Organization and maintenance of Drosophila telomeres: the roles of terminin and non-terminin proteins.

Drosophila telomeres are elongated by occasional transposition of specialized retroelements rather than telomerase activity, and are assembled independently of the sequence of the DNA termini. Drosophila telomeres are capped by terminin, a complex formed by the HOAP, Moi, Ver and HipHop proteins that localize exclusively at telomeres and protect them from fusion events. Other proteins required to prevent end-to-end fusion include HP 1 Eff/UbcD 1, ATM, the components of the Mrel 1-Rad50-Nbs (MRN) complex, and the Woc transcription factor. The terminin proteins are encoded by fast-evolving genes and are not evolutionarily conserved outside the Drosophila species. In contrast, the non-terminin telomere capping proteins are not fast-evolving, do not localize only at telomeres and are conserved from yeasts to mammals. We propose that following telomerase loss, Drosophila rapidly evolved terminin to bind chromosome ends in a sequence-independent manner, and that non-terminin proteins did not evolve as rapidly as terminin because of the functional constraints imposed by their involvement in diverse cellular processes. This hypothesis suggests that the Drosophila non-terminin proteins might correspond to ancestral telomere-associated proteins with homologues in other organisms including humans.