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

Abstract

Many Drosophila non-long terminal repeat (LTR) retrotransposons actively transpose into internal, gene-rich regions of chromosomes but do not transpose onto chromosome ends. HeT-A and TART are remarkable exceptions; they form telomeres of Drosophila by repeated transpositions onto the ends of chromosomes and never transpose to internal regions of chromosomes. Both telomeric and nontelomeric, non-LTR elements transpose by target-primed reverse transcription, and their targets are not determined simply by DNA sequence, so it is not clear why these two kinds of elements have nonoverlapping transposition patterns. To explore roles of retrotransposon-encoded proteins in transposition, we analyzed intracellular targeting of Gag proteins from five non-LTR retrotransposons, HeT-A , TART , jockey , Doc , and I factor. All were expressed as green fluorescent protein-tagged proteins in cultured Drosophila cells. These Gag proteins have high levels of sequence similarity, but they have dramatic differences in intracellular targeting. As expected, HeT-A and TART Gags are transported efficiently to nuclei, where they show specific patterns of localization. These patterns are cell cycle-dependent, disappearing during mitosis. In contrast, only a fraction of jockey Gag moves into nuclei, whereas neither Doc nor I factor Gag is detected in the nucleus. Gags of the nontelomeric retrotransposons form characteristic clusters 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 interactions reflect mechanisms by which the cell influences the level of transposition of an element.

Non-LTR retrotransposons encode noncanonical RRM domains in their first open reading frame

“…For the last twenty years, NLR ORF1p proteins were studied in the absence of detailed structural information, and it was rather obscure how ORF1p would bind RNA. The present identification of RRM modules opens a new perspective and relates the observed cytoplasmic RNPs (16,19,20,(25)(26)(27) to cellular hnRNPs and mRNPs rather than to viral nucleocapsid-like RNPs. Apart from their structural role in RNP formation the RRM domains in NLR ORF1p proteins likely have specialized functions as well, assisted by accessory domains like the CTD or the CCHC zinc knuckles.…”

“…These proteins frequently contain CCHC zinc knuckles that are also found in the retroviral nucleocapsid protein Gag (30). This has fueled speculations on a more general functional (25,26,31) and structural (27,28) similarity between NLR ORF1p and retroviral nucleocapsid proteins. Clearly, structural information is required to gain general insight into common ORF1p functions, into the phylogenetic relations among NLRs and into their differences to LTR retrotransposons and retroviruses.…”

“…Many of these clades contain ORF1p proteins that are unrelated to mammalian L1ORF1p but have similar functional properties (25)(26)(27)(28)(29). These proteins frequently contain CCHC zinc knuckles that are also found in the retroviral nucleocapsid protein Gag (30).…”

HeT-A elements in Drosophila virilis : Retrotransposon telomeres are conserved across the Drosophila genus

“…The most obvious difference is the targeting of transposition: HeT-A and TART transpose only onto chromosome ends, whereas the other members of the clade (generally considered parasitic elements) transpose into many parts of the genome but are never found in these telomere regions. This targeting appears to involve Gag proteins because Gags from both HeT-A and TART move efficiently to specific intranuclear sites and associate with chromosome ends, whereas Gags from other members of the clade remain almost entirely in the cytoplasm (2). Another difference is the large amount of noncoding sequence (5Ј and 3Ј UTR) in HeT-A and TART in D. melanogaster and Drosophila yakuba; the other elements, like almost all other retroelements, have very little sequence that does not code for proteins involved in their own transposition.…”

Telomere Maintenance in Organisms without Telomerase

“…Evidence supports the hypothesis that the Gag protein binds preferentially to the transcript that encoded it, because while many HeT-A elements in the terminal array are 5' truncated or otherwise lack an ORF, newly transposed HeT-A elements have a complete Gag ORF (Biessmann et al, 1994). Unlike Gag proteins for closely related parasitic retrotransposons, the HeT-A and TART Gag proteins are transported efficiently into the nucleus (Rashkova et al, 2002b). Unlike other non-LTR elements telomere specific elements do not require nicked DNA, because they are reverse transcribed directly onto the end of the chromosome.…”