Involvement of the ubiquitin/proteasome system in sorting of the interleukin 2 receptor beta chain to late endocytic compartments

Down-regulation of cell surface growth factor receptors plays a key role in the tight control of cellular responses. Recent reports suggest that the u biquitin system, in addition to participating in degradation by the proteas ome of cytosolic and nuclear proteins, might also be involved in the down-r egulation of various membrane receptors. We have previously characterized a signal in the cytosolic part of the interleukin 2 receptor beta chain (IL2 R beta) responsible for its targeting to late endosomes/lysosomes. In this report, the role of the ubiquitin/proteasome system on the intracellular fa te of IL2R beta was investigated. Inactivation of the cellular ubiquitinati on machinery in ts20 cells, which express a thermolabile ubiquitin-activati ng enzyme E1, leads to a significant decrease in the degradation rate of IL 2R beta, with little effect on its internalization. In addition, we show th at a fraction of IL2R beta can be monoubiquitinated. Furthermore, mutation of the lysine residues of the cytosolic region of a chimeric receptor carry ing the IL2R beta targeting signal resulted in a decreased degradation rate . When cells expressing IL2R beta were treated either by proteasome or lyso some inhibitors, a significant decrease in receptor degradation was observe d. Our data show that ubiquitination is required for the sorting of IL2R be ta toward degradation. They also indicate that impairment of proteasome fun ction might more generally affect intracellular routing.