Complete primary structure of chicken cardiac C-protein (MyBP-C) and its expression in developing striated muscles.

C-protein (MyBP-C) is a myosin binding protein of about 140 kDa which is known to modulate myosin assembly in striated muscles. A cardiac-type isoform of C-protein appears not only in cardiac muscle but also in skeletal muscle before skeletal muscle-type isoforms become detectable during myogenesis, suggesting that the cardiac isoform is involved in the early phase of myofibrillogenesis (Bähler et al., 1985; Kawashima et al., 1986). In this study, in order to understand the structure and functional domains of the cardiac-type C-protein, we cloned and sequenced full-length cDNAs encoding chicken cardiac C-protein from lambda gt11 cDNA libraries which were prepared with poly (A)+ RNA from embryonic chicken cardiac muscle as well as embryonic chicken skeletal muscle by using antibodies specific for cardiac C-protein. Two cDNA variants, probably generated by alternative RNA splicing and encoding different C-protein isoforms, were detected. As judged by the cDNA sequences determined, overall homology of the peptide sequence between cardiac and skeletal muscle C-proteins (Einheber et al., 1990; Fürst et al., 1992, Weber et al., 1994) was about 50-55%. Like other myosin binding proteins, skeletal C-proteins, 86 kDa protein and M-protein, cardiac C-protein contains several copies of fibronectin type III motifs and immunoglobulin C2 motifs in the molecule, but their number and arrangements differed somewhat from those in the other proteins. Northern blot analysis with the cloned cDNA as a probe demonstrated that mRNA of 5.0 kb is transcribed in both cardiac and embryonic skeletal muscle, and that it is specifically expressed in cardiac muscle among adult tissues.