The role of c-Src in platelet-derived growth factor alpha receptor internalization.

We used two approaches to investigate the role of Src family kinases (SFKs) in ligand-stimulated internalization of the platelet-derived growth factor alpha receptor (alphaPDGFR). In cells that normally express SFKs, the internalization rate of the F72/74 mutant alphaPDGFR (which is unable to recruit or activate SFKs) was slower than that of the wild-type (WT) alphaPDGFR. When expressed in cells lacking SFKs (SYF cells), internalization of the WT and mutant receptors was indistinguishable, which indicated that there was not an inherent defect in the mutant receptor's ability to undergo ligand-driven internalization. The internalization difference between the WT and mutant receptors was seen again when c-Src was expressed in the SYF cells. Surprisingly, c-Src slowed the internalization of the mutant receptor but had little effect on WT receptor. We propose the following working hypothesis to explain these findings. In resting cells SFKs suppress internalization of the alphaPDGFR by phosphorylating a hypothetical protein X. This suppression is relieved when cells are exposed to PDGF because SFKs are recruited to the activated WT alphaPDGFR and thereby no longer actively phosphorylate protein X. The internalization of the mutant receptor is slow because it is unable to recruit SFKs and thereby fails to relieve the suppression of internalization. Our findings suggest a role for SFKs in regulating the permissiveness for internalization of the alphaPDGFR.