HipHop interacts with HOAP and HP1 to protect Drosophila telomeres in a sequence-independent manner

Abstract

Telomeres prevent chromosome ends from being repaired as double-strand breaks (DSBs). Telomere identity in Drosophila is determined epigenetically with no sequence either necessary or sufficient. To better understand this sequence-independent capping mechanism, we isolated proteins that interact with the HP1/ORC-associated protein (HOAP) capping protein, and identified HipHop as a subunit of the complex. Loss of one protein destabilizes the other and renders telomeres susceptible to fusion. Both HipHop and HOAP are enriched at telomeres, where they also interact with the conserved HP1 protein. We developed a model telomere lacking repetitive sequences to study the distribution of HipHop, HOAP and HP1 using chromatin immunoprecipitation (ChIP). We discovered that they occupy a broad region >10 kb from the chromosome end and their binding is independent of the underlying DNA sequence. HipHop and HOAP are both rapidly evolving proteins yet their telomeric deposition is under the control of the conserved ATM and Mre11-Rad50-Nbs (MRN) proteins that modulate DNA structures at telomeres and at DSBs. Our characterization of HipHop and HOAP reveals functional analogies between the Drosophila proteins and subunits of the yeast and mammalian capping complexes, implicating conservation in epigenetic capping mechanisms.

Terminin: A protein complex that mediates epigenetic maintenance ofDrosophilatelomeres

“…These events include X-ray induced breaks in the Dp(1;f)1187 mini-chromosome, 16 the mobilization of a P element located near the telomere, 17 breakage of dicentric chromosomes generated by site specific recombination, 18,19 and induction of an enzymatic cut in an I-Sce1 site placed within a P element construct inserted near the telomere. 20 Collectively, these results demonstrate that the HTT elements are not required for fly telomere assembly and that virtually any DNA sequence has the ability to form the nucleoprotein complex that protects the ends of Drosophila chromosomes.…”

Preferential Breakpoints in the Recovery of Broken Dicentric Chromosomes inDrosophila melanogaster

“…For generation of dicentric chromosomes, nosGal4 (Van Doren et al 1998) drove UASFLP (Duffy et al 1998) expression in the male germline. In many crosses, the UAShiphop construct HRH008H (Gao et al 2010) was included because in other experiments we have seen that it can increase the recovery of broken-and-healed chromosomes. Males were mated singly to two to three C(1)DX/Y; eyFLP23A/SM6, Cy females or as four males by five females.…”

“…In some crosses, we also included a UAShiphop (Gao et al 2010) construct because we have seen that it can increase If flies with eyFLP also carry a ring chromosome with an FRT, the dicentric chromosomes that are produced in the developing eye make adults with small, rough eyes . However, FLP expression has no deleterious effect in flies that carry an FRT on a linear X chromosome.…”

Telomere Maintenance in Organisms without Telomerase

“…The telomere-specific components of the cap in Drosophila are collectively termed 'terminin' by analogy to the shelterin protein complex at mammalian telomeres (Raffa et al, 2009). The terminin proteins differ from the shelterin proteins, in part because the TRF1 and TRF2 components of shelterin bind specifically to the canonical telomeric repeat, while the formation of the telomere cap in Drosophila is sequence independent, and in part because many of the terminin proteins are among the fastest evolving proteins in Drosophila (Gao et al, 2010;Raffa et al, 2010;Schmid & Tautz, 1997). There is no direct evidence that the cap in Drosophila plays a role in maintaining chromosome length.…”