Figure 5.

Telomere protection by protein complexes. (a) The six core units of shelterin [272] form a complex coating distal telomeres, although stoichiometries of subunits may vary. TRF2 forms T-loops and binds the double-stranded vertebrate telomeric sequence. TRF1 assists this binding, TIN2 and TPP1 form the core of the complex and control other protein-protein interactions and POT1 can bind single-stranded telomeric repeats to stabilize the T-loop. (b) Fission yeast shelterin [286] is analogous to vertebrates but differs in stoichiometries of proteins. (c) Drosophila terminin [95] has a similar function to shelterin although the precise roles of components that share little homology with shelterin components are speculative. (d) Budding yeast telosomes [287]. Rap1 binds telomeric DNA and can be complexed into dimers by Rif2 or tetramers by Rif1. The entire assembly is proposed to form a velcro-like coating of telomeres although to date structural studies of this complex are on dsDNA only, so any interaction with 3′ overhangs is speculative.