Nucleolar localization of mouse mammary tumor virus proteins in T-cell lymphomas

To characterize novel proteins expressed in lymphoma cells, monoclonal antibodies (MAbs) were raised against variant S49 mouse lymphoma cells. Immunoperoxidase analysis with a specific MAb, named M-66, revealed nuclear localization with prominent staining in the nucleoli of both tumorigenic (T-63) cells and nontumorigenic, immunogenic (T-25-Adh) cells. Weak signals were also observed in the cytoplasm and plasma membrane of both cells. Western blot analysis with M-66 antibody revealed a 14-kDa protein in nuclear extracts of both T-25-Adh and T-63 cells. An additional nuclear 21-kDa protein was evident only in T-63 cells. M-66 identified several clones from a T-25-Adh cDNA expression library. These clones demonstrated extensive homology (approximately 95% identity throughout their length) to the mouse mammary tumor virus (MMTV) env and LTR regions. Extensive amino acid sequence homology (approximately 90% identity) between the clones and the env protein was observed. M-66 identified the 14-kDa protein in another MMTV bearing T-cell lymphoma, EL-4. Immunoperoxidase analysis of EL-4 cells with M-66 also revealed prominent nucleolar staining. MMTV-negative cells and MMTV-positive cells of nonlymphocytic origin were devoid of both 14- and 21-kDa proteins. Moreover, an anti-MMTV gp52 (env) antibody precipitated the 21-kDa protein in T-63 cells. We thus suggest that MMTV bearing T-cell lymphomas express nucleolar proteins translated from the env region of MMTV.