The leader peptide of MMTV Env precursor localizes to the nucleoli in MMTV-derived T cell lymphomas and interacts with nucleolar protein B23.

We have previously described two nucleolar proteins, named p14 and p21, in MMTV-induced T cell lymphomas. These proteins were identified by a monoclonal antibody (M-66) generated from a nontumorigenic, immunogenic variant of S49 T cell lymphoma. While p14 was common to several MMTV-derived T cell lymphomas, p21 was found only in highly tumorigenic variants of S49 cells. Here we report that p14 is the leader peptide of the MMTV env precursor. The epitope recognized by M-66 contains a putative nuclear localization signal. Actinomycin D was found to induce redistribution of p14/p21 from the nucleolus to the nucleoplasm. p14 coimmunoprecipitated and colocalized with the cellular protein, B23. Association with B23 has been previously reported for other auxiliary nucleolar retroviral proteins, such as Rev (HIV) and Rex (HTLV).