Differential processing and turnover of the oncogenically activated neu/erb B2 gene product and its normal cellular counterpart.

In nontransformed DHFR/G-8 cells (NIH 3T3 cells transfected with normal rat neu gene), the normal neu gene product was initially synthesized as a 170-kDa protein bearing endoglycosidase H-sensitive oligosaccharide chains and was then processed to a 175-kDa mature form with endoglycosidase H-resistant, endoglycosidase F-sensitive oligosaccharide chains. Most of this 175-kDa mature form appeared on the cell surface 2 h following synthesis and showed a half-life of approximately 3 h. In the presence of a growth factor(s) partially purified from bovine kidney, the half-life of this 175-kDa normal neu gene product was shortened to less than 30 min. In B104-1-1 cells (NIH 3T3 cells transfected with neu gene activated oncogenically by a point mutation that changes a valine residue to a glutamic acid residue in the putative transmembrane region), the oncogenically activated neu gene product was also synthesized as a 170-kDa precursor with endoglycosidase H-sensitive oligosaccharide chains. However, this 170-kDa precursor diminished very fast and was only partially processed to a 185-kDa mature form which exhibited a half-life of less than 30 min. The 185-kDa activated neu gene product possessed an unidentified post-translational modification in addition to N-linked oligosaccharide chains. Both the precursor and mature forms of the mutationally activated neu gene product showed increased tyrosine-specific phosphorylation as compared with those of their normal counterparts in DHFR/G-8 cells. The mutationally activated neu gene product in B104-1-1 cells shared several features which have been reported previously for the ligand-activated platelet-derived growth factor receptor in v-sis- or c-sis-transformed cells. These properties include: 1) accelerated turnover of the precursor and mature forms compared with the rates of turnover of its normal counterparts, 2) insensitivity of this rapid turnover to lysosomotropic amines, and 3) increased in vivo tyrosine-specific phosphorylation of both the precursor and mature forms. These findings suggest that the mutationally activated neu gene product may transform the cells by mimicking ligand-induced activation.