Evolutionary mode for the functional preservation of fast-evolving Drosophila telomere capping proteins

DNA end protection is fundamental for the long-term preservation of the genome. In vertebrates the Shelterin protein complex protects telomeric DNA ends, thereby contributing to the maintenance of genome integrity. In the Drosophila genus, this function is thought to be performed by the Terminin complex, an assembly of fast-evolving subunits. Considering that DNA end protection is fundamental for successful genome replication, the accelerated evolution of Terminin subunits is counterintuitive, as conservation is supposed to maintain the assembly and concerted function of the interacting partners. This problem extends over Drosophila telomere biology and provides insight into the evolution of protein assemblies. In order to learn more about the mechanistic details of this phenomenon we have investigated the intra- and interspecies assemblies of Verrocchio and Modigliani, two Terminin subunits using in vitro assays. Based on our results and on homology-based three-dimensional models for Ver and Moi, we conclude that both proteins contain Ob-fold and contribute to the ssDNA binding of the Terminin complex. We propose that the preservation of Ver function is achieved by conservation of specific amino acids responsible for folding or localized in interacting surfaces. We also provide here the first evidence on Moi DNA binding.