The earliest form of C-protein expressed during striated muscle development is immunologically the same as cardiac-type C-protein.

A monoclonal antibody (C-315) specific for cardiac-type C-protein was prepared and, in combination with other antibodies specific for fast and slow skeletal muscle C-proteins, it was used to investigate the expression of C-protein isoforms in developing striated muscle cells in vivo and in vitro. During embryonic development of skeletal muscles, a C-protein recognized by C-315 appeared first but only transiently, it being replaced subsequently by two other isoforms recognized by the antibodies to slow and fast skeletal muscle C-proteins in a fiber-type specific manner as previously demonstrated (Obinata et al. (1984) Develop. Biol. 101, 116-124). In contrast, only cardiac-type C-protein was detected in cardiac muscle throughout the developmental stages. When myogenesis in vitro was monitored using the same antibodies, C-315 binding appeared first in multinucleated myotubes as in vivo which was followed by the sequential expression of two other C-protein variants. The reactivity of C-315 as well as that of anti-slow and anti-fast skeletal C-protein antibodies persisted during muscle development in culture. Thus, this study demonstrates that the earliest form of C-protein expressed in striated muscles may either be a cardiac-type isoform or a unique embryonic protein containing an epitope in common with the adult cardiac-type protein, and that transitions of C-protein isoform expression characteristic of each fiber-type occur during muscle development in vivo but not in vitro.