TPP1 is a homologue of ciliate TEBP-β and interacts with POT1 to recruit telomerase

Abstract

Telomeres, the tips of linear chromosomes, are protected by various binding proteins including, in ciliates, the telomere-binding complex TBPα/β. Humans have a TBPα homologue, POT1, but TBPβ has not been found outside of ciliates. Now two groups have separately identified the elusive TBPβ homologue in humans as TPP1. Surprisingly, when the POT1–TPP1 complex binds to telomeric DNA, it does not inhibit telomerase activity, as other telomere binding proteins do. Instead, it stimulates telomerase activity and processivity, the rate of nucleotide addition by the core telomerase enzyme. The structure of TPP1's oligonucleotide binding fold contains features that are particularly characteristic of TEBPβ. When the POT1–TPP1 complex is bound to telomeric DNA, it does not inhibit telomerase activity, as other telomere binding proteins do. Instead, the presence of POT1–TPP1 unexpectedly stimulates telomerase activity and processivity. Telomere dysfunction may result in chromosomal abnormalities, DNA damage responses, and even cancer1. Early studies in lower organisms have helped to establish the crucial role of telomerase and telomeric proteins in maintaining telomere length and protecting telomere ends2,3,4,5,6,7. In Oxytricha nova, telomere G-overhangs are protected by the TEBP-α/β heterodimer3,4. Human telomeres contain duplex telomeric repeats with 3′ single-stranded G-overhangs, and may fold into a t-loop structure that helps to shield them from being recognized as DNA breaks8,9. Additionally, the TEBP-α homologue, POT1, which binds telomeric single-stranded DNA (ssDNA)10, associates with multiple telomeric proteins (for example, TPP1, TIN2, TRF1, TRF2 and RAP1) to form the six-protein telosome/shelterin11,12 and other subcomplexes. These telomeric protein complexes in turn interact with diverse pathways to form the telomere interactome13 for telomere maintenance. However, the mechanisms by which the POT1-containing telosome communicates with telomerase to regulate telomeres remain to be elucidated. Here we demonstrate that TPP1 is a putative mammalian homologue of TEBP-β and contains a predicted amino-terminal oligonucleotide/oligosaccharide binding (OB) fold. TPP1–POT1 association enhanced POT1 affinity for telomeric ssDNA. In addition, the TPP1 OB fold, as well as POT1–TPP1 binding, seemed critical for POT1-mediated telomere-length control and telomere-end protection in human cells. Disruption of POT1–TPP1 interaction by dominant negative TPP1 expression or RNA interference (RNAi) resulted in telomere-length alteration and DNA damage responses. Furthermore, we offer evidence that TPP1 associates with the telomerase in a TPP1-OB-fold-dependent manner, providing a physical link between telomerase and the telosome/shelterin complex. Our findings highlight the critical role of TPP1 in telomere maintenance, and support a yin–yang model in which TPP1 and POT1 function as a unit to protect human telomeres, by both positively and negatively regulating telomerase access to telomere DNA.