THE HUMAN T-CELL LYMPHOTROPIC VIRUS TYPE-1 TOF PROTEIN CONTAINS A BIPARTITE NUCLEAR-LOCALIZATION SIGNAL THAT IS ABLE TO FUNCTIONALLY REPLACE THE AMINO-TERMINAL DOMAIN OF REX

The X region of human T-cell lymphotropic virus type 1 (HTLV-1) encode s two nucleolar/nuclear proteins, the posttranscriptional regulator of mRNA expression Rex and a protein of unknown function named Tof. To g ain insight into the possible biological role of Tof, we investigated the mechanism governing its intracellular trafficking and identified i ts nucleolar/nuclear localization signal (NLS). Mutational analysis of Tof revealed that its NLS was located between amino acids 71 and 98 a nd contained two arginine-rich domains that functioned in an interdepe ndent manner. Studies of Tof-Rex hybrid proteins showed that the Tof N LS could functionally replace the NLS of Rex at the level of nuclear t argeting. As the NLS of Rex is known to mediate its interaction with i ts RNA target, the Rex-responsive element (RXRE), we tested whether th e NLS of Tof could replace that of Rex in mediating activation of a RX RE-containing mRNA. Results showed that the NLS of Tof was indeed able to mediate activation of RXRE-containing mRNAs, suggesting that Tof i tself may function as a regulator of RNA expression and utilization. A comparison of their compartmentalization in response to actinomycin D treatment indicated that Tof did not share Rex's shuttling pathway. E xpression of Tof from its natural multiply spliced mRNA required the p resence of Rex, suggesting that Tof may regulate viral or cellular mRN A expression during the later stages of viral replication.