Abstract
Hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs) is an endosomal protein essential for the efficient sorting of activated growth factor receptors into the lysosomal degradation pathway. Hrs undergoes ligand-induced tyrosine phosphorylation on residues Y329 and Y334 downstream of epidermal growth factor receptor (EGFR) activation. It has been difficult to investigate the functional roles of phosphoHrs, as only a small proportion of the cellular Hrs pool is detectably phosphorylated. Using an HEK 293 model system, we found that ectopic expression of the protein Cbl enhances Hrs ubiquitination and increases Hrs phosphorylation following cell stimulation with EGF. We exploited Cbl's expansion of the phosphoHrs pool to determine whether Hrs tyrosine phosphorylation controls EGFR fate. In structure-function studies of Cbl and EGFR mutants, the level of Hrs phosphorylation and rapidity of apparent Hrs dephosphorylation correlated directly with EGFR degradation. Differential expression of wild-type versus Y329,334F mutant Hrs in Hrs-depleted cells revealed that one or both tyrosines regulate ligand-dependent Hrs degradation, as well as EGFR degradation. By modulating Hrs ubiquitination, phosphorylation, and protein levels, Cbl may control the composition of the endosomal sorting machinery and its ability to target EGFR for lysosomal degradation.