Phosphorylation of the human T-cell leukemia virus type 1 transactivator tax on adjacent serine residues is critical for tax activation

The Tax transactivator protein of human T-cell leukemia virus type 1 (HTLV- 1) plays a central role in the activation of viral gene expression. In addi tion, Tax is capable of activating the expression of specific cellular gene s and is involved in the transformation of T-lymphocytes resulting in the d evelopment of adult T-cell leukemia. Tax is a phosphoprotein that colocaliz es in nuclear bodies with RNA polymerase II, splicing complexes, and specif ic transcription factors including members of the ATF/CREB and NF-KB famili es. In this study, we identified adjacent serine residues at positions 300 and 301 in the carboxy terminus of Tax as the major sites for phosphorylati on. Phosphorylation of at least one of these serine residues is required fo r Tax localization in nuclear bodies and for Tax-mediated activation of gen e expression via both the ATF/CREB and NF KB pathways, Introduction of amin o acid substitutions which are phosphoserine mimetics at positions 300 and 301 restored the ability of a phosphorylation-defective Tax mutant to form nuclear bodies and to activate gene expression, These studies define sites for regulatory phosphorylation events in Tax which are critical for its abi lity to activate gene transcription.