9.A.37 The Nuclear Import Tax Protein (Tax) Family
Tax is a nuclear import protein of human T-Lymphotropic Virus Type I (HTLV-1). HTLV-1 is the etiologic agent of the adult T cell leukemia-lymphoma (ATLL). The viral regulatory protein Tax plays a central role in leukemogenesis as a transcriptional transactivator of both viral and cellular gene expression, and this requires Tax activity in both the cytoplasm and the nucleus. The N-terminal 60 amino acids of Tax, and residues within the zinc finger motif in this region, are important for nuclear localization.(Tsuji et al., 2007). The transport of Tax to the nucleus required neither energy nor carrier proteins. Specific and direct binding occurs between Tax and p62, a nucleoporin with which the importin beta family of proteins interact. The entry of Tax into the nucleus involves a direct interaction with nucleoporins within the nuclear pore complex (NPC; TC #9.A.14). The nuclear export of Tax is also carrier independent. Tax itself has carrier function, and the NF-kappaB subunit p65 can be imported into the nucleus by Tax. Tax may alter the nucleocytoplasmic distribution of cellular proteins, and this could contribute to the deregulation of cellular processes observed in HTLV-1 infection.
The transport reactions catalyzed by Tax is:
Tax ± carrier protein (cytoplasm) Tax ± carrier protein (nucleoplasm)