Telomeres are protein-DNA complexes that protect chromosome ends from illicit ligation and resection. Telomerase is a ribonucleoprotein enzyme that synthesizes telomeric DNA to counter telomere shortening. Human telomeres are composed of complexes between telomeric DNA and a six-protein complex known as shelterin. The shelterin proteins TRF1 and TRF2 provide the binding affinity and specificity for double-stranded telomeric DNA, while the POT1-TPP1 shelterin subcomplex coats the single-stranded telomeric G-rich overhang that is characteristic of all our chromosome ends. By capping chromosome ends, shelterin protects telomeric DNA from unwanted degradation and end-to-end fusion events. Structures of the human shelterin proteins reveal a network of constitutive and context-specific interactions. The shelterin protein-DNA structures reveal the basis for both the high affinity and DNA sequence specificity of these interactions, and explain how shelterin efficiently protects chromosome ends from genome instability. Several protein-protein interactions, many provided by the shelterin component TIN2, are critical for upholding the end-protection function of shelterin. A survey of these protein-protein interfaces within shelterin reveals a series of "domain-peptide" interactions that allow for efficient binding and adaptability towards new functions. While the modular nature of shelterin has facilitated its part-by-part structural characterization, the interdependence of subunits within telomerase has made its structural solution more challenging. However, the exploitation of several homologs in combination with recent advancements in cryo-EM capabilities has led to an exponential increase in our knowledge of the structural biology underlying telomerase function. Telomerase homologs from a wide range of eukaryotes show a typical retroviral reverse transcriptase-like protein core reinforced with elements that deliver telomerase-specific functions including recruitment to telomeres and high telomere-repeat addition processivity. In addition to providing the template for reverse transcription, the RNA component of telomerase provides a scaffold for the catalytic and accessory protein subunits, defines the limits of the telomeric repeat sequence, and plays a critical role in RNP assembly, stability, and trafficking. While a high-resolution definition of the human telomerase structure is only beginning to emerge, the quick pace of technical progress forecasts imminent breakthroughs in this area. Here, we review the structural biology surrounding telomeres and telomerase to provide a molecular description of mammalian chromosome end protection and end replication.

Telomerase is a large ribonucleoprotein complex responsible for the addition of telomeric DNA repeats to chromosomal ends. Telomerase is composed of core and accessory components that work in coordination to ensure telomere length is maintained during development and in specific cell types. Telomerase activity is tightly regulated and is strongly increased in most tumor cells. On the other hand, loss-of-function mutations either in accessory factors or in core components of the complex impact telomere maintenance and cause a large spectrum of severe phenotypes, typically described as telomere biology disorders. A central element for efficient telomerase function is the proper biogenesis and assembly of the holoenzyme. Here, we discuss our current understanding of these processes and how they modulate telomerase efficiency. We consider how these processes are influenced by the specific subcellular localization of different telomerase components during different stages of the assembly of the holoenzyme. We describe the tremendous progress made in this area over the last decade and how recently discovered aspects of telomerase biogenesis can be exploited clinically, to actively benefit patients suffering from telomere biology disorders.

Abstract ED04-01 Telomeres are composed of repetitive DNA sequences at the end of linear chromosomes. Human telomeres contain thousands of repeats of the six nucleotide sequence, TTAGGG (1). Telomere length is maintained by a balance between processes that lengthen telomeres [telomerase] and processes that shorten telomeres [the end-replication problem]. Telomerase (TEE-LÓM-ER-ACE) is a ribonucleoprotein enzyme complex (a cellular reverse transcriptase) that stabilizes telomere length by adding hexameric (TTAGGG) repeats onto the telomeric ends of the chromosomes, thus compensating for the erosion of telomeres that occurs in its absence. The enzyme is expressed in adult reproductive cells, but is undetectable in normal somatic cells except for proliferative cells of renewal tissues (e.g. bone marrow cells, basal cells of the epidermis, proliferative endometrium, and intestinal crypt cells). In all non-reproductive cells, including proliferative cells of renewal tissues, progressive telomere shortening is observed, eventually leading to greatly shortened telomeres and to a limited ability to continue to divide. Telomere shortening is a molecular clock mechanism that counts the number of times a cell has divided and when telomeres are short, cellular senescence (growth arrest) occurs (2-3). It is believed that shortened telomeres in mitotic (dividing) cells functions as an initial brake against the development of cancer (4-6) and may be responsible for some of the changes we a