Size and charge heterogeneity of C-protein isoforms in avian skeletal muscle. Expression of six different isoforms in chicken muscle

Abstract

Summary C-protein is an abundant protein, of unknown function, found in the striated muscles of all vertebrates (Offeret al., 1973). Based on differences in size, charge, antigenicity and sarcomere distribution, at least three different isoforms of this protein have been identified (Callaway & Bechtel, 1981; Yamamoto & Moos, 1983; Reinachet al., 1982; Dhootet al., 1985). These have been termed fast-, slow-and cardiac-type isoforms, relative to their distribution in adult striated muscles. Each of these isoforms appears to be expressed sequentially during the development of the chicken pectoralis muscle (Obinataet al., 1984; Obinata, 1985). To better characterize the various isoforms of C-protein, we have reexamined itsin vivo expression during avian myogenesis using a combination of 1-and 2-dimensional gel electrophoresis, cell-free translation and immunoblotting procedures. In this manuscript we demonstrate for the first time that at least four major C-protein isoforms can be distinguished in adult chicken muscles. These include a fast-type isoform in the pectoralis (PECT) muscle (C_nf), a slow-type isoform in the anterior latissimus dorsi (ALD) muscle (C_s3), a second slow-type isoform in the posterior latissimus dorsi (PLD) muscle (C_s4) and a cardiac-type in the ventricle (C_c). During embryonic development of the PECT muscle two additional isoforms can be resolved. These are both slow-type isoforms based on their reactivities with ALD66, a monoclonal antibody specific for adult slow-type C-protein. These latter isoforms have been termed C_s1 and C_s2. Several of the isoforms, particularly C_ns1 ands C_s3, exhibit two or more spots of different charge but identical molecular weight on 2-D gels. This observation suggests the possibility that these isoforms are post-translationally modified and possibly phosphorylated. Our data show the C-protein family in avian striated muscles to be highly complex. Additional genetic analyses and primary sequence studies will be required to distinguish transcriptional from post-transcriptional variants.