Potential Roles for Gag Proteins in Retroelement Targeting

HeT-A, TART, and TAHRE transpose only to chromosome ends, including ends of broken chromosomes. Drosophila has many othernon-LTR retrotransposons that transpose to many sites, including gene-rich regions where they may cause mutations, yet these are never found in the telomere arrays. All non-LTR elements transpose by the same mechanism, target primed reverse transcription, yet there is no evidence that any specific DNA sequence defines the unique transposition site of HeT-A, TART, and TAHRE; therefore their chromosomal distribution must be directed by other mechanisms. We have found that the Gag protein encoded by the telomere-specific retrotranspons is a major factor in targeting their transposition to chromosome ends. We analyzed intracellular targeting of Gag proteins from six non-LTR retrotransposons, HeT-A, TART, TAHRE, and three non-telomeric elements, jockey, Doc, and I factor. These Gag proteins have high levels of sequence similarity, but they have dramatic differences in intracellular targeting. As expected, HeT-A and TART Gags were efficiently transported to nuclei where HeT-A Gag localized at telomeres and directed both TART and TAHRE Gags to telomeres. TART Gag’s role in this cooperative effort may be supplying reverse transcriptase, an enzyme HeT-A lacks.

In contrast to the telomeric retrotransposons easy entry into the nucleus, Gags of non-telomeric retrotransposons remained in the cytoplasm. These experiments demonstrate that closely related retrotransposon Gag proteins can have different intracellular localizations, presumably because they interact differently with cellular components. We suggest that these localizations reflect the cell’s attempts to hinder transposition of the parasitic non-telomeric elements while enabling efficient transport of their telomere elements.