Role of magnesium binding to myosin in controlling the state of cross-bridges in skeletal rabbit muscle

The effect of Mg2+ on the disposition of myosin cross-bridges was studied on myofibrils and synthetic myosin and rod filaments by employing chymotryptic digestion and chemical cross-linking methods. In the presence of low Mg2+ concentrations (0.1 mM), the proteolytic susceptibility at the heavy meromyosin/light meromyosin (HMM/LMM) junction in these three systems sharply increases over the pH range from 7.0 to 8.2. Such a change has been previously associated with the release of myosin cross-bridges from the filament surface [Ueno, H., & Harrington, W.F. (1981) J. Mol. Biol. 149, 619-640]. Millimolar concentrations of Mg2+ block or reverse this charge-dependent transition. Rod filaments show the same behavior as myosin filaments, indicating that the low-affinity binding sites for Mg2+ are located on the rod portion of myosin. The interpretation of these results in terms of Mg2+-mediated binding of cross-bridges to the filament backbone is supported by cross-linking experiments. The normalized rate of S-2 cross-linking in rod filaments at pH 8.0, kS-2/kLMM, increases upon addition of Mg2+ from 0.30 to 0.65 and approaches the cross-linking rate measured at pH 7.0 (0.75), when the cross-bridges are close to the filament surface. In rod filaments prepared from oxidized rod particles, chymotryptic digestion proceeds both at the S-2/LMM junction and at a new cleavage site located in the N-terminal portion of the molecule. Kinetic analysis of digestion rates at these two sites reveals that binding of Mg2+ to oxidized myosin rods has a similar effect at both sites over the pH range from 7.0 to 8.0.(ABSTRACT TRUNCATED AT 250 WORDS)     

Regulation of the actin-activated ATPase activity of Acanthamoeba myosin II by copolymerization with phosphorylated and dephosphorylated peptides derived from the carboxyl-terminal end of the heavy chain

Myosin II from Acanthamoeba castellanii is a conventional myosin composed of two heavy chains and two pairs of light chains. The amino-terminal approximately 90 kDa of each heavy chain form a globular head that contains the ATPase site and an ATP-sensitive actin-binding site. The carboxyl-terminal approximately 80 kDa of both heavy chains interact to form a coiled coil, helical rod (through which the molecules self-associate into bipolar filaments) ending in a short nonhelical tailpiece. Phosphorylation of 3 serine residues at the tip of the tail (at positions 11, 16, and 21 from the carboxyl terminus) inactivates the actin-activated Mg2(+)-ATPase activity of myosin II filaments. Previous work had indicated that the activity of each myosin II molecule in a filament reflects the global state of phosphorylation of the filament rather than the phosphorylation state of the molecule itself. We have now purified the approximately 28-kDa carboxyl-terminal region of the heavy chain lacking the last two phosphorylation sites, and we have shown that this peptide copolymerizes with and regulates the actin-activated Mg2(+)-ATPase activities of native dephosphorylated and phosphorylated myosin II. It can be concluded from these studies that the biologically relevant enzymatic activity of myosin II is regulated by a phosphorylation-dependent conformational change in the myosin filaments.     

The regulatory light chain is required for folding of smooth muscle myosin

Light chain phosphorylation causes the folded monomeric form of myosin to extend and assemble into filaments. This observation established the involvement of the 20-kDa regulatory light chain (LC20) in conformational transitions of smooth muscle myosin. To further assess the role of this subunit in the intramolecular folding of myosin, LC20 was removed from turkey gizzard myosin at elevated temperatures in the presence of EDTA through the use of an antibody affinity column. Metal-shadowed images showed that LC20-deficient myosin had a tendency to aggregate through the neck region. When MgATP was added to filaments formed from this myosin, less than 10% of the myosin was solubilized, indicating that myosin could not fold in the absence of light chain. Readdition of native regulatory light chain restored the myosin to its original solubility properties, thus establishing reversibility. Addition of foreign light chains from skeletal muscle myosin or a chymotryptic-cleaved gizzard light chain produced the same amount of monomeric myosin in high salt that was obtained by recombination with the homologous light chain. However, the ability of the hybrid myosins to assume the folded conformation was impaired, and only a partially folded species was obtained. Single-headed myosin, like rod and light chain-deficient myosin, remained filamentous in the presence of MgATP. These results are consistent with the hypothesis that the regulatory light chain in the neck region of myosin contributes to a binding site for the myosin tail.     

Preparation of subfragment-1 from abalone smooth muscle myosin

Myosin subfragment-1 (S1), which has one heavy chain (HC) (93 kDa) and two light chains (LC1 and LC2), was prepared by papain digestion of myosin from abalone-smooth muscle in the presence of Ca2+. The Ca-sensitivity of abalone S1 itself was not lost completely (about 30%). The tryptic digestion of S1 showed that in the presence of EDTA, S1 HC was split into 68, 55, and 23 kDa fragments, as in the presence of Ca2+, but 23 kDa was further degraded into 19 kDa. In contrast to the result in the presence of Ca2+, LCs disappeared in the early stage of reaction and Ca-ATPase activity decreased rapidly to about 70% of that of intact S1. This rapid decrease of Ca-ATPase activity seemed to be accompanied with the digestion of LCs. Therefore, LCs contribute to the protection of 23 kDa fragment from further digestion, to the maintenance of Ca-ATPase activity by stabilizing the structure of S1 to some extent in the presence of Ca2+. Since F-actin suppressed the cleavage of S1 HC to 68 and 23 kDa during tryptic digestion, it might be that 23 and 68 kDa corresponded to 20 kDa (C-terminal fragment) and to 50 + 25 kDa (N-terminal fragment) of skeletal myosin S1, respectively.     

Localization of the actin-binding sites of Acanthamoeba myosin IB and effect of limited proteolysis on its actin-activated Mg2+-ATPase activity

Acanthamoeba myosin IB contains a 125-kDa heavy chain that has high actin-activated Mg2+-ATPase activity when 1 serine residue is phosphorylated. The heavy chain contains two F-actin-binding sites, one associated with the catalytic site and a second which allows myosin IB to cross-link actin filaments but has no direct effect on catalytic activity. Tryptic digestion of the heavy chain initially produces an NH2-terminal 62-kDa peptide that contains the ATP-binding site and the regulatory phosphorylation site, and a COOH-terminal 68-kDa peptide. F-actin, in the absence of ATP, protects this site and tryptic cleavage then produces an NH2-terminal 80-kDa peptide. Both the 62- and the 80-kDa peptides retain the (NH+4,EDTA)-ATPase activity of native myosin IB and both bind to F-actin in an ATP-sensitive manner. However, only the 80-kDa peptide retains a major portion of the actin-activated Mg2+-ATPase activity. This activity requires phosphorylation of the 80-kDa peptide by myosin I heavy chain kinase but, in contrast to the activity of intact myosin IB, it has a simple, hyperbolic dependence on the concentration of F-actin. Also unlike myosin IB, the 80-kDa peptide cannot cross-link F-actin filaments indicating the presence of only a single actin-binding site. These results allow the assignment of the actin-binding site involved in catalytic activity to the region near, and possibly on both sides of, the tryptic cleavage site 62 kDa from the NH2 terminus, and the second actin-binding site to the COOH-terminal 45-kDa domain. Thus, the NH2-terminal 80 kDa of the myosin IB heavy chain is functionally similar to the 93-kDa subfragment 1 of muscle myosin and most likely has a similar organization of functional domains.     

Phosphorylation of myorod (catchin) by kinases tightly associated to molluscan and vertebrate smooth muscle myosins

Myorod, also known as catchin, a newly discovered component of molluscan smooth muscle thick filaments, is an alternative product of the myosin heavy chain gene. It contains a C-terminal rod part that is identical to that part of myosin and a unique N-terminal domain that is very small relative to the myosin head domain. The role of myorod in contraction or relaxation of this muscle type is unknown. In the present study we demonstrated that myorod was phosphorylated not only by a kinase endogenous to molluscan myosin and twitchin but also to vertebrate smooth muscle myosin light chain kinase (MLCK). The rates and maximal levels of phosphorylation were up to threefold higher than those observed by protein kinase A with clear optima at the physiological salt concentrations. Using a mild digestion with chymotrypsin we isolated an 11 kDa phosphopeptide and showed that the phosphorylation site was located at the N-terminal domain of myorod at Thr 141 position. The sequence around this site exhibited a high degree of similarity to that expected for the substrate recognition site of MLCK. The phosphorylation rates strongly depended on the ionic conditions indicating that this site could be readily sterically blocked during myorod polymerization. Another component of the thick filaments involved in regulation of the catch state, twitchin, was phosphorylated by MLCK and exhibited endogenous myorod kinase and MLCK activities. A possible role of these phosphorylation reactions in the regulation of molluscan smooth muscles is discussed.     

Filament assembly and regulation of the actin-activated ATPase activity of thymus myosin

The effects of light chain phosphorylation on the actin-activated ATPase activity and filament assembly of calf thymus cytoplasmic myosin were examined under a variety of conditions. When unphosphorylated and phosphorylated thymus myosins were monomeric, their MgATPase activities were not activated or only very slightly activated by actin, but when they were filamentous, their MgATPase activities were stimulated by actin. The phosphorylated myosin remained filamentous at lower Mg2+ concentrations and higher KC1 concentrations than did the unphosphorylated myosin, and the myosin concentration required for filament assembly was lower for phosphorylated myosin than for unphosphorylated myosin. By varying the myosin concentration, it was possible to have under the same assay conditions mostly monomeric myosin or mostly filamentous myosin; under these conditions, the actin-activated ATPase activities of the filamentous myosins were much greater than those of the monomeric myosins. The addition of phosphorylated myosin to unphosphorylated myosin promoted the assembly of unphosphorylated myosin into filaments. These results suggest that phosphorylation may regulate the actomyosin-based motile activities in vertebrate nonmuscle cells by regulating myosin filament assembly.     

Analysis of the metal-induced conformational change in myosin with a monoclonal antibody to light chain two

A monoclonal antibody capable of detecting a conformational change in myosin light chain two (LC2) was characterized in detail. The antibody was shown to bind only to myosin LC2 when tested against fast skeletal myosin (chicken pectoralis muscle). With cardiac or slow muscle myosins, the antibody exclusively recognized their first light chains (LC1). Staining of myofibrils by the monoclonal antibody could be observed only after their irreversible denaturation by acetone or ethanol, or after incubation of the myofibrils in divalent metal chelators. This latter effect was shown to be fully reversible. The metal effect was independent of ionic strength although the affinity of the antibody for myosin was depressed at high salt concentrations. Similar metal effects were detected in the binding of antibody to cardiac or slow myosins. Neither the metal nor the ionic strength-related inhibition of antibody binding were detected with denatured myosin. The antibody binding site overlaps one of the alpha-chymotryptic sites in LC2 protected by divalent metals. Electron microscopic observations of myosin-antibody complexes demonstrated that the antibody binding site is located near the head-rod junction of myosin. Since the binding site of this monoclonal antibody has been mapped by recombinant DNA methods to the junction of the first alpha-helical domain with the calcium binding site of LC2, the location of the calcium binding site must also be located near the head-tail junction of myosin. A model for conformational changes at the myosin head-tail junction is proposed to account for the metal-induced blockage of antibody binding and the inhibition of alpha-chymotryptic digestion of LC2.     

Localization of a new proteolytic site accessible in oxidized myosin rod

We have compared the proteolysis pattern of reduced and oxidized myosin rods in which the five pairs of SH-groups were interchain crosslinked by employing CuCl2 or 5,5'-dithiobis-2-nitrobenzoate. In the tryptic digest of oxidized rod three new fragments appeared on SDS-polyacrylamide gel electrophoresis (chain masses of 100, 45, and 25 kDa). Based on the N-terminal sequences of the isolated peptides, it is concluded that oxidation creates a new cleavage site 102 residues away from the N-terminus of the rod, in the vicinity of one of the modified SH-groups (Cys-108). This observation indicates that S-S crosslinking of myosin rod leads to a local unfolding of the coiled-coil structure.     

Two chymotrypsin-susceptible sites of myosin rod from chicken gizzard

The rod prepared from chicken gizzard myosin has been found to have two sites sensitive to limited digestion with chymotrypsin; these sites were located at a subfragment 2/light meromyosin junction (site 1), and at a site 10 kDa remote from either C-terminal or N-terminal of light meromyosin (site 2). The site 1 was more sensitive to the digestion than the site 2. The cleavage at site 2 of the light meromyosin yielded a 74-kDa fragment that was soluble in a low ionic strength solution, contrary to the insolubility of the parent light meromyosin in the same solution. Studies on the effects of MgCl2, ATP and pH on the susceptibilities of these sites to chymotrypsin have given following results. (a) Millimolar concentrations of MgCl2 protected site 1 and site 2 from the chymotryptic cleavage. (b) The cleavage at site 1 of myosin rod in the low salt solution free of Mg2+ at pH 7.0 and pH 8.5, was not affected by the presence of 5 mM ATP. However, MgCl2-induced protection of site 1 was relieved by addition of ATP. On the other hand, the cleavage at site 2 was stimulated by addition of ATP, irrespective of the presence or absence of MgCl2. (c) The alkaline condition of pH 8.5 was more favorable for the chymotryptic cleavages at both site 1 and site 2 than the neutral condition of pH 7.0. These results suggest that myosin rod contains two flexible regions, the structures of which are influenced by such an ambient factor as MgCl2, ATP or pH.     

Actin-activation of unphosphorylated gizzard myosin

The effect of light chain phosphorylation on the actin-activated ATPase activity and filament stability of gizzard smooth muscle myosin was examined under a variety of conditions. When unphosphorylated and phosphorylated gizzard myosins were monomeric, their MgATPase activities were not activated or only very slightly activated by actin, and when they were filamentous, their MgATPase activities could be stimulated by actin. At pH 7.0, the unphosphorylated myosin in the presence of ATP required 2-3 times as much Mg2+ for filament formation as did the phosphorylated myosin. The amount of stimulation of the unphosphorylated myosin filaments depended upon pH, temperature, and the presence of tropomyosin. At pH 7.0 and 37 degrees C and at pH 6.8 and 25 degrees C, the MgATPase activity of filamentous, unphosphorylated, gizzard myosin was stimulated 10-fold by actin complexed with gizzard tropomyosin. These tropomyosin-actin-activated ATPase activities were 40% of those of the phosphorylated myosin. Under other conditions, pH 7.5 and 37 degrees C and pH 7.0 and 25 degrees C, even though the unphosphorylated myosin was mostly filamentous, its MgATPase activity was stimulated only 4-fold by tropomyosin-actin. Thus, both unphosphorylated and phosphorylated gizzard myosin filaments appear to be active, but the cycling rate of the unphosphorylated myosin is less than that of the phosphorylated myosin. Active unphosphorylated myosin may help explain the ability of smooth muscles to maintain tension in the absence of myosin light chain phosphorylation.     

Influence of the regulatory light chain of fast skeletal muscle myosin on its interaction with actin in the presence and absence of ATP

The effect of myosin LC2 modifications (phosphorylation or selective proteolytic removal of a seven-residue N-terminal peptide) and partial or complete removal of the whole LC2 was studied under various conditions. (1) Actin binding in the absence of ATP is not influenced by the nature of the myosin species (phosphorylated, dephosphorylated or devoid of LC2). (2) A 50% inhibition of K+/EDTA-ATPase was obtained with actin concentrations hardly different when phosphorylated and dephosphorylated myosins were compared (of the order of 5 microM), whereas both myosin devoid of LC2 and myosin in which the LC2 N-terminal peptide has been removed required significantly higher concentrations of actin (13.0 +/- 2 and 12.0 +/- 2.0 microM, respectively). (3) Dissociation of the actomyosin complex at high ionic strength with nucleotides is not influenced by phosphorylation. (4) Actin activation of Mg2+-ATPase is enhanced when LC2 is phosphorylated; no activation enhancement is observed with myosin devoid of LC2. (5) Translational diffusion coefficient measurements of myosin in high-ionic-strength solutions indicate a tendency for LC2-deprived myosin to form autoassociation oligomers. It thus appears that a structural modification (partial cleavage or removal of LC2) induces important structural changes in myosin, pointing to a role for LC2 in the intrinsic conformation of the molecule and its interaction potentialities. Effects of LC2 removal at high ionic strength are best explained by interactions bearing no relationship to physiological functions. A physiologically significant effect of LC2 phosphorylation requires a minimum degree of organization (actomyosin complex) to be expressed in which LC2 could play the role of a return-spring in the cross-bridge mechanism.     

Stepping between Membrane Microdomains

In isolated thick filaments from many types of muscle, the two head domains of each myosin molecule are folded back against the filament backbone in a conformation called the interacting heads motif (IHM) in which actin interaction is inhibited. This conformation is present in resting skeletal muscle, but it is not known how exit from the IHM state is achieved during muscle activation. Here, we investigated this by measuring the in situ conformation of the light chain domain of the myosin heads in relaxed demembranated fibers from rabbit psoas muscle using fluorescence polarization from bifunctional rhodamine probes at four sites on the C-terminal lobe of the myosin regulatory light chain (RLC). The order parameter 〈P₂〉 describing probe orientation with respect to the filament axis had a roughly sigmoidal dependence on temperature in relaxing conditions, with a half-maximal change at ∼19°C. Either lattice compression by 5% dextran T500 or addition of 25 μM blebbistatin decreased the transition temperature to ∼14°C. Maximum entropy analysis revealed three preferred orientations of the myosin RLC region at 25°C and above, two with its long axis roughly parallel to the filament axis and one roughly perpendicular. The parallel orientations are similar to those of the so-called blocked and free heads in the IHM and are stabilized by either lattice compression or blebbistatin. In relaxed skeletal muscle at near-physiological temperature and myofilament lattice spacing, the majority of the myosin heads have their light chain domains in IHM-like conformations, with a minority in a distinct conformation with their RLC regions roughly perpendicular to the filament axis. None of these three orientation populations were present during active contraction. These results are consistent with a regulatory transition of the thick filament in skeletal muscle associated with a conformational equilibrium of the myosin heads.     

A new, smaller actin-activatable myosin subfragment 1 which lacks the 20-kDa, SH1 and SH2 peptide

The actin-dependent ATPase activity of myosin is retained in the separated heads (S1) which contain the NH2-terminal 95-kDa heavy chain fragment and one or two light chains. The S1 heavy chain can be degraded further by limited trypsin treatment into characteristic 25-, 50-, and 20-kDa peptides, in this order from the NH2-terminal end. The 20-kDa peptide contains an actin-binding site and SH1 and SH2, two thiols whose modification dramatically affects ATPase activity. By treating myosin filaments with trypsin at 4 degrees C in the presence of 2 mM MgCl2, we have now obtained preferential cleavage at the 50-20-kDa heavy chain site without any cleavage at the head-rod junction and hinge region in the rod. Incubation of these trypsinized filaments at 37 degrees C in the presence of MgATP released a new S1 fraction which lacked the COOH-terminal 20-kDa heavy chain peptide region. This fraction, termed S1'(75K), has more than 50% of the actin-activated Mg2+-ATPase activity of S1 and the characteristic Ca2+-ATPase and K+-EDTA ATPase activities of myosin. These results show that SH1 and SH2 are not essential for ATPase activity and that binding of actin to the 20-kDa region is not essential for the enhancement of the Mg2+-ATPase activity.     

Actin and myosin: control of filament assembly

Actin filaments, assembled from highly purified actin from either skeletal muscle or Dictyostelium amoebae, are very stable under physiological ionic conditions. A small and limited amount of exchange of actin filament subunits for unpolymerized actin or subunits in other filaments has been measured by three techniques: fluorescence energy transfer, incorporation of 35S-labelled actin monomers into unlabelled actin filaments, and exchange of [14C]ATP with filament-bound ADP. A 40 kDa protein purified from amoebae destabilizes these otherwise stable filaments in a Ca2+-dependent manner. Myosin purified from Dictyostelium amoebae is phosphorylated both in the tail region of the heavy chain and in one of the light chains. Phosphorylation appears to regulate myosin thick-filament formation.     

Expression in Escherichia coli of a functional Dictyostelium myosin tail fragment

The amino acid sequence of the myosin tail determines the specific manner in which myosin molecules are packed into the myosin filament, but the details of the molecular interactions are not known. Expression of genetically engineered myosin tail fragments would enable a study of the sequences important for myosin filament formation and its regulation. We report here the expression in Escherichia coli of a 1.5- kb fragment of the Dictyostelium myosin heavy chain gene coding for a 58-kD fragment of the myosin tail. The expressed protein (DdLMM-58) was purified to homogeneity from the soluble fraction of E. coli extracts. The expressed protein was found to be functional by the following criteria: (a) it appears in the electron microscope as a 74-nm-long rod, the predicted length for an alpha-helical coiled coil of 500 amino acids; (b) it assembles into filamentous structures that show the typical axial periodicity of 14 nm found in muscle myosin native filaments; (c) its assembly into filaments shows the same ionic strength dependence as Dictyostelium myosin; (d) it serves as a substrate for the Dictyostelium myosin heavy chain kinase which phosphorylates myosin in response to chemotactic signaling; (e) in its phosphorylated form it has the same phosphoamino acids and similar phosphopeptide maps to those of phosphorylated Dictyostelium myosin heavy chain; (f) it competes with myosin for the heavy chain kinase. Thus, all the information required for filament formation and phosphorylation is contained within this expressed protein.     

Measurement of the fraction of myosin heads bound to actin in rabbit skeletal myofibrils in rigor

Tryptic digestion of rabbit skeletal myofibrils at physiological ionic strength and pH results in cleavage of the myosin heavy chain at one site giving two bands (M_r = 200,000 and 26,000) on sodium dodecyl sulfate/polyacrylamide gels. Following addition of sodium pyrophosphate (to 1 mm) to dissociate the myosin heads from actin, tryptic proteolysis results in production of three bands, 160K², 51K and 26K, with a 74K band appearing as a precursor of the 51K and 26K species. Under these conditions, there is insignificant cleavage of heavy chain to the heavy and light meromyosins. Trypsin-digested myofibrils yield the same amount of rod as native myofibrils when digested with papain. These results indicate that actin blocks tryptic cleavage of the myosin heavy chain at a site 74K from the N terminus. From measurements of the amount of 51K species formed by digestion of rigor fibers at various sarcomere lengths, we estimate that at least 95% of the myosin heads are bound to actin at 100% overlap of thick and thin filaments. Hence all myosin molecules can bind to actin, and consequently both heads of a myosin molecule can interact simultaneously with actin filaments under rigor conditions.     

Physical integrity of smooth muscle myosin filaments is enhanced by phosphorylation of the regulatory myosin light chain.

BACKGROUND AND AIMS: Smooth muscle myosin monomers self-assemble in solution to form filaments. Phosphorylation of the 20-kD regulatory myosin light chain (MLC20) enhances filament formation. It is not known whether the phosphorylated and non-phosphorylated filaments possess the same structural integrity. METHODS: We purified myosin from bovine trachealis to form filaments, in ATP-containing zero-calcium solution during a slow dialysis that gradually reduced the ionic strength. Sufficient myosin light chain kinase and phosphatase, as well as calmodulin, were retained after the myosin purification and this enabled phosphorylation of MLC20 within 20-40s after addition of calcium to the filament suspension. The phosphorylated and non-phosphorylated filaments were then partially disassembled by ultrasonification. The extent of filament disintegration was visualized and quantified by atomic force microscopy. RESULTS: MLC20 phosphorylation reduced the diameter of the filaments and rendered the filaments more resistant to ultrasonic agitation. Electron microscopy revealed a similar reduction in filament diameter in intact smooth muscle when the cells were activated. CONCLUSION: Modification of the structural and physical properties of myosin filaments by MLC20 phosphorylation may be a key regulation step in smooth muscle where formation and dissolution of the filaments are required in the cells' adaptation to different cell length.     

Regulation of the actin-activated ATPase and in vitro motility activities of monomeric and filamentous Acanthamoeba myosin II.

The actin-activated Mg(2+)-ATPase activity of filamentous Acanthamoeba myosin II is inhibited by phosphorylation of 3 serine residues at the tip of the tail of each heavy chain. From previous studies, it had been concluded that the activity of each molecule in the filament was regulated by the global state of phosphorylation of the filament and was independent of its own phosphorylation state. The actin-activated Mg(2+)-ATPase activity of monomeric phosphorylated myosin II was not known because it polymerizes under the ionic conditions necessary for the expression of this activity. We have now found conditions to maintain myosin II monomeric and active during the enzyme assay. The actin-activated Mg(2+)-ATPase activities of monomeric dephosphorylated and phosphorylated myosin II were found to be the same as the activity of filamentous dephosphorylated myosin II. These results support the conclusion that phosphorylation regulates filamentous myosin II by affecting filament conformation. Consistent with their equivalent enzymatic activities, monomeric and filamentous dephosphorylated myosin II were equally active in an in vitro motility assay in which myosin adsorbed to a surface drives the movement of F-actin. In contrast to their very different enzymatic activities, however, filamentous and monomeric phosphorylated myosin II had similar activities in the in vitro motility assay; both were much less active than monomeric and filamentous dephosphorylated myosin II. One interpretation of these results is that the rate-limiting steps in the two assays are different and that, while the rate-limiting step for actin-activated Mg(2+)-ATPase activity is regulated only at the level of the filament, the rate-limiting step for motility can also be regulated at the level of the monomer.     

Control of filament length by the regulatory light chains in skeletal and cardiac myosins

The possible role of the regulatory light chains (LC2) in in vitro assembly of rabbit skeletal and dog cardiac myosins was examined by formation of minifilaments and synthetic thick filaments. After LC2 was removed, the resulting myosin preparations exhibited little aggregation in 0.5 M KCl and 0.05 M potassium phosphate (pH 6.5). Minifilaments migrated as a single, hypersharp peak during sedimentation velocity, but electron microscopic analysis revealed a more destabilized structure for LC2-deficient minifilaments. Thick filaments were formed in buffers containing 0.15 M KCl and the following: 20 mM imidazole; 20 mM imidazole, 5 mM ATP; or 20 mM imidazole, 5 mM ATP, and 5 mM MgCl2, all at pH 7.0. Skeletal and cardiac myosin filaments formed in imidazole buffer alone were bipolar, tapered at both ends, and about 1.6 micron long. Removal of LC2 resulted in the formation of shorter thick filaments (1.2 micron long). This effect could be reversed by reassociation with LC2. Inclusion of ATP in the buffer disrupted the filament structure, resulting in irregular, short filaments (less than 0.6 micron); addition of both ATP and MgCl2 largely reversed the effects of ATP alone. In cardiac myosin filaments, the bare zone diameter increased from 16 nm as measured in control and LC2-recombined samples to 20 nm in LC2-deficient myosin assemblies. These results implicate LC2 in an active role in controlling synthetic thick filament length in both skeletal and cardiac muscles.