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

Non-muscle myosin 2 can incorporate into established filaments in cells without an assembly competence domain

Myosin 2 dynamically assembles into filaments that exert force on the actin cytoskeleton. To form filaments, myosin 2 monomers transition between folded and unfolded states. Monomer unfolding exposes an extended coiled-coil that interacts with other monomers in parallel and antiparallel fashions, enabling bipolar filament formation. A C-terminal domain of the coiled-coil, termed assembly competence domain (ACD), has been repeatedly identified as necessary for filament assembly. Here, we revisit ACD contribution when full-length filaments are present. Non-muscle myosin 2A lacking the ACD (ΔACD) initially appears diffuse, but triton extraction of cytosolic fraction reveals cytoskeletal association. Disruption of the folded monomer enhances the cytoskeletal fraction, while inhibition of endogenous filament assembly appears to reduce it. Finally, high resolution imaging of endogenous and exogenous bipolar filamentous structures reveals highly coincident signal, suggesting ΔACD constructs co-assemble with endogenous myosin 2A filaments. Our data demonstrate that while the ACD is required for de novo filament assembly, it is not required for monomers to recognize and associate with established filaments in cells. More broadly, this highlights the existence of distinct mechanisms governing myosin 2 monomer assembly into nascent filaments, and monomer recognition and association with established filaments to maintain steady-state contractile networks.

Cryo-EM structure of the autoinhibited state of myosin-2

We solved the near-atomic resolution structure of smooth muscle myosin-2 in the autoinhibited state (10S) using single-particle cryo–electron microscopy. The 3.4-Å structure reveals the precise molecular architecture of 10S and the structural basis for myosin-2 regulation. We reveal the position of the phosphorylation sites that control myosin autoinhibition and activation by phosphorylation of the regulatory light chain. Further, we present a previously unidentified conformational state in myosin-2 that traps ADP and Pi produced by the hydrolysis of ATP in the active site. This noncanonical state represents a branch of the myosin enzyme cycle and explains the autoinhibition of the enzyme function of 10S along with its reduced affinity for actin. Together, our structure defines the molecular mechanisms that drive 10S formation, stabilization, and relief by phosphorylation of the regulatory light chain.

Nonmuscle Myosin II Regulation Directs Its Multiple Roles in Cell Migration and Division

Nonmuscle myosin II (NMII) is a multimeric protein complex that generates most mechanical force in eukaryotic cells. NMII function is controlled at three main levels. The first level includes events that trigger conformational changes that extend the complex to enable its assembly into filaments. The second level controls the ATPase activity of the complex and its binding to microfilaments in extended NMII filaments. The third level includes events that modulate the stability and contractility of the filaments. They all work in concert to finely control force generation inside cells. NMII is a common endpoint of mechanochemical signaling pathways that control cellular responses to physical and chemical extracellular cues. Specific phosphorylations modulate NMII activation in a context-dependent manner. A few kinases control these phosphorylations in a spatially, temporally, and lineage-restricted fashion, enabling functional adaptability to the cellular microenvironment. Here, we review mechanisms that control NMII activity in the context of cell migration and division. Expected final online publication date for the Annual Review of Cell and Developmental Biology, Volume 37 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Distinct Roles of Smooth Muscle and Non-muscle Myosin Light Chain-Mediated Smooth Muscle Contraction

Both smooth muscle (SM) and non-muscle (NM) myosin II are expressed in hollow organs such as the bladder and uterus, but their respective roles in contraction and corresponding physiological functions remain to be determined. In this report, we assessed their roles by analyzing mice deficient of Myl9, a gene encoding the SM myosin regulatory light chain (SM RLC). We find that global Myl9-deficient bladders contracted with an apparent sustained phase, despite no initial phase. This sustained contraction was mediated by NM myosin RLC (NM RLC) phosphorylation by myosin light chain kinase (MLCK). NM myosin II was expressed abundantly in the uterus and young mice bladders, of which the force was accordingly sensitive to NM myosin inhibition. Our findings reveal distinct roles of SM RLC and NM RLC in SM contraction.

Tyrosine Phosphorylation of the Myosin Regulatory Light Chain Controls Non-muscle Myosin II Assembly and Function in Migrating Cells

Active non-muscle myosin II (NMII) enables migratory cell polarization and controls dynamic cellular processes, such as focal adhesion formation and turnover and cell division. Filament assembly and force generation depend on NMII activation through the phosphorylation of Ser19 of the regulatory light chain (RLC). Here, we identify amino acid Tyr (Y) 155 of the RLC as a novel regulatory site that spatially controls NMII function. We show that Y155 is phosphorylated in vitro by the Tyr kinase domain of epidermal growth factor (EGF) receptor. In cells, phosphorylation of Y155, or its phospho-mimetic mutation (Glu), prevents the interaction of RLC with the myosin heavy chain (MHCII) to form functional NMII units. Conversely, Y155 mutation to a structurally similar but non-phosphorylatable amino acid (Phe) restores the more dynamic cellular functions of NMII, such as myosin filament formation and nascent adhesion assembly, but not those requiring stable actomyosin bundles, e.g., focal adhesion elongation or migratory front-back polarization. In live cells, phospho-Y155 RLC is prominently featured in protrusions, where it prevents NMII assembly. Our data indicate that Y155 phosphorylation constitutes a novel regulatory mechanism that contributes to the compartmentalization of NMII assembly and function in live cells.

The central role of the tail in switching off 10S myosin II activity

Myosin II is a motor protein with two heads and an extended tail that plays an essential role in cell motility. Its active form is a polymer (myosin filament) that pulls on actin to generate motion. Its inactive form is a monomer with a compact structure (10S sedimentation coefficient), in which the tail is folded and the two heads interact with each other, inhibiting activity. This conformation is thought to function in cells as an energy-conserving form of the molecule suitable for storage as well as transport to sites of filament assembly. The mechanism of inhibition of the compact molecule is not fully understood. We have performed a 3-D reconstruction of negatively stained 10S myosin from smooth muscle in the inhibited state using single-particle analysis. The reconstruction reveals multiple interactions between the tail and the two heads that appear to trap ATP hydrolysis products, block actin binding, hinder head phosphorylation, and prevent filament formation. Blocking these essential features of myosin function could explain the high degree of inhibition of the folded form of myosin thought to underlie its energy-conserving function in cells. The reconstruction also suggests a mechanism for unfolding when myosin is activated by phosphorylation.

Using the SpyTag SpyCatcher system to label smooth muscle myosin II filaments with a quantum dot on the regulatory light chain

The regulatory light chain (RLC) of myosin is commonly tagged to monitor myosin behavior in vitro, in muscle fibers, and in cells. The goal of this study was to prepare smooth muscle myosin (SMM) filaments containing a single head labeled with a quantum dot (QD) on the RLC. We show that when the RLC is coupled to a QD at Cys-108 and exchanged into SMM, subsequent filament assembly is severely disrupted. To address this, we used a novel approach for myosin by implementing the SpyTag002 SpyCatcher002 system to prepare SMM incorporated with RLC constructs fused to SpyTag or SpyCatcher. We show that filament assembly, actin-activated steady-state ATPase activities, ability to be phosphorylated, and selected enzymatic and mechanical properties were essentially unaffected if either SpyTag or SpyCatcher were fused to the C-terminus of the RLC. Crucially for our application, we also show that a QD coupled to SpyCatcher can be covalently attached to a RLC-Spy incorporated into a SMM filament without disrupting the filament, and that the filaments can move along actin in vitro.

Nonmuscle myosin II folds into a 10S form via two portions of tail for dynamic subcellular localization

Nonmuscle myosin II forms a folded conformation (10S form) in the inactivated state; however, the physiological importance of the 10S form is still unclear. To investigate the role of 10S form, we generated a chimeric mutant of nonmuscle myosin IIB (IIB-SK1·2), in which S1462-R1490 and L1551-E1577 were replaced with the corresponding portions of skeletal muscle myosin heavy chain. The IIB-SK1·2 mutant did not fold into a 10S form under physiological condition in vitro. IIB-SK1·2 was less dynamic by stabilizing the filamentous form and accumulated in the posterior region of migrating cells. IIB-SK1·2 functioned properly in cytokinesis but altered migratory properties; the rate and directional persistence were increased by IIB-SK1·2 expression. Surprisingly, endogenous nonmuscle myosin IIA was excluded from the posterior region of migrating cells expressing IIB-SK1·2, which may underlie the change of the cellular migratory properties. These results suggest that the 10S form is necessary for maintaining nonmuscle myosin II in an unassembled state and for recruitment of nonmuscle myosin II to a specific region of the cell.

Role of the tail in the regulated state of myosin 2

Myosin 2 from vertebrate smooth muscle or non-muscle sources is in equilibrium between compact, inactive monomers and thick filaments under physiological conditions. In the inactive monomer, the two heads pack compactly together, and the long tail is folded into three closely packed segments that are associated chiefly with one of the heads. The molecular basis of the folding of the tail remains unexplained. By using electron microscopy, we show that compact monomers of smooth muscle myosin 2 have the same structure in both the native state and following specific, intramolecular photo-cross-linking between Cys109 of the regulatory light chain (RLC) and segment 3 of the tail. Nonspecific cross-linking between lysine residues of the folded monomer by glutaraldehyde also does not perturb the compact conformation and stabilizes it against unfolding at high ionic strength. Sequence comparisons across phyla and myosin 2 isoforms suggest that the folding of the tail is stabilized by ionic interactions between the positively charged N-terminal sequence of the RLC and a negatively charged region near the start of tail segment 3 and that phosphorylation of the RLC could perturb these interactions. Our results support the view that interactions between the heads and the distal tail perform a critical role in regulating activity of myosin 2 molecules through stabilizing the compact monomer conformation.

Direct evidence for functional smooth muscle myosin II in the 10S self-inhibited monomeric conformation in airway smooth muscle cells

The 10S self-inhibited monomeric conformation of myosin II has been characterized extensively in vitro. Based upon its structural and functional characteristics, it has been proposed to be an assembly-competent myosin pool in equilibrium with filaments in cells. It is known that myosin filaments can assemble and disassemble in nonmuscle cells, and in some smooth muscle cells, but whether or not the disassembled pool contains functional 10S myosin has not been determined. Here we address this question using human airway smooth muscle cells (hASMCs). Using two antibodies against different epitopes on smooth muscle myosin II (SMM), two distinct pools of SMM, diffuse, and stress-fiber-associated, were visualized by immunocytochemical staining. The two SMM pools were functional in that they could be interconverted in two ways: (i) by exposure to 10S- versus filament-promoting buffer conditions, and (ii) by exposure to a peptide that shifts the filament-10S equilibrium toward filaments in vitro by a known mechanism that requires the presence of the 10S conformation. The effect of the peptide was not due to a trivial increase in SMM phosphorylation, and its specificity was demonstrated by use of a scrambled peptide, which had no effect. Based upon these data, we conclude that hASMCs contain a significant pool of functional SMM in the 10S conformation that can assemble into filaments upon changing cellular conditions. This study provides unique direct evidence for the presence of a significant pool of functional myosin in the 10S conformation in cells.

Multiple regulatory steps control mammalian nonmuscle myosin II assembly in live cells

To better understand the mechanism controlling nonmuscle myosin II (NM-II) assembly in mammalian cells, mutant NM-IIA constructs were created to allow tests in live cells of two widely studied models for filament assembly control. A GFP-NM-IIA construct lacking the RLC binding domain (DeltaIQ2) destabilizes the 10S sequestered monomer state and results in a severe defect in recycling monomers during spreading, and from the posterior to the leading edge during polarized migration. A GFP-NM-IIA construct lacking the nonhelical tailpiece (Deltatailpiece) is competent for leading edge assembly, but overassembles, suggesting defects in disassembly from lamellae subsequent to initial recruitment. The Deltatailpiece phenotype was recapitulated by a GFP-NM-IIA construct carrying a mutation in a mapped tailpiece phosphorylation site (S1943A), validating the importance of the tailpiece and tailpiece phosphorylation in normal lamellar myosin II assembly control. These results demonstrate that both the 6S/10S conformational change and the tailpiece contribute to the localization and assembly of myosin II in mammalian cells. This work furthermore offers cellular insights that help explain platelet and leukocyte defects associated with R1933-stop alleles of patients afflicted with human MYH9-related disorder.

The N-terminal lobes of both regulatory light chains interact with the tail domain in the 10 S-inhibited conformation of smooth muscle myosin

In the presence of ATP, unphosphorylated smooth muscle myosin can form a catalytically inactive monomer that sediments at 10 Svedbergs (10 S). The tail of 10 S bends into thirds and interacts with the regulatory domain. ADP-P(i) is "trapped" at the active site, and consequently the ATPase activity is extremely low. We are interested in the structural basis for maintenance of this off state. Our prior photocross-linking work with 10 S showed that tail residues 1554-1583 are proximal to position 108 in the C-terminal lobe of one of the two regulatory light chains ( Olney, J. J., Sellers, J. R., and Cremo, C. R. (1996) J. Biol. Chem. 271, 20375-20384 ). These data suggested that the tail interacts with only one of the two regulatory light chains. Here we present data, using a photocross-linker on position 59 on the N-terminal lobe of the regulatory light chain (RLC), demonstrating that both regulatory light chains of a single molecule can cross-link to the light meromyosin portion of the tail. Mass spectrometric data show four specific cross-linked regions spanning residues 1428-1571 in the light meromyosin portion of the tail, consistent with cross-linking two RLC to one light meromyosin. In addition, we find that position 59 can cross-link internally to residues 42-45 within the same RLC subunit. The internal cross-link only forms in 10 S and not in unphosphorylated heavy meromyosin (lacking the light meromyosin), suggesting a structural rearrangement within the RLC attributed to the interaction of the tail with the head.

Regulation of myosin-VI targeting to endocytic compartments

Myosin-VI has been implicated in endocytic trafficking at both the clathrin-coated and uncoated vesicle stages. The identification of alternative splice forms led to the suggestion that splicing defines the vesicle type to which myosin-VI is recruited. In contrast to this hypothesis, we find that in all cell types examined, myosin-VI is associated with uncoated endocytic vesicles, regardless of splice form. GIPC, a PDZ-domain containing adapter protein, co-assembles with myosin-VI on these vesicles. Myosin-VI is only recruited to clathrin-coated vesicles in cells that express high levels of Dab2, a clathrin-binding adapter protein. Overexpression of Dab2 is sufficient to reroute myosin-VI to clathrin-coated pits in cells where myosin-VI is normally associated with uncoated vesicles. In normal rat kidney (NRK) cells, which express high endogenous levels of Dab2, splicing of the globular tail domain further modulates targeting of ectopically expressed myosin-VI. Although myosin-VI can be recruited to clathrin-coated pits, we find no requirement for myosin-VI motor activity in endocytosis in NRK cells. Instead, our data suggest that myosin-VI recruitment to clathrin-coated pits may be an early step in the recruitment of GIPC to the vesicle surface.

Novel sensors of the regulatory switch on the regulatory light chain of smooth muscle Myosin

Smooth muscle myosin can be switched on by phosphorylation of Ser-19 of the regulatory light chain. Our previous photocross-linking results suggested that an element of the structural mechanism for the regulatory switch was a phosphorylation-induced motion of the regulatory light chain N terminus (Wahlstrom, J. L., Randall, M. A., Jr., Lawson, J. D., Lyons, D. E., Siems, W. F., Crouch, G. J., Barr, R., Facemyer, K. C., and Cremo, C. R. (2003) J. Biol. Chem. 278, 5123-5131). Here we used three different approaches to test this notion, which are reactivity of cysteine thiols, pyrene and acrylodan spectral analysis, and pyrene fluorescence quenching. All methods detected significant differences between the unphosphorylated and phosphorylated regulatory light chain N termini in heavy meromyosin, a double-headed subfragment with an intact regulatory switch. These differences were not observed for subfragment-1, a single-headed, unregulated subfragment. In the presence of either ATP or ADP, phosphorylation increased the solvent exposure and decreased the polarity of the environment about position 23 of the regulatory light chain of heavy meromyosin. These phosphorylation-induced structural changes were not as evident in the absence of nucleotides. Nucleotide binding to unphosphorylated heavy meromyosin caused a decrease in exposure and an increase in polarity of the N terminus, whereas the effects of nucleotide on phosphorylated heavy meromyosin were the opposite. We showed a direct correlation between the kinetics of nucleotide binding/turnover and the conformational change reported by acrylodan at position 23 of the regulatory light chain. Acrylodan-A23C also reports the heads up (extended) to flexed (folded) transition in unphosphorylated heavy meromyosin. This is the first demonstration of direct coupling of nucleotide binding to conformational changes in the N terminus of the regulatory light chain.

Cryo-atomic force microscopy of unphosphorylated and thiophosphorylated single smooth muscle myosin molecules

The purpose of this study was to determine whether steric blockage of one head by the second head of native two-headed myosin was responsible for the inactivity of nonphosphorylated two-headed myosin compared with the high activity of single-headed myosin, as suggested on the basis of electron microscopy of two-dimensional crystals of heavy meromyosin (Wendt, T., Taylor, D., Messier, T., Trybus, K. M., and Taylor, K. A. (1999) J. Cell Biol. 147, 1385-1390; and Wendt, T., Taylor, D., Trybus, K. M., and Taylor, K. (2001) Proc. Natl. Acad. Sci. U. S. A. 98, 4361-4366). Our earlier cryo-atomic force microscopy (cryo-AFM) (Zhang, Y., Shao, Z., Somlyo, A. P., and Somlyo, A. V. (1997) Biophys. J. 72, 1308-1318) indicates that thiophosphorylation of the regulatory light chain increases the separation of the two heads of a single myosin molecule, but the thermodynamic probability of steric hindrance by strong binding between the two heads was not determined. We now report this probability determined by cryo-AFM of single whole myosin molecules shown to have normal low ATPase activity (0.007 s-1). We found that the thermodynamic probability of the relative head positions of nonphosphorylated myosin was approximately equal between separated heads as compared with closely apposed heads (energy difference of 0.24 kT (where k is a Boltzman constant and T is the absolute temperature)), and thiophosphorylation increased the number of molecules having separated heads (energy advantage of -1.2 kT (where k is a Boltzman constant and I is the absolute temperature)). Our results do not support the suggestion that strong binding of one head to the other stabilizes the blocked conformation against thermal fluctuations resulting in steric blockage that can account for the low activity of nonphosphorylated two-headed myosin.

Extraction and reconstitution of calponin and consequent contractile ability in permeabilized smooth muscle fibers

We demonstrate reduction and restoration of contractile ability in response to protein extraction and reconstitution in Triton X-100/glycerol-permeabilized smooth muscle fibers. Through significant reduction in the content of caldesmon (CaD), calponin (CaP), and the 20-kDa regulatory light chain (RLC) of myosin, but not other contractile proteins in "chemically skinned" fibers, we substantially reduced the contractile ability of these fibers, as measured by their ability to generate isometric force and to hydrolyze ATP by actomyosin Mg2+ ATPase. When the protein-depleted fibers were then reconstituted (either with a mixture of purified protein standards of CaD, CaP, and myosin RLC or with a protein extract from the demembranized muscle fibers containing CaD, CaP, and myosin RLC plus several low-molecular-mass proteins), all proteins used for reincorporation returned nearly to control levels, as did isometric force generation and rate of ATP hydrolysis. The fact that the low-molecular-mass proteins do not affect contractility in this model system indicates that our methods for reversible modulation of the content of CaP and CaD may provide a valuable tool for studying the thin-filament-based regulation of contractility.

Structural model of the regulatory domain of smooth muscle heavy meromyosin

The goal of this study was to provide structural information about the regulatory domains of double-headed smooth muscle heavy meromyosin, including the N terminus of the regulatory light chain, in both the phosphorylated and unphosphorylated states. We extended our previous photo-cross-linking studies (Wu, X., Clack, B. A., Zhi, G., Stull, J. T., and Cremo, C. R. (1999) J. Biol. Chem. 274, 20328-20335) to determine regions of the regulatory light chain that are cross-linked by a cross-linker attached to Cys(108) on the partner regulatory light chain. For this purpose, we have synthesized two new biotinylated sulfhydryl reactive photo-cross-linking reagents, benzophenone, 4-(N-iodoacetamido)-4'-(N-biotinylamido) and benzophenone, 4-(N-maleimido)-4'-(N-biotinylamido). Cross-linked peptides were purified by avidin affinity chromatography and characterized by Edman sequencing and mass spectrometry. Labeled Cys(108) from one regulatory light chain cross-linked to (71)GMMSEAPGPIN(81), a loop in the N-terminal half of the regulatory light chain, and to (4)RAKAKTTKKRPQR(16), a region for which there is no atomic resolution data. Both cross-links were to the partner regulatory light chain and occurred in unphosphorylated but not phosphorylated heavy meromyosin. Using these data, data from our previous study, and atomic coordinates from various myosin isoforms, we have constructed a structural model of the regulatory domain in an unphosphorylated double-headed molecule that predicts the general location of the N terminus. The implications for the structural basis of the phosphorylation-mediated regulatory mechanism are discussed.

A fluorescent resonant energy transfer-based biosensor reveals transient and regional myosin light chain kinase activation in lamella and cleavage furrows

Approaches with high spatial and temporal resolution are required to understand the regulation of nonmuscle myosin II in vivo. Using fluorescence resonance energy transfer we have produced a novel biosensor allowing simultaneous determination of myosin light chain kinase (MLCK) localization and its [Ca2+]4/calmodulin-binding state in living cells. We observe transient recruitment of diffuse MLCK to stress fibers and its in situ activation before contraction. MLCK is highly active in the lamella of migrating cells, but not at the retracting tail. This unexpected result highlights a potential role for MLCK-mediated myosin contractility in the lamella as a driving force for migration. During cytokinesis, MLCK was enriched at the spindle equator during late metaphase, and was maximally activated just before cleavage furrow constriction. As furrow contraction was completed, active MLCK was redistributed to the poles of the daughter cells. These results show MLCK is a myosin regulator in the lamella and contractile ring, and pinpoints sites where myosin function may be mediated by other kinases.

Functional regions in the essential light chain of smooth muscle myosin as revealed by the mutagenesis approach

The endogenous essential light chain (LC17) of myosin from intestine smooth muscle was replaced with mutated essential light chains prepared using recombinant techniques. Complete exchange was observed with histidine-tagged derivatives of LC17a, LC17b and E122A-LC17a (LC17a and LC17b are the usual constituants of smooth muscle myosin), with small changes in the ATPase activity of reconstituted myosins. Much less exchange was observed with the light-chain derivative lacking the last 12 amino acid residues, demonstrating the importance of this segment, which may act as one arm of a pair of pincers to bind the myosin heavy chain.

Calponin binds to the 20-kilodalton regulatory light chain of myosin

Calponin (CaP) is a 34 kDa smooth muscle-specific protein that has been implicated in regulation of smooth muscle contractility. Two CaP binding sites on smooth muscle myosin rod have been recently described [Szymanski and Tao (1997) J.Biol.Chem. 272, 11142-11146]. We used a combination of cosedimentation, overlay, and fluorescence assays to determine the interaction between CaP and both subfragment 1 of myosin and isolated 20 kDa regulatory light chain of myosin (RLC). Subfragment 1, which was generated by cleavage of myosin with Staphylococcus aureus protease (myosin S1SA) inhibits cosedimentation of CaP with myosin filaments. Fluorescence assay showed that CaP labeled with fluorescent label (DAN-CaP) interacts with myosin S1SA in solution via a single class of binding sites. The binding constant (kaff) of this interaction at 50 mM NaCl is (2. 1 +/- 0.2) x 10(6) M-1 (n = 3). The interaction between DAN-CaP and myosin S1SA depends on ionic strength, and the EC50 of inhibition of this interaction occurs at about 130 mM NaCl. In contrast, the subfragment 1 that was generated by papain digestion (myosin S1PA), which cleaves RLC 4 kDa away from the NH2-terminal end of the molecule, does not interact with DAN-CaP. Overlay and fluorescent assay in solution showed that CaP binds to isolated RLC, suggesting that the interaction between CaP and subfragment 1 of myosin is due to a direct binding of CaP to RLC. CaP binding to myosin S1SA is stronger than to subfragment 2 in physiological salt concentrations. CaP binding to myosin head strengthened upon phosphorylation of RLC by Ca2+/calmodulin-dependent myosin light chain kinase. We suggest that CaP binds to subfragment 1 of myosin, exclusively via the NH2-terminal end of RLC, and this interaction could play a role in regulation of the actin-myosin interaction in smooth muscle contractility.

The contractile apparatus of airway smooth muscle. Biophysics and biochemistry

Qualitatively the mechanical, structural, and biochemical properties of airway smooth muscles resemble those of all other smooth muscle. However, one important distinguishing feature of airway smooth muscle is that the major portion of isotonic shortening is completed within the first 3 s in a muscle whose contraction is 10 s. This indicates the importance of focusing on the changes that occur in these 3 s and also the limiting role of the maximum velocity of shortening in determining shortening data. There is evidence that the maximum capacity and velocity of shortening in human bronchial smooth muscle from patients with asthma are significantly greater than those obtained from healthy siblings. In the demonstration in which cells in culture are arrested by withdrawing all fetal calf serum, the cells alter their phenotype to cells that are very long (more than 200 micrometers) and shorten twice as much as cells freshly isolated when the tissue is new. Speculatively, if such cells developed in vivo they could account for the increased contractility of asthmatic airway smooth muscle. These cultured cells could also be excellent models for study of airway smooth muscle contractility.

Skeletal muscle myosin monomer in equilibrium with filaments forms a folded conformation

Rabbit skeletal myosin forms stable filaments under physiological conditions, and only a small amount stays as a monomer in equilibrium with filaments. The myosin monomers were observed in two conformational states, as extended and folded forms upon electron microscopy and gel filtration high performance liquid chromatography. The fraction of monomers in the folded conformation increased with a decrease in the concentration of NaCl below 0.2 M, and the conformational state was affected neither by the presence of ATP nor by the phosphorylation of regulatory light chain. In most of the folded monomers, the tail bent back toward the heads at one region, 45 nm apart from the head-tail junction, and the remaining tail portion containing the C-terminal tip appeared to interact with the head-tail junction. Only a small percentage of the folded monomers was in a more compact conformation close to the 10 S conformation of vertebrate smooth muscle and non-muscle myosins. The folded monomers, however, may not trap the products of ATP hydrolysis as assessed by single turnover experiments. The percentage of monomers in the 10 S-like conformation was increased by the exchange of a regulatory light chain with the smooth muscle light chain, indicating the participation of head-tail junction, including the regulatory light chain in the formation of folded conformation. The folded conformation may be common to various myosin IIs, suggestive of common roles for the folded monomers.

Asymmetry in the distribution of free versus cytoskeletal myosin II in locomoting microcapillary endothelial cells

Myosin II is required for normal amoeboid locomotion. In order to understand how myosin II elicits its effects on locomotive behavior, we have mapped myosin II-cytoskeleton interactions in locomoting endothelial cells. Bovine microcapillary endothelial cells were microinjected with fluorescently labeled myosin II, and the distribution of myosin II was imaged in the living cells by fluorescence microscopy. The same cells were then permeabilized with Triton X-100 and imaged again. The second set of images showed only myosin II that was associated with detergent-insoluble cytoskeleton. Dividing the image of retained myosin II by that of total myosin II produced a map of the extent to which myosin II was associated with the detergent-resistant cytoskeleton at any point in the cell. In cells migrating at the edge of a scrape wound, myosin II was preferentially retained in a region approximately 10 microm wide located just behind the cells' leading lamellipodia. Relatively little myosin II was retained in perinuclear cytoplasm. A vector representation of the distribution of total versus retained myosin II demonstrated that myosin II retention was sharply polarized with respect to locomotion, favoring the front of migrating cells. Myosin II-enriched cytoskeleton in this region may help polarize protrusive activity and/or move cytoplasmic bulk forward. Patches of myosin II retention were also observed in adherent tails of many cells, consistent with a role in pulling the rear of the cell forward.

Substitution mutations in the myosin essential light chain lead to reduced actin-activated ATPase activity despite stoichiometric binding to the heavy chain

Myosin essential light chain (ELC) wraps around an alpha-helix that extends from the myosin head, where it is believed to play a structural support role. To identify other role(s) of the ELC in myosin function, we have used an alanine scanning mutagenesis approach to convert charged residues in loops I, II, III, and helix G of the Dictyostelium ELC into uncharged alanines. Dictyostelium was used as a host system to study the phenotypic and biochemical consequences associated with the mutations. The ELC carrying loop mutations bound with normal stoichiometry to the myosin heavy chain when expressed in ELC-minus cells. When expressed in wild type cells these mutants competed efficiently with the endogenous ELC for binding, suggesting that the affinity of their interaction with the heavy chain is comparable to that of wild type. However, despite apparently normal association of ELC the cells still exhibited a reduced efficiency to undergo cytokinesis in suspension. Myosin purified from these cells exhibited 4-5-fold reduction in actin-activated ATPase activity and a decrease in motor function as assessed by an in vitro motility assay. These results suggest that the ELC contributes to myosin's enzymatic activity in addition to providing structural support for the alpha-helical neck region of myosin heavy chain.

The motor domain and the regulatory domain of myosin solely dictate enzymatic activity and phosphorylation-dependent regulation, respectively

While the structures of skeletal and smooth muscle myosins are homologous, they differ functionally from each other in several respects, i.e., motor activities and regulation. To investigate the molecular basis for these differences, we have produced a skeletal/smooth chimeric myosin molecule and analyzed the motor activities and regulation of this myosin. The produced chimeric myosin is composed of the globular motor domain of skeletal muscle myosin (Met1-Gly773) and the C-terminal long alpha-helix domain of myosin subfragment 1 as well as myosin subfragment 2 (Gly773-Ser1104) and light chains of smooth muscle myosin. Both the actin-activated ATPase activity and the actin-translocating activity of the chimeric myosin were completely regulated by light chain phosphorylation. On the other hand, the maximum actin-activated ATPase activity of the chimeric myosin was the same as skeletal myosin and thus much higher than smooth myosin. These results show that the C-terminal light chain-associated domain of myosin head solely confers regulation by light chain phosphorylation, whereas the motor domain determines the rate of ATP hydrolysis. This is the first report, to our knowledge, that directly determines the function of the two structurally separated domains in myosin head.

Structure and function of the 10 S conformation of smooth muscle myosin

Smooth myosin regulatory light chain (RLC) was exchanged with RLC labeled with benzophenone-4-iodoacetamide at Cys-108. Irradiation under conditions that favor the folded (10 S) conformation resulted in 10 S cross-linked myosin that could not unfold. Purified 10 S cross-linked myosin was cross-linked between the RLC of one head to light meromyosin between leucine 1554 and glutamate 1583, adjacent to a predicted noncoiled region, approximately 60 nm from the tip of the tail. At high ionic strength without actin, product release from one-half of the heads was slow (like 10 S) whereas the other half were activated. This suggests that tail binding to the RLC carboxyl-terminal domain stabilizes ionic interactions important to slow nucleotide release. With actin, product release from both (un)phosphorylated 10 S cross-linked myosin was from one slow population similar to unphosphorylated filaments. 10 S cross-linked myosin weakly bound actin (dissociation constant > 500 microM) and did not move actin in vitro. Single-headed myosin did not fold or trap nucleotide. These and other data suggest that "trapping" occurs only with both heads and the tail binds to a newly formed site, which includes the RLC carboxyl-terminal domain, once trapping has occurred.

Roles of light chains in the activity and conformation of smooth muscle myosin

The 20-kDa regulatory (LC20) and 17-kDa essential (LC17) light chain subunits could be removed from porcine aorta smooth muscle myosin by the use of trifluoperazine and ammonium chloride. The isolated heavy chain rebound both light chains, resulting in the restoration of native properties. Experiments on reconstitution of the isolated heavy chain with LC17 and/or LC20 showed that both light chains were required for folding into the 10 S conformation and thus for the phosphorylation-dependent filament formation of smooth muscle myosin. However, LC17 was not essential for the phosphorylation-dependent regulation of actin-activated ATPase activity and superprecipitation but was required for full regulation. LC17 and phosphorylated LC20 were found to act as activators, and dephosphorylated LC20 was found to act as a repressor of the motor activities of smooth muscle myosin.

The effect of cross-linking of the two heads of porcine aorta smooth muscle myosin on its conformation and enzymic activity

The two heads of porcine aorta smooth muscle myosin can be cross-linked by a disulfide bridge between the two 17-kDa essential light chains with 5,5'-dithiobis(2-nitrobenzoic acid) [Katoh, T., Tanahashi, K., Hasegawa, Y. & Morita, F. (1995) Eur. J. Biochem. 227, 459-465]. When the cross-linked myosin sample was visualized by rotary shadowing, the two heads of myosin molecules appeared predominantly to adhere to each other. The cross-linking of dephosphorylated myosin in the presence of ATP was greatly inhibited by a decrease in the concentration of NaCl from 0.4 M to 0.15 M, suggesting that the cross-linking of the two heads was suppressed in 10S myosin. However, the fraction of dephosphorylated myosin in a filamentous state at 0.1 M NaCl in the presence of 1 mM ATP was increased from 33% to 83% by the cross-linking. The cross-linking of the two heads might inhibit the formation of the 10S conformation, leading to the increase in the fraction of filamentous myosin. The filaments of the cross-linked myosin sample were visualized by electron microscopy and appeared morphologically similar to those of uncross-linked myosin. The ATPase activity of the cross-linked dephosphorylated myosin sample was more than three times as high as that of an uncross-linked control. The increase in the activity may be related to the increase in the fraction of filamentous myosin caused by the cross-linking. The ATPase activity of dephosphorylated myosin in the presence of actin was increased more than twofold by the cross-linking, but the activity of phosphorylated myosin was affected only slightly. The degree of phosphorylation-dependent regulation of actin-activated ATPase activity decreased with an increase in the degree of cross-linking and was extrapolated to zero at 100% cross-linking. Superprecipitation of acto-cross-linked dephosphorylated myosin was activated, while that of acto-cross-linked phosphorylated myosin was inhibited only slightly. These results suggest that the freedom of each head in myosin molecules may be required to keep the ATPase activity and superprecipitation of acto-dephosphorylated myosin low but not for keeping these activity levels high in acto-phosphorylated myosin.

Structure-function relationships in smooth muscle: the missing links

Smooth muscle cells have developed a contractile machinery that allows them to exert tension on the surrounding extracellular matrix over their entire length. This has been achieved by coupling obliquely organized contractile filaments to a more-or-less longitudinal framework of cytoskeletal elements. Earlier structural data suggested that the cytoskeleton was composed primarily of intermediate filaments and played only a passive role. More recent findings highlight the segregation of actin isotypes and of actin-associated proteins between the contractile and cytoskeletal domains and raise the possibility that the cytoskeleton performs a more active function. Current efforts focus on defining the relative contributions of myosin cross-bridge cycling and actin-associated protein interactions to the maintenance of tension in smooth muscle tissue.

Drosophila nonmuscle myosin II is required for rapid cytoplasmic transport during oogenesis and for axial nuclear migration in early embryos

The X-linked Drosophila gene spaghetti squash (sqh) encodes the regulatory light chain of nonmuscle myosin II. To assess the requirement for myosin II in oogenesis and early embryogenesis, we induced homozygous germline clones of the hypomorphic mutation sqh1 in otherwise heterozygous mothers. Developing oocytes in such sqh1 germline clones often failed to attain full size due to a defect in ‘dumping’, the rapid phase of cytoplasmic transport from nurse cells. In contrast to other dumpless mutants described to date, sqh1 egg chambers showed no evidence of ring canal obstruction, and no obvious alteration in the actin network. However the distribution of myosin II was abnormal. We conclude that the molecular motor responsible for cytoplasmic dumping is supplied largely, if not exclusively, by nurse cell myosin II and we suggest that regulation of myosin activity is one means by which cytoplasmic transport may be controlled during oocyte development. The eggs resulting from sqh1 clones, though smaller than normal, began development but exhibited an early defect in axial migration of cleavage nuclei towards the posterior pole of the embryo, in a similar manner to that seen in early cleavage eggs in which the actin cytoskeleton is disrupted. Thus both nurse cell dumping and axial migration require a maternally supplied myosin II.

Unfolding domains in smooth muscle myosin rod

Gizzard smooth muscle myosin rod, an alpha-helical coiled coil, exhibits two cooperative thermal or denaturant-induced helix unfolding transitions in solutions containing 0.6 M NaCl at neutral pH, when monitored by circular dichroism at 222 nm. The first smaller transition unfolds part of the subfragment 2 (S2) domain, and the main transition unfolds the remaining helix including the light meromyosin (LMM) domain. These unfolding domains were identified by monitoring the fluorescence of acrylodan, an environmentally sensitive fluorescence probe, and the ESR signal of a maleimide spin-label, sensitive to motion, both specifically attached to Cys 43 in the S2 region of the rod sequence. The identities of the domains were verified by studying the unfolding of the S2 and LMM coiled-coil peptides obtained by proteolytic cleavage of spin-labeled and unlabeled rod. The fluorescence of acrylodan-labeled rod indicated that although the S2 intermediate is unfolded, it is not in a random-coil conformation. The unfolded S2 region stabilized the LMM domain against unfolding, possibly by a direct interaction with the LMM region. Such an interaction may be involved in the salt- and phosphorylation-dependent 6S to 10S shift in configuration of the myosin molecule.

Two heads are required for phosphorylation-dependent regulation of smooth muscle myosin

Recent structural evidence (Rayment, I., Holden, H. M., Whittaker, M., Yohn, C. B., Lorenz, M., Holmes, K. C., and Milligan, R. A. (1993) Science 261, 58-65) suggests that the two heads of skeletal muscle myosin interact when the protein is bound to filamentous actin. Direct chemical cross-linking experiments show that the two heads of smooth muscle myosin interact in the presence of filamentous actin and the absence of ATP (Onishi, H., Maita, T., Matsuda, G., and Fujiwara, K. (1992) Biochemistry 31, 1201-1210). Head-head interactions may be important in the mechanism of phosphorylation-dependent regulation of smooth muscle myosin. To explore the structural elements essential for phosphorylation-dependent regulation, we purified a proteolytic fragment of chicken gizzard myosin containing only one head attached to an intact tail. This molecule contained a partially digested regulatory light chain, which was replaced with exogenously added intact light chain in either the thiophosphorylated or the unphosphorylated state. Control experiments showed that this replacement was nearly quantitative and did not alter the actin-activated ATPase of this myosin. Electron micrographs confirmed that the single-headed preparation contained an intact form of single-headed myosin. The unphosphorylated single-headed myosin hydrolyzed ATP rapidly and moved actin filaments in an in vitro motility assay. Phosphorylation had minimal effects upon these properties. Therefore, we conclude that phosphorylation-dependent regulation in this myosin requires two heads. These findings may have important implications in studies of other regulated motor proteins that contain two motor domains.

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

Light chain binding domains of rabbit skeletal, turkey gizzard and scallop myosin comprised of equimolar amounts of a short heavy chain fragment, essential light chain, and regulatory light chain have been obtained following extensive tryptic digestion. These complexes that are analogous to the regulatory domain prepared previously from scallop myosin by digestion with clostripain resist proteolysis due to the mutual protection of the heavy chain and the light chains, and are common structural features of the myosins studied. Specific Ca(2+)-binding by the regulatory domains reflects the behaviour of intact myosin; only scallop regulatory domain has a specific Ca(2+)-binding site. The heavy chain fragments of the different regulatory domains have been isolated under denaturing conditions and reconstituted with scallop essential light chain and scallop regulatory light chain or turkey gizzard regulatory light chain to yield regulatory domain hybrids. Hybrids containing the turkey gizzard regulatory light chain were used in Ca(2+)-binding studies since they were far more stable than their counterparts with the scallop regulatory light chain. The gizzard hybrid binds Ca2+ with a comparable specificity but somewhat lower affinity than native scallop regulatory domain. The rabbit regulatory domain hybrid also binds Ca2+, although with a reduced affinity and specificity. The results indicate that Ca(2+)-binding ability is determined by the light chains and modified by the heavy chains.

Mutagenesis of the phosphorylation site (serine 19) of smooth muscle myosin regulatory light chain and its effects on the properties of myosin

A full-length cDNA of smooth muscle regulatory light chain was obtained and the recombinant regulatory light chain was expressed in an Escherichia coli expression system. The recombinant regulatory light chain was introduced into myosin or HMM using a subunit exchange strategy [Morita, J., Takashi, R., & Ikebe, M. (1991) Biochemistry 30, 9539-9545]. The recombinant wild-type regulatory light chain exhibited the same biological properties as the natural isolate, i.e., phosphorylation at Ser-19 by myosin light-chain kinase and phosphorylation-activated actomyosin ATPase activity. To clarify whether or not the activation of the ATPase by phosphorylation is simply due to the introduction of negative charge, we produced three mutant light chains. Two of them contain Ser-19 substituted by either Asp or Ala and the third contains Asp substituted for both Thr-18 and Ser-19. Incorporation of the Asp mutant partially activated actomyosin ATPase activity but the activation level was significantly lower than that by phosphorylation. The Asp/Asp mutant further activated actomyosin ATPase activity. On the other hand, the Ala mutant did not affect the ATPase activity. Incorporation of Asp mutant slightly affected the 10S-6S conformational transition and filament formation of myosin. The Asp/Asp mutant more significantly affected the 10S-6S conformational transition and filament formation of myosin. These results suggested that the activation of smooth muscle myosin requires the introduction of negative charge in the defined spacial position. Using Ser-19 deficient mutants, the effects of Thr-18 phosphorylation on myosin function was also studied. Actin-activated ATPase activity of myosin was significantly activated by phosphorylation of Thr-18.(ABSTRACT TRUNCATED AT 250 WORDS)

Smooth muscle myosin subfragment-1 is a kinetic analogue for heavy meromyosin in the extended conformation

The 10S-->6S (Flexed-->Extended) transition in smooth muscle myosin is related to increased ATPase activity, but there is controversy over whether the analogous 9S-->7S transition in HMM is also associated with ATPase activity. We therefore studied the association of ionic strength, phosphorylation, and ATPase activity for HMM as compared to S1 which has no apparent flexed conformation. In addition, we performed both steady state and single turnover analyses, to control for artifacts due to multiple subfragment populations that might skew steady state results. At low ionic strength where myosin and HMM are in the flexed conformation, HMM had a near zero ATPase activity while S-1 had a high ATPase rate (0.07 s-1). At 400 mM ionic strength, where both myosin and HMM are in the extended conformation, S1 and HMM had the same ATPase rate (0.04 s-1). Phosphorylation did not affect S1 significantly, but shifted the HMM curve to higher rates at lower ionic strengths. Both steady state and single turnover experiments gave the same results, indicating that steady state results were not skewed by multiple subfragment populations. These data indicate that HMM has a conformation-ATPase relation similar to that observed with myosin. Furthermore, these findings suggest that the S1 ATPase rate corresponds to that of HMM in the extended conformation.

Role of gizzard myosin light chains in calcium binding

The contraction of molluscan and vertebrate smooth muscles is regulated by myosin. Although the myosin and its associated two subunits, the regulatory light chain and the essential light chain, constitute the Ca2+ regulatory system in both types of muscles, the mechanisms by which Ca2+ signal is transduced are quite different. In molluscan muscles, the direct binding of Ca2+ to the regulatory system triggers muscle contraction. In vertebrate smooth muscles, however, phosphorylation of the regulatory light chain is the major triggering mechanism. We measured Ca2+ binding in gizzard myosin and in hybrids of scallop myosin containing gizzard regulatory light chain or in hybrids of scallop regulatory domain containing gizzard essential light chain. Isolated chicken gizzard myosin did not bind Ca2+ in the range of pCa 8.0 to 5.0 in the presence of 2 mM MgCl2, supporting the lack of the specific Ca(2+)-binding site in gizzard myosin. Phosphorylation of the regulatory light chain did not generate a specific (Ca2+)-binding site. The hybrid scallop myosin containing gizzard regulatory light chain showed a similar Ca2+ binding as native scallop myosin with a one to one stoichiometry of Ca2+ to myosin head saturating at about pCa 6.0 at pH 7.6. In contrast, the hybrid scallop regulatory domain containing gizzard essential light chain did not bind Ca2+ either at pCa 6.0 or at pCa 8.0. Control preparations reconstituted with scallop essential light chains bound 0.69 mol per mol Ca2+ at pCa 6.0 with no binding at pCa 8.0.(ABSTRACT TRUNCATED AT 250 WORDS)

Polylysine activates smooth muscle actin-myosin interaction without LC20 phosphorylation

Phosphorylation/dephosphorylation of the 20-kDa light chain of smooth muscle myosin is a major regulator of actin-myosin interaction. Phosphatase inhibitors have thus been shown to enhance contraction in smooth muscle. The activity of type II phosphatase against phosphorylated myosin light chains is inhibited by polylysine. Thus we studied the effects of polylysine (10-13 kDa) on actin-myosin interaction in permeabilized guinea pig taenia coli fibers and in bovine aortic actomyosin. Addition of polylysine (10-20 microM) to Ca-ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid buffered solution ([Ca2+] less than 0.01 microM) elicited a contraction in fibers of 40 +/- 8% (n = 6) of maximally stimulated contractions ([Ca2+] congruent to 1.5 microM). Untreated fibers did not generate any significant force in parallel control experiments. Similarly, polylysine stimulated the ATPase activity both in fibers and actomyosin in a dose-dependent manner. This stimulation could be completely inhibited and abolished upon addition of heparin, a negatively charged heteropolysaccharide. In actomyosin previously phosphorylated with ATP gamma S, polylysine in a concentration range of 2-13 microM did not further stimulate enzyme activity. These increases in activity were not connected with significant changes in the phosphorylation of 20-kDa myosin light chain nor could any incorporation of 32P associated with polylysine stimulation be detected in both skinned fibers and actomyosin by autoradiography of SDS gels. Our data indicate that polylysine increases actin-myosin interaction in both smooth muscle model systems by directly influencing contractile proteins. As such, polylysine may be a useful probe for the mechanism of activation of smooth muscle.

LC20 and kinetics of gizzard myosin subfragment-1: digestion with papain vs. S. aureus protease

Previous reports have shown that papain-digested gizzard subfragment-1 (PAP-S1) has a cleaved regulatory light chain (LC20), and Vmax similar to phosphorylated heavy meromyosin (HMM) (Greene et al., Biochemistry 22:530-535, 1983; Sellers et al., J. Biol. Chem. 257:13880-13883, 1982; Umemoto et al., J. Biol. Chem. 264:1431-1436, 1989], while S. aureus protease-digested S-1 (SAP-S1) has intact LC20, but Vmax closer to that of unphosphorylated HMM [Ikebe and Hartshorne, 1985]. To determine whether intact LC20 inhibits ATPase activity for subfragment-1 (S1), we compared the kinetic properties and structures of unphosphorylated PAP-S1 and SAP-S1. SDS-PAGE showed that SAP-S1 had 68 and 24 KDa heavy chain and 20 and 17 KDa light chain components. PAP-S1 (15 minutes digestion at 20 degrees C) also had 68 and 17 KDa bands, but the single 24 KDa band (24HC) was replaced by a group of 22-24 KDa fragments and LC20 was cleaved to a 16 KDa fragment. At 13 mM ionic strength, both PAP-S1 and SAP-S1 had Vmax similar to phosphorylated HMM (1.1-1.5 s-1). SAP-S1 had the same KATPase as phosphorylated HMM (38 microM actin), but KATPase for PAP-S1 was 3-fold stronger (11 microM actin). Subsequent digestion of SAP-S1 with papain did not significantly change Vmax, but as LC20 and 24HC were cleaved, both KATPase and Kbinding strengthened 3- to 5-fold. Thus, intact LC20 did not inhibit, and cleavage of LC20 did not increase Vmax for S1. Rather, papain cleavage of LC20 and 24HC was associated with strengthened actin binding.

Formation and properties of smooth muscle myosin 20-kDa light chain-skeletal muscle myosin hybrids and photocrosslinking from the maleimidylbenzophenone-labeled light chain to the heavy chain

Experimental conditions which permit the exchange of smooth muscle 20-kDa light chain into skeletal muscle myosin are described. The hybridization does not result in the regulation of actin-activated ATPase activity of the hybrid myosin by smooth light chain phosphorylation. Further, the KCl dependence of the Mg-ATPase activity of the hybrid was similar to that of skeletal muscle myosin. The dephosphorylation of the smooth light chain in the hybrid did not induce a conformational change in the hybrid from the 6 S to the 10 S state, thereby indicating that the conformational transition is dependent also on the nature of the heavy chain subunit. Exchange of the smooth light chain premodified at its Cys-108 by photolabile 4-(N-maleimido)benzophenone and photolysis resulted in crosslinking to the heavy chain subunit. Immunopeptide mapping using a monoclonal antibody against residues 1-23 at the N-terminus of the skeletal muscle myosin heavy chain identified the location of the photocrosslinking site to be beyond 92 kDa away from the N-terminus.

A folded (10 S) conformer of myosin from a striated muscle and its implications for regulation of ATPase activity

Myosin from the striated adductor muscle of the scallop Pecten maximus is shown to fold into a compact 10 S conformer under relaxing conditions, as has been characterized for smooth and non-muscle myosins. The folding transition is accompanied by the trapping of nucleotide at the active site to give a species with a half-life of about an hour at 20 degrees C. Ca2+ binding to the specific, regulatory sites on a myosin head promotes unfolding to the extended 6 S conformer and activates product release by 60-fold. The unfolding transition, however, remains much slower than the contraction-relaxation cycle of scallop striated muscle and could not play a role in the regulation of these events. The dissociation of products from myosin heads in native thick filaments is Ca2(+)-regulated, but under relaxing conditions the nucleotide is released at least an order of magnitude faster than from the 10 S monomeric myosin, at a rate similar to that observed with heavy meromyosin. Thus, there is no evidence for any intermolecular interaction between neighbouring molecules in the filament analogous to the head-neck intramolecular interaction in the 10 S conformer. It is possible that the 10 S myosin state represents an inert form involved in the control of filament assembly during muscle growth and development. Removal of regulatory light chains or labelling the reactive heavy chain thiol of myosin prevents, or at least disfavours, formation of the folded 10 S conformer and allows separation of the modified protein from the native molecules.

Reaction of thiol groups of gizzard myosin heavy chains with 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole

Chicken gizzard myosin treated with 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) resulted in a 65% inhibition of the K(+)-ATPase (myosin ATP phosphohydrolase (actin translocating), EC 3.6.1.32) activity and 3.5 mol of the reagent was bound per 4.7 x 10(5) g protein. The labeling was limited to the heavy chain region and none of the light chains were lost. MgATP had no effect on the inactivation or labeling pattern. Thiolysis of NBD-myosin with dithiothreitol restored the K(+)-ATPase activity and concurrently, 1 mol of the NBD group was removed from the heavy chain region. Cysteine residues were modified in NBD-myosin at sites other than the active site when the enzyme activity was inhibited. There was a difference in the extent of NBD-Cl modification of gizzard myosin at 0.6 M KCl (6 S elongated state) when compared to that at 0.15 M KCl (10 S folded state). This was also seen in the heavy meromyosin-like chymotryptic fragments and tryptic fragments of NBD-myosin. The reagent NBD-Cl can detect changes in the conformation of gizzard myosin by way of its reaction with thiol groups of the heavy chain region.