Ubiquitin-independent binding of Hrs mediates endosomal sorting of the interleukin-2 receptor β-chain

Several lines of evidence have revealed that ubiquitylation of membrane proteins serves as a signal for endosomal sorting into lysosomes or lytic vacuoles. The hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs) interacts with ubiquitylated cargoes through its ubiquitin-interacting-motif domain (UIM domain), and plays an essential early role in endosomal sorting. Here, we show that the C-terminal region of Hrs, which does not contain the UIM domain, can bind to interleukin-2 receptor beta (IL-2R beta). We found a direct interaction between bacterially expressed IL-2R beta and Hrs in GST pull-down assays, indicating that their binding is independent of ubiquitin. Trafficking and degradation assays revealed that, similarly to wild-type IL-2R beta, an IL-2R beta mutant lacking all the cytoplasmic lysine residues is sorted from Hrs-positive early endosomes to LAMP1-positive late endosomes, resulting in degradation of the receptor. By contrast, an IL-2R beta mutant lacking the Hrs-binding region passes through early endosomes and is mis-sorted to compartments positive for the transferrin receptor. The latter mutant exhibits attenuated degradation. Taken together, these results indicate that precise sorting of IL-2R beta from early to late endosomes is mediated by Hrs, a known sorting component of the ubiquitin-dependent machinery, in a manner that is independent of UIM-ubiquitin binding.