The Drosophila Mre11/Rad50 Complex Is Required to Prevent Both Telomeric Fusion and Chromosome Breakage

Abstract

The MRN complex consists of the two evolutionarily conserved components Mre11 and Rad50 and the third less-conserved component Nbs1/Xrs2. This complex mediates telomere maintenance in addition to a variety of functions in response to DNA double-strand breaks, including homologous recombination, nonhomologous end joining (NHEJ), and activation of DNA damage checkpoints. Mutations in the Mre11 gene cause the human ataxia-telangiectasia-like disorder (ATDL). Here, we show that null mutations in the Drosophila mre11 and rad50 genes cause both telomeric fusion and chromosome breakage. Moreover, we demonstrate that these mutations are in the same epistasis group required for telomere capping and mitotic chromosome integrity. Using an antibody against Rad50, we show that this protein is uniformly distributed along mitotic chromosomes, and that Rad50 is unstable in the absence of its binding partner Mre11. To define the roles of rad50 and mre11 in telomere protection, mutant chromosome preparations were immunostained for both HP1 and HOAP, two proteins that protect Drosophila telomeres from fusion. Cytological analysis revealed that mutations in rad50 and mre11 drastically reduce accumulation of HOAP and HP1 at telomeres. This suggests that the MRN complex protects Drosophila telomeres by facilitating recruitment of HOAP and HP1 at chromosome ends.

Three distinct chromatin domains in telomere ends of polytene chromosomes inDrosophila melanogaster Telmutants

“…In mutants, the lack of proteins encoded by the genes Su (var)205,tefu (ATM), mre11 and rad50 leads to a dramatic increase in frequency of telomeric fusions in diploid dividing cells. Since these associations of telomeres do not break in mitotic anaphase, this observation suggests that these proteins play an important role in protecting the telomeres from fusions (Fanti et al, 1998;Oikemus et al, 2004;Ciapponi et al, 2004). The important differences observed between the polytene and the mitotically dividing cells are most probably due to the fact that salivary gland differentiation takes place in early embryogenesis.…”

“…One candidate is HOAP, which forms a complex with HP1 (Shareef et al, 2001) and is present in cap regions of Su(var)205 null mutants (Cenci et al, 2003). A number of additional proteins appear to contribute to the stability of the HOAP/HP1 complex since, in tefu (ATM) (Oikemus et al, 2004), mre11 and rad50 mutants (Ciapponi et al, 2004), HP1 and HOAP fail to accumulate in cap regions in polytene chromosomes.…”

Drosophila telomeres: an exception providing new insights

“…Other proteins necessary for capping function are also major components of the DNA damage response (11), as has been found in mammals (34). In particular, members of the Mre11/Rad50/ Nbs (MRN) complex are required to maintain HOAP at the telomere and prevent telomere fusions (35)(36)(37)(38)(39), as are the ATM and ATR kinases (39)(40)(41)(42)(43). The ATM and ATR DNA damage response pathways are partially redundant.…”

The Ku Protein Complex Is Involved in Length Regulation of Drosophila Telomeres

“…Finally, the Drosophila homologs of the mre/rad50 comdeficiency strains suggest easier accessibility of the end, predicting the accumulation of longer retrotransposon plex and ATM kinase, which play important roles at yeast and mammalian telomeres, are also required for arrays at chromosome tips over time. This has been found for mutations in Su(var)205, the gene that entelomere capping in Drosophila (Queiroz-Machado et al 2001;Bi et al 2004;Ciapponi et al 2004; Oikemus codes the telomere-binding protein, HP1 (Eissenberg et al 1990(Eissenberg et al ). et al 2004Silva et al 2004).…”