ctivation domain by functional exchange. J. Virol. 66: 2583–2587. 31. Wen, W., J. L. Meinkoth, R. Y. Tsien, and S. S. Taylor. 1995. Identi?cation of a signal for rapid export of proteins from the nucleus. Cell 82: 463–473. 32. Zacksenhaus, E., R. Bremner, R. A. Phillips, and B. L. Gallie. 1993. A bipartite nuclear localization signal in the retinoblastoma gene product and its importance for biological activity. Mol. Cell. Biol. 13: 4588–4599. 33. Zapp, M. L., T. J. Hope, T. G. Parslow, and M. R. Green. 1991. Oligomer- ization and RNA binding domains of the type 1 human immunode?ciency virus Rev protein: a dual function for an arginine-rich binding motif. Proc. Natl. Acad. Sci. USA 88: 7734–7738. V OL. 16, 1996 NES OF HTLV-1 Rex 5155 on March 24, 2016 by guest http://mcb./ Downloaded from