title: Constitutive proteolysis of the ErbB-4 receptor tyrosine kinase by a unique, sequential mechanism.
abstract: The heregulin receptor tyrosine kinase ErbB-4 is constitutively cleaved, in the presence or absence of ligand, by an exofacial proteolytic activity producing a membrane-anchored cytoplasmic domain fragment of 80 kD. Based on selective sensitivity to inhibitors, the proteolytic activity is identified as that of a metalloprotease. The 80-kD product is tyrosine phosphorylated and retains tyrosine kinase activity. Importantly, the levels of this fragment are controlled by proteasome function. When proteasome activity is inhibited for 6 h, the kinase-active 80-kD ErbB-4 fragment accumulates to a level equivalent to 60% of the initial amount of native ErbB-4 (approximately 10(6) receptors per cell). Hence, proteasome activity is essential to prevent the accumulation of a significant level of ligand-independent, active ErbB-4 tyrosine kinase generated by metalloprotease activity. Proteasome activity, however, does not act on the native ErbB-4 receptor before the metalloprotease-mediated cleavage, as no ErbB-4 fragments accumulate when metalloprotease activity is blocked. Although no ubiquitination of the native ErbB-4 is detected, the 80-kD fragment is polyubiquitinated. The data, therefore, describe a unique pathway for the processing of growth factor receptors, which involves the sequential function of an exofacial metalloprotease and the cytoplasmic proteasome.
authors.affiliation: Department of Biochemistry and Department of Medicine, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-0146, USA.
authors.forename: M
authors.initials: M
authors.lastname: Vecchi
authors.forename: G
authors.initials: G
authors.lastname: Carpenter
mesh_terms: D016475:3T3 Cells; D000818:Animals; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D004789:Enzyme Activation; D066246:ErbB Receptors / Q000378:metabolism*; D006801:Humans; D006868:Hydrolysis; D008666:Metalloendopeptidases / Q000378:metabolism*; D051379:Mice; D008970:Molecular Weight; D009097:Multienzyme Complexes / Q000378:metabolism*; D010446:Peptide Fragments / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex; D011499:Protein Processing, Post-Translational; D066247:Receptor, ErbB-4; D014452:Ubiquitins / Q000378:metabolism
chemical_list: D009097:Multienzyme Complexes; D010446:Peptide Fragments; D014452:Ubiquitins; C587689:ERBB4 protein, human; D066246:ErbB Receptors; C587690:Erbb4 protein, mouse; D066247:Receptor, ErbB-4; D003546:Cysteine Endopeptidases; D008666:Metalloendopeptidases; D046988:Proteasome Endopeptidase Complex