The structurally conserved TELR region on shelterin protein TPP1 is essential for telomerase processivity but not recruitment

Abstract

In addition to mediating telomerase recruitment, shelterin protein TPP1 also stimulates telomerase processivity. Assessing the in vivo significance of the latter role of TPP1 has been difficult, as TPP1 mutations that perturb telomerase function tend to abolish both telomerase recruitment and processivity. We sought to separate the two activities of TPP1 in regulating telomerase by considering a structure-guided mutagenesis study on the S. cerevisiae telomerase-associated Est3 protein, which revealed a TELR surface region on Est3 that regulates telomerase function via an unknown mechanism without affecting the interaction between Est3 and telomerase (1). Here, we show that mutations within the structurally conserved TELR region on TPP1 impaired telomerase processivity while leaving telomerase recruitment unperturbed, hence uncoupling the two roles of TPP1 in regulating telomerase. Telomeres in cell lines containing homozygous TELR mutations progressively shortened to a critical length that caused cellular senescence, despite the presence of abundant telomerase in these cells. Our findings not only demonstrate that telomerase processivity can be regulated by TPP1, in a process separable from its role in recruiting telomerase to telomeres, but also establish that the in vivo stimulation of telomerase processivity by TPP1 is critical for telomere length homeostasis and long-term cell viability. SignificanceTelomerase directs the synthesis of new telomeric repeats at chromosome ends, enabling cells to overcome the end replication problem and continue to divide. The shelterin protein TPP1 interacts with telomerase, promoting both telomerase recruitment and processivity (the addition of multiple telomeric repeats after a single substrate binding event). Here we show the identification of separation-of-function mutants of TPP1 that eliminate telomerase processivity but leave the telomerase recruitment function intact. When introduced into human cells in a homozygous manner, these mutations can induce critical telomere shortening and cellular senescence. Our observations therefore provide the first demonstration that telomerase processivity, in addition to telomerase recruitment, is a key regulatory step in vivo for continued human cell proliferation.