Proviral Insertional Activation of c-erbB: Differential Processing of the Protein Products Arising from Two Alternate Transcripts

Abstract

Proviral insertional activation of c-erbB results in the expression of two alternate transcripts (ENV⁺ and ENV^-). We used cDNA clones representing the two alternate transcripts to generate stably transformed quail fibroblast cell lines which express the products of these transcripts independently. Analysis of the co- and posttranslational processing of the insertionally activated c-erbB products expressed in these cell lines revealed that the protein products of the ENV⁺ and ENV^- transcripts were processed differently. The ENV⁺ transcript produced a primary translation product which was rapidly cotranslationally cleaved near the amino terminus to form a 79,000-M^r product. This protein product was efficiently converted to a higher-molecular-weight form, of between 82,000 and 88,000 (gp82-88), which was terminally glycosylated and expressed on the cell surface. A small portion of the ENV⁺ primary translation product underwent a second proteolytic cleavage to generate an unglycosylated 53,000-M^r species. In contrast, the primary translation product of the ENV^- transcript, p80, was not proteolytically processed; this precursor form was rapidly converted to two discrete glycosylation intermediates, gp82 and go84. Only a small portion (<10%) of the total ENV^- insertionally activated c-erbB product was slowly converted to the terminally glycosylated cell surface form, gp85-88. The processing differences that distinguished the ENV⁺ and ENV^- products were similar to processing differences that we observed in parallel studies on the viral erbB products of the avian erythroblastosis viruses AEV-H and AEV-R, respectively. Since all four erbB protein products shared the same number, position, and sequence context of potential N-linked glycosylation sites, yet differed in the extent of their carbohydrate maturation, these data suggest that the mechanisms used by these truncated receptor molecules to associate with cellular membranes may be distinct.