A mechanical study of regulation in the striated adductor muscle of the scallop

The ultrastructure and contractile properties of a fast-acting, obliquely striated, myosin-regulated muscle: the funnel retractor of squids

We investigated the ultrastructure, contractile properties, and in vivo length changes of the fast-acting funnel retractor muscle of the long-finned squid Doryteuthis pealeii. This muscle is composed of obliquely striated, spindle-shaped fibers ~3 mum across that have an abundant sarcoplasmic reticulum, consisting primarily of membranous sacs that form 'dyads' along the surface of each cell. The contractile apparatus consists of 'myofibrils' approximately 0.25-0.5 microm wide in cross section arrayed around the periphery of each cell, surrounding a central core that contains the nucleus and large mitochondria. Thick myofilaments are approximately 25 nm in diameter and approximately 2.8 microm long. 'Dense bodies' are narrow, resembling Z lines, but are discontinuous and are not associated with the cytoskeletal fibrillar elements that are so prominent in slower obliquely striated muscles. The cells approximate each other closely with minimal intervening intercellular connective tissue. Our physiological experiments, conducted at 17 degrees C, showed that the longitudinal muscle fibers of the funnel retractor were activated rapidly (8 ms latent period following stimulation) and generated force rapidly (peak twitch force occurred within 50 ms). The longitudinal fibers had low V(max) (2.15 +/-0.26 L(0) s(-1), where L(0) was the length that generated peak isometric force) but generated relatively high isometric stress (270+/-20 mN mm(-2) physiological cross section). The fibers exhibited a moderate maximum power output (49.9 W kg(-1)), compared with vertebrate and arthropod cross striated fibers, at a V/V(max) of 0.33+/-0.044. During ventilation of the mantle cavity and locomotion, the funnel retractor muscle operated in vivo over a limited range of strains (+0.075 to -0.15 relative to resting length, L(R)) and at low strain rates (from 0.16 to 0.91 L(R) s(-1) ), corresponding to a range of V/V(max) from 0.073 to 0.42. During the exhalant phase of the jet the range of strains was even narrower: maximum range less than +/-0.04, with the muscle operating nearly isometrically during ventilation and slow, arms-first swimming. The limited length operating range of the funnel retractor muscles, especially during ventilation and slow jetting, suggests that they may act as muscular struts.

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.

Coordination of the two heads of myosin during muscle contraction

We have used luminescence resonance energy transfer between regulatory light chains (RLC) to detect structural changes within the dimeric myosin molecule in contracting muscle fibers. Fully functional scallop muscle fibers were prepared such that each myosin molecule contained a terbium-labeled (luminescent donor) RLC on one head and a rhodamine-labeled (acceptor) RLC on the other. Time-resolved luminescence energy transfer between the two heads increased upon the transition from relaxation (ATP) to contraction (ATP plus Ca) and increased further in rigor (no ATP). Combined with experiments on mutant RLCs labeled specifically at other sites, these results support a model in which the force-generating weak-to-strong transition causes one myosin LC domain to tilt through a 30 degrees angle toward the other, thus acting as a coordinated lever arm.

Mechanochemical coupling in spin-labeled, active, isometric muscle

Observed effects of inorganic phosphate (P(i)) on active isometric muscle may provide the answer to one of the fundamental questions in muscle biophysics: how are the free energies of the chemical species in the myosin-catalyzed ATP hydrolysis (ATPase) reaction coupled to muscle force? Pate and Cooke (1989. Pflugers Arch. 414:73-81) showed that active, isometric muscle force varies logarithmically with [P(i)]. Here, by simultaneously measuring electron paramagnetic resonance and the force of spin-labeled muscle fibers, we show that, in active, isometric muscle, the fraction of myosin heads in any given biochemical state is independent of both [P(i)] and force. These direct observations of mechanochemical coupling in muscle are immediately described by a muscle equation of state containing muscle force as a state variable. These results challenge the conventional assumption mechanochemical coupling is localized to individual myosin heads in muscle.

Myosin light-chain domain rotates upon muscle activation but not ATP hydrolysis

We have studied the correlation between myosin structure, myosin biochemistry, and muscle force. Two distinct orientations of the myosin light-chain domain were previously resolved using electron paramagnetic resonance (EPR) spectroscopy of spin-labeled regulatory light chains in scallop muscle fibers. In the present study, we measured isometric force during EPR spectral acquisition, in order to define how these two light-chain domain orientations are coupled to force and the myosin ATPase cycle. When muscle fibers are partially activated with increasing amounts of calcium, the distribution between the two light-chain domain orientations shifts toward the one associated with strong actin binding. This shift in distribution is linearly related to the increase in force, suggesting that rotation of the light-chain domain is coupled to strong actin binding. However, when nucleotide analogues are used to trap myosin in the pre- and posthydrolysis states of its ATPase cycle in relaxed muscle, there is no change in the distribution between light-chain domain orientations, showing that the rotation of the light-chain domain is not directly coupled to the ATP hydrolysis step. Instead, it is likely that in relaxed muscle the myosin thick filament stabilizes two light-chain domain orientations that are independent of the nucleotide analogue bound at the active site. We conclude that a large and distinct rotation of the light-chain domain of myosin is responsible for force generation and is coupled to strong actin binding but is not coupled to a specific step in the myosin ATPase reaction.

Microsecond rotational dynamics of spin-labeled myosin regulatory light chain induced by relaxation and contraction of scallop muscle

We have used saturation transfer electron paramagnetic resonance (ST-EPR) to study the rotational dynamics of spin-labeled regulatory light chain (RLC) in scallop (Placopecten magellanicus) muscle fibers. The single cysteine (Cys 51) in isolated clam (Mercenaria) RLC was labeled with an indanedione spin label (InVSL). RLC was completely and specifically extracted from scallop striated muscle fibers, eliminating the Ca sensitivity of ATPase activity and isometric force, which were both completely restored by stoichiometric incorporation of labeled RLC. The EPR spectrum of the isolated RLC revealed nanosecond rotational motions within the RLC, which were completely eliminated when the labeled RLC was bound to myosin heads in myofibrils or fibers in rigor. This is the most strongly immobilized RLC-bound probe reported to date and thus offers the most reliable detection of the overall rotational motion of the LC domain. Conventional EPR spectra of oriented fibers indicated essentially complete probe disorder, independent of ATP and Ca, eliminating orientational dependence and thus making this probe ideal for unambiguous measurement of microsecond rotational motions of the LC domain by ST-EPR. ST-EPR spectra of fibers in rigor indicated an effective rotational correlation time (taureff) of 140 +/- 5 microseconds, similar to that observed for the same spin label bound to the catalytic domain. Relaxation by ATP induced microsecond rotational motion (taureff = 70 +/- 4 microseconds), and this motion was slightly slower upon Ca activation of isometric contraction (taureff = 100 +/- 5 microseconds). These motions in relaxation and contraction are similar to, but slower than, the motions previously reported for the same spin label bound to the catalytic domain. These results support a model for force generation involving rotational motion of the LC domain relative to the catalytic domain and dynamic disorder-to-order transitions in both domains.

A large and distinct rotation of the myosin light chain domain occurs upon muscle contraction

For more than 30 years, the fundamental goal in molecular motility has been to resolve force-generating motor protein structural changes. Although low-resolution structural studies have provided evidence for force-generating myosin rotations upon muscle activation, these studies did not resolve structural states of myosin in contracting muscle. Using electron paramagnetic resonance, we observed two distinct orientations of a spin label attached specifically to a single site on the light chain domain of myosin in relaxed scallop muscle fibers. The two probe orientations, separated by a 36 degrees +/- 5 degrees axial rotation, did not change upon muscle activation, but the distribution between them changed substantially, indicating that a fraction (17% +/- 2%) of myosin heads undergoes a large (at least 30 degrees) axial rotation of the myosin light chain domain upon force generation and muscle contraction. The resulting model helps explain why this observation has remained so elusive and provides insight into the mechanisms by which motor protein structural transitions drive molecular motility.

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.

Modification of the bi-directional sliding movement of actin filaments along native thick filaments isolated from a clam

The properties of bi-directional sliding of F-actin prepared from rabbit skeletal muscle moving along clam thick filaments have been characterized in the presence of agents known to modify unloaded shortening velocity in muscle to determine if the sliding characteristics of actin are similar in the two directions of movement. Actin filaments moved at a fast velocity towards the central bare zone (11.1 +/- 0.2 microns s-1) and at a slower velocity away from the bare zone (3.9 +/- 0.3 microns s-1). Movement of filaments at the slow sliding velocity is thought to be sustained by a change in orientation of the myosin head. The Michaelis Menten constant (Km values) of approximately 0.3 mM in the presence of MgATP concentrations of 0.01-2.0 mM at an ionic strength of 43.5 mM were reduced to approximately 0.1 mM at low ionic strength (18.5 mM) although the Km values at the fast and slow sliding velocities at each ionic strength were similar. In the presence of constant concentrations of MgATP, increasing the MgADP concentrations from 0.5 to 2mM, decreased the bi-directional sliding velocity of actin. The data were well fitted with an equation described by Michaelis Menten kinetics yielding mean absolute Km and Ki values of 0.41 +/- 0.01 and 0.44 +/- 0.05 mM for the fast velocity and 0.29 +/- 0.07 and 0.45 +/- 0.02 mM for the slow velocity of sliding, respectively. The Km and Ki values were not significantly different from each other at either the fast or slow sliding velocities. The actin filament sliding velocity appeared to be controlled through the thick filament as actin was devoid of regulatory proteins and the presence of Ca2+ modified the MgATP dependent movement of actin. The pCa value for half maximal sliding velocity was 7.0 for both fast and slow velocities. The Km and Ki values and the Ca2+ sensitivity of the actin movement at the fast and slow sliding velocity are similar suggesting that no major biochemical changes have occurred in the myosin head as a result of a change in orientation.

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.

Charge replacement near the phosphorylatable serine of the myosin regulatory light chain mimics aspects of phosphorylation

Phosphorylation of the myosin regulatory light chains (RLCs) activates contraction in smooth muscle and modulates force production in striated muscle. RLC phosphorylation changes the net charge in a critical region of the N terminus and thereby may alter interactions between the RLC and myosin heavy chain. A series of N-terminal charge mutations in the human smooth muscle RLC has been engineered, and the mutants have been evaluated for their ability to mimic the phosphorylated form of the RLC when reconstituted into scallop striated muscle bundles or into isolated smooth muscle myosin. Changing the net charge in the region from Arg-13 to Ser-19 potentiates force in scallop striated muscle and maintains smooth muscle myosin in an unfolded filamentous state without affecting ATPase activity or motility of smooth muscle myosin. Thus, the effect of RLC phosphorylation in striated muscle and its ability to regulate the folded-to-extended conformational transition in smooth muscle may be due to a simple reduction of net charge at the N terminus of the light chain. The ability of phosphorylation to regulate smooth muscle myosin's ATPase activity and motility involves a more complex mechanism.

Amino acid sequences of myosin essential and regulatory light chains from two clam species: comparison with other molluscan myosin light chains

We have determined the amino acid sequences of the essential light chains (ELC) and regulatory light chains (RLC) of myosin from two species of clam, Mercenaria mercenaria and Macrocallista nimbosa, using protein chemistry methods. The N-termini of all four proteins were blocked, and sequencing was carried out on various chemically and enzymatically produced peptide fragments. Cleavage of either Mercenaria RLC (MRLC) or Macrocallista RLC (VLC) at its 3 Arg yielded four peptides, three of which could not be sequenced directly, due to an N-terminal blocking group and 2 Arg-Gln bonds in these proteins. The fourth peptide was partially and specifically cleaved at an unusually reactive residue, Met-64, which is invariant in all known RLC sequences. A comparison of all available molluscan ELC and RLC sequences was carried out in search of clues to functionally important features of these proteins in muscles which are regulated by a Ca(2+)-sensitive myosin. By analogy with other RLCs, VRLC and MRLC may be phosphorylated at Ser-11 by an endogenous kinase. All myosin light chains, like troponin C and calmodulin, contain four homologous regions, I to IV, each of which contains a twelve-residue potential Ca(2+)-binding loop flanked on either side by a pair of helices. All RLCs, including those from Ca(2+)-insensitive myosins, contain a divalent cation-binding site in region I. Clam and other molluscan ELCs contain a single Ca(2+)-binding site in region III. This site is present only in the ELCs of myosins that are regulated by direct binding of Ca2+. The ELC site III is likely to play a key role in the regulation of molluscan muscle contraction.

Dictyostelium discoideum essential myosin light chain: gene structure and characterization

We have used a Dictyostelium essential myosin light chain (EMLC) cDNA clone to isolate additional cDNA clones which supply a different 3' sequence from that previously described. The revised cDNA sequence encodes a polypeptide of 150 amino acids. Amino acid residues 147-167 of the previously reported sequence are replaced by new residues 147 to 150. The new cDNA encodes a polypeptide with 66% amino acid sequence identity with the Physarum polycephalum EMLC, and approximately 30% identity with mammalian EMLC sequences. These new cDNA clones were used to isolate two genomic DNA fragments which contain the entire EMLC gene. The Dictyostelium EMLC gene contains a single intron located immediately 3' of the translation initiation codon and encodes a product most similar to MLC3 isoform of vertebrates. Primer extension analysis places the transcription initiation site approximately 90 nucleotides upstream of the translation initiation site. A DNA fragment containing 350 bases of sequence upstream of the putative transcription initiation site is sufficient to drive expression of a reporter gene upon reintroduction into growing Dictyostelium cells. In addition, the CAT reporter mRNA produced by this construct showed a pattern of developmental regulation similar to that previously reported for the endogenous EMLC mRNA. Based on comparison with published EMLC sequences from a variety of sources, the Dictyostelium EMLC shows slightly higher similarity to vertebrate EMLCs from striated muscle sources than nonmuscle sources. While Dictyostelium and human nonmuscle sequences display only 28% identity over their entire sequence, the region from residue 88 to 108 shows much higher identity (67%). The high evolutionary conservation of this region of the EMLC suggests it may play an important role in EMLC function, and as such, represents a good target for future mutagenesis studies.

The calcium ion dependence of scallop myosin ATPase activity

The ATPase activity of scallop (Pecten maximus) striated adductor myosin and heavy meromyosin (HMM) have been investigated as a function of [Ca2+] using formycin triphosphate (FTP) as a fluorescent ATP analogue. The FTPase activity of the regulated fraction of these preparations was activated steeply over the range of 0.1 to 1 microM [Ca2+], implying the existence of a form of cooperativity that is intrinsic to the myosin heads. In addition to the previously characterised heterogeneity with respect to an unregulated fraction, the regulated fraction of HMM was resolved into two populations whose activities showed a slightly different dependency on [Ca2+]. This was revealed unambiguously at intermediate levels of activation where, in some experiments, the product release rate constants differed for the two populations by more than fivefold. At maximum relaxation or maximum activation, these rate constants differed by two- to three-fold and were not clearly resolved by the multiexponential fitting procedure. The populations might arise as a consequence of isoenzymes, modification during preparation or slowly interconverting conformers; Ca2+ binding itself being a rapid equilibrium process in both populations. FTP turnover by myosin could not be analysed in such detail because of the technical problems of measuring the fluorescence of a suspension of filaments, but the rates of the elementary steps appeared similar to those of HMM. The fraction of unregulated molecules in myosin preparations was comparable to that of HMM indicating that if it is a consequence of preparative damage, the modification must occur prior to tryptic digestion.

Regulation of scallop myosin by mutant regulatory light chains

Scallop adductor myosin is regulated by its subunits; the regulatory light chain (R-LC) and essential light chain (E-LC). Myosin light chains suppress muscle activity in the absence of calcium and are responsible for relaxation. The binding of Ca2+ to the myosin triggers contraction by releasing the inhibition imposed on myosin by the light chains. To map the functional domains of the R-LC, we have carried out mutagenesis followed by bacterial expression. Both wild-type and mutant proteins were hybridized to scallop myosin heavy chain/E-LC to map the regions of the light chain that are responsible for the binding to the myosin heavy chain/E-LC, for restoring the specific calcium-binding site, and controlling the myosin ATPase activity. The R-LC is expressed in Escherichia coli using the pKK223-3 (Pharmacia) expression vector and has been purified to greater than 90% purity. E. coli-expressed wild-type R-LC differs from the native R-LC by having the initiating methionine residue and an unblocked NH2 terminus. The wild-type R-LC restores Ca2+ binding and Ca2+ sensitivity when hybridized to scallop myosin. A point mutation of the sixth Ca2(+)-liganding position of domain I (Asp39----Ala39) results in a R-LC that binds more weakly to the heavy chain/E-LC and restores the specific Ca2(+)-binding site but not regulation of the actin-activated Mg2+ ATPase. A second mutation was produced by substituting the last 11 residues of the COOH terminus with 15 different residues. This mutant restores the specific Ca2(+)-binding site, but does not restore Ca2+ regulation to the actin-activated ATPase activity. Several other point mutations do not alter light chain function. The experiments directly establish that the divalent cation-binding site of domain I is functionally distinct from the specific Ca2(+)-binding site. The results indicate that an intact domain I and the COOH terminus are required to suppress the myosin ATPase activity. The fact that the domain I mutation and the COOH-terminal mutation disrupt regulation but do not affect Ca2(+)-binding indicates that these two aspects of regulation are separable and, therefore, the R-LC has distinct functional regions.

Striated scallop muscle relaxation: fast force transients produced by photolysis of Diazo-2

Relaxation of the myosin regulated striated adductor muscles of Pecten maximus was initiated by the photolysis of the caged Ca2+ chelator, Diazo-2. The fibres relaxed to approximately 30% of the maximum tension with a mean half-time of 17.9 +/- 1.6 ms (n = 7, temp 12 degrees C), much faster than the rates observed in intact muscle at the same temperature. This indicates that in the intact adductor muscle the slower relaxation rate is determined by the speed of Ca2+ removal from the sarcoplasm. The faster rate of relaxation of scallop muscle in vitro, compared with frog skeletal muscle may reflect different mechanisms of regulation of the crossbridge cycle.

Arrangement of myosin heads on Limulus thick filaments

The two myosin heads with a single surface subunit on thick filaments from chelicerate arthropod muscle may originate from the same, or from axially sequential molecules, as suggested by three-dimensional reconstructions. The resolution attained in the reconstructions, however, does not permit one to distinguish unequivocally between these two possible arrangements. We examined the effect of 0.6 M KCl on relaxed thick filaments separated from Limulus muscle and filaments in which nearest myosin heads were cross-linked by the bifunctional agent, 3,3'-dithio-bis[3'(2')-O-[6-propionylamino)hexanoyl]adenosine 5'-triphosphate (bis22ATP), in the presence of vanadate (Vi). In high salt, surface myosin dissolved from both native, relaxed filaments and those exposed to 1-2 mM dithiothreitol after cross-linking, but was retained on filaments with cross-linked heads. Since bis22ATP must form intermolecular bonds between myosin heads within each subunit to prevent myosin solubilization in high salt, we conclude that each of these heads originates from a different myosin molecule, as was previously predicted by the reconstructions.

The effect of low ATP concentrations on relaxation in the myosin regulated myofibrils from scallop

Troponin-tropomyosin-regulated myofibrils show a significant increase in ATPase activity and contract in the absence of calcium when the ATP concentration falls significantly below the saturation level. By contrast, the ATPase of the myosin-regulated myofibrils of scallop striated muscle was not activated in the absence of calcium when the ATP concentration was lowered to 10mM. Nevertheless, a very small fraction of crossbridges were active at 10mM ATP resulting in very slow myofibrillar shortening. In contrast to the behaviour of rabbit contractile proteins there was no correlation between myofibrillar shortening and ATP induced turbidity changes of actomyosin taken from scallop.

The mechanism of regulatory light chain dissociation from scallop myosin

The dissociation of the regulatory light chains from scallop myosin subfragments, on addition of EDTA, was investigated by using the fluorophore 8-anilinonaphthalene-1-sulphonate as a probe. The rate of this process (0.014 s-1) was partially limited by the rate of Mg2+ dissociation (0.058 s-1) from the non-specific high-affinity site. The dissociation of the regulatory light chain subfragment 1 was less extensive than from heavy meromyosin. Reassociation of the scallop regulatory light chain was induced on addition of Mg2+, but it appeared to be limited by a first-order step. The nature of this step was revealed by the kinetics of Mercenaria regulatory light chain association. Scallop heavy meromyosin, denuded of its regulatory light chains, exists in a refractory state, whose reversal to the nascent state limits the rate of light chain association (0.006 s-1). The formation of the refractory state is the driving force for the net dissociation of regulatory light chains from scallop heavy meromyosin. This mechanism is discussed with reference to existing structural information on light-chain-denuded myosin.