Nuclear import of the retrotransposon Tf1 is governed by a nuclear localization signal that possesses a unique requirement for the FXFG nuclear pore factor Nup124p

Retroviruses, such as human immunodeficiency virus, that infect nondividing cells generate integration precursors that must cross the nuclear envelope to reach the host genome. As a model for retroviruses, we investigated the nuclear entry of Tf1, a long-terminal-repeat-containing retrotransposon of the fission yeast Schizosaccharomyces pombe. Because the nuclear envelope of yeasts remains intact throughout the cell cycle, components of Tf1 must be transported through the envelope before integration can occur. The nucle ar localization of the Gag protein of Tf1 is different from that of other p roteins tested in that it has a specific requirement for the FXFG nuclear p ore factor, Nup124p. Using extensive mutagenesis, we found that Gag contain ed three nuclear localization signals (NLSs) which, when included individua lly in a heterologous protein, were sufficient to direct nuclear import. In the context of the intact transposon, mutations in the NLS that mapped to the first 10 amino acid residues of Gag significantly impaired Tf1 retrotra nsposition and abolished nuclear localization of Gag. Interestingly, this N LS activity in the heterologous protein was specifically dependent upon the presence of Nup124p. Deletion analysis of heterologous proteins revealed t he surprising result that the residues in Gag with the NLS activity were in dependent from the residues that conveyed the requirement for Nup124p. In f act, a fragment of Gag that lacked NLS activity, residues 10 to 30, when fu sed to a heterologous protein, was sufficient to cause the classical NLS of simian virus 40 to require Nup124p for nuclear import. Within the context of the current understanding of nuclear import, these results represent the novel case of a short amino acid sequence that specifies the need for a pa rticular nuclear pore complex protein.