Monoclonal antibodies against chicken tropomyosin isoforms: production, characterization, and application

Eight mouse monoclonal antibodies, CH1, CH106, CH291, CL2, CG1, CG3, CG beta 2 and CG beta 6, against chicken tropomyosin isoforms have been prepared and characterized. The antigens recognized by these isoform-specific monoclonal antibodies were identified by both solid-phase radioimmunoassay and protein immunoblotting. To some extent, most antibodies showed isoform-specific, but one (CG3) recognized all isoforms of tropomyosin from chicken materials. The effects of monoclonal antibodies on the binding of cardiac tropomyosin to F-actin were investigated. Antibodies CH1, CH106, and CH291 had the ability to interfere with the binding of tropomyosin to F-actin, whereas others appeared to have no effect. Monoclonal antibody CL2 was able to distinguish the skeletal muscle tropomyosin-enriched microfilaments from the fibroblastic tropomyosin-enriched microfilaments of differentiating muscle cells. This antibody will be most useful for studying the compartmentalization of microfilaments and microfilament-associated proteins, particularly actin and tropomyosin isoforms during muscle differentiation. Immunofluorescence microscopy with CG1 antibody which recognized CEF tropomyosin isoforms 1 and 3 revealed the continuous staining of stress fibers in some populations of CEF cells. On the other hand, both periodic fluorescent staining and continuous staining of stress fibers were observed with CG3 antibody in all CEF cells.

Relationship between the concentration of myoglobin and parvalbumin in various types of muscle tissues from chickens

The concentration of parvalbumin was determined in various types of chicken muscle by immunological analysis and was compared with that of myoglobin. Parvalbumin was present specifically in skeletal muscle and absent in cardiac and gizzard muscle; exceptionally, neither parvalbumin nor myoglobin was detected in white breast muscle. The wing and leg red muscles, which had larger amounts of myoglobin, contained smaller quantities of parvalbumin. In these muscles, the concentration of parvalbumin was inversely related to that of myoglobin (correlation coefficient = -0.69). Both myoglobin and parvalbumin were observed in the legs of 18-d-old embryos; the parvalbumin content exceeded that of myoglobin until the birds were 4 to 6 weeks old, but the relationship was reversed thereafter. Myoglobin in gizzard muscle was present in 18-d-old embryos and increased markedly at hatching; it was already present in cardiac muscle at an early embryonic stage, increasing gradually until 14 weeks after hatching.

Inhibition of smooth muscle actin-activated myosin Mg2+-ATPase activity by caldesmon

Caldesmon, a major calmodulin- and actin-binding protein of smooth muscle (Sobue, K., Muramoto, Y., Fujita, M., and Kakiuchi, S. (1981) Proc. Natl. Acad. Sci. U. S. A. 78, 5652-5655), has been obtained in highly purified form from chicken gizzard by a modification of a previously published procedure (Ngai, P. K., Carruthers, C. A., and Walsh, M. P. (1984) Biochem. J. 218, 863-870) and was found to cause a significant inhibition of both superprecipitation and actin-activated myosin Mg2+-ATPase activity in a system reconstituted from the purified contractile and regulatory proteins without influencing the phosphorylation state of myosin. This inhibitory effect was seen both in the presence and absence of tropomyosin. A Ca2+-and calmodulin-dependent kinase which catalyzed phosphorylation of caldesmon was identified in chicken gizzard; this kinase is distinct from myosin light-chain kinase. Caldesmon prepared by calmodulin-Sepharose affinity chromatography was contaminated with caldesmon kinase activity and was unable to inhibit actomyosin ATPase activity or superprecipitation. Phosphatase activity capable of dephosphorylating caldesmon was also identified in smooth muscle. These results indicate that caldesmon can inhibit smooth muscle actomyosin ATPase activity in vitro, and this function may itself be subject to regulation by reversible phosphorylation of caldesmon.

Identical myoglobin is present in both skeletal and smooth muscles of chicken

Chicken gizzard has been considered to be an exceptional organ of smooth musculature in which a myoglobin is present. Since the characterization of the gizzard myoglobin has to date, been very incomplete, we studied the structures and functions in detail. The main component, which constituted roughly 90% of the protein, isolated by chromatofocusing, was homogeneous by electrophoretic and ultracentrifugal analyses. The molecular weight was consistently 1.8 X 10(4) by equilibrium sedimentation and iron analysis, and the isoelectric point was 7.8. Spectroscopic properties of the oxy-, carboxy- and deoxy-derivatives were typical of myoglobin. The oxygenation equilibria were also typical of myoglobin, showing neither homotropic nor heterotropic allosteric interactions, and the temperature-dependence (delta H0) was estimated as -16.6 kcal/mol. All these characteristics of the gizzard myoglobin were identical with those of the protein from the skeletal muscles. The amino acid composition and peptide mapping results also concluded that identical myoglobin was present in the gizzard, skeletal and probably cardiac muscles.

Rapid proteolytic generation of neurotensin-related peptide(s) and biologic activity during extraction of rat and chicken gastric tissues

Using a radioimmunoassay directed toward the COOH-terminal, biologically active region of mammalian neurotensin (NT), the rapid (within seconds) generation of immunoreactive NT (iNT) during acid extraction of mammalian and avian gastric tissues has been demonstrated. Levels of iNT were shown to increase 25-200-fold in time. The reaction occurred in 0.1 N HCl and 2 N acetic acid and was prevented by raising the pH above 5 or by adding acetone. The temperature and pH dependence and the ability of pepstatin A to inhibit the reaction suggested the involvement of a pepsin-related acid protease. Furthermore, the reaction could be mimicked by incubating a stabilized gastric extract with hog pepsin at pH 2. Size exclusion chromatography demonstrated the presence of a precursor-like substance with an apparent Mr of 60,000. Although iNT generated in avian and mammalian gastric extracts could be distinguished chromatographically from NT in that species, the partially purified gastric iNT was active in a bioassay for NT which quantitates changes in vascular permeability after intradermal injection into rats. One might suggest that iNT serves as a signal within the gastric lumen, being generated at low pH by secreted pepsin. It is also possible that iNT could be formed within blood or gastric interstitial fluid by the action of pepsin-related (cathepsin or renin-like) enzymes at normal physiologic pH.

Dinitrophenylation of phosphorylated gizzard myosin

Chymotrypsin and papain digestion of 3H-labelled dinitrophenylated chicken gizzard myosin, pretreated with the myosin light chain catalyzed phosphorylating system, released a subfragment 1 devoid of its light chains but containing all of the label associated with the thiols of the heavy chain region. This was also the case when myosin was incubated in the presence of the phosphorylating system but without ATP. Dinitrophenylation of a reconstituted actomyosin made from phosphorylated myosin occurred mainly on -SH groups of the heavy chains, in contrast to the predominant modification of the light chains of myosin from a control actomyosin that was treated similarly. Conformational changes in myosin may govern, in part, actin-myosin interaction through the phosphorylation of the light chain of Mr 20.000.

Discrimination of assembled and disassembled forms of gizzard myosin by papain

Chicken gizzard myosin in 0.15 M or 0.5 M NaCl was cleaved at two sites of heavy chain with 2-10 micrograms/ml papain. MgATP inhibited these cleavages of myosin in 0.15 M NaCl but not in 0.5 M NaCl. The protective effect of ATP was observed at concentrations as low as 10 microM and increased in proportion to ATP concentration to a maximum at 1 mM. ADP was as effective as ATP, while adenosine 5'-[beta, gamma-imido]triphosphate, an unhydrolyzable ATP analogue, was less effective than ATP or ADP. AMP had no protective effect on the digestion of myosin and GTP inhibited slightly the digestion. When the papain-insensitive myosin in 0.15 M NaCl and 2.5 mM MgATP was phosphorylated by Ca2+/calmodulin-dependent myosin light-chain kinase, the myosin restored the vulnerability to papain. However, the two papain-susceptible forms, nonphosphorylated form in the absence of MgATP and phosphorylated form in the presence of MgATP, yielded very similar but distinct proteolytic fragments upon the digestion. When the extent of myosin assembly was estimated by the turbidimetry of myosin suspension in 0.15 M NaCl, nonphosphorylated myosin in the absence and presence of MgATP was assembled and disassembled, respectively, and phosphorylated myosin in the presence of MgATP was assembled. These results suggest that, at physiological ionic strength, papain as a probe distinguishes disassembled myosin and assembled myosin as papain-insensitive and papain-sensitive forms, respectively.

Correlation between the papain digestibility and the conformation of 10s-myosin from chicken gizzard

In our previous reports, ATP was shown to induce a drastic change in the conformation of gizzard myosin molecules. For example, the sedimentation constant of unphosphorylated myosin (UM) increased from 6S to 10S although an ATP-induced change in the sedimentation constant did not occur with phosphorylated myosin (Suzuki et al. (1978) J. Biochem. 84, 1529). We now report the finding that the ATP-induced formation of 10S-myosin is associated with a drastic change in the papain digestibility of gizzard UM. With 10S-myosin, the cleavage by papain was strongly inhibited at two regions on heavy chains and at one region on light chains; that is, the junction between the 72K dalton and 22K dalton fragments (i.e., a cleavable site in myosin head), the one between the 22K dalton and 130K dalton fragments (i.e., a head-tail junction), and the one between the 3K dalton and 17K dalton fragments of 20K dalton light chains. An even more intimate correlation between the myosin conformation and the papain digestibility of myosin was demonstrated by using thiophosphorylated myosin (thioPM); the cleavages by papain at the 72K-22K dalton junction and the 22K-130K dalton junction were not inhibited when thioPM was digested.

Electron microscopic studies on myosin molecules from chicken gizzard muscle III. Myosin dimers

Previously, we (Onishi, H. & Wakabayashi, T. (1982) J. Biochem. 92, 871) reported that the ATP-induced disassembly of chicken gizzard "thick filaments" resulted in myosin monomers with "looped" tails. In the present study, we found that these monomers assembled themselves into antiparallel dimers when they were placed in a medium of low ionic strength (approximately 2 mM).

alpha-Actinin and vinculin in normal and thrombasthenic platelets

Recently, the contractile protein alpha-actinin was identified in normal human platelets by its antigenic cross-reaction with a monospecific antibody to purified muscle alpha-actinin. In this study, we extend that preliminary identification of platelet alpha-actinin. Amino acid analysis, one-dimensional peptide maps, and silver stain analysis on polyacrylamide gels demonstrate that human platelet alpha-actinin shows a greater degree of similarity to smooth muscle alpha-actinin than to striated muscle alpha-actinin. There is no evidence to suggest that alpha-actinin is a glycoprotein. In addition, we find that thrombasthenic platelets, which are deficient in glycoproteins IIb and IIIa (GPIIb and GPIIIa) contain normal amounts of alpha-actinin, confirming the recent finding that alpha-actinin and GPIIIa are different proteins in human platelets. We demonstrate that both normal and thrombasthenic platelets also contain vinculin, a 130,000-dalton polypeptide found in many cell types at sites of end-on attachment of microfilaments to the plasma membrane. Thus, the thrombasthenic defect in GPIIb and GPIIIa does not diminish the content of either alpha-actinin or vinculin.

Amino acid sequences of the regulatory light chains of striated adductor muscle myosins from Ezo giant scallop and Akazara scallop

Amino acid sequences of the regulatory light chains of striated adductor muscle myosin from Ezo giant scallop (Patinopecten yessoensis) and akazara scallop (Chlamys nipponensis akazara) were determined. Tryptic peptides of each light chain were isolated and sequenced. The alignment of the tryptic peptides in each chain was deduced from the amino acid compositions and the partial sequences of peptic peptides of the Ezo giant scallop light chain. The light chains both consist of 156 residues. Heterogeneous residues, glutamic acid and aspartic acid were observed at the 155th position in the sequence of the akazara scallop light chain. A comparison of the Ezo giant scallop light chain with the glutamic acid-containing and aspartic acid-containing akazara light chains revealed 6 and 7 amino acid substitutions, respectively. When the presented sequences were compared with those of the regulatory light chains of gizzard, cardiac and skeletal muscle myosins, a strong homology was observed in the calcium binding region, but there were considerable heterogeneities in the N- and C-terminal regions.

Epinemin: a new protein associated with vimentin filaments in non-neural cells

In this report I describe a new protein, defined by a monoclonal antibody, which is associated with vimentin filaments in a variety of cultured cells and in skeletal muscle. By immunofluorescence it is absent in smooth muscle, in cells without vimentin, and in neural vimentin containing cells. This protein has a molecular weight of 44,500, a pl of 5, a two-dimensional tryptic peptide fingerprint pattern different from vimentin, is unrelated to actin by Cleveland peptide analysis and by light and electron microscopy, and is not recognized by either a polyclonal antivimentin antibody (Frank, E.D., and L. Warren, 1981, Proc. Natl. Acad. Sci. USA, 78:3020-3024) or a monoclonal antibody against all classes of intermediate filaments (Pruss, R.M., R. Mirsky, M.C. Raff, R. Thorpe, A.J. Dowding, and B.H. Anderton, 1981, Cell, 27:419-428). The protein is resistant to nonionic detergent extraction, is soluble in high salt and can thus be removed from vimentin filaments, but fragments with vimentin in either low salt or anionic detergent and collapses with vimentin in colchicine-treated cells. By light microscopy, the distribution of the protein is indistinguishable from vimentin filaments and appears uniform along them. In contrast, immunoferritin electron microscopy reveals that the molecule is distributed in an intermittent pattern on vimentin filaments. Adopting the terminology of Granger and Lazarides (1980, Cell, 30:263-275), the molecule is called epinemin, meaning "upon filaments."

Classification of tropomyosin components into an alpha-like or a beta-like family by partial peptide mapping

Tropomyosins can be classified as belonging to an alpha-like or a beta-like family depending on the absence or presence, respectively in their protease-V8 digestion pattern of two peptides with an apparent molecular mass of 21 kDa. Chicken cardiac tropomyosin and the 43 kDa component from gizzard tropomyosin are accordingly classified as alpha-like tropomyosins, while the 33 kDa gizzard tropomyosin component is a beta-like tropomyosin. The 21 kDa peptides have an overall charge which is more positive than that of the intact tropomyosin or any other tropomyosin peptide and probably contain the -NH2 half of the molecule.

Selective purification of the 20,000-Da light chains of smooth muscle myosin

The 20,000-Da light chains of gizzard smooth muscle myosin have been purified to homogeneity. Actomyosin, prepared by MgATP extraction of myofibrils, was denatured in 8 M urea, 1 M guanidine HCl, and 0.05% sodium dodecyl sulfate. Myosin heavy chains were precipitated with ethanol and the light chain enriched fraction was dialyzed and subjected to chromatography on DEAE-Sephacel. Fractions containing the 20,000-Da light chains were further purified by hydrophobic chromatography on phenyl-Sepharose. The 20,000-Da light chains eluted at low ionic strength from the phenyl-Sepharose column were judged to be greater than 95% pure by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and contained only 0.04 mol of phosphate/mol of light chain. The yield of light chains was calculated to be 219 +/- 17 mg/kg of starting gizzard smooth muscle. This method may be useful for preparation of homogeneous 20,000-Da smooth muscle myosin light chains in the quantities necessary for study of contractile systems.

Engineering of site-directed antisera against vertebrate calmodulin by using synthetic peptide immunogens containing an immunoreactive site

Site-directed antisera against vertebrate calmodulin were elicited in rabbits by injection of a synthetic immunogen containing the pentadecapeptide Gly-Gln-Val-Asn-Tyr-Glu-Glu-Phe-Val-Gln-Met-Met-Thr-Ala-Lys-OH, which corresponds to residues 134-148 of vertebrate calmodulin. A major immunoreactive region (residues 127-144) of calmodulin is found in the COOH-terminal structural domain and an immunoreactive site for one antiserum is contained in the heptapeptide Asn-Tyr-Glu-Glu-Phe-Val-Gln-NH2, which corresponds to residues 137-143 of vertebrate calmodulin. This immunoreactive heptapeptide was conjugated to a carrier protein by adding a cysteine residue to the NH2 terminus of the peptide and coupling the Cys-heptapeptide to the carrier through the thiol group of the cysteine residue. Injection of this Cys-heptapeptide-protein conjugate into rabbits yielded antisera that react with the heptapeptide but not with native calmodulin. Thus, the immunoreactive heptapeptide that is exposed on the surface of calmodulin is immunogenic, but it is not sufficient to elicit antibodies that react with native calmodulin. However, when the Cys-pentadecapeptide corresponding to residues 134-148 and containing the immunoreactive heptapeptide sequence was conjugated to a carrier protein and injected into rabbits, antisera were elicited that react with the intact calmodulin molecule. The affinities and specificities of these antisera for calmodulin are similar to those of antisera elicited by injection of the intact protein and are sufficient for their use in radioimmunoassays. These results indicate that the successful engineering of site-directed antisera against proteins by using synthetic peptide immunogens may require an appropriate intramolecular environment that allows the peptide region to closely approximate the spatial orientation it adopts in the intact protein.

Structural analysis of the C-terminal region of chicken gizzard desmin

The C-terminal sequence of chicken gizzard desmin has been examined by computer programs based on Chou-Fasman prediction of secondary structure, auto-correlation and Fourier analysis of hydrophobic residue distribution, and cross-correlation analysis of acidic and basic residues. These analyses indicate that, although parts of the desmin sequence are alpha-helical, the helical regions may not be as extensive as predicted previously. Detailed analysis of the distribution of the charged residues in the desmin sequence strongly suggests that it cannot form a tropomyosin-like coiled-coil double-helix because of the almost complete absence of stabilizing salt-bridge interactions between proximal pairs of acidic and basic residues. Although the desmin sequence does contain the tropomyosin-like hydrophobic residue distribution, differences exist in the overall distributions of hydrophobes, with clusters of these residues being found in certain regions of the desmin sequence. Analyses of the sequences of porcine desmin and vimentin reveal very similar structural characteristics for these molecules, although vimentin is predicted to have a different structure at its C-terminus.

Vinculin localization in cardiac muscle

Vinculin isolated from chicken cardiac muscle crossreacts with antibodies against smooth muscle vinculin. Antibodies to vinculin were used for localization of vinculin in cardiac muscle by indirect immunofluorescence method. In cardiac muscle vinculin was localized in intercalated discs and near plasma membrane at the cell periphery between external myofibrils and sarcolemma. It was suggested that vinculin plays an important role in myofibril-sarcolemma interaction in cardiac muscle.

Developmental change of protein constituents in chicken gizzards

Developmental change of protein constituents of chick gizzard smooth muscle was described by the fluorescent antibody technique and two-dimensional polyacrylamide gel electrophoresis. Myosin heavy chain, tropomyosin, and desmin were immunohistologically detected in 5-day-old gizzard primordia, but myoglobin was detected after 19 days of incubation. Two-dimensional polyacrylamide gel electrophoresis revealed that most structural proteins including beta- and gamma-actin are synthesized almost simultaneously in the primordium, and accumulate in three patterns by which the proteins examined are classified: (1) gradually increasing protein (gamma-actin, tropomyosin, desmin), (2) abruptly increasing protein at a certain stage (myosin, myoglobin), (3) decreasing or constantly kept protein (tubulin, beta-actin). Based on the quantitative analysis of protein constituents, the nature of regulatory system of protein synthesis in smooth muscle and the possible functional difference between beta- and gamma-actin are discussed.

Dependence on Ca2+ and tropomyosin of the actin-activated ATPase activity of phosphorylated gizzard myosin in the presence of low concentrations of Mg2+

Ca2+ and tropomyosin are required for activation of ATPase activity of phosphorylated gizzard myosin by gizzard actin at less than 1 mM Mg2+, relatively low Ca2+ concentrations (1 microM), producing half-maximal activation. At higher concentrations, Mg2+ will replace Ca2+, 4 mM Mg2+ increasing activity to the same extent as does Ca2+ and abolishing the Ca2+ dependence. Above about 1 mM Mg2+, tropomyosin is no longer required for activation by actin, activity being dependent on Ca2+ between 1 and 4 mM Mg2+, but independent of [Ca2+] above 4 mM Mg2+. Phosphorylation of the 20,000-Da light chain of gizzard myosin is required for activation of ATPase activity by actin from chicken gizzard or rabbit skeletal muscle at all concentrations of Mg2+ employed. The effect of adding or removing Ca2+ is fully reversible and cannot be attributed either to irreversible inactivation of actin or myosin or to dephosphorylation. After preincubating in the absence of Ca2+, activity is restored either by adding micromolar concentrations of this cation or by raising the concentration of Mg2+ to 8 mM. Similarly, the inhibition found in the absence of tropomyosin is fully reversed by subsequent addition of this protein. Replacing gizzard actin with skeletal actin alters the pattern of activation by Ca2+ at concentrations of Mg2+ less than 1 mM. Full activation is obtained with or without Ca2+ in the presence of tropomyosin, while in its absence Ca2+ is required but produces only partial activation. Without tropomyosin, the range of Mg2+ concentrations over which activity is Ca2+-dependent is restricted to lower values with skeletal than with gizzard actin. The activity of skeletal muscle myosin is activated by the gizzard actin-tropomyosin complex without Ca2+, although Ca2+ slightly increases activity. The Ca2+ sensitivity of reconstituted gizzard actomyosin is partially retained by hybrid actomyosin containing gizzard myosin and skeletal actin, but less Ca2+ dependence is retained in the hybrid containing skeletal myosin and gizzard actin.

The use of monoclonal antibodies to fragments of chicken type IV collagen in structural and localization studies

In previous experiments, three pepsin-resistant fragments of type IV collagen were isolated from chicken gizzards and designated 7S, F3, and (F1)2F2 (Mayne, R., and Zettergren, J. G. (1980) Biochemistry 19, 4065-4072). In the present experiments, a series of monoclonal antibodies to type IV collagen were prepared, each one of which recognized an epitope present in only one of the three fragments. A high molecular weight fraction of type IV collagen (designated 7S + arms (215 nm)) was isolated after agarose gel filtration and characterized by electron microscopy after rotary shadowing and by gel electrophoresis. Analysis of 7S + arms (215 nm) by inhibition enzyme-linked immunosorbent assay demonstrated the presence of the epitopes for 7S and F3 but not for (F1)2F2. This result, therefore, provides additional evidence that the order of the pepsin-resistant fragments of chicken type IV collagen is 7S-F3-(F1)2F2.

Caldesmon, a calmodulin-binding, F actin-interacting protein, is present in aorta, uterus and platelets

Caldesmon, a protein originally found in chicken gizzard, was concluded also to be present in bovine aorta, uterus, and human platelets by demonstration of a protein with the following properties: (a) Ca2+-dependent calmodulin-binding; (b) binding to F actin in such way that the binding was broken on Ca2+-dependent binding of calmodulin; (c) cross-reactivity in immune blotting procedures with affinity-purified antibody against gizzard caldesmon; (d) similar subunit Mr-values on SDS-gel to those of gizzard caldesmon. Like gizzard caldesmon, platelet caldesmon was composed of two polypeptide bands of Mr 150 000 and 147 000, but caldesmon in aorta and uterus gave a single band of Mr 150 000. A polypeptide of Mr 165 000 that was immunologically distinct from caldesmon but, like caldesmon, bound to calmodulin and F actin in a flip-flop fashion, was also demonstrated in aorta and uterus.

Preparation of the alkali and P light chains of chicken gizzard myosin. Amino acid sequence of the alkali light chain

1. A simple method is described for the purification of the alkali and P light chains from chicken gizzard myosin. 2. The sequence of the alkali light chain has been unequivocally determined, except for the N-terminal dipeptide, by using the tryptic and CNBr peptides. 3. No evidence was obtained for any specific high-affinity Ca2+-binding sites on the alkali light chain. 4. Detailed evidence on which the sequence is based has been deposited as Supplementary Publication SUP 50120 (14 pages) at the British Library Lending Division, Boston Spa, Wetherby, West Yorkshire LS23 7QB, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1983) 209, 5.

Studies on the effect of phosphorylation of the 20,000 Mr light chain of vertebrate smooth muscle myosin

Myosin was rapidly prepared from turkey gizzard muscle to a high level of purity, in high yield and in a non-phosphorylated state. It was consistently observed that the actin-activated Mg2+ ATPase activity of this myosin was dependent on the level of phosphorylation of the 20,000 Mr light chain, for example, in the non-phosphorylated state, the myosin Mg2+ ATPase activity was not activated by actin whereas, when the light chains were phosphorylated, the Mg2+ ATPase activity of the myosin was activated approximately ninefold by actin. Using the "desensitized" scallop myosin test system (Kendrick-Jones et al., 1976; Sellers et al., 1980) it was further demonstrated that phosphorylation of the 20,000 Mr gizzard light chain has a regulatory role. These results also suggest that the regulatory mechanisms mediated by smooth muscle myosin light chains and molluscan myosin regulatory light chains are similar, i.e. in the absence of Ca2+, both types of light chain inhibit myosin interaction with actin and this inhibition is relieved by either phosphorylation in smooth muscle or by direct calcium binding in molluscan myosins. The basis of regulation exerted by these light chains is therefore repression derepression. Using a variety of techniques, i.e. turbidity measurements, quantitative high speed centrifugation, electron microscopy and dark field light microscopy, it was observed that the stability of gizzard myosin filaments at approximately physiological conditions (0.15 M-NaCl, 1 mM-MgATP, pH 7.0) was dependent on the level of light chain phosphorylation. Using purified calmodulin-dependent light chain kinase and phosphatase, it was further shown that these gizzard myosin filaments can be reversibly assembled and disassembled as a result of phosphorylation-dephosphorylation of the 20,000 Mr light chain.

Purification of the intermediate filament-associated protein, synemin, from chicken smooth muscle. Studies on its physicochemical properties, interaction with desmin, and phosphorylation

Synemin, a 230,000-dalton protein associated with desmin- and vimentin-containing intermediate filaments (Granger, B. L., and Lazarides, E. (1980) Cell 22, 727-738), has been purified from gizzard smooth muscle and biochemically characterized. Purification was achieved by extracting the salt-insoluble pellet of muscle protein with 6 M urea and chromatography of the urea extract on columns of hydroxylapatite, DEAE-Sephacel, and phosphocellulose. The soluble form of synemin is a globular tetramer of 980,000 daltons with a S20,w of 22.4 +/- 3.2. Synemin has a pI of 5.34, in agreement with its high content in glutamic acid (20%), and is rich in serine (11%) and poor in cysteine (0.4%). Synemin is phosphorylated in smooth muscle and is one of the muscle proteins with the highest capacity to incorporate exogenously added [32P]phosphate. Of the [32P] phosphate incorporated into synemin, 95% is bound to serine and only 5% to threonine. The phosphorylation of synemin is enhanced by the cyclic AMP analog, 8-Br-cyclic AMP. Immunofluorescence studies using anti-synemin antibodies show that purified synemin binds to filaments of desmin assembled in vitro. Synemin specifically inhibits the immunoprecipitation of purified soluble desmin by anti-desmin antibodies, indicating that synemin interacts in vitro with soluble desmin.