures proper conformational switching of troponin-tropomyosin. Loss of actin-sensing function within this domain, by pathological proteolysis or cardiomyopathic mutation, may be sufficient to perturb tropomyosin conformation.

Structural basis for the activation of muscle contraction by troponin and tropomyosin

The molecular regulation of striated muscle contraction couples the binding and dissociation of Ca(2+) on troponin (Tn) to the movement of tropomyosin on actin filaments. In turn, this process exposes or blocks myosin binding sites on actin, thereby controlling myosin crossbridge dynamics and consequently muscle contraction. Using 3D electron microscopy, we recently provided structural evidence that a C-terminal extension of TnI is anchored on actin at low Ca(2+) and competes with tropomyosin for a common site to drive tropomyosin to the B-state location, a constrained, relaxing position on actin that inhibits myosin-crossbridge association. Here, we show that release of this constraint at high Ca(2+) allows a second segment of troponin, probably representing parts of TnT or the troponin core domain, to promote tropomyosin movement on actin to the Ca(2+)-induced C-state location. With tropomyosin stabilized in this position, myosin binding interactions can begin. Tropomyosin appears to oscillate to a higher degree between respective B- and C-state positions on troponin-free filaments than on fully regulated filaments, suggesting that tropomyosin positioning in both states is troponin-dependent. By biasing tropomyosin to either of these two positions, troponin appears to have two distinct structural functions; in relaxed muscles at low Ca(2+), troponin operates as an inhibitor, while in activated muscles at high Ca(2+), it acts as a promoter to initiate contraction.

Analysis of tarantula skeletal muscle protein sequences and identification of transcriptional isoforms

Background

Tarantula has been used as a model system for studying skeletal muscle structure and function, yet data on the genes expressed in tarantula muscle are lacking.

Results

We constructed a cDNA library from Aphonopelma sp. (Tarantula) skeletal muscle and got 2507 high-quality 5'ESTs (expressed sequence tags) from randomly picked clones. EST analysis showed 305 unigenes, among which 81 had more than 2 ESTs. Twenty abundant unigenes had matches to skeletal muscle-related genes including actin, myosin, tropomyosin, troponin-I, T and C, paramyosin, muscle LIM protein, muscle protein 20, a-actinin and tandem Ig/Fn motifs (found in giant sarcomere-related proteins). Matches to myosin light chain kinase and calponin were also identified. These results support the existence of both actin-linked and myosin-linked regulation in tarantula skeletal muscle.

We have predicted full-length as well as partial cDNA sequences both experimentally and computationally for myosin heavy and light chains, actin, tropomyosin, and troponin-I, T and C, and have deduced the putative peptides. A preliminary analysis of the structural and functional properties was also carried out. Sequence similarities suggested multiple isoforms of most myofibrillar proteins, supporting the generality of multiple isoforms known from previous muscle sequence studies. This may be related to a mix of muscle fiber types.

Conclusion

The present study serves as a basis for defining the transcriptome of tarantula skeletal muscle, for future in vitro expression of tarantula proteins, and for interpreting structural and functional observations in this model species.

Ca2+ -induced tropomyosin movement in scallop striated muscle thin filaments

Striated muscle contraction in most animals is regulated at least in part by the troponin-tropomyosin (Tn-Tm) switch on the thin (actin-containing) filaments. The only group that has been suggested to lack actin-linked regulation is the mollusks, where contraction is regulated through the myosin heads on the thick filaments. However, molluscan gene sequence data suggest the presence of troponin (Tn) components, consistent with actin-linked regulation, and some biochemical and immunological data also support this idea. The presence of actin-linked (in addition to myosin-linked) regulation in mollusks would simplify our general picture of muscle regulation by extending actin-linked regulation to this phylum as well. We have investigated this question structurally by determining the effect of Ca(2+) on the position of Tm in native thin filaments from scallop striated adductor muscle. Three-dimensional reconstructions of negatively stained filaments were determined by electron microscopy and single-particle image analysis. At low Ca(2+), Tm appeared to occupy the "blocking" position, on the outer domain of actin, identified in earlier studies of regulated thin filaments in the low-Ca(2+) state. In this position, Tm would sterically block myosin binding, switching off filament activity. At high Ca(2+), Tm appeared to move toward a position on the inner domain, similar to that induced by Ca(2+) in regulated thin filaments. This Ca(2+)-induced movement of Tm is consistent with the hypothesis that scallop thin filaments are Ca(2+) regulated.

Head-head interaction characterizes the relaxed state of Limulus muscle myosin filaments

Regulation of muscle contraction via the myosin filaments occurs in vertebrate smooth and many invertebrate striated muscles. Studies of unphosphorylated vertebrate smooth muscle myosin suggest that activity is switched off through an intramolecular interaction between the actin-binding region of one head and the converter and essential light chains of the other, inhibiting ATPase activity and actin interaction. The same interaction (and additional interaction with the tail) is seen in three-dimensional reconstructions of relaxed, native myosin filaments from tarantula striated muscle, suggesting that such interactions are likely to underlie the off-state of myosin across a wide spectrum of the animal kingdom. We have tested this hypothesis by carrying out cryo-electron microscopy and three-dimensional image reconstruction of myosin filaments from horseshoe crab (Limulus) muscle. The same head-head and head-tail interactions seen in tarantula are also seen in Limulus, supporting the hypothesis. Other data suggest that this motif may underlie the relaxed state of myosin II in all species (including myosin II in nonmuscle cells), with the possible exception of insect flight muscle. The molecular organization of the myosin tails in the backbone of muscle thick filaments is unknown and may differ between species. X-ray diffraction data support a general model for crustaceans in which tails associate together to form 4-nm-diameter subfilaments, with these subfilaments assembling together to form the backbone. This model is supported by direct observation of 4-nm-diameter elongated strands in the tarantula reconstruction, suggesting that it might be a general structure across the arthropods. We observe a similar backbone organization in the Limulus reconstruction, supporting the general existence of such subfilaments.

Three-dimensional reconstruction of tarantula myosin filaments suggests how phosphorylation may regulate myosin activity

Muscle contraction involves the interaction of the myosin heads of the thick filaments with actin subunits of the thin filaments. Relaxation occurs when this interaction is blocked by molecular switches on these filaments. In many muscles, myosin-linked regulation involves phosphorylation of the myosin regulatory light chains (RLCs). Electron microscopy of vertebrate smooth muscle myosin molecules (regulated by phosphorylation) has provided insight into the relaxed structure, revealing that myosin is switched off by intramolecular interactions between its two heads, the free head and the blocked head. Three-dimensional reconstruction of frozen-hydrated specimens revealed that this asymmetric head interaction is also present in native thick filaments of tarantula striated muscle. Our goal in this study was to elucidate the structural features of the tarantula filament involved in phosphorylation-based regulation. A new reconstruction revealed intra- and intermolecular myosin interactions in addition to those seen previously. To help interpret the interactions, we sequenced the tarantula RLC and fitted an atomic model of the myosin head that included the predicted RLC atomic structure and an S2 (subfragment 2) crystal structure to the reconstruction. The fitting suggests one intramolecular interaction, between the cardiomyopathy loop of the free head and its own S2, and two intermolecular interactions, between the cardiac loop of the free head and the essential light chain of the blocked head and between the Leu305-Gln327 interaction loop of the free head and the N-terminal fragment of the RLC of the blocked head. These interactions, added to those previously described, would help switch off the thick filament. Molecular dynamics simulations suggest how phosphorylation could increase the helical content of the RLC N-terminus, weakening these interactions, thus releasing both heads and activating the thick filament.

Blebbistatin stabilizes the helical order of myosin filaments by promoting the switch 2 closed state

Blebbistatin is a small-molecule, high-affinity, noncompetitive inhibitor of myosin II. We have used negative staining electron microscopy to study the effects of blebbistatin on the organization of the myosin heads on muscle thick filaments. Loss of ADP and Pi from the heads causes thick filaments to lose their helical ordering. In the presence of 100 microM blebbistatin, disordering was at least 10 times slower. In the M.ADP state, myosin heads are also disordered. When blebbistatin was added to M.ADP thick filaments, helical ordering was restored. However, blebbistatin did not improve the order of thick filaments lacking bound nucleotide. Addition of calcium to relaxed muscle homogenates induced thick-thin filament interaction and filament sliding. In the presence of blebbistatin, filament interaction was inhibited. These structural observations support the conclusion, based on biochemical studies, that blebbistatin inhibits myosin ATPase and actin interaction by stabilizing the closed switch 2 structure of the myosin head. These properties make blebbistatin a useful tool in structural and functional studies of cell motility and muscle contraction.

Understanding the organisation and role of myosin binding protein C in normal striated muscle by comparison with MyBP-C knockout cardiac muscle

Myosin binding protein C (MyBP-C) is a component of the thick filament of striated muscle. The importance of this protein is revealed by recent evidence that mutations in the cardiac gene are a major cause of familial hypertrophic cardiomyopathy. Here we investigate the distribution of MyBP-C in the A-bands of cardiac and skeletal muscles and compare this to the A-band structure in cardiac muscle of MyBP-C-deficient mice. We have used a novel averaging technique to obtain the axial density distribution of A-bands in electron micrographs of well-preserved specimens. We show that cardiac and skeletal A-bands are very similar, with a length of 1.58 ± 0.01 μm. In normal cardiac and skeletal muscle, the distributions are very similar, showing clearly the series of 11 prominent accessory protein stripes in each half of the A-band spaced axially at 43-nm intervals and starting at the edge of the bare zone. We show by antibody labelling that in cardiac muscle the distal nine stripes are the location of MyBP-C. These stripes are considerably suppressed in the knockout mouse hearts as expected. Myosin heads on the surface of the thick filament in relaxed muscle are thought to be arranged in a three-stranded quasi-helix with a mean 14.3-nm axial cross bridge spacing and a 43 nm helix repeat. Extra “forbidden” meridional reflections, at orders of 43 nm, in X-ray diffraction patterns of muscle have been interpreted as due to an axial perturbation of some levels of myosin heads. However, in the MyBP-C-deficient hearts these extra meridional reflections are weak or absent, suggesting that they are due to MyBP-C itself or to MyBP-C in combination with a head perturbation brought about by the presence of MyBP-C.

Millisecond time-resolved changes occurring in Ca2+-regulated myosin filaments upon relaxation

Contraction of many muscles is activated in part by the binding of Ca(2+) to, or phosphorylation of, the myosin heads on the surface of the thick filaments. In relaxed muscle, the myosin heads are helically ordered and undergo minimal interaction with actin. On Ca(2+) binding or phosphorylation, the head array becomes disordered, reflecting breakage of the head-head and other interactions that underlie the ordered structure. Loosening of the heads from the filament surface enables them to interact with actin filaments, bringing about contraction. On relaxation, the heads return to their ordered positions on the filament backbone. In scallop striated adductor muscle, the disordering that takes place on Ca(2+) binding occurs on the millisecond time scale, suggesting that it is a key element of muscle activation. Here we have studied the reverse process. Using time-resolved negative staining electron microscopy, we show that the rate of reordering on removal of Ca(2+) also occurs on the same physiological time scale. Direct observation of images together with analysis of their Fourier transforms shows that activated heads regain their axial ordering within 20 ms and become ordered in their final helical positions within 50 ms. This rapid reordering suggests that reformation of the ordered structure, and the head-head and other interactions that underlie it, is a critical element of the relaxation process.

Head-head and head-tail interaction: a general mechanism for switching off myosin II activity in cells

Intramolecular interaction between myosin heads, blocking key sites involved in actin-binding and ATPase activity, appears to be a critical mechanism for switching off vertebrate smooth-muscle myosin molecules, leading to relaxation. We have tested the hypothesis that this interaction is a general mechanism for switching off myosin II-based motile activity in both muscle and nonmuscle cells. Electron microscopic images of negatively stained myosin II molecules were analyzed by single particle image processing. Molecules from invertebrate striated muscles with phosphorylation-dependent regulation showed head-head interactions in the off-state similar to those in vertebrate smooth muscle. A similar structure was observed in nonmuscle myosin II (also phosphorylation-regulated). Surprisingly, myosins from vertebrate skeletal and cardiac muscle, which are not intrinsically regulated, undergo similar head-head interactions in relaxing conditions. In all of these myosins, we also observe conserved interactions between the 'blocked' myosin head and the myosin tail, which may contribute to the switched-off state. These results suggest that intramolecular head-head and head-tail interactions are a general mechanism both for inducing muscle relaxation and for switching off myosin II-based motile activity in nonmuscle cells. These interactions are broken when myosin is activated.

Structural basis for the regulation of muscle contraction by troponin and tropomyosin

The molecular switching mechanism governing skeletal and cardiac muscle contraction couples the binding of Ca2+ on troponin to the movement of tropomyosin on actin filaments. Despite years of investigation, this mechanism remains unclear because it has not yet been possible to directly assess the structural influence of troponin on tropomyosin that causes actin filaments, and hence myosin-crossbridge cycling and contraction, to switch on and off. A C-terminal domain of troponin I is thought to be intimately involved in inducing tropomyosin movement to an inhibitory position that blocks myosin-crossbridge interaction. Release of this regulatory, latching domain from actin after Ca2+ binding to TnC (the Ca2+ sensor of troponin that relieves inhibition) presumably allows tropomyosin movement away from the inhibitory position on actin, thus initiating contraction. However, the structural interactions of the regulatory domain of TnI (the "inhibitory" subunit of troponin) with tropomyosin and actin that cause tropomyosin movement are unknown, and thus, the regulatory process is not well defined. Here, thin filaments were labeled with an engineered construct representing C-terminal TnI, and then, 3D electron microscopy was used to resolve where troponin is anchored on actin-tropomyosin. Electron microscopy reconstruction showed how TnI binding to both actin and tropomyosin at low Ca2+ competes with tropomyosin for a common site on actin and drives tropomyosin movement to a constrained, relaxing position to inhibit myosin-crossbridge association. Thus, the observations reported reveal the structural mechanism responsible for troponin-tropomyosin-mediated steric interference of actin-myosin interaction that regulates muscle contraction.

Three-dimensional structure of vertebrate cardiac muscle myosin filaments

Contraction of the heart results from interaction of the myosin and actin filaments. Cardiac myosin filaments consist of the molecular motor myosin II, the sarcomeric template protein, titin, and the cardiac modulatory protein, myosin binding protein C (MyBP-C). Inherited hypertrophic cardiomyopathy (HCM) is a disease caused mainly by mutations in these proteins. The structure of cardiac myosin filaments and the alterations caused by HCM mutations are unknown. We have used electron microscopy and image analysis to determine the three-dimensional structure of myosin filaments from wild-type mouse cardiac muscle and from a MyBP-C knockout model for HCM. Three-dimensional reconstruction of the wild-type filament reveals the conformation of the myosin heads and the organization of titin and MyBP-C at 4 nm resolution. Myosin heads appear to interact with each other intramolecularly, as in off-state smooth muscle myosin [Wendt T, Taylor D, Trybus KM, Taylor K (2001) Proc Natl Acad Sci USA 98:4361-4366], suggesting that all relaxed muscle myosin IIs may adopt this conformation. Titin domains run in an elongated strand along the filament surface, where they appear to interact with part of MyBP-C and with the myosin backbone. In the knockout filament, some of the myosin head interactions are disrupted, suggesting that MyBP-C is important for normal relaxation of the filament. These observations provide key insights into the role of the myosin filament in cardiac contraction, assembly, and disease. The techniques we have developed should be useful in studying the structural basis of other myosin-related HCM diseases.

FSH stimulates ovarian cancer cell growth by action on growth factor variant receptor

A number of FSH receptor (FSH-R) isoforms with distinct structural motifs and signaling paradigms have been described, including a single transmembrane domain variant that functions as a growth factor type receptor (FSH-R3). This study tested the hypothesis that FSH can stimulate ovarian cancer cell proliferation by acting on FSH-R3, using the tumorigenic mouse ovarian surface epithelial cell (MOSEC) line ID8. FSH enhanced ID8 proliferation in a concentration-dependent fashion. Moreover, FSH-treatment of ID8 elicited intracellular events consistent with activation of FSH-R3 and distinct from those associated with activation of the canonical G-protein coupled FSH-R isoform (FSH-R1). Specifically, the FSH-R3 signaling pathway included cAMP-independent activation of ERK downstream of an SNX-482 sensitive component likely to be the Cav2.3 calcium channel. Northern analysis using probes specific for exons 7 and 11 of FSH-R identified consistently only one 1.9kb transcript. Immunoblot analysis confirmed expression of FSH-R3 but not FSHR-1 in ID8. Together, these data suggest that FSH-R3 signaling promotes proliferation of ovarian cancer cells.

Fast skeletal muscle regulatory light chain is required for fast and slow skeletal muscle development

In skeletal muscle, the myosin molecule contains two sets of noncovalently attached low molecular weight proteins, the regulatory (RLC) and essential (ELC) light chains. To assess the functional and developmental significance of the fast skeletal isoform of the RLC (RLC-f), the murine fast skeletal RLC gene (Mylpf) was disrupted by homologous recombination. Heterozygotes containing an intronic neo cassette (RLC-/+) had approximately one-half of the amount of the RLC-f mRNA compared to wild-type (WT) mice but their muscles were histologically normal in both adults and neonates. In contrast, homozygous mice (RLC-/-) had no RLC-f mRNA or protein and completely lacked both fast and slow skeletal muscle. This was likely due to interference with mRNA processing in the presence of the neo cassette. These RLC-f null mice died immediately after birth, presumably due to respiratory failure since their diaphragms lacked skeletal muscle. The body weight of newborn RLC-f null mice was decreased 30% compared to heterozygous or WT newborn mice. The lack of skeletal muscle formation in the null mice did not affect the development of other organs including the heart. In addition, we found that WT mice did not express the ventricular/slow skeletal RLC isoform (RLC-v/s) until after birth, while it was expressed normally in the embryonic heart. The lack of skeletal muscle formation observed in RLC-f null mice indicates the total dependence of skeletal muscle development on the presence of RLC-f during embryogenesis. This observation, along with the normal function of the RLC-v/s in the heart, implicates a coupled, diverse pathway for RLC-v/s and RLC-f during embryogenesis, where RLC-v/s is responsible for heart development and RLC-f is necessary for skeletal muscle formation. In conclusion, in this study we demonstrate that the Mylpf gene is critically important for fast and slow skeletal muscle development.

Using annotated peptide mass spectrum libraries for protein identification

A system for creating a library of tandem mass spectra annotated with corresponding peptide sequences was described. This system was based on the annotated spectra currently available in the Global Proteome Machine Database (GPMDB). The library spectra were created by averaging together spectra that were annotated with the same peptide sequence, sequence modifications, and parent ion charge. The library was constructed so that experimental peptide tandem mass spectra could be compared with those in the library, resulting in a peptide sequence identification based on scoring the similarity of the experimental spectrum with the contents of the library. A software implementation that performs this type of library search was constructed and successfully used to obtain sequence identifications. The annotated tandem mass spectrum libraries for the Homo sapiens, Mus musculus, and Saccharomyces cerevisiae proteomes and search software were made available for download and use by other groups.

Plasma concentrations of parathyroid hormone-related protein in dogs with potential disorders of calcium metabolism

The plasma concentrations of total calcium, ionised calcium, albumin, parathyroid hormone and parathyroid hormone-related protein (PTHrp) were measured in 25 dogs with lymphoma, nine dogs with primary hyperparathyroidism and seven dogs with adenocarcinoma of the apocrine gland of the anal sac. Plasma total calcium, ionised calcium, albumin and parathyroid hormone-related protein were measured in 18 clinically normal control dogs. The concentration of PTHrp was high in 12 of the 14 dogs that were hypercalcaemic because of an underlying malignancy but was within the reference range in all the control dogs, in the 17 normocalcaemic dogs with lymphoma and in the seven dogs which were hypercalcaemic because of a parathyroid adenoma.

A comparison of muscle thin filament models obtained from electron microscopy reconstructions and low-angle X-ray fibre diagrams from non-overlap muscle

The regulation of striated muscle contraction involves changes in the interactions of troponin and tropomyosin with actin thin filaments. In resting muscle, myosin-binding sites on actin are thought to be blocked by the coiled-coil protein tropomyosin. During muscle activation, Ca2+ binding to troponin alters the tropomyosin position on actin, resulting in cyclic actin-myosin interactions that accompany muscle contraction. Evidence for this steric regulation by troponin-tropomyosin comes from X-ray data [Haselgrove, J.C., 1972. X-ray evidence for a conformational change in the actin-containing filaments of verterbrate striated muscle. Cold Spring Habor Symp. Quant. Biol. 37, 341-352; Huxley, H.E., 1972. Structural changes in actin and myosin-containing filaments during contraction. Cold Spring Habor Symp. Quant. Biol. 37, 361-376; Parry, D.A., Squire, J.M., 1973. Structural role of tropomyosin in muscle regulation: analysis of the X-ray diffraction patterns from relaxed and contracting muscles. J. Mol. Biol. 75, 33-55] and electron microscope (EM) data [Spudich, J.A., Huxley, H.E., Finch, J., 1972. Regulation of skeletal muscle contraction. II. Structural studies of the interaction of the tropomyosin-troponin complex with actin. J. Mol. Biol. 72, 619-632; O'Brien, E.J., Gillis, J.M., Couch, J., 1975. Symmetry and molecular arrangement in paracrystals of reconstituted muscle thin filaments. J. Mol. Biol. 99, 461-475; Lehman, W., Craig, R., Vibert, P., 1994. Ca2+-induced tropomyosin movement in Limulus thin filaments revealed by three-dimensional reconstruction. Nature 368, 65-67] each with its own particular strengths and limitations. Here we bring together some of the latest information from EM analysis of single thin filaments from Pirani et al. [Pirani, A., Xu, C., Hatch, V., Craig, R., Tobacman, L.S., Lehman, W. (2005). Single particle analysis of relaxed and activated muscle thin filaments. J. Mol. Biol. 346, 761-772], with synchrotron X-ray data from non-overlapped muscle fibres to refine the models of the striated muscle thin filament. This was done by incorporating current atomic-resolution structures of actin, tropomyosin, troponin and myosin subfragment-1. Fitting these atomic coordinates to EM reconstructions, we present atomic models of the thin filament that are entirely consistent with a steric regulatory mechanism. Furthermore, fitting the atomic models against diffraction data from skinned muscle fibres, stretched to non-overlap to preclude crossbridge binding, produced very similar results, including a large Ca2+-induced shift in tropomyosin azimuthal location but little change in the actin structure or apparent alteration in troponin position.

The globular tail domain of myosin Va functions as an inhibitor of the myosin Va motor

The actin-activated ATPase activity of full-length mammalian myosin Va is well regulated by Ca2+, whereas that of truncated myosin Va without the C-terminal globular tail domain (GTD) is not. Here, we have found that exogenous GTD is capable of inhibiting the actin-activated ATPase activity of GTD-deleted myosin Va. A series of truncated constructs of myosin Va further showed that the entire length of the first coiled-coil (coil-1) of the tail domain is critical for GTD-dependent regulation of myosin Va and that deletion of 58 residues from the C-terminal end of coil-1 markedly hampered regulation. Negative staining electron microscopy revealed that GTD-deleted myosin Va formed a "Y"-shaped structure, which was converted to a triangular shape, similar to the structure of full-length myosin Va in the inhibited state, by addition of exogenous GTD. In contrast, the triangular shape was not observed when the C-terminal 58 residues of coil-1 were deleted, even in the presence of exogenous GTD. Based on these results, we propose a model for the formation of the inhibited state of myosin Va. GTD binds to the C-terminal end of coil-1. The neck-tail junction of myosin Va is flexible, and the long neck enables the head domain to reach the GTD associated with the end of coil-1. Once the head interacts with the GTD, the triangular inhibited conformation is stabilized. Consistent with this model, we found that shortening of the neck of myosin Va by two IQ motifs abolished the regulation by GTD, whereas regulation was partially restored by shortening of coil-1 by an amount comparable to that of the two IQ motifs.

Structure and function of myosin filaments

Myosin filaments interact with actin to generate muscle contraction and many forms of cell motility. X-ray and electron microscopy (EM) studies have revealed the general organization of myosin molecules in relaxed filaments, but technical difficulties have prevented a detailed description. Recent studies using improved ultrastructural and image analysis techniques are overcoming these problems. Three-dimensional reconstructions using single-particle methods have provided many new insights into the organization of the myosin heads and tails. Docking of atomic structures into cryo-EM density maps suggests how regulated myosin filaments are 'switched off', bringing about muscle relaxation. Additionally, sequence analysis suggests probable interactions between myosin tails in the backbone, whereas crystallographic and EM studies are starting to reveal tail interactions directly in three dimensions.