Evolution of CST function in telomere maintenance

Telomeres consist of an elaborate, higher-order DNA architecture, and a suite of proteins that provide protection for the chromosome terminus by blocking inappropriate recombination and nucleolytic attack. In addition, telomeres facilitate telomeric DNA replication by physical interactions with telomerase and the lagging strand replication machinery. The prevailing view has been that two distinct telomere capping complexes evolved, shelterin in vertebrates and a trimeric complex comprised of Cdc13, Stn1 and Ten1 (CST) in yeast. The recent discovery of a CST-like complex in plants and humans raises new questions about the composition of telomeres and their regulatory mechanisms in multicellular eukaryotes. In this review we discuss the evolving functions and interactions of CST components and their contributions to chromosome end protection and DNA replication

The Pot1a-associated proteins Tpt1 and Pat1 coordinate telomere protection and length regulation in Tetrahymena

We have identified two new Pot1a-associated telomere proteins, Pat1 and Tpt1, from Tetrahymena. Tpt1 is required to prevent telomere elongation and appears to be the Tetrahymena equivalent of vertebrate TPP1. Pat1 depletion causes gradual telomere shortening, indicating that it is needed for telomerase to gain access to the DNA terminus

Structural and functional analysis of the human POT1-TPP1 telomeric complex

POT1 and TPP1 are part of the shelterin complex and are essential for telomere length regulation and maintenance. Naturally occurring mutations of the telomeric POT1–TPP1 complex are implicated in familial glioma, melanoma and chronic lymphocytic leukaemia. Here we report the atomic structure of the interacting portion of the human telomeric POT1– TPP1 complex and suggest how several of these mutations contribute to malignant cancer. The POT1 C-terminus (POT1C) forms a bilobal structure consisting of an OB-fold and a holiday junction resolvase domain. TPP1 consists of several loops and helices involved in extensive interactions with POT1C. Biochemical data shows that several of the cancerassociated mutations, partially disrupt the POT1–TPP1 complex, which affects its ability to bind telomeric DNA efficiently. A defective POT1–TPP1 complex leads to longer and fragile telomeres, which in turn promotes genomic instability and cancer

Structure of human telomerase holoenzyme with bound telomeric DNA

Telomerase adds telomeric repeats at chromosome ends to compensate for telomere loss caused by incomplete genome end replication1. In humans, telomerase is upregulated during embryogenesis and in cancers, while mutations that compromise its function result in diseases2. Our previous 8 Å human telomerase structure revealed vertebrate-specific composition and architecture3, consisting of a catalytic core flexibly tethered to an H/ACA ribonucleoprotein (RNP) lobe by telomerase RNA. To effectively modulate telomerase activity as a therapeutic approach against cancers and diseases, high-resolution structural information is necessary. Here we present the structure of human telomerase holoenzyme bound to a telomeric DNA, determined by cryo-electron microscopy (cryo-EM) at 3.4 Å resolution for the H/ACA RNP and 3.8 Å resolution for the catalytic core. The structure reveals crucial DNA/RNA binding interfaces in telomerase active site and locations of mutations that alter telomerase activity. We identified a histone H2A-H2B dimer within the holoenzyme bound to an essential telomerase RNA motif, suggesting an unexpected role for histones in telomerase RNA folding and function. Furthermore, the first high-resolution structure of a eukaryotic H/ACA RNP reveals the molecular recognition of conserved RNA and protein motifs and new interactions crucial for understanding the molecular pathology of many disease mutations. Our findings illuminate unanticipated structural details of human telomerase assembly and active site, paving the way for the development of telomerase-targeting therapeutics