Proximity of alkali light chains to 27K domain of the heavy chain in myosin subfragment 1

Probing myosin light chain 1 structure with monoclonal antibodies

Five monoclonal antibodies that react with different regions of myosin light chain 1 from human ventricular myocardial muscle were used to obtain information on interactions between the light chain 1 and heavy chains and generally on the tertiary structure of the light chain 1 within the myosin head. We performed Western blot assays of the five antibodies with myosins from different cardiac and skeletal muscles, with different proteolytic fragments of bovine ventricular myosin light chain 1 (LC1) and to different recombinant fragments of human ventricular LC1 and rat fast skeletal light chain LC1/LC3. The five antibodies were mapped in three different regions of the light chain 1: two antibodies mapped within the first eight amino-terminal residues, two between residues 71 and 74, and one between residues 129 and 134. The apparent dissociation constants of the last three antibodies, determined by antibody-antigen equilibria in solution, were lower than when isolated light chains were used as antigens. It is probable that the corresponding amino acids involved in the antibody epitopes were either involved in interactions between the light and heavy myosin subunits, or somehow hindered by the myosin heavy chain bulk. In contrast, the apparent dissociation constants measured for both other antibodies were higher when myosin, rather than isolated light chains, was used as antigen. Thus LC1 fixation to heavy chains within the myosin molecule induced conformation changes at the amino-terminal end of the light chain 1. No difference in the accessibility of this mobile LC1 segment was detected in the presence of actin. Finally, observed differences in epitope accessibility on the light chain LC1 in myosin, as compared with chymotryptic subfragment 1 (SF1), indicated conformational differences between native myosin and extensively studied SF1 molecules.

Influence of nucleotide on chemical crosslinking between alkali light chains and the heavy chain of myosin subfragment 1

When chymotryptic myosin subfragment 1 (S1) of fast skeletal muscle myosin is treated with dithiobis(succinimidylpropionate) (DSP), the alkali light chains A1 and A2 become intramolecularly crosslinked to the N-terminal 27 kDa fragment of the S1 heavy chain (Labbé et al. (1981) Biochem. Biophys. Res. Commun. 102, 466-475). The results presented here show that in the presence of MgATP the efficiency of the crosslinking is markedly reduced. The results may indicate a nucleotide-induced structural rearrangement within the myosin head. It was also observed that crosslinking depressed the nucleotide-promoted tryptic conversion of the 27 kDa fragment to its 22 kDa derivative, suggesting that the crosslinks are in the vicinity of the additional tryptic cleavage site in the 27 kDa fragment or that the crosslinking prevents nucleotide-induced conformational changes in this region of the S1 heavy chain.

Dictyostelium discoideum myosin: isolation and characterization of cDNAs encoding the essential light chain

We used an antibody specific for Dictyostelium discoideum myosin to screen a lambda gt11 cDNA expression library to obtain cDNA clones which encode the Dictyostelium essential myosin light chain (EMLC). The amino acid sequence predicted from the sequence of the cDNA clone showed 31.5% identity with the amino acid sequence of the chicken EMLC. Comparisons of the Dictyostelium EMLC, a nonmuscle cell type, with EMLC sequences from similar MLCs of skeletal- and smooth-muscle origin, showed distinct regions of homology. Much of the observed homology was localized to regions corresponding to consensus Ca2+-binding of E-F hand domains. Southern blot analysis suggested that the Dictyostelium genome contains a single gene encoding the EMLC. Examination of the pattern of EMLC mRNA expression showed that a significant increase in EMLC message levels occurred during the first few hours of development, coinciding with increased actin expression and immediately preceding the period of maximal chemotactic activity.

Dictyostelium discoideum myosin: isolation and characterization of cDNAs encoding the essential light chain

We used an antibody specific for Dictyostelium discoideum myosin to screen a lambda gt11 cDNA expression library to obtain cDNA clones which encode the Dictyostelium essential myosin light chain (EMLC). The amino acid sequence predicted from the sequence of the cDNA clone showed 31.5% identity with the amino acid sequence of the chicken EMLC. Comparisons of the Dictyostelium EMLC, a nonmuscle cell type, with EMLC sequences from similar MLCs of skeletal- and smooth-muscle origin, showed distinct regions of homology. Much of the observed homology was localized to regions corresponding to consensus Ca2+-binding of E-F hand domains. Southern blot analysis suggested that the Dictyostelium genome contains a single gene encoding the EMLC. Examination of the pattern of EMLC mRNA expression showed that a significant increase in EMLC message levels occurred during the first few hours of development, coinciding with increased actin expression and immediately preceding the period of maximal chemotactic activity.

Anti-actin antibodies. An immunological approach to the myosin-actin and the tropomyosin-actin interfaces

The topography of the rigor complex between subfragment-1 (S-1) of myosin and actin was investigated by using several specific antibodies directed to well-located sequences in actin. A major contact area for S-1 was characterized in the hydrophilic 18-28 constant sequence, and the variable 1-7 sequence was only found to be in close proximity to the interface. The C-terminal extremity of actin situated around Cys-374 appeared to be included in a region close to the S-1 heavy chain and the N-terminal part of actin. The interaction between tropomyosin and actin was also studied. Neither of the terminal parts of actin were involved in this interaction. Thus, the regions involved in the interactions of S-1 and tropomyosin with actin do not overlap.

Evidence that the N-terminal region of A1-light chain of myosin interacts directly with the C-terminal region of actin. A proton magnetic resonance study

Earlier 1H-NMR experiments on the myosin subfragment-1 (S1) light chain isoenzymes from rabbit fast muscle, containing either the A1 or the A2 alkali light chains [S1(A1) or S1(A2)], have shown that the 41-residue N-terminal extension of A1, rich in proline, alanine and lysine residues, is freely mobile in solution but that this mobility is constrained in the acto-S1(A1) complex [Prince et al. (1981) Eur. J. Biochem. 121, 213-219]. It is now established that this N-terminal region of the A1-light chain interacts directly with the C-terminal region of actin in the acto-S1(A1) complex. This was shown by covalently labelling the Cys-374 residue of actin with a spin-label and observing the enhanced relaxation this paramagnetic centre induced in the 1H-NMR spectrum of S1(A1). In particular, the signal arising from the -N+(CH3)3 protons of alpha-N-trimethylalanine (Me3Ala) were monitored as this residue is uniquely sited at the N-terminus of the A1 light chain [Henry et al. (1982) FEBS Lett. 144, 11-15]. Experiments using complexes of actin with either the N-terminal 37-residue peptide of A1, S1(A1) or heavy meromyosin indicate that the N-terminal region of A1 is binding in a similar manner to actin in each case, with the N-terminal Me3Ala residue within 1.5 nm of the spin label introduced to Cys-374 of actin. A similar strategy was adopted to show that the Me3Ala residue can also be found close (less than 1.5 nm) to the fast-reacting SH1 thiol group on the S1 heavy chain. These data, together with published work, have been used to suggest a possible organisation for the polypeptide chains in the myosin head.

Properties of the alkali light-chain-20-kilodalton fragment complex from skeletal myosin heads

We have developed a rapid and reproducible procedure widely applicable to the preparation of pure aqueous solutions of the complex between an alkali light chain and the COOH-terminal heavy-chain fragments of skeletal myosin chymotryptic subfragment 1 (S-1) split by various proteases. It was founded on the remarkable ethanol solubility of these complexes. A systematic study of the ethanol fractionation of the tryptic (27K-50K-20K)-S-1 (A2) showed the NH2-terminal 27K fragment to behave like a specific protein entity being quantitatively precipitated at a relatively low ethanol concentration. Only the 20K peptide-A2 complex remained in solution when the S-1 derivative was treated with exactly 4 volumes of ethanol in the presence of 6 M guanidinium chloride. At a lower ethanol concentration, a soluble mixture of 50K and 20K peptides together with the light chain was obtained. The isolated 20K fragment-A2 system containing a 1:1 molar ratio of each component was investigated by biochemical and 1H nuclear magnetic resonance (NMR) techniques to highlight its structure and the interaction of the 20K heavy-chain segment with F-actin and with the light chain. During the treatment of the complex with alpha-chymotrypsin, only the 20K peptide was fragmented in contrast to its stability within the whole S-1. The binding of F-actin to the complex led, however, to a strong inhibition of its chymotryptic degradation. 1-Ethyl-3-[3-(dimethylamino)propyl]carbodiimide cross-linking of F-actin to the complex produced covalent actin-20K peptide only, the amount of which was lower relative to that observed with the entire split S-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Sequence analysis of mutations that affect the synthesis, assembly and enzymatic activity of the unc-54 myosin heavy chain of Caenorhabditis elegans

We have sequenced 11 representative mutations of the unc-54 myosin heavy chain gene of Caenorhabditis elegans that affect the synthesis, assembly or enzymatic activity of the encoded myosin heavy chain. Six of the sequenced unc-54 mutations cause premature termination of protein synthesis. Four mutations (e1092, e1115, e1213, e1328) were ochre mutations, one mutation (e903) was a frameshift, which caused premature termination at a nearby UGA terminator, and one mutation (e190) was a deletion that altered the reading frame and caused termination at an ochre codon. Two mutations (e675 and s291) were inphase deletions, which resulted in a shortened myosin rod segment. These aberrant myosins fail to assemble into normal thick filaments. The sequence alterations of the missense mutations (e1152, s74, s95) indicated amino acid residues that are critical for myosin function. The mutation e1152 causes the production of a myosin heavy chain that fails to assemble into thick filaments. It had two adjacent amino acid substitutions at the extreme amino terminus of the rod, indicating a role for subfragment-2 in thick filament assembly. Mutants homozygous for s74 or s95 are very slow-moving, although they make myosin heavy chains that assemble normally. The encoded amino acid substitutions of s95 and s74 are in the 23 X 10(3) Mr and 50 X 10(3) Mr domains of the myosin head, flanking the ATP binding site. The sequenced mutations are distributed throughout the gene in the order predicted from genetic fine-structure mapping experiments. Seven of eight point mutations isolated following ethylmethane sulphonate mutagenesis were G X C to A X T transitions. A single X-ray-induced allele proved to be a deletion of two adjacent thymidine residues. The three deletion mutations were found in a region of the myosin rod with numerous direct and inverted nucleotide sequence repeats, but their origin cannot be accounted for by homologous recombination. Instead, a comparison of the deletion junctions suggests that the deletions arose by a site-specific mechanism.

Effect of tryptic cleavage on the stability of myosin subfragment 1. Isolation and properties of the severed heavy-chain subunit

The procedure of thermal ion-exchange chromatography has been used to examine the effect of prior tryptic cleavage on the stability of myosin subfragment 1 (SF1). Although it is found that digestion does destabilize the subunit interactions at physiological temperatures, the heavy-chain subunit can be isolated either as an equimolar complex comprised of 50K, 27K, and 21K fragments or as one comprised of 50K, 27K, and 18K peptides. Thus, the interactions within the heavy chain are considerably more stable than those between the two subunits. Both forms of the free severed heavy chain exhibit ATPase properties similar to those of the parent tryptic SF1. The Vmax for the actin-activated MgATPase of the free severed heavy chain is the same as that for both undigested and tryptic SF1 (A2). Since its Km for actin is similar to that of tryptic SF1(A2), it may be concluded that changes in the affinity of SF1 for actin induced by trypsin [Botts, J., Muhlrad, A., Takashi, R., & Morales, M. F. (1982) Biochemistry 21, 6903-6905] are not dependent on the presence of the associated alkali light chain. Furthermore, the communication between the SH1 site and the ATPase site is also shown to be independent of the associated alkali light chain, and it persists despite the cleavages present in the free heavy chain. Studies on the ability of these severed heavy chains to reassociate with free A1 and A2 chains indicate that the binding site is retained in the 21K-severed heavy chain but is lost in the 18K form.

Incorporation of 6-carboxyfluorescein into myosin subfragment 1

We describe for the first time the introduction of a label into the "50K" domain of myosin subfragment 1 (S-1), and we investigate the properties of this fluorescent modification in relation to the ATPase and actin-binding activities, both residing in the myosin head. The labeling consists of a major incorporation of 6-carboxyfluorescein into the "50K" domain of S-1. Using different conditions for tryptic digestion that allowed a fragmentation of the "50K" domain with a loss of 5 kilodaltons (kDa) leading to a final product of 45 kDa, we have shown that the fluorescent dye remains in the 45-kDa final product. By studying cross-linking as a function of time, we have demonstrated that the "50K" domain and the 45-kDa fluorescent peptide are equally cross-linkable to actin. We have also investigated the K+EDTA-, Ca2+-, Mg2+-, and actin-activated ATPase activities of this modified S-1 and after purification observed no enzymatic changes.

The interaction of skeletal myosin subfragment 1 with the polyanion, heparin

The association between chymotryptic skeletal muscle myosin subfragment 1 (S1) and the polyanion, heparin, was investigated as an experimental approach in probing the functional importance of the cationic sites on S1 and their involvement in ionic interactions within the myosin head during energy transduction. The direct binding of heparin, used at micromolar concentrations, and its influence on the structural and functional properties of S1 were followed by gel chromatography, electron microscopy, chemical cross-linking techniques and limited digestion studies. 1. The limited tryptic digestion of S1 showed that the presence of heparin, as well as of the homopolymer, poly-(L-glutamic acid) causes a specific structural change in the 50-kDa heavy chain region of S1 and accelerates the breakdown of this segment into a 45-kDa species by a proteolytic cleavage restricted to its COOH-terminal portion. Under similar experimental conditions, the binding of MgATP and MgADP to S1 led also to the 50-kDa----45-kDa conversion, suggesting that the S1-nucleotide interactions exhibit some resemblances to the polyanion-S1 binding of polyanionic ligands to S1. This particular area is adjacent to the actin site containing the 45-kDa and 20-kDa segments of the S1 heavy chain. On the other hand, the polyanions as well as nucleotides induced changes in the interface between the heavy chain and the alkali light chains. 2. Moreover, the binding of heparin to S1 resulted in the self-association of the enzyme and the production of stable small S1 oligomers, most likely dimers, which were demonstrated by the alteration of the size of the S1 particles examined by electron microscopy and their freezing by chemical cross-linking agents. These findings are relevant to the recently reported property of skeletal chymotryptic S1 to form dimers under convenient ionic conditions, in particular in the presence of Mg-nucleotides. The interaction of cationic sites on S1 and possibly on the 50-kDa region of the heavy chain with polyanions promotes the dimerization of the S1 molecules. The binding of S1 to F-actin abolished S1 aggregation.

Proteolysis and the domain organization of myosin subfragment 1

Because the proteolytic cleavage of a folded polypeptide depends not only on the specificity of the protease but on the nature of the folding, we investigated the cleavage of (chymotryptically produced) subfragment 1 (designated "S-1") or "head" segment of myosin by seven proteases with different specificities. All seven produced approximately the same three fragments of S-1--namely, fragments (from the NH2 terminus) of 27, 50, and 20 kilodaltons, suggesting that in intact S-1 these fragments are distinct domains. The same proteases were used to hydrolyze the MgADP complex of S-1. All failed to do so except trypsin, which, as found earlier [Hozumi, T. (1983) Biochemistry 22, 799-804], makes two additional cleavages. This result suggests that the conformational change induced by MgADP opens up only a small stretch of polypeptide chain, which stretch happens to be vulnerable to trypsin.

Crosslinking by thiol disulfide interchange of 5,5'-dithiobis(2-nitrobenzoic acid)-treated light chain and heavy chain of rabbit skeletal myosin

Interchain disulfide crosslinks between the heavy-chain fragment in heavy meromyosin and myosin light chain 2, generated by 5,5'-dithiobis(2-nitrobenzoic acid (Nbs2), are formed under appropriate ionic conditions at neutral pH as revealed by liberation of the chromogenic 2-nitro-5-thiobenzoic acid. The presence of the original or of a slightly digested light chain 2 reduces the rate of the reaction of heavy meromyosin with Nbs2-modified light chain 2 by 32 - 39%, if Ca2+ is present. Dodecyl sulfate/polyacrylamide gel electrophoresis in absence of reducing agents shows that Nbs2-modified light chain 2 attaches to the heavy chain in the region of the 21-kDa fragment of heavy meromyosin, which contains the essential thiol groups and which has been located at the subfragment 1/subfragment 2 junction of myosin [Balint, M., Wolf, I., Tarcsafalvi, A., Gergely, J. and Sreter, F. A. (1978) Arch. Biochem. Biophys. 190, 793-799]. Modification of thiol-1 groups with iodoacetamide as well as crosslinking the thiol-1 and thiol-2 groups by the bifunctional reagent p-N,N'-phenylenedimaleimide prior to incubation with Nbs2-modified light chain 2 has no substantial effect on the crosslinking reaction. This indicates that other thiol groups are involved in the binding of Nbs2-modified light chain 2 to the heavy chain. An examination of K+, Ca2+, Mg2+ and actin-activated Mg2+ ATPase activities of heavy meromyosin that had been crosslinked with Nbs2-modified light chain 2 shows only a slight change in comparison with intact heavy meromyosin, indicating that crosslinking had not altered significantly the hydrolytic site. Crosslinking of Nbs2-modified light chain 2 to light-chain-2-deficient heavy meromyosin restored the original light-chain-2-dependent Ca2+ sensitivity of the tryptic fragmentation of heavy meromyosin, suggesting that crosslinking takes place at the proper binding site for light 2.