cDNA cloning and sequence comparisons of human and chicken muscle C-protein and 86kD protein

Thick filaments in vertebrate striated muscles are composed of myosin heavy chain (MHC) and myosin light chains (MLCs) plus at least eight other proteins: C-protein, 86kD protein (birds) or H-protein (mammals), M-protein, myomesin, titin, MM-creatine kinase, skelemin, and AMP-deaminase. Except for CPK and AMP deaminase, none have well defined functions. Analysis of cDNA clones encoding chicken C-protein and 86kD protein has revealed a high degree of shared amino acid identity, particularly in the C-terminal 40kD. To identify functionally significant regions, the human counterpart of each protein was cloned, sequenced and analysed. Two human C-protein cDNAs were isolated with significant homology to chicken fast C-protein. Clone H75, with 69% identity to chicken fast C-protein, shows the same pattern of hybridization as the chicken fast C-protein in chicken muscles. The other clone, H8 with 60% identity, shows a pattern of hybridization in chicken muscles which is consistent with the expression of chicken slow C-protein. The human 86kD protein shares 66% DNA sequence identity with the chicken 86kD protein. Assuming that essential sequences would be conserved during evolution, we compared the chicken and human proteins using PALIGN. Chicken and human fast C-proteins possess 66% peptide identity over their deduced length plus 10% conservative substitutions. Human slow C-protein and chicken fast C-protein share 44% peptide sequence identity, plus 16% conservative substitutions. Chicken and human 86kD proteins are also very similar: 54% peptide identity plus 20% conservative substitutions. This high degree of sequence identity between chicken and human C- and 86kD proteins suggests selective pressure on the primary sequence. Recent primary sequence analyses of projectin and mini-titins from Drosophila, twitchin from C. elegans, C-protein, smMLCK, 86kD protein, and M-protein from the chicken, titin from the rabbit, and skelemin from the mouse reveals that all these proteins possess multiple internal repeats of approximately 100 amino acids. These repeating domains are of two types: one is homologous to the internal repeats which define the C-2 subset of the immunoglobulin superfamily, the other is related to the fibronectin type III repeat. Both human C-proteins possess comparable internal repeats and preliminary evidence suggests the presence of the same repeats in human 86kD. This duality of repeat structure is found in many extracellular proteins and is typified by the N-CAMs.(ABSTRACT TRUNCATED AT 400 WORDS)