ErbB3 interacts with Hrs and is sorted to lysosomes for degradation.

The ErbB family of receptor tyrosine kinases mediates activation of a wide network of signaling pathways. ErbB3 has weak kinase activity, but its six docking sites for the p85 subunit of phosphoinositide 3-kinase make it an important contributor to proliferative signaling. ErbB3 has a relatively short half-life but the exact mechanisms controlling its turnover are unclear as contradictory reports exist. ErbB-mediated signaling is, however, negatively regulated by endocytosis of the receptors, followed by either recycling or degradation. Our previous studies showed that ErbB3 can be endocytosed and degraded in the absence of its ligand heregulin. However, binding of heregulin increased the degradation rate. In the current study we have investigated in more detail the trafficking and degradation of ErbB3 in the presence or absence of heregulin. We report that ErbB3 is internalized by clathrin-mediated endocytosis both in the presence and absence of heregulin. Moreover, we show that both proteasomal and lysosomal activity regulate ErbB3 degradation. Although steady-state expression of ErbB3 is regulated by proteasomal activity to a large extent, probably linked to a previously identified ER-localized quantity control, the results indicate that internalization, both constitutive and ligand-induced, causes lysosomal degradation of ErbB3. Furthermore, we show that ErbB3 interacts with the ESCRT-0 subunit Hrs both in the presence and absence of heregulin. This indicates an ESCRT-mediated sorting of ErbB3 to late endosomes and lysosomes, and in line with this we show that impaired ESCRT function leads to an endosomal accumulation of ErbB3.