CONSTITUTIVE PROTEOLYSIS OF THE ERBB-4 RECEPTOR TYROSINE KINASE BY A UNIQUE, SEQUENTIAL MECHANISM

The heregulin receptor tyrosine kinase ErbB-4 is constitutively cleave d, in the presence or absence of ligand, by an exofacial proteolytic a ctivity producing a membrane-anchored cytoplasmic domain fragment of 8 0 kD. Based on selective sensitivity to inhibitors, the proteolytic ac tivity is identified as that of a metalloprotease. The 80-kD product i s tyrosine phosphorylated and retains tyrosine kinase activity. Import antly, the levels of this fragment are controlled by proteasome functi on. When proteasome activity is inhibited for 6 h, the kinase-active 8 0-kD ErbB-4 fragment accumulates to a level equivalent to 60% of the i nitial amount of native ErbB-4 (similar to 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 kin ase generated by metalloprotease activity. Proteasome activity, howeve r, does not act on the native ErbB-4 receptor before the metalloprotea se-mediated cleavage, as no ErbB-4 fragments accumulate when metallopr otease activity is blocked. Although no ubiquitination of the native E rbB-4 is detected, the 80-kD fragment is polyubiquitinated. The data, therefore, describe a unique pathway for the processing of growth fact or receptors, which involves the sequential function of an exofacial m etalloprotease and the cytoplasmic proteasome.