Ligand-independent activation of the platelet-derived growth factor β receptor: Requirements for bovine papillomavirus E5-induced mitogenic signaling

The E5 protein of bovine papillomavirus type 1 binds to and activates the endogenous platelet-derived growth factor (PDGF) β receptor in fibroblasts, resulting in cell transformation. We have developed a functional assay to test the ability of PDGF β receptor mutants to mediate a mitogenic signal initiated by the E5 protein. Lymphoid Ba/F3 cells are strictly dependent on interleukin-3 for growth, but coexpression of the wild-type PDGF β receptor and the E5 or v-sis-encoded protein generated a mitogenic signal which allowed Ba/F3-derived cells to proliferate in the absence of interleukin-3. In these cells, the E5 protein bound to and caused increased tyrosine phosphorylation of both the mature and the precursor forms of the wild-type PDGF β receptor. The tyrosine kinase activity of the receptor was necessary for E5-induced receptor tyrosine phosphorylation and mitogenic activity but not for complex formation with the E5 protein. In contrast, the PDGF-binding domain of the receptor was not required for complex formation with the E5 protein, E5-induced tyrosine phosphorylation or mitogenic activity, demonstrating that E5-mediated receptor activation is ligand independent. Analysis of receptor mutants lacking various combinations of tyrosine phosphorylation sites revealed that the E5 and v-sis-encoded proteins display similar requirements for signaling and suggested that the wild-type PDGF β receptor can generate multiple independent mitogenic signals. Importantly, these mutants dissociated two activities of the PDGF β receptor tyrosine kinase, both of which are required for sustained mitogenic signaling: (i) receptor autophosphorylation and creation of binding sites for SH2 domain- containing proteins and (ii) phosphorylation of substrates other than the receptor itself.