The Requirement of Tyrosines 579 and 581 for Maximal Ligand-Dependent Activation of the βPDGFR Is Influenced by Noncytoplasmic Regions of the Receptor

Abstract

Mutating tyrosines 579 and 581 of the β platelet-derived growth factor receptor (βPDGFR) tyrosine kinase to phenylalanines (the F2 mutation) impair activation of the receptor in response to ligand, but mutation of the analogous tyrosines in the αPDGFR has no effect on ligand-dependent receptor activation. We have found that the F2 mutation has only a modest effect on ligand-dependent activation of a chimeric PDGFR composed of the extracellular and transmembrane domains of the αPDGFR and the cytoplasmic domain of the βPDGFR by three measures: (1) the ability to phosphorylate endogenous and exogenous protein substrates in vitro, (2) phosphorylation of tyrosine 857, and (3) binding of the effector proteins PLCγ, RasGAP, and SHP-2. Conversely, the F2 mutation substantially impairs ligand-dependent activation of chimeric PDGFRs that consist of either the extracellular domain alone or the extracellular and transmembrane domains of the βPDGFR and all remaining sequence from the αPDGFR by two measures: (1) phosphorylation of endogenous protein substrates in vitro and (2) binding of PLCγ and SHP-2. Our results indicate that the requirement of tyrosines 579 and 581 for maximal activation of the βPDGFR in response to ligand is primarily determined by noncytoplasmic regions of the receptor.