2 DIFFERENT GENES-CODING FOR PROCESSABLE AND NONPROCESSABLE FORMS OF A VIRAL ENVELOPE PROTEIN CAN ACCOUNT FOR THE APPARENT HORMONAL-STIMULATION OF PROTEIN PROCESSING IN W7MG1 LYMPHOMA-CELLS

In the mouse mammary tumor virus (MMTV)-infected mouse T-lymphoma cell line W7MG1, glucocorticoid hormone regulates two aspects of MMTV gene expression: hormone stimulates MMTV gene transcription and increases the ratio of mature envelope proteins to envelope precursor protein produced. To separate these two effects and determine the mechanism by which hormone regulates the conversion of the envelope precursor Pr74 to the mature cleaved products gp52 and gp33, we constructed expression vectors in which the envelope gene is constitutively transcribed. Surprisingly, the envelope precursor protein Pr74 encoded by two independently isolated, allelic envelope genes behaved differently. Pr74-P (encoded by the ENV/P gene) was processed efficiently to the mature products gp52 and gp33, independently of the level of expression, hormonal induction of cellular genes, or the presence of other MMTV proteins. In contrast, under the same conditions, Pr74-N (encoded by the ENV/N gene) was not processed further despite being relatively stable. In sucrose gradient analyses, Pr74-P sedimented as monomers, whereas Pr74-N was found in high mol wt aggregates of heterogeneous size. Coimmunoprecipitation analysis determined that Pr74-N associated with BiP, whereas Pr74-P did not. This is indicative of improper folding of Pr74-N in the endoplasmic reticulum. We propose that the apparent hormonal regulation of MMTV envelope protein processing in the W7MG1 cell line is a result of differential expression of two different classes of envelope genes: the first encodes a defective non-processable envelope precursor, similar in behavior to Pr74-N, and is expressed in both the absence and presence of glucocorticoid; the second encodes a wild-type processable envelope protein precursor similar to Pr74-P, which is expressed only after transcriptional activation by glucocorticoid.