The C-terminus of the kinase-defective neuregulin receptor ErbB-3 confers mitogenic superiority and dictates endocytic routing

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Abstract
Signaling by the epidermal growth factor (EGF) family and the neuregulin group of ligands is mediated by four ErbB receptor tyrosine kinases, that form homo‐ and heterodimeric complexes. Paradoxically, the neuregulin receptor ErbB‐3 is devoid of catalytic activity, but its heterodimerization with other ErbBs, particularly the ligand‐less ErbB‐2 oncoprotein of carcinomas, reconstitutes superior mitogenic and transforming activities. To understand the underlying mechanism we constructed a chimeric EGF‐receptor (ErbB‐1) whose autophosphorylation C‐terminal domain was replaced by the corresponding portion of ErbB‐3. Consistent with the possibility that this domain recruits a relatively potent signaling pathway(s), the mitogenic signals generated by the recombinant fusion protein were superior to those generated by ErbB‐1 homodimers and comparable to the proliferative activity of ErbB‐2/ErbB‐3 heterodimers. Upon ligand binding, the chimeric receptor recruited an ErbB‐3‐specific repertoire of signaling proteins, including Shc and the phosphatidylinositol 3‐kinase, but excluding the ErbB‐1‐specific substrate, phospholipase Cγ1. Unlike ErbB‐1, which is destined to lysosomal degradation through a mechanism that includes recruitment of c‐Cbl and receptor poly‐ubiquitination, the C‐terminal tail of ErbB‐3 shunted the chimeric protein to the ErbB‐3‐characteristic recycling pathway. These observations attribute the mitogenic superiority of ErbB‐3 to its C‐terminal tail and imply that the flanking kinase domain has lost catalytic activity in order to restrain the relatively potent signaling capability of the C‐terminus.