Serine phosphorylation of the ligand-activated beta-platelet-derived growth factor receptor by casein kinase I-gamma 2 inhibits the receptor's autophosphorylating activity

Platelet-derived growth factor (PDGF) receptors (PDGFRs) are membrane prote in-tyrosine kinases that, upon activation, become tyrosine-phosphorylated a nd associate with numerous SH2 domain-containing molecules involved in medi ating signal transduction. In Rat-2 fibroblasts, we have characterized the phosphorylation of the beta-PDGFR following its activation by PDGF, In cont rast to tyrosine phosphorylation, which was transient and returned to near basal levels by 30 min, PDGF-stimulated Ser/Thr phosphorylation of the beta -PDGFR was increased by 5 min and remained elevated after 30 min. In vivo, after 5 min of PDGF stimulation, serine phosphorylation of the beta-PDGFR w as greatly reduced by CKI-7, a specific inhibitor of casein kinase I (CKI), In vitro, recombinant CKI-gamma 2 phosphorylated the ligand-activated beta -PDGFR on serine residues in a CKI-7-sensitive manner and resulted in a mar ked inhibition of the receptor's autophosphorylating activity. Furthermore, in Rat-2 fibroblasts, expression of hemagglutinin epitope-tagged active CK I-gamma 2 resulted in a dramatic decrease in the tyrosine phosphorylation s tate of the beta-PDGFR in response to PDGF, consistent with receptor inacti vation. Our data suggest that upon PDGF stimulation, CKI-gamma 2 is activat ed and/or translocated in proximity to the beta-PDGFR, whereby it phosphory lates the beta-PDGFR on serine residues and negatively regulates its tyrosi ne kinase activity, leading to receptor inactivation.