Regulatory domains of myosins: influence of heavy chain on Ca(2+)-binding

Visualizing key hinges and a potential major source of compliance in the lever arm of myosin

We have determined the 2.3-Å-resolution crystal structure of a myosin light chain domain, corresponding to one type found in sea scallop catch ("smooth") muscle. This structure reveals hinges that may function in the "on" and "off" states of myosin. The molecule adopts two different conformations about the heavy chain "hook" and regulatory light chain (RLC) helix D. This conformational change results in extended and compressed forms of the lever arm whose lengths differ by 10 Å. The heavy chain hook and RLC helix D hinges could thus serve as a potential major and localized source of cross-bridge compliance during the contractile cycle. In addition, in one of the molecules of the crystal, part of the RLC N-terminal extension is seen in atomic detail and forms a one-turn alpha-helix that interacts with RLC helix D. This extension, whose sequence is highly variable in different myosins, may thus modulate the flexibility of the lever arm. Moreover, the relative proximity of the phosphorylation site to the helix D hinge suggests a potential role for conformational changes about this hinge in the transition between the on and off states of regulated myosins.

Mechanism of catch force: tethering of thick and thin filaments by twitchin

Catch is a mechanical state occurring in some invertebrate smooth muscles characterized by high force maintenance and resistance to stretch during extremely slow relaxation. During catch, intracellular calcium is near basal concentration and myosin crossbridge cyctng rate is extremely slow. Catch force is relaxed by a protein kinase A-mediated phosphorylation of sites near the N- and C- temini of the minititin twitchin (~526 kDa). Some catch force maintenance car also occur together with cycling myosin crossbridges at submaximal calcium concentrations, but not when the muscle is maximally activated. Additionally, the link responsible for catch can adjust during shortening of submaximally activated muscles and maintain catch force at the new shorter length. Twitchin binds to both thick and thin filaments, and the thin filament binding shown by both the N- and Cterminal portions of twitchin is decreased by phosphorylation of the sites that regulate catch. The data suggest that the twitchin molecule itself is the catch force beanng tether between thick and thin filaments. We present a model for the regulation of catch in which the twitchin tether can be displaced from thin filaments by both (a) the phosphorylation of twitchin and (b) the attachment of high force myosin crossbridges.

The on-off switch in regulated myosins: different triggers but related mechanisms

In regulated myosin, motor and enzymatic activities are toggled between the on-state and off-state by a switch located on its lever arm domain, here called the regulatory domain (RD). This region consists of a long alpha-helical "heavy chain" stabilized by a "regulatory" light chain (RLC) and an "essential" light chain (ELC). The on-state is activated by phosphorylation of the RLC of vertebrate smooth muscle RD or by direct binding of Ca(2+) to the ELC of molluscan RD. Crystal structures are available only for the molluscan RD. To understand in more detail the pathway between the on-state and the off-state, we have now also determined the crystal structure of a molluscan (scallop) RD in the absence of Ca(2+). Our results indicate that loss of Ca(2+) abolishes most of the interactions between the light chains and may increase the flexibility of the RD heavy chain. We propose that disruption of critical links with the C-lobe of the RLC is the key event initiating the off-state in both smooth muscle myosins and molluscan myosins.

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.

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.

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.

Amino-acid sequence of squid myosin heavy chain

This work describes the determination of the cDNA sequence encoding the myosin heavy chain (MHC) of the squid, Loligo pealei. To date, the amino-acid sequence of the MHC of calcium-regulated myosins is known only for two closely related species of scallops. We have determined the sequence of the entire coding region of the muscle MHC of squid, a cephalopod, and compared it with the MHC of scallops, which are pelecypods, and to other regulated and non-regulated myosins. Residues present in the MHC of only regulated myosins have been identified. The 6504 base pair (bp) sequence contains an open reading frame of 5805 nucleotides, which encodes 1935 amino acids. The sequence includes 697 bps of 3' untranslated sequence and 2 bps of 5' untranslated sequence. The deduced amino-acid sequence shows the squid MHC to be 72-73% identical and 86-87% similar to the calcium-regulated scallop MHCs cloned previously. In contrast, the squid MHC sequence is only 54-55% identical and 74% similar to skeletal MHCs of non-regulated myosins such as human fast skeletal embryonic and human perinatal skeletal muscle, and 39-40% identical and 60-62% similar to smooth muscle MHC of rabbit uterus muscle and chicken gizzard muscle, respectively. We have also detected two isoforms of the MHC in squid that appear to be spliced variants of a single myosin gene. These isoforms differ in the sequence encoding the surface loop at the nucleotide binding site. Taken together, our data may help to identify more precisely the residues that are crucial in regulated myosins.

Single-headed scallop myosin and regulation

Single-headed scallop myosin (shM) was prepared by papain digestion of filamentous scallop myosin and purified by hydrophobic interaction chromatography. The shM preparation consisted of equimolar amounts of polypeptides corresponding to an intact heavy chain, rod chain, essential light chain, and regulatory light chain. In electron micrographs the shape of shM showed the presence of a single head domain to which a normal looking rod was attached. Myosin and shM bound Ca2+ with association constants of 5 x 10(6) and 11 x 10(6) M-1, respectively. The ATPase activity of shM was activated about 3-fold by Ca2+. Both heads of myosin and shM had comparable ATPase activities in the presence of Ca2+. The activation of the ATPase activity of single-headed scallop myosin by Ca2+ paralleled closely the Ca2+ binding, in sharp contrast to the activation of intact myosin by Ca2+, which is highly cooperative. Single turnover experiments of myosin with radioactive ATP gave a half-life for the ATPase cycle of approximately 3 min in the presence of EGTA, whereas that of single-headed myosin was shorter than approximately 30 s, which was the resolution time of these measurements. The results suggest that the presence of two heads, as well as the attachment of the head to the coiled coil rod, contribute to the regulation of scallop myosin by Ca2+.

Sequence variations in the surface loop near the nucleotide binding site modulate the ATP turnover rates of molluscan myosins

The muscle and species-specific differences in enzymatic activity between Placopecten and Argopecten striated and catch muscle myosins are attributable to the myosin heavy chain. To identify sequences that may modulate these differences, we cloned and sequenced the cDNA encoding the myosin heavy chains of Placopecten striated and catch muscle. Deduced protein sequences indicate two similar isoforms in catch and striated myosins (97% identical); variations arise by differential RNA splicing of five alternative exons from a single myosin heavy chain gene. The first encodes the phosphate-binding loop; the second, part of the ATP binding site; the third, part of the actin binding site; the fourth, the hinge in the rod; and the fifth, a tailpiece found only in the catch muscle myosin heavy chain. Both Placopecten myosin heavy chains are 96% identical to Argopecten myosin heavy chaina isoforms. Because subfragment-1 ATPase activities reflect the differences observed in the parent myosins, the motor domain is responsible for the variations in ATPase activities. In addition, data show that differences are due to Vmax and not actin affinity. The sequences of all four myosin heavy chain motor domains diverge only in the flexible surface loop near the nucleotide binding pocket. Thus, the different ATPase activities of four molluscan muscle myosins are likely due to myosin heavy chain sequence variations within the flexible surface loop that forms part of the ATP binding pocket of the motor domain.

Regulation of contraction by calcium binding myosins

Contraction of molluscan muscles is triggered by binding of Ca2+ to myosin. Molluscan myosins are regulated molecules, their light chains serve as regulatory subunits. They differ from myosins of skeletal muscles in requiring Ca2+ for activity and having a specific high-affinity Ca2+ binding site. As all conventional myosins molluscan myosins also consist of two heavy chains, two regulatory and two essential light chains. Scallop myosin is particularly suitable for studying light chain function since its regulatory light chains readily dissociate in the absence of divalent cations and its essential light chains can be exchanged with foreign light chains. The structural, mutational and biochemical studies presented here are aimed to elucidate the role of the light chains in regulation, to describe the interactions between the myosin subunits and to locate the regions and the amino acids responsible for the differences between functional and non-functional light chains.

Role of essential light chain EF hand domains in calcium binding and regulation of scallop myosin

The specific Ca2+ binding site that triggers contraction of molluscan muscle requires the presence of an essential light chain (ELC) from a Ca2+ binding myosin. Of the four EF hand-like domains in molluscan ELCs, only domain III has an amino acid sequence predicted to be capable of binding Ca2+. In this report, we have used mutant ELCs to locate the Ca2+ binding site in scallop myosin and to probe the role of the ELC in regulation. Point mutations in domain III of scallop ELC have no effect on Ca2+ binding. Interestingly, scallop and rat cardiac ELC chimeras support Ca2+ binding only if domain I is scallop. These results are nevertheless in agreement with structural studies on a proteolytic fragment of scallop myosin, the regulatory domain. Furthermore, Ca2+ sensitivity of the scallop myosin ATPase requires scallop ELC domain I: ELCs containing cardiac domain I convert scallop myosin to an unregulated molecule whose activity is no longer repressed in the absence of Ca2+. Despite its unusual EF hand domain sequence, our data indicate that the unique and required contribution of molluscan ELCs to Ca2+ binding and regulation of molluscan myosins resides exclusively in domain I.

Role of myosin light chains

All conventional myosin IIs, whether isolated from skeletal, smooth, or invertebrate muscle sources, have two heads attached to an extended 16 nm alpha-helical coiled-coil tail. The head can be divided into a globular motor domain of approximately 770 amino acids that contains the catalytic and actin binding sites, and a neck region of approximately 70 amino acids which binds one essential and one regulatory light chain (ELC and RLC). The neck region with its associated LCs plays both structural and regulatory roles. While the mechanism and extent of regulation by the LCs varies for different myosins, the structural role may be a more fundamental feature of myosin II motors. Our understanding of the neck region has advanced rapidly in recent years primarily because of two types of information: (1) the high resolution structures of the LC binding domain from the thick-filament regulated scallop myosin (Xie et al., 1994) and of the head of unregulated skeletal myosin (Rayment et al., 1993), and (2) the ability to remove and/or mutate portions of both the heavy and light chains for analysis by in vitro motility assays.