Telomere protection in Drosophila: functional analysis of the terminin complex. Grazia D. Raffa, Emanuela Micheli, Fiammetta Verni, Domenico Raimondo, Alessandro Cicconi, Laura Ciapponi, Giovanni Cenci, Stefano Cacchione, Maurizio Gatti. Biology and Biotechnology, Sapienza Universita' di Roma, Rome, Italy.

   Drosophila telomeres are epigenetically determined, sequence-independent structures that are not maintained by telomerase, but by transposition to chromosome ends of specialized retroelements. Genetic and biochemical analyses have recently shown that fly telomeres are capped by terminin, a complex that includes at least four proteins: HOAP, HipHop, Modigliani (Moi) and Verrocchio (Ver). With the exception of Ver, which exhibits a structural homology with Stn1, the terminin proteins are not conserved outside the Drosophilidae and are all encoded by fast-evolving genes. Terminin localizes and appears to function only at telomeres just like shelterin, suggesting that terminin is a functional analogue of shelterin. We have now analyzed the structure of terminin using suitable protein truncations and DNA binding assays. HOAP binds double stranded (ds) and Ver single stranded (ss) DNA; Moi does not bind DNA but interacts directly with HOAP and Ver forming a bridge between the two proteins. Thus, the architecture of terminin is similar to that found in other telomere capping complexes including shelterin, where the ss DNA-binding protein Pot1 is connected to the TRF1/TRF2 ds DNA-associated proteins by the non-DNA-binding factor TPP1. Our data further suggest that Ver and Moi mask ss DNA at Drosophila telomeres, just like TPP1-Pot1 at human telomeres: When chromosome ends lack either Moi or Ver, telomeres form DNA repair foci that contain the phosphorylated form of the H2Av histone. Collectively, our results reinforce the idea that the basic mechanisms of telomere capping are conserved from yeast to flies and humans, and support a unifying model for telomere protection.