Mutational analysis of the beta-type platelet-derived growth factor receptor defines the site of interaction with the bovine papillomavirus type 1 E5 transforming protein.

The E5 polypeptide of bovine papillomavirus type 1 is a small membrane-bound protein which induces the transformation of immortalized fibroblasts, apparently via the formation of a ternary complex with the platelet-derived growth factor receptor (PDGFR) and the 16-kDa V-ATPase protein. This interaction seems to be mediated, at least in part, by their respective transmembrane domains. E5 also cooperates with transfected beta PDGFR to induce interleukin-3 (IL-3)-independent growth of a mouse myeloid precursor cell line (32D) which normally lacks expression of most known tyrosine kinase growth factor receptors. Cell proliferation induced by beta PDGFR and E5 is also highly specific, since the highly conserved alpha PDGFR and other related receptors did not physically or functionally interact with E5 in these cells. In the current study, analysis of chimeric alpha and beta PDGFRs confirmed that a short region encompassing the beta PDGFR transmembrane domain was sufficient for complex formation with E5, receptor autophosphorylation, and sustained proliferation of 32D cells in the absence of IL-3. Furthermore, a deletion mutant lacking the entire extracellular domain efficiently bound E5 and induced IL-3-independent growth. These data provide direct evidence that the interaction between E5 and the beta PDGFR involves amino acids 531 to 556 of the receptor transmembrane region and that this specific interaction is critical for activation of the PDGFR signaling complex.