NATIVE AVIAN C-ERBB GENE EXPRESSES A SECRETED PROTEIN PRODUCT CORRESPONDING TO THE LIGAND-BINDING DOMAIN OF THE RECEPTOR

A primer-directed cDNA library was used to obtain cDNA clones corresponding to the 5' end (i.e., the ligand-binding domain) of the avian c-erbB gene. Bacterial c-erbB fusion proteins were synthesized and used to obtain polyclonal antisera specific for the ligand-binding domain of the avian receptor. These antisera and antisera specific for the carboxyl terminal domain of the chicken c-erbB gene product have been used to study the native protein products of the c-erbB locus in primary cell cultures by in vivo labeling and immunoprecipitation. Our studies reveal that three c-erbB gene products of M(r) 300,000, M(r) 170,000, and M(r) 95,000 are synthesized in uninfected chicken embryo fibroblasts. Only the M(r) 300,000 and M(r) 170,000 species can be precipitated by using antisera specific for the cytoplasmic domain of the c-erbB product. The 95,000 species is not recognized by the antiserum directed against the carboxyl-terminal domain of c-erbB and is specifically released into the culture medium. Northern transfer studies reveal a lower molecular weight transcript of almost-equal-to 2.6 kilobases that selectively hybridizes to the ligand-binding domain of the avian c-erbB gene product but does not hybridize with probes specific for the cytoplasmic kinase domain of c-erbB. An additional cDNA clone corresponding to this transcript has been isolated, and its sequence suggests it may arise via alternative processing. Together, these data suggest that a truncated form of this growth factor receptor-i.e., a M(r) 95,000 species-is synthesized from a low molecular weight c-erbB transcript that exclusively encodes the ligand-binding domain of the receptor. Secretion of truncated growth factor receptors has been reported recently in several systems, and our results are discussed in the light of these findings.