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

Molecular cloning and sequence of Sparus aurata skeletal myosin light chains expressed in white muscle: developmental expression and thyroid regulation

Two full-length cDNA clones encoding the skeletal myosin light chain 2 (MLC2; 1452bp) and myosin light chain 3 (MLC3; 972bp) were isolated from a cDNA library prepared from gilthead sea bream Sparus aurata larvae. The MLC2 cDNA encoded a predicted protein of 170 residues that was 79% identical to rabbit MLC2 over the entire length and 87% identical within the Ca2+-binding region. The deduced amino acid sequence of MLC3 was 153 residues in length and was 91% and 69% identical to the zebrafish and rabbit MLC3, respectively. Northern blot analysis revealed that in adults both transcripts were expressed in fast white muscle only. MLC2 appeared earlier in development: MLC2 transcripts were detectable from the beginning of segmentation, whereas MLC3 transcripts did not appear until 27h post-fertilisation. At this developmental stage, a second MLC2 transcript of 0.89 kilobase-pairs was present. MLCs exhibited a different age-related pattern of response to varied thyroidal states, which were experimentally induced by the administration of 1μgg−1bodymass of thyroxine (T4) or triiodothyronine (T3), or 5ngg−1bodymass of the hypothyroidal compound thiourea; MLC3 expression was not significantly affected, whereas levels of MLC2 transcripts were significantly elevated in the white muscle only of juvenile sea bream after administration of T4. Although the mechanism of thyroidal regulation of MLC expression remains unknown, the present results suggest that different regulatory mechanisms exist for different MLCs.

Myosin isoforms in red and white muscles of some marine teleost fishes

The myosin content from red and white muscles of three marine fish species, saithe (Pollachius virens. L.), haddock (Melanogrammus aeglefinus, L.), both members of the family Gadidae, and capeline (Mallotus villosus, M.) of the family Osmeridae, was analyzed electrophoretically. Analysis of the native myosin by electrophoresis under non-dissociating conditions revealed two isoforms in red muscles, and three or four in white muscles. The white muscles of the two closely related species had a similar pattern of isoforms. Myosin from the slow red muscles had two types of light chain, LC1S and LC2S, and myosin from the fast white muscles three, LC1F, LC2F, and LC3F. The pattern of light chains in both types of muscles was species-dependent. All the light chains from fish myosins were more acidic than those of the rat diaphragm used as standard. One main type of heavy chain was detected in each kind of muscle. In white muscles of saithe there was an extra band, present in minor amounts. The heavy chains from white muscle myosin had lower electrophoretic mobilities than those from red muscle, and the mobilities of all of them were intermediate between those of the heavy chains type IIa and I of rat diaphragm myosin. In our opinion, there are probably more isomyosins in fish muscles than those detected in the present work and their presence is obscured by comigration with the main types.