Identification of the domain in ErbB2 that restricts ligand-induced degradation.

Ligand-induced receptor degradation is an important process for down-regulation of plasma membrane receptors. While epidermal growth factor receptor (EGFR) is rapidly internalised and degraded upon ligand stimulation, ErbB2, the closest member to EGFR in ErbB receptor family, is resistant in ligand-induced degradation. To understand the molecular mechanisms underlying the impairment in ligand-induced degradation of ErbB2, we attempted to determine structural factor in ErbB2 that restricts the degradation. By analysis of ligand-induced degradation of EGFR/ErbB2 chimeras, we have identified a region between amino acid residues F1030 and L1075 in ErbB2 as the domain that restricts the ligand-induced degradation. We designated this domain as the Blocking ErbB2 Degradation or the BED domain. Replacement of the BED domain in an EGFR/ErbB2 chimera with the corresponding region of EGFR changed this chimera from a non-degradable to a degradable receptor, indicating that the BED domain is the factor restricting the ligand-induced degradation of ErbB2. In addition, we found that a non-degradable EGFR/ErbB2 chimera was not defective in tyrosine phosphorylation, ubiquitination and interaction with c-Cbl, rather, was defective in ligand-induced internalisation, suggesting that the endocytosis defect is the cause restricting the degradation of ErbB2, and that c-Cbl-catalysed mono-ubiquitination is not involved in the impairment in ligand-induced degradation of ErbB2.