Erratum: Identification of the gene for fly non-muscle myosin heavy chain: Drosophila myosin heavy chains are encoded by a gene family

[This corrects the article on p. 913 in vol. 8, PMID: 2498088.].     

Suppression of muscle hypercontraction by mutations in the myosin heavy chain gene of Drosophila melanogaster

The indirect flight muscles (IFM) of Drosophila melanogaster provide a good genetic system with which to investigate muscle function. Flight muscle contraction is regulated by both stretch and Ca(2+)-induced thin filament (actin + tropomyosin + troponin complex) activation. Some mutants in troponin-I (TnI) and troponin-T (TnT) genes cause a "hypercontraction" muscle phenotype, suggesting that this condition arises from defects in Ca(2+) regulation and actomyosin-generated tension. We have tested the hypothesis that missense mutations of the myosin heavy chain gene, Mhc, which suppress the hypercontraction of the TnI mutant held-up(2) (hdp(2)), do so by reducing actomyosin force production. Here we show that a "headless" Mhc transgenic fly construct that reduces the myosin head concentration in the muscle thick filaments acts as a dose-dependent suppressor of hypercontracting alleles of TnI, TnT, Mhc, and flightin genes. The data suggest that most, if not all, mutants causing hypercontraction require actomyosin-produced forces to do so. Whether all Mhc suppressors act simply by reducing the force production of the thick filament is discussed with respect to current models of myosin function and thin filament activation by the binding of calcium to the troponin complex.     

A canonical sequence organization at the 5'-end of the myosin heavy chain genes

The sequences encoding the 5'-ends of three chicken fast-white myosin heavy chain (MHC) genes have been determined. When compared with the sequences of two other MHC genes it is apparent that both the exon and intron positions are conserved. All exon sequences are highly conserved; there is absolute amino acid conservation in the second and third exons. In addition, while the first and third introns diverge among the genes, the second intron is highly conserved between the five. This intron contains a 24-bp sequence that is repeated twice in one of the introns and once in the other four. Analyses indicate that this sequence, which is partially homologous to 7SL RNA, appears to be largely restricted to the MHC gene family. Analysis of the 5'-flanking sequences show that while small homologies are present between some of the genes, they have extensively diverged in this region.     

A novel beta thalassemia gene with a single base mutation in the conserved polypyrimidine sequence at the 3' end of IVS 2

An adult Algerian patient with homozygous beta thalassemia was found to have a unique beta thalassemia gene. Cloning and sequencing revealed that the only abnormality present in this beta gene is a transversion in the polypyrimidine stretch at the 3' end of the large intervening sequence (IVS 2) six bases 5' to the consensus AG dinucleotide sequence (CCGCCCACAG instead of CCTCCCACAG). In addition, digestion of the cloned fragment by the enzyme Mnl I demonstrates the disappearance of a restriction site as expected. This is the first example of a defect in the consensus sequence at the 3' end of an IVS leading to a thalassemia phenotype presumably due to decreased splicing.     

Comparative genomic analysis of the arthropod muscle myosin heavy chain genes allows ancestral gene reconstruction and reveals a new type of 'partially' processed pseudogene

Background  

Alternative splicing of mutually exclusive exons is an important mechanism for increasing protein diversity in eukaryotes. The insect Mhc (myosin heavy chain) gene produces all different muscle myosins as a result of alternative splicing in contrast to most other organisms of the Metazoa lineage, that have a family of muscle genes with each gene coding for a protein specialized for a functional niche.     

Smooth muscle myosin light chain kinase. Amino acid sequence at the site phosphorylated by adenosine cyclic 3',5'-phosphate-dependent protein kinase whether or not calmodulin is bound

Turkey gizzard smooth muscle myosin light chain kinase is a calmodulin-dependent enzyme containing 2 serine residues that can be phosphorylated by cAMP-dependent protein kinase. One of these sites can be phosphorylated only when calmodulin is not bound to the enzyme; the amino acid sequence around this site has been reported recently (Lukas, T. J., Burgess, W. H., Prendergast, F. G., Lau, W., and Watterson, D. M. (1986) Biochemistry 25, 1458-1464). Here we report the sequence around the site that is phosphorylated by cAMP-dependent protein kinase whether or not calmodulin is bound: Lys-Ala-Ser(P)-Gly-Ser-Ser-Pro-Thr-Ser-Pro-Ile-Asn-Ala-Asp-Lys-Val-Glu-A sn-Glu- . This sequence conforms to the previously defined criteria for substrates of cAMP-dependent protein kinase.     

The heavy chain genes of a lupus anti-DNA autoantibody are encoded in the germ line of a nonautoimmune strain of mouse and conserved in strains of mice polymorphic for this gene locus

The variable region of the heavy chain of a prototypic anti-DNA autoantibody from the lupus-prone mouse, MRL-lpr/lpr, was cloned and sequenced. The VH and JH genes expressed by this antoantibody were found to be identical to germ line genes from the nonautoimmune mouse strain, BALB/c. The D gene of this autoantibody differed by one nucleotide from several members of the germ line SP2 family, but has been found in expressed D genes from several strains of mice. These results show that a normal mouse strain contains all of the structural information necessary for the expression of the heavy chain variable region of a lupus autoantibody. A fragment that is present in both BALB/c and MRL mice is highly homologous in both coding and flanking sequences to the autoantibody VH gene (VH130) and is the same size as the BALB/c germ line gene. This suggests that these two strains may share the same allele of this VH gene, despite the fact that they are polymorphic for this VH gene family. Other mouse strains that are polymorphic for this locus contained one to three VH genes that were highly related to VH130 in both coding and flanking regions. Thus, VH genes that may be allelic to the antibody VH gene or that may have arisen by gene conversion, unequal crossing over or gene duplication, are conserved in many mouse strains.