A common myosin light chain is expressed in chicken embryonic skeletal, cardiac, and smooth muscles and in brain continuously from embryo to adult

The light chain composition of chicken brain myosin-Va: calmodulin, myosin-II essential light chains, and 8-kDa dynein light chain/PIN

Class V myosins are a ubiquitously expressed family of actin-based molecular motors. Biochemical studies on myosin-Va from chick brain indicate that this myosin is a two-headed motor with multiple calmodulin light chains associated with the regulatory or neck domain of each heavy chain, a feature consistent with the regulatory effects of Ca(2+) on this myosin. In this study, the identity of three additional low molecular weight proteins of 23-,17-, and 10 kDa associated with myosin-Va is established. The 23- and 17-kDa subunits are both members of the myosin-II essential light chain gene family, encoded by the chicken L23 and L17 light chain genes, respectively. The 10-kDa subunit is a protein originally identified as a light chain (DLC8) of flagellar and axonemal dynein. The 10-kDa subunit is associated with the tail domain of myosin-Va.

Alterations of myocardial contraction associated with a structural heart defect in embryonic chicks

Ablation of cardiac neural crest at stages 8-10 produces a structural heart defect (persistent truncus arteriosus, PTA) in embryonic chicks. PTA is associated with decreased myocardial contractility, as indicated by decreased left ventricular ejection fraction. We compared the force of small ventricular strips from normal and defective chick hearts. In intact muscle, ablation of the neural crest leads to a 30-50% decrease in twitch force at any level of extracellular Ca2+ (0.45-20 mM) at embryonic days (ED) 7 and 15, relative to sham-operated controls. These differences could reflect defects at the level of the contractile apparatus and/or in the excitation-contraction coupling process. To distinguish changes of the contractile apparatus, we used detergent skinned preparations. The maximal Ca(2+)-activated force (Fmax) at ED15 was not significantly different between control and experimental embryos. At ED 7, however, Fmax was reduced by 36% in experimental preparations. Electron-micrographs showed that the organization and orientation of the myofibrils was similar in experimental and control ventricles. At ED 14, however, the average myofibrillar diameter was significantly increased in experimental ventricles. The content of the major myofibrillar proteins (myosin heavy chain, actin, and tropomyosin), determined from polyacrylamide gel electrophoresis and Coomassie Blue staining, normalized to total protein, was not statistically different in experimental and control ventricles at ED7. At ED15, however, content of these proteins was doubled in experimental ventricles. These data suggest a possible defect of the contractile apparatus at both ED 7 and 15, since the ratio of Fmax/myosin is reduced in the experimental hearts.

Evolutionarily conserved promoter region containing CArG*-like elements is crucial for smooth muscle myosin heavy chain gene expression

In recent years, significant progress has been made toward understanding skeletal muscle development. However, the mechanisms that regulate smooth muscle development and differentiation are presently unknown. To better understand smooth muscle-specific gene expression, we have focused our studies on the smooth muscle myosin heavy chain (SMHC) gene, a highly specific marker of differentiated smooth muscle cells. The goal of the present study was to isolate and characterize the mouse SMHC gene promoter, since the mouse promoter would be particularly suited for in vivo promoter analyses in transgenic mice and would serve as a tool for targeting genes of interest into smooth muscle cells. We report here the isolation and characterization of the mouse SMHC promoter and its 5' flanking region. DNA sequence analysis of a 2.6-kb portion of the promoter identified several potential binding sites for known transcription factors. Transient transfection analysis of promoter deletion constructs in primary cultures of smooth muscle cells showed that the region between -1208 and -1050 bp is critical for maximal SMHC promoter activity. A comparison of SMHC promoter sequences from mouse, rat, and rabbit revealed the presence of a highly conserved region located between -967 and -1208 bp. This region includes three CArG/CArG*-like elements, two SP-1 binding sites, a NF-1-like element, an Nkx2-5 binding site, and an Elk-1 binding site. Gel mobility shift assay and DNase I footprinting analyses show that all three CArG/CArG*-like elements can form DNA-protein complexes with nuclear extract from vascular smooth muscle cells. Protein binding to the CArG* elements can be competed out by either serum response element or by an authentic CArG element from the cardiac alpha-actin gene. Using a serum response factor (SRF) antibody, we demonstrate that SRF is part of the protein complex. In addition, we show that cotransfection with the SRF dominant-negative mutant expression vector abolishes SMHC promoter activity, suggesting that SRF protein plays a critical role in SMHC gene regulation.

Tissue-specific and non-tissue-specific heavy-chain isoforms of myosin in the brain as revealed by monoclonal antibodies

Four types of monoclonal antibody (BM-1, BM-2, BM-3 and BM-4) each having distinctive tissue specificity were obtained by immunizing mice with purified bovine cerebrum myosin. Both BM-1 and BM-2 reacted most efficiently with cerebrum myosin and less efficiently with myosins from other limited nonmuscle tissues, the tissue specificity of BM-1 being much narrower than that of BM-2. BM-3 reacted more efficiently with several other nonmuscle myosins than with cerebellar or cerebral myosin. BM-4 recognized various nonmuscle and smooth muscle myosins with a nearly equal efficiency. Cerebral myosin as well a cerebellar myosin contained two or more electrophoretic variants of the heavy chains. BM-1 and BM-3 as well as BM-2 and BM-3 were found to recognize selectively these distinct heavy-chain isoforms. The antigenic sites of the three tissue-specific antibodies (BM-1, BM-2 and BM-3) were all localized near the head/tail junction of the myosin molecules, while that of non-tissue-specific antibody BM-4 was near the center of the tail. These and additional results indicate that mammalian brain tissues as well as several other nonmuscle tissues contain multiple heavy-chain isoforms of myosin, the levels of which differed considerably from one tissue to another.

Human embryonic/atrial myosin alkali light chain gene: characterization, sequence, and chromosomal location

We have isolated and sequenced the gene encoding the human embryonic/atrial myosin alkali light chain isoform (MLC-1emb/A). The gene is split into seven exons by six introns; the last exon, as in all MLC isoform genes sequenced to date, is completely 3' untranslated sequence. Comparison of the MLC-1emb/A isoform gene with the other MLC-1 genes showed that the exon-intron arrangement of the human MLC-1emb/A isoform gene is analogous to that of the other MLC-1 type isoform genes. We have also mapped the human MLC-1emb/A isoform gene to the long arm of chromosome 17; the corresponding mouse gene has been mapped to chromosome 11. This gene, together with a number of others such as the collagen(I) alpha 1, galactokinase, and thymidine kinase genes, is part of the largest syntenic group between mouse and man.

Evolution of EF-hand calcium-modulated proteins. I. Relationships based on amino acid sequences

The relationships among 153 EF-hand (calcium-modulated) proteins of known amino acid sequence were determined using the method of maximum parsimony. These proteins can be ordered into 12 distinct subfamilies--calmodulin, troponin C, essential light chain of myosin, regulatory light chain, sarcoplasmic calcium binding protein, calpain, aequorin, Stronglyocentrotus purpuratus ectodermal protein, calbindin 28 kd, parvalbumin, alpha-actinin, and S100/intestinal calcium-binding protein. Eight individual proteins--calcineurin B from Bos, troponin C from Astacus, calcium vector protein from Branchiostoma, caltractin from Chlamydomonas, cell-division-cycle 31 gene product from Saccharomyces, 10-kd calcium-binding protein from Tetrahymena, LPS1 eight-domain protein from Lytechinus, and calcium-binding protein from Streptomyces--are tentatively identified as unique; that is, each may be the sole representative of another subfamily. We present dendrograms showing the relationships among the subfamilies and uniques as well as dendrograms showing relationships within each subfamily. The EF-hand proteins have been characterized from a broad range of organismal sources, and they have an enormous range of function. This is reflected in the complexity of the dendrograms. At this time we urge caution in assigning a simple scheme of gene duplications to account for the evolution of the 600 EF-hand domains of known sequence.

Prediction of the secondary structure of myosin light chains from comparison of homologous sequences. Implications for the interaction between myosin heavy and light chains

The primary sequences of seventeen essential and seventeen regulatory myosin light chains were analyzed and compared, using algorithms based on the different structural properties of their amino acid residues. This process allowed estimation of the structural homology between the proteins studied, and improved the prediction of their mean secondary structure and functionally important segments or residues. On the basis of the crystal structure of troponin C, a model of the myosin essential light chain with a fairly compact form is proposed. The possible sites of interaction between myosin light and heavy chains from rabbit skeletal muscle were also investigated by a complementarity method adapted to helix-rich proteins. Segments 139-149 and 65-75 in the essential light chain and segments 27-37, 67-77 and 97-107 in the regulatory light chain are suggested to constitute some of these sites, as most of them were found to have the features of surface-seeking helices.

Isolation of the chick myosin alkali light chain gene expressed in embryonic gizzard muscle and transitional expression of the light chain gene family in vivo

A chick embryonic myosin alkali light chain L23 gene that is expressed transiently at embryonic stages in chick skeletal, cardiac and smooth muscles and in brain continuously from embryo to adult stages, was isolated and characterized. Sequence analysis showed that the exonic sequence of this gene was identical with that of embryonic myosin light chain mRNA except for one base replacement. This gene is a single gene of 5200 bases, which is divided into seven exons by six introns, and the positions of inserts of all the introns are well-conserved as in the skeletal and cardiac muscle myosin alkali light chain genes. Therefore, this embryonic myosin light chain gene can be classified as a member of the myosin alkali light chain gene family, and these three genes may have originated from a common ancestral gene. Transcription of the embryonic light chain gene starts from the same initiation site 33 bases upstream from ATG in embryonic muscle tissues and brain. Comparison of the nucleotide sequence around the promotor region of the embryonic myosin light chain gene with the corresponding regions of the skeletal and cardiac myosin light chain genes showed that the 11-base consensus sequence (TCCTATTTATAG) is present about 100 bases upstream from the transcription initiation site in each gene.

Complexity of myosin species in the avian posterior latissimus dorsi muscle

The myosin content of the avian posterior latissimus dorsi muscle, a small fast-twitch muscle similar in fibre type to the much-studied pectoralis major muscle (type IIB), has been explored using high resolution chromatography of the proteolytic fragment known as subfragment-1 and of the products of its limited tryptic digestion, followed by N-terminal sequencing of selected peptides. The complexity of species found greatly exceeds that anticipated from the fibre-type homogeneity of the muscle and from previous studies (Bandman et al., Cell 29 (1982) 645-50; Lowey et al., J. Musc. Res. Cell Motility 4 (1983) 695-716; Crow & Stockdale Dev. Biol. 118 (1986) 333-42). A minimum of four heavy chain species were identified. One form, approximately 40% of the heavy chain complement, appears to be identical to the well-characterized type IIB isoform of the pectoralis major muscle. The remaining species differ from the pectoralis major form in primary sequence. None is identical to the post-hatch isoform of the pectoralis major muscle.

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.

Single chicken cardiac myosin alkali light-chain gene generates two different mRNAs by alternative splicing of a complex exon

We have isolated and characterized two kinds of cDNA for the chicken cardiac myosin alkali light chain. The sequences of the two cDNAs are identical, except for a notable divergence in part of the 3' untranslated sequence. By analysis of isolated genomic clones, it was shown that the genomic sequences corresponding to the different sequences in the 3' untranslated regions of the two mRNAs were arranged within a limited part of a single stretch of DNA; also the two distinct 3' untranslated regions of the two mRNAs shared part of the last exon, which was 0.6 x 10(3) base-pairs long. There are two canonical acceptor sites available for RNA splicing in the last exon, the first being located at the 5' end of the exon, and the second at 370 base-pairs downstream from this end. Together with analysis by S1 nuclease mapping, the foregoing results lead us to conclude that, by the differential use of these two acceptor sites, a single gene generates two distinct mRNAs of 1.45 x 10(3) base-pairs and 1.1 x 10(3) base-pairs with or without the 5' half of the last exon. The two mRNAs appear to utilize the same modified poly(A) signal, AGTAAA, rather than the authentic AATAAA sequence present about 30 base-pairs downstream from the poly(A) attachment sites. This is probably because another consensus G + T-rich sequence is present at an appropriate distance from the AGTAAA sequence, but not from the AATAAA sequence. The gene for the cardiac myosin alkali light chain has proved to be expressed in ventricular muscle and in atrial and anterior latissimus dorsi muscles, the last of these being characteristic of slow skeletal muscle. In these muscles, two kinds of mRNA for the cardiac myosin alkali light chain, identical with those in ventricular muscle, were expressed and their relative amount in each tissue was almost the same as that in ventricular muscle.