Interaction between vertebrate skeletal and uterine muscle myosins and light meromyosins

Nonmuscle myosin IIB, a sarcomeric component in the extraocular muscles

Extraocular muscles (EOMs) are categorized as skeletal muscles; however, emerging evidence indicates that their gene expression profile, metabolic characteristics and functional properties are significantly different from the prototypical members of this muscle class. Gene expression profiling of developing and adult EOM suggest that many myofilament and cytoskeletal proteins have unique expression patterns in EOMs, including the maintained expression of embryonic and fetal isoforms of myosin heavy chains (MyHC), the presence of a unique EOM specific MyHC and mixtures of both cardiac and skeletal muscle isoforms of thick and thin filament accessory proteins. We demonstrate that nonmuscle myosin IIB (nmMyH IIB) is a sarcomeric component in approximately 20% of the global layer fibers in adult rat EOMs. Comparisons of the myofibrillar distribution of nmMyHC IIB with sarcomeric MyHCs indicate that nmMyH IIB co-exists with slow MyHC isoforms. In longitudinal sections of adult rat EOM, nmMyHC IIB appears to be restricted to the A-bands. Although nmMyHC IIB has been previously identified as a component of skeletal and cardiac sarcomeres at the level of the Z-line, the novel distribution of this protein within the A band in EOMs is further evidence of both the EOMs complexity and unconventional phenotype.

Role of the C-terminal extremities of the smooth muscle myosin heavy chains: implication for assembly properties

The two light meromyosin isoforms from rabbit smooth muscle were prepared as recombinant proteins in Escherichia coli. These species which differed only by their C-terminal extremity showed the same circular dichroism spectra and endotherms in measurements of differential scanning calorimetry. Their solubility properties were different at pH 7.0 in the absence of monovalent salts. Their paracrystals formed at low pH differed by their aspect and number. These data suggest a role for the C-terminal extremity of myosin heavy chains in the assembly of myosin molecules in filaments and consequently in the contractility of smooth muscles.

Expression of smooth muscle myosin heavy chains and unloaded shortening in single smooth muscle cells

The functional significance of the variable expression of the smooth muscle myosin heavy chain (SM-MHC) tail isoforms, SM1 and SM2, was examined at the mRNA level (which correlates with the protein level) in individual permeabilized rabbit arterial smooth muscle cells (SMCs). The length of untethered single permeabilized SMCs was monitored during unloaded shortening in response to increased Ca2+ (pCa 6.0), histamine (1 microM), and phenylephrine (1 microM). Subsequent to contraction, the relative expression of SM1 and SM2 mRNAs from the same individual SMCs was determined by reverse transcription-polymerase chain reaction amplification and densitometric analysis. Correlational analyses between the SM2-to-SM1 ratio and unloaded shortening in saponin- and alpha-toxin-permeabilized SMCs (n = 28) reveal no significant relationship between the SM-MHC tail isoform ratio and unloaded shortening velocity. The best correlations between SM2/SM1 and the contraction characteristics of untethered vascular SMCs were with the minimum length attained following contraction (n = 20 and r = 0.72 for alpha-toxin, n = 8 and r = 0.78 for saponin). These results suggest that the primary effect of variable expression of the SM1 and SM2 SM-MHC tail isoforms is on the cell final length and not on shortening velocity.

Segregated assembly of muscle myosin expressed in nonmuscle cells

Skeletal muscle myosin cDNAs were expressed in a simian kidney cell line (COS) and a mouse myogenic cell line to investigate the mechanisms controlling early stages of myosin filament assembly. An embryonic chicken muscle myosin heavy chain (MHC) cDNA was linked to constitutive promoters from adenovirus or SV40 and transiently expressed in COS cells. These cells accumulate hybrid myosin molecules composed of muscle MHCs and endogenous, nonmuscle, myosin light chains. The muscle myosin is found associated with a Triton insoluble fraction from extracts of the COS cells by immunoprecipitation and is detected in 2.4 +/- 0.8-micron-long filamentous structures distributed throughout the cytoplasm by immunofluorescence microscopy. These structures are shown by immunoelectron microscopy to correspond to loosely organized bundles of 12-16-nm-diameter myosin filaments. The muscle and nonmuscle MHCs are segregated in the transfected cells; the endogenous nonmuscle myosin displays a normal distribution pattern along stress fibers and does not colocalize with the muscle myosin filament bundles. A similar assembly pattern and distribution are observed for expression of the muscle MHC in a myogenic cell line. The myosin assembles into filament bundles, 1.5 +/- 0.6 micron in length, that are distributed throughout the cytoplasm of the undifferentiated myoblasts and segregated from the endogenous nonmuscle myosin. In both cell lines, formation of the myosin filament bundles is dependent on the accumulation of the protein. In contrast to these results, the expression of a truncated MHC that lacks much of the rod domain produces an assembly deficient molecule. The truncated MHC is diffusely distributed throughout the cytoplasm and not associated with cellular stress fibers. These results establish that the information necessary for the segregation of myosin isotypes into distinct cellular structures is contained within the primary structure of the MHC and that other factors are not required to establish this distribution.

Incorporation of nascent myosin heavy chains into thick filaments of cardiac myocytes in thyroid-treated rabbits

A monoclonal antibody (mAb 37) specific for alpha-myosin heavy chain (alpha-MHC) is used to follow the spatial and temporal incorporation of alpha-MHC into rabbit left ventricular myocytes. The expression of the two adult cardiac MHC genes, alpha and beta, is regulated by manipulating the thyroid hormone level of the animal. 10 wk on a propylthiouracil diet down-regulates expression of alpha-MHC to near 0%. alpha-MHC gene expression is up-regulated by injecting L-triiodothyronine (100 micrograms/kg per d) for 1-4 d. This protocol provides a means by which to follow the redistribution pattern of alpha-MHC within the myocyte in vivo. A uniform distribution of immunofluorescent signal is seen within every myocyte throughout the left ventricle. Ultracryomicrotomy without fixation is used to obtain sections for immunogold-electron microscopy. To quantify the immunogold method the density of gold-labeled antibody per unit of area tissue is determined for various regions of the sarcomere. Tissue from normal and 2-wk baby has a uniform distribution of gold density along the length of the A band. The average gold density of the A band increases with days of thyroid injection from 38 +/- 4 grains/micron 2 (n = 2 animals) (mean +/- SE) at day 1 to 182 +/- 59 grains (n = 2 animals) at day 4. There is a nonuniform incorporation of the newly synthesized alpha-MHC within the A band of thyroid-treated animals since 50% more of the alpha-MHC is found at the end of the A band while the center of the A band has 40% less than the average alpha-MHC content (grains/micron 2, n = 7 animals). These results support a thick filament assembly model that allows every myosin in a thick filament to be exchanged with new myosin. However, in the intact functioning myocyte, there is greater exchange of new myosin at the ends than in the central region of the thick filament.

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.

Probing myosin head structure with monoclonal antibodies

Monoclonal antibodies that react with defined regions of the heavy and light chains of chicken skeletal muscle myosin have been used to provide a correlation between the primary and the tertiary structures of the head. Electron microscopy of rotary shadowed antibody-myosin complexes shows that the sites for three epitopes in the 25,000 Mr tryptic fragment (25k) of subfragment-1, including one within 4000 Mr of the amino terminus of the myosin heavy chain, are clustered 145(+/- 20) A from the head-rod junction. An epitope in the 50,000 Mr fragment maps even further out on the head. These antibodies bind to the head in several orientations, suggesting that each of the heads can rotate can rotate 180 degrees about the head-rod junction. The epitopes are accessible on subfragment-1 bound to actin when they were probed with Fab fragments; therefore, none of these heavy chain sites is is on the contact surface between the head and actin. Two of the anti-25k antibodies affect the K+-EDTA-and Ca2+-ATPase activities of myosin in a manner that mimics the effect on activity of the modification of the reactive thiol, SH-1. These two antibodies also inhibit the actin-activated ATPase non-competitively with respect to actin. None of the other eight antibodies tested had any marked effect on activity. A monoclonal antibody that reacts with an epitope in the amino-terminal third of myosin light chain 2 maps close to the head-rod junction. A polyclonal antibody specific for the amino terminus of light chain 3 binds further up in the "neck region" of the head, indicating that these portions of the two classes of light chains are located at different sites.

Electric birefringence study of rabbit skeletal myosin subfragments HMM, LMM, and rod in solution

Electric birefringence measurements and depolarized light scattering experiments were performed with HMM, LMM, and rod, the three fragments of myosin, under conditions (0.3 M KCl, 0.02 M PO4, pH 7.3) the medium currently used for biochemical assays of myosin in its native state as well as of its subfragments. The comparison of myosin and rod relaxation times (17.2 and 22.8 microseconds, respectively) suggests that the average bend angle in the tail is sharper in intact myosin (90 degrees) whereas rod, when detached from the heads, is a more elongated species with an average bend angle of 120-135 degrees. The LMM relaxation time (6.4 microseconds) is consistent with a rigid linear stick model of length 78 nm. Flexibility in myosin tail is thus confirmed as located in the HMM-LMM hinge. LMM and rod did not exhibit any significant variation of their apparent relaxation times with concentration and the decay curves were best fitted by a single exponential, evidence that the concentration of parallel staggered dimers was negligible in the concentration range studied here (0-7 g/l). This observation lends support to previous results obtained with myosin. Respective HMM, LMM, and rod molecular weights and homogeneity as evaluated by SDS-PAGE analysis were correlated to the Kerr constants of their solutions. Large variations in LMM Kerr constants could be related to the loss of a COOH-terminal peptide on prolonged chymotryptic digestion. Electric birefringence combined with depolarized light scattering is presented as a potential method for net charge distribution studies.

Assembly and kinetic properties of myosin light chain isozymes from fast skeletal muscle

Myosin from chicken pectoralis muscle consists of isozymes that differ in their alkali light chains. It is possible to isolate alkali 1 (A1) and alkali 2 (A2) homodimers of native myosin by immunoadsorption methods, and to compare their steady-state kinetics as well as their assembly into synthetic filaments under a variety of ionic conditions. Bipolar filaments of the isozymes formed at low salt concentrations had a narrow length distribution and did not differ from controls made from unfractionated myosin. Chicken myosin also assembles into highly homogeneous minifilaments similar to those formed by rabbit myosin in a citrate/Tris buffer. Analytical ultracentrifugation and electron microscopy showed that A1-homodimer, A2-homodimer and unfractionated myosin assembled into 0.3 micron short, bipolar minifilaments, which were indistinguishable from one another in size and shape. The steady-state myosin ATPase activity of the two homodimeric isozymes was identical in K+(EDTA) and Ca2+ assay media. The actomyosin Mg2+ ATPase measured at 25 and 55 mM-KCl (pH 8.0) showed only minor differences in both Vmax and Kapp. Actomyosin activity was also determined for the more homogeneous minifilament preparations of the isozymes and these, as well, produced essentially indistinguishable kinetic parameters. Thus we find no evidence to support the hypothesis that a particular alkali light chain of myosin can affect either the structure of the filaments or the steady-state rate of ATP hydrolysis.

Effects of taxol and Colcemid on myofibrillogenesis

To determine the relationship between thin filaments, Z-bands, microtubules, intermediate filaments (IFs), T-tubules, and sarcoplasmic reticulum (SR) during myofibrillogenesis, myotubes were selectively depleted of their myofibrils with 12-tetradecanoylphorbol 13-acetate (TPA) and then were allowed to regenerate in (i) normal medium, (ii) taxol, and (iii) Colcemid. Myofibrils assembled in normal medium formed typical A-, I-, Z-, M-, and H-bands and associated IFs, T-tubules, and SR. Myofibrils assembled in taxol formed "A-bands" of aligned thick filaments interdigitating with long microtubules and "I-bands" consisting only of microtubules. These unprecedented sarcomeres lacked thin filaments, Z-bands, and associated IFs and SR. "Solitary A-bands," consisting exclusively of laterally aligned bipolar thick filaments 1.6 microM in length without either thin filaments or microtubules, were observed. Myofibrils assembled in Colcemid formed all myofibrillar components in the absence of microtubules but these did not achieve rigorous lateral alignment. Colcemid and taxol induced the formation of patchy Z-bands that invariably served as insertion sites for thin filaments, irrespective of the presence or absence of adjacent thick filaments. Z-bands may function as actin-organizing centers for each sarcomere.