AN ALTERNATIVELY PROCESSED MRNA FROM THE AVIAN C-ERBB GENE ENCODES A SOLUBLE, TRUNCATED FORM OF THE RECEPTOR THAT CAN BLOCK LIGAND-DEPENDENT TRANSFORMATION

At least four major transcripts are produced by the avian c-erbB/epidermal growth factor receptor gene. cDNAs corresponding to the smallest one, a 2.6-kb transcript, were isolated from an adult chicken liver cDNA library. Sequence analysis revealed that the 3' end of one cDNA clone diverged from the known sequence of the extracellular ligand-binding domain (LBD) of the full-length receptor. A genomic DNA subfragment that contained this unique 3' divergent end was isolated. Sequence analysis of this genomic DNA fragment revealed that the 2.6-kb c-erbB transcript is produced by alternative processing. Translation of this 2.6-kb transcript would produce a secreted, truncated receptor molecule which contains the amino-terminal three-fourths of the extracellular LBD of the native receptor. COS1 cells and primary chicken embryo fibroblast cells were transfected with expression vectors that contained the 2.6-kb c-erbB cDNA. Conditioned medium from these transfected cells contained a 70-kDa protein that was specifically immunoprecipitated by a polyclonal antiserum directed against the LBD of the avian c-erbB gene product. The 70-kDa truncated receptor could be coimmunoprecipitated from conditioned medium of transfected COS1 cells that was supplemented with recombinant human transforming growth factor alpha (TGF-alpha) by a monoclonal antibody against human TGF-alpha. Additionally, transfected chicken embryo fibroblast cells that overexpressed the 70-kDa truncated receptor were blocked in their ability to form TGF-alpha-dependent colonies in soft agar. These data suggest that the secreted, truncated receptor encoded by the 2.6-kb c-erbB transcript can bind to TGF-alpha and may play an important growth-regulatory function in vitro.