ALTERED SUBCELLULAR LOCATION OF AN ACTIVATED AND TUMOR-ASSOCIATED EPIDERMAL GROWTH-FACTOR RECEPTOR

The epidermal growth factor (EGF) receptor is a membrane bound tyrosin e kinase whose activity is initiated by ligand binding, The malignant brain tumour glioblastoma frequently shows amplification and rearrange ments of the EGF receptor gene that are associated with the synthesis of a constitutively activated tyrosine kinase, lacking amino acids 6-2 73 near the protein's N-terminus, When expressed in Chinese hamster ov ary (CHO) cells, this mutant receptor (p140(EGFR)) displays ligand-ind ependent tyrosine kinase activity, stimulates DNA synthesis, and promo tes cell proliferation, Here, we investigate the subcellular location of p140(EGFR) in CHO cell transfectants as web as in human glioblastom a tumours, p140(EGFR) had an intracellular location that contrasted sh arply with the plasma membrane location of the wild-type EGF receptor, Endoglycosidase H sensitivity analysis and the pattern of p140(EGFR) immunoreactivity suggested that the aberrant tyrosine kinase resided p rimarily in the endoplasmic reticulum, The half-life of p140(EGFR) in, the endoplasmic reticulum was extended several-fold over that of the ligand-activated wild-type receptor, The altered subcellular location of p140(EGFR) in combination with its prolonged half-life suggest that this activated tyrosine kinase may escape the regulatory mechanisms u tilized for the attenuation of wild-type receptor signaling, Therefore , the previously reported growth stimulatory property of the ligand-in dependent p140(EGFR) may be attributed to a sustained tyrosine kinase activity resulting from an altered subcellular location.