Analysis of amino acid signals within the human T cell leukemia virus type I transactivator protein Tax that control nuclear export, cytoplasmic intracellular localization, and secretion

Human T cell leukemia virus type I (HTLV-I) is the etiologic agent of adult T cell leukemia (ATL) and HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The HTLV-I transactivator protein Tax interfaces with numerous transcription factor families, including ATF/CREB and NF-κB, requiring Tax to localize to both the nuclear and cytoplasmic compartments. However, studies of Tax intracellular localization have focused on its localization to the nucleus because of its primary role as transcriptional transactivator of viral and cellular genes. Recently, results have suggested that Tax localization to the cytoplasm also plays a large role in the interaction of Tax not only with the NF-κB signaling pathway, but also the cell secretory pathway and cell cycle checkpoint control. Additionally, the amount of Tax localized to the cytoplasm is cell type dependent, suggesting that the effects of Tax resulting from cytoplasmic localization may be different between cell types. The purpose of this Thesis was to determine the mechanism of Tax cytoplasmic localization as well as to begin an analysis of the cytoplasmic proteins and cellular pathways Tax interacts with. In terms of the ability of Tax to exit from the nucleus, Tax was demonstrated to contain a leucine-rich nuclear export signal (NES). Unlike other proteins containing a leucine-rich NES, Tax nuclear export was not sensitive to leptomycin B (LMB), the specific inhibitor of the CRM-1 nuclear export pathway. However, studies herein indicated that the calcium binding protein calreticulin may play a role in Tax nuclear export. Studies concerning the localization of Tax within the cytoplasm focused on Tax entry into the cellular secretory pathway. Cytoplasmic Tax was determined to localize to the endoplasmic reticulum (ER) and Golgi complex (GC); organelles associated with the cellular secretory process. Additionally, secreted Tax occurred as a full-length protein, indicating that Tax secretion involved a leaderless pathway. With these results concerning Tax nuclear export and cellular secretion, these studies begin to provide a link between cytoplasmic Tax and the pathogenesis demonstrated in both ATL and HAM/TSP patients.