Complete sequence of human fast-type and slow-type muscle myosin-binding-protein C (MyBP-C). Differential expression, conserved domain structure and chromosome assignment.

Myosin-binding-protein C (MyBP-C) is a myosin-associated protein of unknown function found in the cross-bridge-bearing zone (C region) of A bands in striated muscle. Using a cDNA clone encoding the fast-type isoform of chicken MyBP-C, we screened a human fetal muscle cDNA library and isolated clones encoding the full-length human fast-type isoform of MyBP-C. cDNA clones encoding the slow-type isoform of human MyBP-C, were also isolated and fully sequenced. Northern-blot analysis demonstrated skeletal muscle-specific expression of these gene products. Using human/hamster somatic-cell hybrids, we were able to map the slow-type MyBP-C to human chromosome 12, and the fast-type MyBP-C to chromosome 19. The cDNA for human fast-type MyBP-C encodes a polypeptide of 1142 amino acids with an expected molecular mass of 128.1 kDa. Comparison of this cDNA with other members of the MyBP family reveals extensive primary-sequence conservation. Each MyBP-C contains seven immunoglobulin C2 motifs and three fibronectin type-III repeats in the arrangement C2-C2-C2-C2-C2-III-III-C2-III-C2. Regions of high identity shared by the chicken and the two human proteins are not restricted to the immunoglobulin and fibronectin motifs. Sequence comparison of all three proteins has allowed us to map a highly conserved region between the first and second C2 motifs, the only large spacer sequence present between motifs in these proteins.