The proteolytic processing sites of the human immunodeficiency virus type 1 (HIV-1) Gag precursor are cleaved in a sequential manner by the viral protease. We investigated the factors that regulate sequential processing. When full-length Gag protein was digested with recombinant HIV-1 protease in vitro, four of the five major processing sites in Gag were cleaved at rates that differ by as much as 400-fold. Three of these four processing sites were cleaved independently of the others. The CA/p2 site, however, was cleaved approximately 20-fold faster when the adjacent downstream p2/NC site was blocked from cleavage or when the p2 domain of Gag was deleted. These results suggest that the presence of a C-terminal p2 tail on processing intermediates slows cleavage at the upstream CA/p2 site. We also found that lower pH selectively accelerated cleavage of the CA/p2 processing site in the full-length precursor and as a peptide primarily by a sequence-based mechanism rather than by a change in protein conformation. Deletion of the p2 domain of Gag results in released virions that are less infectious despite the presence of the processed final products of Gag. These findings suggest that the p2 domain of HIV-1 Gag regulates the rate of cleavage at the CA/p2 processing site during sequential processing in vitro and in infected cells and that p2 may function in the proper assembly of virions.

The proto-oncogene c-fgr is expressed at high levels in cell lines derived from lymphomas which are infected with Epstein-Barr virus (EBV) (Cheah et al., 1986). mRNA extracted directly from biopsies of EBV-infected tissues was analyzed on Northern blots to determine if c-fgr is expressed during lympho-proliferations induced in vivo by EBV and in nasopharyngeal carcinoma (NPC), the epithelial malignancy associated with the virus. Elevated levels of c-fgr expression were detected in all EBV-positive lympho-proliferations in vivo but not in cell lines established by EBV infection in vitro. This indicates that the induction of the c-fgr proto-oncogene is not an essential component of EBV-induced transformation. Although no c-fgr expression was detected in EBV-positive or -negative epithelial cell lines, the 3.0-kb c-fgr mRNA was detected at low levels in mRNA obtained from NPC biopsy specimens. However, NPC tissue, after passage in nude mice (which eliminates infiltrating lymphoid and myeloid cells) did not contain the 3.0-kb c-fgr mRNA. The absence of expression of c-fgr in the malignant epithelial cells infected with EBV contrasts with the elevated level of the proto-oncogene in EBV-infected lymphoma tissue and cell lines established from lymphomas. This suggests differences in the expression of cellular functions in EBV-induced malignancies of these 2 distinct cell types.

No abstract