CHARACTERIZATION OF NUCLEAR-PROTEIN BINDING TO A SITE IN THE LONG TERMINAL REPEAT OF A MURINE LEUKEMIA-VIRUS - COMPARISON WITH THE NFAT COMPLEX

We previously identified a protein-binding site (MLPal) that is locate d downstream of the enhancer element in the long terminal repeat (LTR) of a mink cell focusing-forming (MCF) murine leukemia virus (F. K. Yo shimura, K. Diem, H. Chen, and J. Tupper, J. Virol. 67:2298-2304, 1993 ). We determined that the MLPal site regulates transcription specifica lly in T cells and affects the lymphomagenicity of the MCF isolate 13 murine leukemia virus with a single enhancer repeat in its LTR. In thi s report, we present evidence that two different proteins, a T-cell-sp ecific protein and a ubiquitous protein, bind the MLPal site in a sequ ence-specific manner. By mutational analysis, we determined that the T -cell-specific and the ubiquitous proteins require different nucleotid es in the MLPal sequence for DNA binding. By competitive electrophoret ic mobility shift assays, we demonstrated that the T-cell-specific pro tein that binds MLPal is identical or similar to a protein from nonact ivable T cells that interacts with the binding site of the nuclear fac tor of activated T cells (NFAT). Unlike the NFAT-binding site, however , the MLPal site does not bind proteins that are inducible by T-cell a ctivation. We observed that the MLPal sequence is conserved in the LTR s of other mammalian retroviruses that cause T-cell diseases. Furtherm ore, the MLPal sequence is present in the transcriptional regulatory r egions of cellular genes that either are expressed specifically in T c ells or are commonly rearranged by provirus integration in thymic lymp homas. Thus, the MLPal-binding proteins may play a role in the transcr iptional regulation not only of the MCF virus LTR but also of cellular genes involved in T-cell development.