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

Abstract

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 nuclear localization of the Gag protein of Tf1 is different from that of other proteins tested in that it has a specific requirement for the FXFG nuclear pore factor, Nup124p. Using extensive mutagenesis, we found that Gag contained three nuclear localization signals (NLSs) which, when included individually 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 retrotransposition and abolished nuclear localization of Gag. Interestingly, this NLS activity in the heterologous protein was specifically dependent upon the presence of Nup124p. Deletion analysis of heterologous proteins revealed the surprising result that the residues in Gag with the NLS activity were independent from the residues that conveyed the requirement for Nup124p. In fact, a fragment of Gag that lacked NLS activity, residues 10 to 30, when fused 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 particular nuclear pore complex protein.

Element-specific localization of Drosophila retrotransposon Gag proteins occurs in both nucleus and cytoplasm

“…A few retroelement Gags have been seen in the nucleus as expected. Gag protein of the fission yeast LTR retrotransposon Tf1 localizes to the nucleus (32). When human foamy Virus infects a cell, its Gag moves rapidly to the centriole, apparently positioning itself for entry into the nucleus when the membrane breaks down at metaphase (33).…”

Introgression of Drosophila simulans Nuclear Pore Protein 160 in Drosophila melanogaster Alone Does Not Cause Inviability but Does Cause Female Sterility

“…Why do NPC protein genes evolve so quickly at the molecular level? The nuclear entry of retroviruses and retrotransposons is mediated by NPC (e.g., Dang and Levin 2000;Petersen et al 2000;von Kobbe et al 2000;Gustin and Sarnow 2001;Sistla et al 2007;Beliakova-Bethell et al 2009), and this host-pathogen interaction may have accelerated the evolution of NPC proteins. Alternatively, the NPC proteins may have evolved so quickly to interact properly with rapidly evolving sequences of the centromere and its surrounding heterochromatin (Sawamura et al 1993b;Henikoff et al 2001;Bayes and Malik 2009;Ferree and Barbash 2009).…”

Retroviruses and yeast retrotransposons use overlapping sets of host genes

“…One possibility is that Ty3 nuclear entry occurs in two stages, with contact mediated by Gag3 and entry by IN. In the case of Tf1, another gypsylike element, the Gag-like protein contains a nuclear localization signal and requires the FG nucleoporin Nup124 for nuclear entry (Dang and Levin 2000).…”