Detection of larger polypeptides structurally and functionally related to type I transforming growth factor

Peptides corresponding to various regions of type I rat transforming growth factor (rTGF) have been chemically synthesized, conjugated to carrier protein, and used to immunized rabbits. An antiserum raised against one of these peptides, corresponding to the carboxyl-terminal 17 amino acids of rTGF, has been used to develop a competitive RIA for the immunizing peptide. In this assay, the antiserum reacts only with a restricted region of the immunizing peptide corresponding to the 11 carboxyl-terminal amino acids of the rTGF molecule. Intact rTGF competes as well as the immunizing peptide on a molar basis, indicating that the cognate sequence in rTGF is recognized by this antiserum. Mouse epidermal growth factor (mEGF) was ineffective as a competitor, even at a 10,000-fold greater concentration, showing that the RIA was capable of distinguishing TGF from functionally related proteins. Several immunoreactive species were detected by gel filtration of conditioned medium from cultured retrovirus-transformed rat cells; both a low molecular weight species that corresponds to rTGF, and a higher molecular weight specie(s) copurify with biologic activity in the EGF radioreceptor assay. Immunoblotting analysis of the higher molecular weight peak fractions revealed three polypeptides (MrS, 24,000, 40,000, and 42,000) that reacted specifically with the antiserum. These findings suggest the existence of a family of larger polypeptides containing both structural and functional determinants of rTGF.