Evolution of Telomere Binding Proteins

Telomere binding proteins provide essential functions for chromosome maintenance in most eukaryotes and consequently are well suited for analysis in the context of evolution. This review focuses on patterns gleaned from structural and functional characteriza­ tion of telomere proteins that reveal contrasting evolutionary histories for double-stranded and single-stranded DNA-bindingprotein families. The myb-like/homeodomain DNA-binding motif is ubiquitous among members of the double-stranded telomere DNA-binding protein family which includes R ap l and Tazl in yeast, TRF1 and TRF2 in vertebrates, as well as putative plant-specific telomere proteins, TBP1, TRP1 and Sm hl. In this myb-motif family, strong purifying selection has preserved amino-acid sequence among distantly related lineages. Accessory domains linked with the myb-like DNA-binding domain define distinct lineages, indicating that the myb motif was probably recruited multiple times from a reserve of more ancestral forms functioning as transcription factors. The oligonucleotide/oligosaccharide/oligopeptide-binding (OB)-fold is universally found in members of the second class of telomere binding proteins that recognize and bind T/G-rich telomere sequences in the form of single-stranded DNA. Examples of proteins in this class include TEBP-a and TEBP-P from ciliated protozoa, C d c l3 from budding yeast, and Potl which is widely distributed in fission yeast, vertebrates and plants. For these OB-fold proteins, rapid divergence at the amino-acid sequence level has all but erased traces of common ancestry. Homology is apparent, however, when comparing three-dimensional structures and functional characters. Sequence alignments consistent with these structural comparisons provide a tentative glimpse of deeply rooted lineages that likely emerged from the general single-stranded DNA-binding proteins dedicated to DNA replication and repair. Interaction networks among telomere DNA-binding proteins and their associated interacting partners hint at further common patterns and innovations encountered during the evolution of telomere capping complexes.