Cloning and sequence analysis of a cDNA for rat transforming growth factor-α

Transforming growth factors (TGFs) are mitogenic polypeptides produced most conspicuously by transformed cells and conferring on normal cells several phenotypic alterations associated with transformation^1,2. TGFs comprise two distinct sets of molecules: TGF-αs are structurally similar to epidermal growth factor (EGF), binding to and inducing the tyrosine phosphorylation of the EGF receptor in a manner indistinguishable from that of EGF³. In addition, the 50-amino acid rat TGF-α⁴ has 33 and 44% homologies with mouse⁵ and human⁶ EGFs, respectively, and shares with EGFs a conserved pattern of three disulphide bridges⁷. Thus, it has been proposed that TGF-αs belong to a family of EGF-like polypeptides⁷. TGF-βs, on the other hand, display no measurable binding to EGF receptors, but potentiate the growth stimulating activities of TGF-α⁸. Here we report the isolation of a complementary DNA clone encoding rat TGF-α. This cDNA hybridizes to a 4.5-kilobase (kb) messenger RNA that is 30 times larger than necessary to code for a 50-amino acid polypeptide and is present not only in retrovirus- transformed rat cells but also at lower levels in normal rat tissues. The nucleotide sequence of the cDNA predicts that TGF-α is synthesized as a larger product and that the larger form may exist as a transmembrane protein. However, unlike many polypeptide hormones (including EGF^9,10), cleavage of the 50-amino acid TGF-α from the larger form does not occur at paired basic residues, but rather between alanine and valine residues, suggesting the role of a novel protease.