Regulation in vitro of brush border myosin by light chain phosphorylation

Combining AFM and acoustic probes to reveal changes in the elastic stiffness tensor of living cells

Knowledge of how the elastic stiffness of a cell affects its communication with its environment is of fundamental importance for the understanding of tissue integrity in health and disease. For stiffness measurements, it has been customary to quote a single parameter quantity, e.g., Young's modulus, rather than the minimum of two terms of the stiffness tensor required by elasticity theory. In this study, we use two independent methods (acoustic microscopy and atomic force microscopy nanoindentation) to characterize the elastic properties of a cell and thus determine two independent elastic constants. This allows us to explore in detail how the mechanical properties of cells change in response to signaling pathways that are known to regulate the cell's cytoskeleton. In particular, we demonstrate that altering the tensioning of actin filaments in NIH3T3 cells has a strong influence on the cell's shear modulus but leaves its bulk modulus unchanged. In contrast, altering the polymerization state of actin filaments influences bulk and shear modulus in a similar manner. In addition, we can use the data to directly determine the Poisson ratio of a cell and show that in all cases studied, it is less than, but very close to, 0.5 in value.

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.

Characterization of myosin-II binding to Golgi stacks in vitro

In addition to important roles near the actin-rich cell cortex, ample evidence indicates that multiple myosins are also involved in membrane movements in the endomembrane system. Nonmuscle myosin-II has been shown to have roles in anterograde and retrograde trafficking at the Golgi. Myosin-II is present on Golgi stacks isolated from intestinal epithelial cells and has been localized to the Golgi in several polarized and unpolarized cell lines. An understanding of roles of myosin-II in Golgi physiology will be facilitated by understanding the molecular arrangement of myosin-II at the Golgi. Salt-washing removes endogenous myosin-II from isolated Golgi and purified brush border myosin-II can bind in vitro. Brush border myosin-II binds to a tightly bound Golgi peripheral membrane protein with a K(1/2) of 75 nM and binding is saturated at 0.7 pmol myosin/microg Golgi. Binding studies using papain cleavage fragments of brush border myosin-II show that the 120-kDa rod domain, but not the head domain, of myosin heavy chain can bind directly to Golgi stacks. The 120-kDa domain does not bind to Golgi membranes when phosphorylated in vitro with casein kinase-II. These results suggest that phosphorylation in the rod domain may regulate the binding and/or release of myosin-II from the Golgi. These data support a model in which myosin-II is tethered to the Golgi membrane by its tail and actin filaments by its head. Thus, translocation along actin filaments may extend Golgi membrane tubules and/or vesicles away from the Golgi complex.

Evidence for a functional interaction between cingulin and ZO-1 in cultured cells

Cingulin, a protein component of the submembrane plaque of tight junctions (TJ), contains globular and coiled-coil domains and interacts in vitro with several TJ and cytoskeletal proteins, including the PDZ protein ZO-1. Overexpression of Xenopus cingulin in transfected Xenopus A6 cells resulted in the disruption of endogenous ZO-1 localization, suggesting that cingulin functionally interacts with ZO-1. Glutathione S-transferase pull-down experiments showed that a conserved ZO-1 interaction motif (ZIM) at the NH(2) terminus of cingulin is required for cingulin-ZO-1 interaction in vitro. An NH(2)-terminal region of cingulin, containing the ZIM, was sufficient, when fused to coiled-coil sequences, to target transfected cingulin to junctions. However, deletion of the ZIM did not abolish junctional localization of transfected cingulin in A6 cells, suggesting that cingulin can be recruited to TJ through multiple protein interactions. Interestingly, the ZIM was required for cingulin recruitment into ZO-1-containing adherens junctions of Rat-1 fibroblasts, indicating that cingulin junctional recruitment does not require the molecular context of TJ. Cingulin coiled-coil sequences enhanced the junctional accumulation of expressed cingulin head region in A6 cells, but purified recombinant cingulin did not form filaments under physiological conditions in vitro, suggesting that the cingulin coiled-coil domain acts primarily by promoting dimerization.

Macrophage caldesmon is an actin bundling protein

A rapid purification procedure was developed for the isolation of caldesmon (CaD) from rabbit alveolar macrophage. The purified protein migrated with an apparent M(r) of 74,000 +/- 4000 on SDS-PAGE and cross-reacted with anti-gizzard CaD antibodies. A higher M(r) isoform was isolated from chicken gizzard. Their actin-binding parameters and effects on actomyosin-ATPase activity were investigated under identical experimental conditions. Electron microscope studies revealed that macrophage CaD was able to cross-link actin filaments into both networks and bundles. Compact F-actin bundles were predominantly or exclusively seen at cross-linker to actin molar ratios in the 1:20 to 1:10 range. Apparent K(a) at extrapolated saturation of the CaD-binding sites on F-actin was 1.2 x 10(6) M(-1) for macrophage CaD and 1.6 x 10(6) M(-1) for chicken gizzard CaD. CaD from either source was able to stimulate the actin-activated ATPase activity of macrophage myosin. Unexpectedly, chicken gizzard CaD also increased the ATPase activity of gizzard myosin. The degree of stimulation was approximately doubled in the presence of a large excess of Ca(2+)-calmodulin but was unaffected by the presence of macrophage tropomyosin. However, macrophage CaD did not behave as a Ca(2+)- and calmodulin-regulated actin-binding protein. These results, together with published data on other well-characterized actin bundling proteins, suggest that nonmuscle CaD could be essentially involved in the formation and organization of actin bundles at adhesion sites and cell surface projections. However, they afforded no evidence that the macrophage isoform might play a specific role in the Ca(2+)-dependent regulation of actin and myosin II interactions.

Molecular mechanisms of nonmuscle myosin-II regulation

Myosin II, the conventional two-headed myosin that forms bipolar filaments, is directly involved in regulating cytokinesis, cell motility and cell morphology in nonmuscle cells. To understand the mechanisms by which nonmuscle myosin-II regulates these processes, investigators are now looking at the regulation of this molecule in vertebrate nonmuscle cells. The identification of multiple isoforms of nonmuscle myosin-II, whose activities and regulation differ from that of smooth muscle myosin-II, suggests that, in addition to regulatory light chain phosphorylation, other regulatory mechanisms control vertebrate nonmuscle myosin-II activity.

Changes in morphology of human skin fibroblasts induced by local anaesthetics: role of actomyosin contraction

Local anaesthetics block action potentials in the membranes of excitable cells but their effects on non-excitable cells are less well known. Some local anaesthetics are applied directly onto the skin, and for this reason the effect of procaine (p-aminobenzoic acid diethylamino-etyl ester hydrochloride) and tetracaine (4-[butylamino]benzoic acid 2-[dimethylamino]ethyl ester) upon the morphology and cytoskeleton organisation of human skin fibroblasts was investigated. The time lapse video recording of fibroblasts cultured in serum-enriched medium revealed that the cells rapidly change shape after the addition of the anaesthetic. These effects were fully reversible. The microscopic observations were confirmed by quantitative analysis of projected cell area and cell shape parameters. Local anaesthetics significantly changed the actin cytoskeleton organisation, inducing total disappearance of stress fibres. Serum-starvation or myosin light chain kinase inhibitors, KT 5926 inhibitor (8R*,9S*,11S*)-(-)-9-hydroxy-9-methoxycarbonyl-8-methyl-14-n-propoxy-2,3 ,9, 10-tetrahydro-8,11-epoxy,1H,8H,11H-2,7b,11a-triazadibenzo[a,g]c ycloocta[cde] trinden-1-one or wortmannin, which induce the 'relaxed' morphology of the cells, prevent both the anaesthetic-induced changes in cell shape and the disassembly of stress fibres. Together, the observations suggest that local anaesthetics affect the actomyosin system, inducing contraction.

Fluorescent analogues of myosin II for tracking the behavior of different myosin isoforms in living cells

Fluorescently labeled smooth muscle myosin II is often used to study myosin II dynamics in non-muscle cells. In order to provide more specific tools for tracking non-muscle myosin II in living cytoplasm, fluorescent analogues of non-muscle myosin IIA and IIB were prepared and characterized. In addition, smooth and non-muscle myosin II were labeled with both cy5 and rhodamine so that comparative, dynamic studies may be performed. Non-muscle myosin IIA was purified from bovine platelets, non-muscle myosin IIB from bovine brain, and smooth muscle myosin II from turkey gizzards. After being fluorescently labeled with tetramethylrhodamine-5-iodoacetamide or with a succinimidyl ester of cy5, they retained the following properties: (1) reversible assembly into thick filaments, (2) actin-activatable MgATPase, (3) phosphorylation by myosin light chain kinase, (4) increased MgATPase upon light-chain phosphorylation, (5) interconversion between 6S and 10S conformations, and (6) distribution into endogenous myosin II-containing structures when microinjected into cultured cells. These fluorescent analogues can be used to visualize isoform-specific dynamics of myosin II in living cells.

Focal adhesions, contractility, and signaling

Focal adhesions are sites of tight adhesion to the underlying extracellular matrix developed by cells in culture. They provided a structural link between the actin cytoskeleton and the extracellular matrix and are regions of signal transduction that relate to growth control. The assembly of focal adhesions is regulated by the GTP-binding protein Rho. Rho stimulates contractility which, in cells that are tightly adherent to the substrate, generates isometric tension. In turn, this leads to the bundling of actin filaments and the aggregation of integrins (extracellular matrix receptors) in the plane of the membrane. The aggregation of integrins activates the focal adhesion kinase and leads to the assembly of a multicomponent signaling complex.

Cellular titin localization in stress fibers and interaction with myosin II filaments in vitro

We previously discovered a cellular isoform of titin (originally named T-protein) colocalized with myosin II in the terminal web domain of the chicken intestinal epithelial cell brush border cytoskeleton (Eilertsen, K.J., and T.C.S. Keller. 1992. J. Cell Biol. 119:549-557). Here, we demonstrate that cellular titin also colocalizes with myosin II filaments in stress fibers and organizes a similar array of myosin II filaments in vitro. To investigate interactions between cellular titin and myosin in vitro, we purified both proteins from isolated intestinal epithelial cell brush borders by a combination of gel filtration and hydroxyapatite column chromatography. Electron microscopy of brush border myosin bipolar filaments assembled in the presence and absence of cellular titin revealed a cellular titin-dependent side-by-side and end-to-end alignment of the filaments into highly ordered arrays. Immunogold labeling confirmed cellular titin association with the filament arrays. Under similar assembly conditions, purified chicken pectoralis muscle titin formed much less regular aggregates of muscle myosin bipolar filaments. Sucrose density gradient analyses of both cellular and muscle titin-myosin supramolecular arrays demonstrated that the cellular titin and myosin isoforms coassembled with a myosin/titin ratio of approximately 25:1, whereas the muscle isoforms coassembled with a myosin:titin ratio of approximately 38:1. No coassembly aggregates were found when cellular myosin was assembled in the presence of muscle titin or when muscle myosin was assembled in the presence of cellular titin. Our results demonstrate that cellular titin can organize an isoform-specific association of myosin II bipolar filaments and support the possibility that cellular titin is a key organizing component of the brush border and other myosin II-containing cytoskeletal structures including stress fibers.

Isolation of the bile canalicular actin-myosin II motor

Cytoskeleton-rich canalicular membranes (CCMs) with preserved cytoskeleton and demembranated CCMs, consisting only of cytoskeletal elements, were used to examine the relationship of pericanalicular microfilaments, myosin II phosphorylation, and canalicular contraction. The components of CCMs were visualized by fluorescence microscopy using the filamentous actin probe rhodamine-phalloidin and by electron microscopy, before and after incubation in 1 microM Ca2+/1 mM ATP (contraction solution). Canalicular contraction (luminal closure) was evaluated by morphometric analysis. Myosin II was extracted from CCMs, purified by immunoprecipitation, and analyzed on Western blots. In sequential experiments, autoradiographs of gels from [gamma-32P]-ATP-treated CCMs in the presence or absence of Ca2+ were examined after 0.25, 0.50, 1, 2, 3, 5, and 10 min, and the effects of W7 (a calmodulin antagonist) and ML9 (a myosin light chain kinase inhibitor) were evaluated. The results showed that phosphorylation of the 20-kDa protein was low in controls but enhanced beginning 0.25-0.50 min after addition of contraction solution. Both W7 and ML9 significantly inhibited this reaction and inhibited canalicular contraction. The results indicate that phosphorylation of the regulatory 20-kDa myosin light chain of canaliculus-associated myosin II coincides with or precedes contraction of the canaliculus. We conclude that the canalicular contractile apparatus is composed of actin filaments and a myosin II motor.

Monoclonal antibody 7H6 reacts with a novel tight junction-associated protein distinct from ZO-1, cingulin and ZO-2

The tight junction is an essential element of the intercellular junctional complex; yet its protein composition is not fully understood. At present, only three proteins, ZO-1 (Stevenson, B. R., J. D. Siliciano, M. S. Mooseker, and D. A. Goodenough. 1986. J. Cell Biol. 103:755-766), cingulin (Citi, S., H. Sabanay, R. Jakes, B. Geiger, and J. Kendrick-Jones. 1988. Nature (Lond.). 333:272-275) and ZO-2 (Gumbiner, B., T. Lowenkopf, and D. Apatira. 1991. Proc. Natl. Acad. Sci. USA. 88:3460-3464) are known to be associated with the tight junction. We have generated a monoclonal antibody (7H6) against a bile canaliculus-rich membrane fraction prepared from rat liver. This 7H6 antigen was preferentially localized by immunofluorescence at the junctional complex regions of hepatocytes and other epithelia, and 7H6-affiliated gold particles were shown electron microscopically to localize at the periphery of tight junctions. Immunoblot analysis of a bile canaliculus-rich fraction of rat liver using 7H6, anti-ZO-1 antibody (R26.4C), and anti-cingulin antibody revealed that 7H6 reacted selectively with a 155-kD protein, whereas R26.4C reacted only with a 225-kD protein. Anti-cingulin antibody reacted solely with 140 and 108-kD proteins, indicating that the protein recognized by 7H6 is immunologically different from ZO-1 and cingulin. Immunoprecipitation of detergent extracts obtained from metabolically labeled MDCK cells with R26.4C coprecipitated a 160-kD protein, which corresponds to ZO-2, with ZO-1. However, 7H6 did not react with the 160-kD protein. These results strongly suggest that the 7H6 antibody recognizes a novel tight junction-associated protein different from ZO-1, cingulin and ZO-2.

Role of the COOH-terminal nonhelical tailpiece in the assembly of a vertebrate nonmuscle myosin rod

A short nonhelical sequence at the COOH-terminus of vertebrate nonmuscle myosin has been shown to enhance myosin filament assembly. We have analyzed the role of this sequence in chicken intestinal epithelial brush border myosin, using protein engineering/site-directed mutagenesis. Clones encoding the rod region of this myosin were isolated and sequenced. They were truncated at various restriction sites and expressed in Escherichia coli, yielding a series of mutant myosin rods with or without the COOH-terminal tailpiece and with serial deletions from their NH2-termini. Deletion of the 35 residue COOH-terminal nonhelical tailpiece was sufficient to increase the critical concentration for myosin rod assembly by 50-fold (at 150 mM NaCl, pH 7.5), whereas NH2-terminal deletions had only minor effects. The only exception was the longest NH2-terminal deletion, which reduced the rod to 119 amino acids and rendered it assembly incompetent. The COOH-terminal tailpiece could be reduced by 15 amino acids and it still efficiently promoted assembly. We also found that the tailpiece promoted assembly of both filaments and segments; assemblies which have different molecular overlaps. Rod fragments carrying the COOH-terminal tailpiece did not promote the assembly of COOH-terminally deleted material when the two were mixed together. The tailpiece sequence thus has profound effects on assembly, yet it is apparently unstructured and can be bisected without affecting its function. Taken together these observations suggest that the nonhelical tailpiece may act sterically to block an otherwise dominant but unproductive molecular interaction in the self assembly process and does not, as has been previously thought, bind to a specific target site(s) on a neighboring molecule.

A high molecular weight centrosomal protein of mammalian cells is antigenically related to myosin II

Available data on the molecular composition of the centrosome, the typical microtubule-organizing center of animal cells, are still fragmentary. To address this important issue we have taken advantage of centrosome isolation from a human lymphoblastic cell line (KE37) to generate a monoclonal antibody (mAb) library. Here we present the characterization of one of these mAbs (CTR56). On the basis of both its immunofluorescence staining pattern and its reactivity with a major 200 kD antigen on immunoblots, CTR56 has been tentatively classified as an anticellular myosin heavy chain. In light of cytological and biochemical data obtained in parallel with two other well-characterized myosin antibodies, it appears that myosin cannot be considered as a genuine centrosomal protein. We have resolved the paradoxical results with CTR56 by showing that in addition to the cellular myosin heavy chain, this antibody also recognizes a high molecular weight protein specifically enriched in centrosomal fractions. The possible biological significance of this finding is discussed in structural and functional terms.

Expression of myosin regulatory light chains in rat brain: characterization of a novel isoform

We have characterized cDNA clones of mRNAs encoding two distinct isoforms of myosin regulatory light chain expressed in rat brain. One clone, isolated from a cultured astrocyte cDNA library, is derived from a 1200-base mRNA that is expressed at high levels in cultured astrocytes, and at higher levels in the embryonic brain than in the adult brain. The nucleotide sequence of this cDNA is essentially identical to a previously reported cDNA encoding a smooth muscle isoform from rat aorta cells (Taubman et al., J. Cell Biol., 104 (1987) 1505-1515). The second clone hybridized to a 1300-base mRNA that is expressed abundantly in the adult brain and is the predominant species in cultured neuroblasts. Both mRNAs are expressed, to varying extents, in other muscle and nonmuscle tissues. The deduced amino acid sequences of the two isoforms differ in 4 residues out of 171. On the basis of the tissue distribution of their mRNAs and a comparison of identities among the known amino acid sequences of myosin regulatory light chains we suggest that both proteins should be considered as non-muscle isoforms. We conclude that there are at least two isoforms of the myosin regulatory light chain expressed in rat brain and that their expression is under both cell-specific and developmental regulation.

The yeast type II myosin heavy chain: analysis of its predicted polypeptide sequence

We have completed the nucleotide sequence of the yeast MYO1 gene and deduced its amino acid sequence. The gene is 5553 bp long and contains no introns. Analysis of the sequence, as well as its comparison with other myosins, demonstrate that the yeast protein is a type II myosin heavy chain with characteristic head and tail regions. The latter domain contains six proline residues in two clusters of three, at approximately two thirds from the start of the gene.

Subcellular compartmentalization of myosin isoforms in embryonic chick heart ventricle myocytes during cytokinesis

Embryonic chick heart ventricle myocytes retain the ability to alternate between proliferation and functional differentiation. A cytoplasmic isoform of myosin is present in cleavage furrows of various nonmuscle cells during cytokinesis, whereas one or more of the cardiac myosin isoforms are localized in sarcomeres of beating cardiomyocytes. Antibodies were employed to reveal the subcellular localizations of cytoplasmic and cardiac myosin isoforms in embryonic chick ventricle cardiomyocytes during cytokinesis. Monoclonal anticytoplasmic myosin antibodies were prepared against myosin purified from brains of 1-day-posthatched chickens and shown to react with chick brain myosin heavy chain by Western blots and/or ELISA tests. One monoclonal antibrain myosin antibody also cross-reacted with chick cardiac myosin but not with skeletal or smooth muscle myosins. Two antichick cardiac myosin monoclonal antibodies and one antichick skeletal myosin polyclonal antibody that cross-reacts with cardiac myosin were employed to identify cardiac sarcomeric myosin. Cells were isolated from day 8 embryonic chick heart ventricles, enriched for myocytes, grown in vitro for 3 days, and then examined by immunofluorescence techniques. Monoclonal antibodies against cytoplasmic myosin preferentially localized in the cleavage furrows of both cardiofibroblasts and cardiomyocytes in all stages of cytokinesis. In contrast, antibodies that recognize cardiac myosin were distributed throughout cardiomyocytes during early stages of cytokinesis, but became progressively excluded from the furrow area during middle and late stages of cytokinesis. These data suggest that in cells that contain both cytoplasmic and sarcomeric myosin isoforms, only cytoplasmic myosin isoforms are mobilized to from the contractile ring for cytokinesis.

Localization of myosin in the cytoskeleton of brush border cells using monoclonal antibodies and confocal laser-beam scanning microscopy

Monoclonal antibodies binding to the rod portion of brush border myosin were used to localize myosin in chicken intestinal brush border cells by indirect immunofluorescence. Isolated cells, or cells still attached in a sheet, were analyzed by conventional epifluorescence microscopy, which showed that most of the immunoreactive myosin is localized in the apical brush border (terminal web), and in a basal region. In addition, a weak, diffuse granular and rod-like labeling was detected throughout the cell body. Using the laser-scanning confocal microscope (White et al., 1987), a more precise localization of the myosin within the terminal web and the cell body was obtained. In the terminal web, most of the myosin was concentrated in a circumferential ring, below the plasma membrane, and the remaining myosin was found in the inter-rootlet area. These two populations of myosin were topologically strictly related, since they were found in the same optical sections. In the cell body, as well as in the basal region, the myosin was found to be associated with the outer limiting membrane of the cell, in a cortical location, whereas essentially no myosin was detected in the cytoplasm.

Role of actin and myosin in the control of paracellular permeability in pig, rat and human vascular endothelium

1. We have investigated the endothelial actomyosin system with particular emphasis on its possible role in actively opening a paracellular route for permeability. 2. Actin and myosin comprised 16% of total endothelial protein with a molar actin/myosin ratio of 16.2 which is close to the actin/myosin ratio of muscle (studies on freshly isolated pig pulmonary arterial endothelial cells, PAEC). 3. By immunocytochemistry at the light and electron microscope levels the bulk of actin and myosin was colocalized in close vicinity to the intercellular clefts of both micro- and macrovascular endothelial cells in situ and in vitro. 4. Calcium-ionophore-induced rise in permeability of human umbilical venous endothelial cells (HUVEC) and PAEC monolayers grown on filters in a two-chamber permeability system was caused by opening of intercellular gaps. Gap formation depended on the rise in intracellular Ca2+ and could be blocked by the calmodulin-binding drugs trifluperazine (TFP) and W7. 5. In skinned monolayers of cultured PAEC and in isolated sheets of HUVEC gap formation was shown to require ATP and occurred only when free myosin binding sites were available on endothelial actin filaments (experiments with myosin subfragment 1 modified by N-ethylmaleimide, S1-NEM). 6. These experiments suggest that actin and myosin in endothelial cells play a central role in regulating the width of the intercellular clefts, thereby controlling the paracellular pathway of vascular permeability.

Mammalian nonsarcomeric myosin regulatory light chains are encoded by two differentially regulated and linked genes

The myosin 20,000-D regulatory light chain (RLC) has a central role in smooth muscle contraction. Previous work has suggested either the presence of two RLC isoforms, one specific for nonmuscle and one specific for smooth muscle, or the absence of a true smooth muscle-specific isoform, in which instance smooth muscle cells would use nonmuscle isoforms. To address this issue directly, we have isolated rat RLC cDNAs and corresponding genomic sequences of two smooth muscle RLC based on homology to the amino acid sequence of the chicken gizzard RLC. These cDNAs are highly homologous in their amino acid coding regions and contain unique 3'-untranslated regions. RNA analyses of rat tissue using these unique 3'-untranslated regions revealed that their expression is differentially regulated. However, one cDNA (RLC-B), predominantly a nonmuscle isoform, based on abundant expression in nonmuscle tissues including brain, spleen, and lung, is easily detected in smooth muscle tissues. The other cDNA (RLC-A; see Taubman, M., J. W. Grant, and B. Nadal-Ginard. 1987. J. Cell Biol. 104:1505-1513) was detected in a variety of nonmuscle, smooth muscle, and sarcomeric tissues. RNA analyses comparing expression of both RLC genes with the actin gene family and smooth muscle specific alpha-tropomyosin demonstrated that neither RLC gene was strictly smooth muscle specific. RNA analyses of cell lines demonstrated that both of the RLC genes are expressed in a variety of cell types. The complete genomic structure of RLC-A and close linkage to RLC-B is described.

Alteration of the enzymatic properties of smooth muscle myosin by a monoclonal antibody against subfragment 2

A monoclonal antibody against subfragment 2 (S-2) of smooth muscle myosin, designated MM-9, was generated and characterized. MM-9 potently inhibited subfragment 1 (S-1) release by papain proteolysis of myosin, suggesting that the epitope of MM-9 is at or very close to the S-1/S-2 junction. The depression of Ca2(+)- and Mg2(+)-ATPase activities of myosin at low ionic strength was significantly reduced by MM-9. MM-9 increased the acto dephosphorylated HMM ATPase activity about 3-fold. On the other hand, the antibody had no effect on the KCl-dependence of viscosity of monomeric myosin. These results suggest that the folding of the myosin rod is not the direct determinant of enzymatic activity, and that the subtle conformational change at the S-1/S-2 junction (head-neck region) plays a critical role in determining enzymatic activities.

Phosphorylation of a second site for myosin light chain kinase on platelet myosin

The 20,000-dalton light chain of bovine platelet myosin is phosphorylated at two sites by myosin light chain kinase. The first and second phosphorylation sites are at a serine and a threonine residue, respectively. The location of the phosphorylation sites was determined by using limited proteolysis. The N-terminal sequence of the 17,000-dalton tryptic fragment of platelet myosin 20,000-dalton light chain was found to be identical with that of gizzard 20,000-dalton light chain from Ala-17 to Phe-33. On the basis of these results and the distribution of 32P among the proteolytic fragments, it was concluded that serine-19 and threonine-18 were the two phosphorylation sites. Phosphorylation at the threonine residue markedly increases the actin-activated ATPase activity of myosin. It was found that platelet myosin forms 10S and 6S conformations and its Mg2+-ATPase activity parallels the transition from the 6S to the 10S conformation. The conformational transition was influenced by phosphorylation at both sites, and the phosphorylation at the threonine residue further shifted the equilibrium toward the 6S conformation. The phosphorylation at the threonine residue also induced thick filament formation in the presence of ATP. These results suggest that the phosphorylation at the threonine residue as well as at the serine residue may play an important role in the contractility of nonmuscle cells.

Protein kinase C phosphorylation of thymus myosin

We have examined the effect of protein kinase C phosphorylation on the actin-activated ATPase activity and filament stability of calf thymus myosin. Protein kinase C phosphorylated thymus myosin regulatory light chains, LC20, on two sites which are different from the site phosphorylated by myosin light chain kinase. The light meromyosin part of the thymus myosin heavy chain was also phosphorylated by protein kinase C, but at a rate about 10% that of LC20. Under conditions where both unphosphorylated thymus and myosin light chain kinase-phosphorylated thymus myosin were filamentous and under conditions where myosin light chain kinase phosphorylation induces myosin filament formation, protein kinase C phosphorylation had little effect on the actin-activated ATPase activity or filament stability of unphosphorylated or myosin light chain kinase-phosphorylated myosin. In contrast, protein kinase C phosphorylation has been reported to inhibit the actin-activated ATPase activity of gizzard myosin.

Parallel modulation of brush border myosin conformation and enzyme activity induced by monoclonal antibodies

Monoclonal antibodies binding to distinct epitopes on the tail of brush border myosin were used to modulate the conformation and state of assembly of this myosin. BM1 binds 1:3 of the distance from the tip of the tail to the head and prevents the extended-tail (6S) monomer from folding into the assembly-incompetent folded-tail (10S) state, whereas BM4 binds to the tip of the myosin tail, and induces the myosin to fold into the 10S state. Thus, at physiological ionic strength BM1 promotes and BM4 blocks the assembly of the myosin into filaments. Using BM1 and BM4 together, we were able to prevent both folding and filament assembly, thus locking myosin molecules in the extended-tail 6S monomer conformation at low ionic strength where they normally assemble into filaments. Using these myosin-antibody complexes, we were able to investigate independently the effects of folding of the myosin tail and assembly into filaments on the myosin MgATPase. The enzymatic activities were measured from the fluorescent profiles during the turnover of the ATP analogue formycin triphosphate (FTP). Extended-tail (6S) myosin molecules had an FTPase activity of 1-5 X 10(-3) s-1, either at high ionic strength as a monomer alone or when complexed with antibody, or at low ionic strength as filaments or when maintained as extended-tail monomers by the binding of BM1 and BM4. Folding of the molecules into the 10S state reduced this rate by an order of magnitude, effectively trapping the products of FTP hydrolysis in the active sites.

Correlation of enzymatic properties and conformation of bovine erythrocyte myosin

Myosin was purified from bovine erythrocytes by chromatography on DEAE-cellulose, Sepharose CL-4B, hydroxylapatite, and DEAE-5PW. The yield was about 200 micrograms/L of packed cells. From SDS-polyacrylamide gels, the purity was estimated to be greater than 95%. The bovine erythrocyte myosin is composed of heavy chains of 200 kDa and light chains of 20 and 17 kDa, in a molar stoichiometry of 1. Myosin was also purified from human erythrocytes by the same method. The molecular weights of two light chains were 26K and 19.5K which confirmed the earlier reports [Fowler, V. M., Davis, J. Q., & Bennet, V. (1985) J. Cell Biol. 100, 47-55; Wong, A. J., Kiehart, D. P., & Pollard, T.D. (1985) J. Biol. Chem. 260, 46-49]. Phosphorylation by gizzard myosin light chain kinase, to a level of 1 mol of phosphate/mol of 20-kDa light chain, increased actin-activated ATPase, and the extent of activation was dependent on the MgCl2 concentration. Both Ca2+-ATPase and Mg2+-ATPase activities were dependent on KCl concentration and markedly decreased below 0.3 M KCl. Mg2+-ATPase of phosphorylated myosin, while more resistant to decreasing ionic strength, was also decreased below 0.2 M KCl. These results are similar to those obtained with smooth muscle myosin and suggest that the 10S-6S transition occurs. In confirmation of this, gel filtration, viscosity, and electron microscopy (rotary shadowing) show that erythrocyte myosin forms extended and folded conformations in high and low salt, respectively. It is proposed that each conformation is characterized by distinct enzymatic properties.(ABSTRACT TRUNCATED AT 250 WORDS)

In situ localization of myosin and actin in dendritic spines with the immunogold technique

The in situ detection of macromolecules by means of immunoelectron microscopy provides information about their ultrastructural localization in cellular compartments. With this technique, we have demonstrated that the contractile proteins actin and myosin are both localized in dendritic spines at densities exceeding those of other neuronal compartments. Myosin was associated with actin filaments, with spine plasma membrane, and with membranes of the spine apparatus. Given the dynamic properties of actin and myosin, these data suggest that these proteins may be involved in the mechanism of synaptic plasticity in general and in morphometric change resulting from intense synaptic activation in particular.

Active site trapping of nucleotide by smooth and non-muscle myosins

The folded 10 S monomer conformation of smooth muscle myosin traps the hydrolysis products ADP and Pi in its active sites. To test the significance of this, we have searched for equivalent trapping in other conformational and assembly states of avian gizzard and brush border myosins, using formycin triphosphate (FTP) as an ATP analogue. When myosin monomers were in the straight-tail 6 S conformation, the hydrolysis products were released at about 0.03 s-1. Adoption of the folded 10 S monomer conformation reduced this rate by more than 100-fold, effectively trapping the products FDP and Pi in the active sites. This profound inhibition of product release occurred only on formation of the looped tail monomer conformation. In vitro-assembled myosin filaments released products at a comparable rate to free straight-tail 6 S monomers, and smooth muscle heavy meromyosin, which lacks the C-terminal two-thirds of the myosin tail, also did not trap the products in this way. Phosphorylation of the myosin regulatory light chain had no effect on the rate of product release from straight-tail 6 S myosin monomers or from myosin filaments. Rather, it allowed actin to accelerate product release. Phosphorylation acted also to destabilize the folded monomer conformation, causing the recruitment of molecules from the pool of folded monomers into the myosin filaments. The two processes of contraction and filament assembly are thus both controlled in vitro by light-chain phosphorylation. A similar linked control in vivo would allow the organization of myosin in the cell to adapt itself continuously to the pattern of contractile activity.

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.

Phosphorylation of brush border myosin at threonine on its 20 kDa light chains by a calmodulin-independent kinase activates its ATPase

A calmodulin-independent kinase isolated from chicken intestinal brush border phosphorylates brush border myosin mainly at an apparently single threonine on its 20 kDa light chains. Phosphorylation to 1.9 mol phosphate/mol myosin activated the myosin actin-activated ATPase about 12-fold, to about 100 nmol/min per mg. Brush border myosin ATPase can thus be activated by phosphorylation either at threonine, by calmodulin-independent kinase, or at serine, by calmodulin-dependent myosin light chain kinase, as previously shown [(1987) FEBS Lett. 223, 262-266].

Brush border myosin filament assembly and interaction with actin investigated with monoclonal antibodies

Monoclonal antibodies binding to epitopes in the rod portion of brush border myosin were used to study the mechanism of filament assembly and its role in myosin interaction with actin. The antibodies and their Fab fragments had specific effects on the size of the filaments assembled in vitro. Two antibodies (BM3 and BM4), directed against the tip of the myosin tail, completely inhibited myosin filament assembly. The other antibodies (BM1, BM2 and BM5), binding to other sites along the myosin rod, only partially blocked filament growth, and short filaments could be assembled. Thiophosphorylated brush border myosin filaments appeared slightly more stable to the effects of the antibodies than those composed of dephosphorylated myosin. Only one (BM3) of the antibodies which completely inhibited the assembly of new filaments was capable of disassembling preformed myosin filaments. The other antibody, BM4, partially disassembled filaments, leaving approximately 0.2-microns long 'cores', suggesting that polymerization in this myosin occurs by a biphasic mechanism, i.e. the formation of a stable nucleus of antiparallely packed molecules, followed by elongation. The antibodies BM1 and BM2 bound to myosin filaments generating a regular transverse pattern with a approximately 14-nm periodicity, and had little effect on the stability of these preformed filaments. Inhibition of filament formation and solubilization of the myosin by the antibodies appeared to be associated with inhibition of myosin interaction with actin, as measured by the actin-activated MgATPase activity. In the presence of the antibodies which completely inhibit filament assembly, we observed a decrease to approximately 20% (BM4-Fab) and to approximately 50% (BM3) of the control actin-activated myosin MgATPase activity, and this activity was kinetically different from that of the soluble myosin S1 fragment, suggesting that the rod has a profound effect on the kinetics of actomyosin interaction.

Cingulin, a new peripheral component of tight junctions

The tight junction (Zonula occludens), a belt-like region of contact between cells of polarized epithelia, serves as a selective barrier to small molecules and as a total barrier to large molecules, and is involved in the separation between lumenal and basolateral compartments of the epithelium. In the electron microscope, tight junctions show focal regions of apparent fusion between the adjoining cell membranes, and freeze-fractured membranes display an elaborate network of branching and anastomosing strands. Very little is known about the molecular composition and architecture of tight junctions. The first specific zonula occludens-associated protein, designated ZO-1, has recently been identified in mammalian epithelial and endothelial cells. Here we describe the identification and purification of a new component of this junctional complex in avian brush-border cells, which we name cingulin. Cingulin is an acidic, heat-stable protein, with a highly elongated shape. Immunofluorescence and immunoelectron microscopy of brush-border cells with anti-cingulin antibodies show that cingulin is localized in the apical zone of the terminal web, at the endofacial surfaces of the zonula occludens.

Developmental organization of the intestinal brush-border cytoskeleton

At the terminal web of chicken intestinal epithelial cell, the actin bundles are cross-linked by a fine filamentous network of actin-associated cross-linkers. Myosin, fodrin, and TW 260/240 have been identified as major components of the cross-linkers. We studied the development of the cross-linkers by quick-freeze, deep-etch electron microscopy, and the expression of cross-linker proteins (myosin, fodrin 240, and TW 260) by immunofluorescence and immunoblotting analysis during the embryogenesis. Microvilli start to form at 5-7 days, and the rootlets begin to elongate at 10 days. At an early stage of the development of the terminal web (13 days), fodrin 240 and a small amount of myosin are expressed, and a few actin-associated cross-linkers are present between the rootlets. However, TW 260 is not expressed at this stage. At an intermediate stage (19 days), the amount of myosin increases, and TW 260 begins to be expressed. The number of cross-linkers associated with the unit length of the rootlets is 24/microns. At the final stage of the terminal web formation (2 days after hatching), the amount of fodrin 240, myosin, and TW 260 is similar to the adult level, and the number of the actin-associated cross-linkers per unit length of the rootlet is 27/microns (approximately 85% of the adult). These results suggest that the synthesis of cross-linker proteins may be intricately regulated to achieve the desired density of cross-linkages at each developmental stage: at early and intermediate stages, sufficient and not an excess of cross-linkages are formed; and at a final stage, a higher complexity of cross-linkages is achieved. In addition, there is a differential expression of the components of the actin-associated cross-linkers: myosin and fodrin could be early components of the cross-linkers involved in the basic stabilization of the terminal web structure, whereas TW 260/240 becomes incorporated later, possibly involved in the stabilization preparatory to the rapid elongation of microvilli, which occurs after the formation of the terminal web.

Polymerization of vertebrate non-muscle and smooth muscle myosins

We investigated how light chain phosphorylation controls the stability of filaments of vertebrate non-muscle myosins (from bovine thymocytes and chicken intestine epithelial brush border cells) and smooth muscle myosin (from chicken gizzard) in vitro. Using a sedimentation assay, the solubilities of the myosins were determined by measuring the amounts of myosin monomers (Cm) and filaments (Cp) present under a given set of conditions as a function of the total myosin concentration (Ct). Below 200 mM-NaCl, each myosin displayed distinct "critical monomer concentrations" (Cc) for polymerization, which were dependent on the salt concentration, the state of light chain phosphorylation and the presence of MgATP. At 150 mM-NaCl, MgATP increased the Cc of non-phosphorylated brush border myosin approximately five to tenfold, thymus myosin approximately 10 to 15-fold, and gizzard myosin approximately 25 to 50-fold. When these myosins were phosphorylated, MgATP had little effect on their solubilities, and their Cc values remained low. Analytical ultracentrifugation and electron microscopy demonstrated that the myosins were present in three different conformational states under the conditions used in the sedimentation assays, i.e. filaments, extended monomer (6 S) and folded monomer (10 S). Since at equilibrium only filaments and monomers were observed, we suggest that the polymerization pathway for these myosins can be analysed in terms of a dynamic monomer-polymer equilibrium (polymer in equilibrium 6 S monomer in equilibrium 10 S monomer). At roughly physiological ionic strength, light chain dephosphorylation (in the presence of MgATP) promotes the folded state (10 S), whereas phosphorylation promotes the extended state (6 S), and thereby favours filament assembly. The relevance of the monomer-polymer equilibrium to the state of organization of the myosin in vivo is discussed.

Effects of light chain phosphorylation and skeletal myosin on the stability of non-muscle myosin filaments

The effect of light chain phosphorylation and the presence of skeletal muscle myosin on the stability of non-phosphorylated non-muscle myosin filaments was investigated. Purified skeletal, brush border and thymus myosins were assembled in vitro into hybrid filaments consisting of varying proportions of (1) non-muscle and skeletal myosins, or (2) phosphorylated and non-phosphorylated non-muscle myosins. The stability of these hetero- and homopolymers in the presence of MgATP was determined using sedimentation, gel electrophoresis and immunochemical techniques. In addition, the effect of a monoclonal antibody, binding to the tip of brush border myosin tail, on the assembly of the homo- and heteropolymers, was tested. Filamentous non-phosphorylated non-muscle myosin was disassembled by MgATP to the same extent whether in homo- or heteropolymers, indicating that skeletal myosin has no stabilising effect on the hybrid filaments. The presence of small amounts of phosphorylated non-muscle myosin was, however, found to prevent the complete disassembly by MgATP of non-phosphorylated non-muscle myosin filaments, indicating that light chain phosphorylation stabilizes co-operatively non-muscle myosin filaments. The monoclonal antibody prevented the assembly of brush border myosin into both homo- and heteropolymers, and its effect on the filaments was compared with that of MgATP.

Phosphorylation of brush border myosin by brush border calmodulin-dependent myosin heavy and light chain kinases

Calmodulin-dependent myosin light chain kinase isolated from chicken intestinal brush border phosphorylates brush border myosin at an apparently single serine identical to that phosphorylated by smooth muscle myosin light chain kinase. Phosphorylation to 1.8 mol phosphate/mol myosin activated the myosin actin-activated ATPase about 10-fold, to about 50 nmol/min per mg. Myosin phosphorylated on its light chains could then be further phosphorylated to a total of 3.2 mol phosphate per mol by brush border calmodulin-dependent heavy chain kinase. Heavy chain phosphorylation did not alter the actin-activated ATPase of either myosin prephosphorylated on its light chains or of unphosphorylated myosin.

Myosin from pancreatic acinar carcinoma cells. Isolation, characterization and demonstration of heavy- and light-chain phosphorylation

Myosin has been identified in a variety of non-muscle cells, and is believed to play a role in maintenance of cell shape, locomotion, cytokinesis, exocytosis and other cellular functions. In this paper we describe the purification of myosin from a pancreatic acinar-cell carcinoma of the rat which forms solid tumours, but retains many differentiated functions. The purified myosin was composed of a 200,000 Da heavy chain and two or three classes of light chains. Electron-microscopic examination of rotary-shadowed preparations revealed that individual molecules had two globular heads and a long tail measuring approx. 149 nm. The myosin was soluble in high-salt buffers and became sedimentable as the ionic strength was lowered. Examination of negative-stained preparations showed that this sedimentable myosin consisted of short, bipolar, thick filaments which had a strong tendency to aggregate in a head-to-head manner. The ATPase activity of the purified myosin was stimulated by EDTA or Ca2+, but not by Mg2+. In low ionic strength the Mg2+-dependent ATPase activity was activated by muscle f-actin. The pancreatic myosin bound to actin and could be dissociated by the addition of MgATP. Myosin purified from cells cultured in media containing [32P]Pi was phosphorylated on one of the light chains as well as the heavy chain. Thus pancreatic acinar cells contain a typical non-muscle myosin, and the subunits of this molecule are subject to post-translational modification by phosphorylation.

Studies on the structure and conformation of brush border myosin using monoclonal antibodies

We have produced and characterised five monoclonal antibodies against myosin isolated from chicken intestinal epithelial brush border cells. The binding sites of the antibodies on the rod portion of brush border myosin were localised using rotary shadowing/electron microscopy of myosin-antibody complexes. Two antibodies were shown to bind to the tip of the myosin tail, two antibodies to sites about two thirds down the length of the rod, and one antibody about one third down the length of the rod. Brush border myosin was digested with papain, trypsin and alpha-chymotrypsin, and they myosin fragments obtained were analysed by western blots with the monoclonal antibodies and polyclonal antiserum, and by gel overlay with 125I-labelled light chains. Using this approach we were able to identify and map the protease cleavage sites and thus characterise the proteolytic substructure of brush border myosin. Solid-phase assays, western blots and immunofluorescence were used to study the cross-reactivity of these monoclonal antibodies against a variety of myosins from different species and cell types, to assess the immunological relatedness between brush border myosin and homologous molecules present in different tissues and species. Finally, we used a competitive solid-phase assay to measure the 'relative affinities' of the antibodies towards the three possible conformational states of brush border myosin, i.e. filament, extended monomer and folded monomer.

Cytoplasmic gels from macrophages. Evidence for the involvement of four proteins in the calcium-dependent solation of actin networks

A method has been devised to study the influence of Ca2+ on the in vitro formation of actin gel networks. Under appropriate conditions low-Ca2+ cytosolic extracts (less than 1 nM) from macrophages rapidly formed a macromolecular complex composed of actin, filamin, alpha-actinin and two new proteins of 70 kDa and 55 kDa. [Pacaud, M. (1986) Eur. J. Biochem. 156, 521-530]. Increasing concentrations of free Ca2+ to 1-2 microM resulted in complete inhibition of the association of 70-kDa protein, a protein which associates actin filaments into parallel arrays. Concentrations of Ca2+ greater than 3 microM caused incorporation of two additional proteins, gelsolin and a 18-kDa polypeptide, with no change in either the actin or alpha-actinin content of the cytoskeletal structures. Use of a polyacrylamide gel overlay technique with 125I-calmodulin revealed that a high-Mr calmodulin-binding protein analogous to spectrin was also associated with these structures when micromolar Ca2+ was present. Similar assays with 45CaCl2 indicated that the 70-kDa protein binds Ca2+ with high affinity. It is thus suggested that Ca2+ might regulate the dynamic assembly of microfilaments through several target proteins, gelsolin, the 70-kDa protein and calmodulin.

Brush border myosin heavy chain phosphorylation is regulated by calcium and calmodulin

Myosin from chicken intestinal brush borders is phosphorylated on its heavy chains at threonine by a kinase isolated from brush borders. In contrast to other heavy chain kinases, the brush border kinase activity is dependent on calcium and calmodulin. The partially purified preparation also phosphorylated myosin on its light chains at serine, but in a calmodulin-independent manner. Phosphorylation of the light chains in the absence of calmodulin or both heavy and light chains in the presence of calmodulin activated its actin-activated ATPase activity about 10-fold, to about 50 nmol/min per mg.

Localisation of light chain and actin binding sites on myosin

A gel overlay technique has been used to identify a region of the myosin S-1 heavy chain that binds myosin light chains (regulatory and essential) and actin. The 125I-labelled myosin light chains and actin bound to intact vertebrate skeletal or smooth muscle myosin, S-1 prepared from these myosins and the C-terminal tryptic fragments from them (i.e. the 20-kDa or 24-kDa fragments of skeletal muscle myosin chymotryptic or Mg2+/papain S-1 respectively). MgATP abolished actin binding to myosin and to S-1 but had no effect on binding to the C-terminal tryptic fragments of S-1. The light chains and actin appeared to bind to specific and distinct regions on the S-1 heavy chain, as there was no marked competition in gel overlay experiments in the presence of 50-100 molar excess of unlabelled competing protein. The skeletal muscle C-terminal 24-kDa fragment was isolated from a tryptic digest of Mg2+/papain S-1 by CM-cellulose chromatography, in the presence of 8 M urea. This fragment was characterised by retention of the specific label (1,5-I-AEDANS) on the SH1 thiol residue, by its amino acid composition, and by N-terminal and C-terminal sequence analyses. Electron microscopical examination of this S-1 C-terminal fragment revealed that: it had a strong tendency to form aggregates with itself, appearing as small 'segment-like' structures that formed larger aggregates, and it bound actin, apparently bundling and severing actin filaments. Further digestion of this 24-kDa fragment with Staphylococcus aureus V-8 protease produced a 10-12-kDa peptide, which retained the ability to bind light chains and actin in gel overlay experiments. This 10-12-kDa peptide was derived from the region between the SH1 thiol residue and the C-terminus of S-1. It was further shown that the C-terminal portion, but not the N-terminal portion, of the DTNB regulatory light chain bound this heavy chain region. Although at present nothing can be said about the three-dimensional arrangement of the binding sites for the two kinds of light chain (regulatory and essential) and actin in S-1, it appears that these sites are all located within a length of the S-1 heavy chain of about 100 amino acid residues.

Proteolytic fragmentation of brain myosin and localisation of the heavy-chain phosphorylation site

The heavy chains and the 19-kDa and 20-kDa light chains of bovine brain myosin can by phosphorylated. To localise the site of heavy-chain phosphorylation, the myosin was initially subjected to digestion with chymotrypsin and papain under a variety of conditions and the fragments thus produced were identified. Irrespective of the ionic strength, i.e. whether the myosin was monomeric or filamentous, chymotryptic digestion produced two major fragments of 68 kDa and 140 kDa; the 140-kDa fragment was further digested by papain to yield a 120-kDa and a 23-kDa fragment. These fragments were characterised by (a) a gel overlay technique using 125I-labelled light chains, which showed that the 140-kDa and 23-kDa polypeptides contain the light-chain-binding sites; (b) using myosin photoaffinity labelled at the active site with [3H]UTP, which showed that the 68-kDa fragment contained the catalytic site, and (c) electron microscopy, using rotary shadowing and negative-staining techniques, which demonstrated that after chymotryptic digestion the myosin head remains attached to the tail whereas on papain digestion isolated heads and tails were observed. Thus the 120-kDa polypeptide derived from the 140-kDa fragment is the tail of the myosin, and the 68-kDa fragment containing the catalytic site and the 23-kDa fragment, with the light-chain-binding sites, form the head (S1) portion of the myosin. When [32P]-phosphorylated brain myosin was digested with chymotrypsin and papain it was shown that the heavy-chain phosphorylation site is located in a 5-kDa peptide at the C-terminal end of the heavy chain, i.e. the end of the myosin tail. Using hydrodynamic and electron microscopic techniques, no significant effect of either light-chain or heavy-chain phosphorylation on the stability of brain myosin filaments was observed, even in the presence of MgATP. Brain myosin filaments appear to be more stable than those of other non-muscle myosins. Light-chain phosphorylation did, however, have an effect on the conformation of brain myosin, for example in the presence of MgATP non-phosphorylated myosin molecules were induced to fold into a very compact folded state.