The myosin alkali light chain proteins and their genes

Regulatory and essential light-chain-binding sites in myosin heavy chain subfragment-1 mapped by site-directed mutagenesis

Site-directed mutagenesis of the cloned subfragment-1 (S-1) region of the unc-54 gene, encoding the myosin heavy chain B (MHC B) from Caenorhabditis elegans, has been used to locate binding sites for the regulatory and essential light chains. MHC B S-1 synthesized in Escherichia coli co-migrated with rabbit skeletal muscle myosin S-1 (Mr 90,000), was recognized by anti-nematode myosin antiserum on immunoblots, and specifically bound to 125I-labelled regulatory and essential light chains in a gel overlay assay. Deletion of 102 residues from the C terminus (mutant 655) reduced regulatory and essential light-chain binding to about 30% and 20% of wild-type levels, respectively. Similar reductions in relative binding of the two light chains were seen with mutant 534, in which 38 residues were deleted from the C terminus. Potential binding sites within 75 residues of the C terminus of S-1 were mapped by construction of five other mutant S-1 clones (398, 399, 400, 409 and 411) containing internal deletions of ten to 12 amino acid residues. These showed up to 30% reductions in their ability to bind essential light chains, but did not differ significantly from wild-type in their ability to bind regulatory light chains. Another mutant, 415, containing a deletion of a conserved acidic hexapeptide, E-D-I-R-D-E, showed enhancement of binding of regulatory and essential light chains to 150% and 165% of wild-type levels. Hence, the major binding sites for both light chains are within 38 amino acid residues of the C terminus.

Sequence and expression of human myosin alkali light chain isoforms

In order to initiate the study of the functional differences between myosin alkali light chain isoforms and to investigate the mechanisms of their differential expression, we have isolated cDNA clones for two human alkali light chain isoforms. Here we report DNA sequence and RNA blotting analyses that demonstrate that these cDNAs represent transcripts encoding human MLC3F and MLC1Sb. The sequence of the human MLC1Sb cDNA offers the first fully characterized example of a slow-fiber skeletal muscle alkali light chain isoform from any species. The sequence analysis of these two cDNAs allows an examination of evolutionarily conserved features of mammalian alkali light chain genes. Examination of the genomic organization of the human alkali light chain isoform genes revealed that, in contrast with some strains of mice, both are single copy genes. RNA blot analysis conclusively demonstrates that the human skeletal muscle MLC1Sb gene is also expressed in the heart ventricle but not the atria. In addition, we examined the expression of alkali light chain isoforms during the in vitro differentiation of a variety of human and rodent myogenic cells and found striking variation in the pattern of alkali light chain isoform gene expression in different myogenic cells.

Correlation among the proton charges and molecular masses of myosin subunits

The molecular masses and isoelectric points of myosin light and heavy chains were calculated from their known primary sequences and their respective distribution in a two-dimensional graph is displayed. Implications for the electrophoretic study of myosin subunits are inferred from this analysis.

Developmental and muscle-specific changes in methylation of the myosin light chain LC1f and LC3f promoters during avian myogenesis

The fast alkali myosin light chains LC1f and LC3f are two contractile protein isoforms encoded for by a single gene complex. Expression of these two isoforms is dependent upon differential initiation of transcription at either of two promoters encoding unique 5' exons for isoform-specific amino termini of these light chains. Studies of protein expression have shown that the two isoforms are first expressed at different stages of development and in the case of the LC3f isoform only in fast twitch muscle fiber types. The molecular mechanisms that regulate the differential transcription of the gene complex are unknown. Experiments reported here demonstrated the direct correlation of isoform protein and mRNA expression with the undermethylation of the DNA in the promoter regions of the gene for each of the expressed myosin light chain isoforms. We find that fast and slow muscles have different patterns of undermethylation of the two promoter regions of the gene. Moreover, changes in methylation of the promoter regions were shown to occur specifically in skeletal muscle tissue, to be developmentally regulated, and to only occur in the LC3f promoter of those muscle groups that express LC3f protein.

Chromosomal assignment of two myosin alkali light-chain genes encoding the ventricular/slow skeletal muscle isoform and the atrial/fetal muscle isoform (MYL3, MYL4)

In all eukaryotes, myosin plays a major role in the maintenance of cell shape and in cellular movement; in association with actin and other contractile proteins it is also a major structural component of the muscle sarcomere. Several isoforms of myosin alkali light chain have been identified, associated with different muscle types. We have recently localized the gene encoding the fast skeletal muscle alkali light-chain isoforms MLC1F and MLC3F (HGM symbol, MYL1) to human chromosome 2q32.1-qter (Cohen-Haguenauer 1988). We present here the chromosomal assignment of two loci encoding the ventricular muscle isoform MLC1V (equivalent to the slow skeletal muscle isoform MLC1Sb) and the atrial muscle isoform MLC1A (equivalent to the fetal isoform MLC1emb) using a panel of 25 independent man-rodent somatic cell hybrids. The MLC1V gene (HGM symbol, MYL3) was mapped to human chromosome 3 using a human full-length cDNA probe that hybridizes to a single major human TaqI2.8-kb fragment. The MLC1A probe (HGM symbol, MYL4) was a 360-bp mouse cDNA fragment that gave a distinct signal with human DNA using low stringency conditions of hybridization and washings and after presaturation of the Southern blots with rodent DNA. A single PstI 7.8-kb fragment gives an intense signal, and its presence correlates with the presence of chromosome 17 among the hybrids. These data are in keeping with the localizations of the MLC1V gene to mouse chromosome 9, and of the MLC1A gene to mouse chromosome 11, which share some markers in common with human chromosomes 3 and 17 respectively.

Chronic stimulation-induced changes of myosin light chains at the mRNA and protein levels in rat fast-twitch muscle

Transitions in the expression of the myosin light chains (LC) were investigated in fast-twitch muscles of the rat during chronic (10 h/day), low-frequency (10 Hz) stimulation. Changes were followed at the mRNA level by Northern blot analysis and in vitro translation, as well as at the protein level by electrophoresis under denaturing and nondenaturing conditions. In vivo synthesis of the light chains was assessed by measuring the incorporation of intramuscularly injected [35S]methionine. Chronic stimulation induced a transition in the isomyosin pattern with an increase of FM3, a concomitant decrease in FM1 and, after longer stimulation periods, the appearance of low concentrations of the slow isomyosin. These changes were accompanied by an elevated LC1f/LC3f ratio and increases in the amounts of both the LC1sb and, to a lesser degree, LC2s proteins. Alterations in the amounts of specific mRNAs were the same whether determined by Northern blot analysis or by in vitro translation of total RNA preparations from the same muscles. Generally, the changes in the relative concentrations of fast and slow light-chain proteins agreed with the changes detected at the mRNA level and the alterations in protein synthesis detected with the use of an in vivo labeling assay. An exception was the elevated tissue content of LC2s where no changes were detectable in the concentration of its mRNA as determined by in vitro translation or in vivo synthesis. The increase in LC2s protein may, therefore, have been due to reduced degradation. In addition, the decrease in LC3f was more pronounced at the protein level than at the mRNA level. This might indicate an increased turnover of LC3f or the existence of additional post-transcriptional regulations of LC3f expression.

Altered expression of myosin light-chain isoforms in chronically stimulated fast-twitch muscle of the rat

Fast-twitch tibialis anterior muscle of the rat was chronically stimulated for periods of 18 days, 28 days and 56 days. Changes in the myosin light-chain (LC) pattern consisted in an increase in LC1f, concomitant with a decrease in LC3f. In contrast to previous findings in chronically stimulated fast-twitch tibialis anterior muscle of the rabbit, no substantial increases occurred in the slow myosin light-chain isoforms. In vivo labeling using [35S]methionine incorporation revealed differences in relative turnover between the fast myosin light chains. The relative turnover of the fast myosin light chains appeared to increase in normal muscle in the order LC2f less than LC1f less than LC3f. As judged from [35S]methionine incorporation, the changes in light-chain tissue content mainly resulted from altered synthesis rates. However, in the case of LC3f the decrease in protein content could not only be explained by a reduced synthesis, but, additionally, appeared to be due to enhanced degradation. Parvalbumin, which was included in the present study, was also found to decrease in the stimulated muscle. However, its decrease appeared to result primarily from reduced synthesis.

Myosin light chains of guinea-pig striated muscles. Similarities and differences with rat myosin light chains

1. Myosin light chains of guinea-pig striated muscles have been screened by two-dimensional gel electrophoresis and compared to rat myosin light chains. 2. The fast type light chains 1F and 3F, slow type light chains 1S and 2S, and embryonic type light chain 1E are shown to differ in the two rodents; only the fast type light chains 2F co-electrophorese on the gel. 3. In guinea-pig, as in rat, ventricle muscle light chains appear the same as the 1S and 2S light chains and atrial light chain type 1 the same as the 1E light chain. We show that this embryonic light chain of guinea-pig myosin is difficult to identify and may be confused with the adult 1F light chain.

A novel human myosin alkali light chain is developmentally regulated. Expression in fetal cardiac and skeletal muscle and in adult atria

We have isolated cDNA recombinant phages encoding the embryonic isoform of the myosin alkali light chain (MLC1emb) from a human fetal skeletal muscle library. The cDNA clones were detected by their weak cross-hybridization to a human MLC1F and MLC3F cDNA clone. Nucleotide sequence analysis of the complete cDNA (GT14) revealed an open reading frame for 197 amino acids. The derived protein sequence constitutes the first structural information on this myosin isoform of any organism. Remarkable structural similarities to other alkali MLC polypeptides, particularly to those of the slow-muscle type, are evident. Under conditions of high stringency, the GT14 clone hybridized to an abundant mRNA species in fetal ventricular muscle and adult atrial muscle, whereas in fetal skeletal muscle only a very weakly hybridizing mRNA component was detected. These mRNAs were indistinguishable by size and the thermal stability of their hybrids formed with the DNA insert of clone GT14. We therefore conclude that identical mRNA is expressed in these tissues, presumably transcribed from the same gene. According to its pattern of mRNA expression, the novel MLC isoform described here was designated as "embryonic and atrial myosin light chain" (MLC1emb/A) in reference to its developmental stage-specific and tissue-specific appearance in embryonic skeletal muscle, fetal ventricle and adult atrium.

Promoter analysis of myosin alkali light chain genes expressed in mouse striated muscle

There are three principal myosin alkali light chain (MLC) genes expressed in mouse striated muscle. The skeletal muscle gene MLC1F/MLC3F, the ventricular muscle/slow skeletal muscle gene MLC1V(MLC1S), and the atrial muscle/foetal striated muscle gene MLC1A(MLC1emb). MLC1V and MLC1A are expressed in both cardiac and skeletal muscle, and we show here that these genes use a single site of initiation of transcription, and therefore the same proximal promoter region, in both muscle types, and in myogenic cell lines in culture. We have previously shown that for the MLC1F/MLC3F gene, 1200bp of upstream sequence from the MLC1F promoter is sufficient to allow tissue specific and developmentally regulated expression. We have therefore isolated, characterised, and sequenced over 1200bp upstream of each of the three MLC genes in order to look for elements which may be involved in their regulation. Detailed comparison of their promoter sequences, as well as those of the cardiac and skeletal muscle alpha-actin genes, reveals a number of common elements. Among these is an "MLC-sequence" (CCTTTTATAG) common to all MLC genes, including those of chick and rat, and a "cardiac sequence" common to the mouse MLC1A, MLC1V and alpha-cardiac actin genes expressed in the heart.

Upstream regulatory region for inducible expression of the chicken skeletal myosin alkali light-chain gene

The expression of the fast type of myosin alkali light chain 1 is induced during the differentiation of muscle cells. To study the mechanism of its gene regulation, we joined the sequence of the 5'-flanking and upstream region of the chicken myosin alkali light-chain gene to the structural gene for chloramphenicol acetyltransferase (CAT). The fusion gene was introduced either into quail myoblasts transformed by a temperature-sensitive mutant of Rous sarcoma virus (tsNY68) or into chicken myoblasts, and the transiently expressed CAT activity was assayed after the differentiation of the myoblasts. From the experiments with the external and internal deletion mutants of the fusion gene, the cis-acting regulatory region responsible for the enhanced expression of the CAT activity in response to the cell differentiation was found to be localized at 2 kilobases upstream of the transcription initiation site. This region of 160 nucleotides contained two pairs of short sequences worthy of note, a direct repeat of 12 nucleotides, and an inverted repeat of 8 nucleotides. The nucleotide sequences of the 5'-flanking sequence up to nucleotide -3381 were determined and compared with those of the upstream activating elements of actin genes.

Evolution of the EF-hand calcium-binding protein family: evidence for exon shuffling and intron insertion

The evolutionary history of the intracellular calcium-binding protein superfamily is well documented. The members of this gene family are all believed to be derived from a common ancestor, which, itself, was the product of two successive gene duplications. In this study, we have compared and analyzed the structures of the recently described genes coding for these proteins. We propose a series of evolutionary events, which include exon shuffling and intron insertion, that could account for the evolutionary origin of all the members of this superfamily. According to this hypothesis, the ancestral gene, a product of two successive duplications, consisted of at least four exons. Each exon coding for a peptide (a calcium-binding domain) was separated by an intron that had mediated the duplication. Each distinct lineage evolved from this ancestor by genomic rearrangement, with insertion of introns being a prominent feature.

Molecular genetics in basic myology: a rapidly evolving perspective

Myology has greatly benefited from the recent unification of concepts in molecular, cellular, and developmental biology. The interplay between intrinsic and extrinsic factors in determining the physiologic characteristics of individual myofibers has emerged as an important theme. Of special note is the manner in which the study of contractile protein gene structure and expression has contributed to our understanding of the development and ultimate plasticity of the contractile apparatus. As mechanistic models of normal myogenesis achieve increasing sophistication, the opportunities for understanding the pathogenesis of progressive muscle disfunction improve. In this article we review recent progress in basic myology which will be of interest to clinicians studying the heritable neuromuscular disorders.

Alkali myosin light chains in man are encoded by a multigene family that includes the adult skeletal muscle, the embryonic or atrial, and nonsarcomeric isoforms

A set of cDNA clones coding for alkali myosin light chains (AMLC) was isolated from fetal human skeletal muscle. Nucleotide sequence analysis and RNA expression patterns of individual clones revealed related sequences corresponding to (i) fast fiber type MLC1 and MLC3; (ii) the embryonic MLC that is also expressed in fetal ventricle and adult atrium (MLCemb); and (iii) a nonsarcomeric MLC isoform that is found in all nonmuscle cell types and smooth muscle. The AMLC gene family in man comprises unique copies for MLC1, MLC3 and MLCemb, and multiple copies for the nonsarcomeric MLC genes. The gene coding for MLC1 and MLC3 is located on human chromosome 2.

Upstream regulatory region for inducible expression of the chicken skeletal myosin alkali light-chain gene

The expression of the fast type of myosin alkali light chain 1 is induced during the differentiation of muscle cells. To study the mechanism of its gene regulation, we joined the sequence of the 5'-flanking and upstream region of the chicken myosin alkali light-chain gene to the structural gene for chloramphenicol acetyltransferase (CAT). The fusion gene was introduced either into quail myoblasts transformed by a temperature-sensitive mutant of Rous sarcoma virus (tsNY68) or into chicken myoblasts, and the transiently expressed CAT activity was assayed after the differentiation of the myoblasts. From the experiments with the external and internal deletion mutants of the fusion gene, the cis-acting regulatory region responsible for the enhanced expression of the CAT activity in response to the cell differentiation was found to be localized at 2 kilobases upstream of the transcription initiation site. This region of 160 nucleotides contained two pairs of short sequences worthy of note, a direct repeat of 12 nucleotides, and an inverted repeat of 8 nucleotides. The nucleotide sequences of the 5'-flanking sequence up to nucleotide -3381 were determined and compared with those of the upstream activating elements of actin genes.

Enzymatic amplification of myosin heavy-chain mRNA sequences in vitro

We have developed a procedure that detects the presence of mRNA coding for human beta-myosin heavy chain in small amounts of total, unfractionated RNA isolated from heart or skeletal muscle. The protocol is based on the enzymatic amplification in vitro of a selected 106-bp myosin isotype-specific subregion of this mRNA. The method, which is a modification of the so-called "polymerase chain reaction," requires two synthetic oligonucleotide primers (20-mers), reverse transcriptase, and DNA polymerase I (Klenow fragment). Two principle steps are involved: (i) the selected mRNA subregion is converted into a double-stranded cDNA, and (ii) this cDNA is amplified in 22 synthetic cycles. After gel electrophoresis and blotting the amplification product is identified by hybridization with a third oligonucleotide recognizing the region between the two primer annealing sites, and by restriction mapping. Only mRNA from muscle tissue promoted formation of the amplified 106-bp fragment. We estimate that less than 30,000 beta-myosin heavy-chain mRNA molecules are sufficient to produce a signal. The procedure is fast, specific, and very sensitive. It may be used in muscle gene expression studies with small numbers of cells or even in single muscle fibers.

Physicochemical and immunological properties of myosin light chains from the ordinary muscle of marine teleost fishes

1. Myosin light chains (A1, A2 and DTNB light chain) were isolated from the ordinary muscle of six marine fishes and the physicochemical and immunological properties were examined. 2. All the isolated light chains were rich in aspartic and glutamic acids, alanine and lysine, but poor in histidine and tyrosine. The contents of alanine, proline and lysine were generally higher in A1 than in A2. Between closely related species such as skipjack and bonito, the amino acid profiles of corresponding light chains were very similar to each other. 3. Structural similarity and difference of A1 light chain were further demonstrated immunologically using anti-A1 antiserum. Precipitation lines were fused with each other among closely related species, whereas strong spurs were observed among phylogenetically distinct species.

Assignment of the human fast skeletal muscle myosin alkali light chains gene (MLC1F/MLC3F) to 2q 32.1-2qter

A DNA probe derived from a mouse intronless pseudogene including coding regions for the myosin fast skeletal muscle alkali light chains, MLC1F/MLC3F (suggested HGM symbol, MYL1), was tested on a panel of 25 independent man-rodent somatic cell hybrids in order to assign the human MLC1F/MLC3F gene to a human chromosome. A 3.7-kb TaqI human fragment was found to correlate with the presence of chromosome 2 in the hybrids, characterized both by cytogenetic analysis and reference enzyme markers. A regional assignment to 2q32.1-qter was possible using hybrids whose human parental strains bore a reciprocal translocation t(X;2) (p22;q32.1). The fact that IDH1 and the MLC1F/MLC3F gene are closely linked on chromosome 1 in the mouse and map to the same region of human chromosome 2 in man indicates, that these chromosomes have a conserved region of homology between them and that the human 3.7-kb TaqI fragment corresponds indeed to a functional gene.

Evidence for distinct phosphorylatable myosin light chains in avian heart and slow skeletal muscle

In mammalian organisms the regulatory or phosphorylatable myosin light chains in heart and slow skeletal muscle have been shown to be identical and presumable constitute the product of a single gene. We analyzed the expression of the avian cardiac myosin light chain (MLC) 2-A in heart and slow skeletal muscle by a combination of experimental approaches, e.g., two-dimensional gel electrophoresis of the protein and hybridization of mRNA to specific MLC 2-A sequences cloned from chicken. The investigations have indicated that, unlike in mammals, in avian organisms the phosphorylatable myosin light chains from heart and slow skeletal muscle are distinct proteins and therefore products of different genes. The expression of MLC 2-A is restricted to the myocardium and no evidence was found that it is shared with slow skeletal muscle.

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.

Amino acid sequence of rabbit ventricular myosin light chain-2: identity with the slow skeletal muscle isoform

Many studies have established a correlation of differences in the activities of various muscle types with differences in the expression of myosin isoforms. In this paper we report the sequence determination of myosin light chain-2 from rabbit slow skeletal (LC2s) and ventricular (LC2v) muscles. We sequenced tryptic peptides from LC2v which account for all except a few terminal amino acid residues. The major part (87 residues) of the rabbit LC2s sequence, obtained from tryptic and cyanogen bromide (CNBr) peptides, was found to be identical to rabbit LC2v. Our results provide the first sequence information on LC2s from any species, and lend strong support to the hypothesis that LC2s and LC2v are identical. Comparisons of rabbit LC2v and LC2s with rabbit LC2f (from fast skeletal muscle), and also with chicken LC2f and LC2v, show clearly that LC2s and LC2v from mammalian and avian species are more closely related to each other than they are to LC2f isoforms from the same species.