It all comes together at the ends: telomerase structure, function, and biogenesis

Abstract

Telomerase is a reverse transcriptase specialized in the addition of telomeric DNA repeats onto the ends of chromosomes. Telomere extension offsets the loss of telomeric repeats from the failure of DNA polymerases to fully replicate linear chromosome ends. Telomerase functions as a ribonucleoprotein, requiring an integral telomerase RNA (TR) component, in addition to the catalytic telomerase reverse transcriptase (TERT). Extensive studies have identified numerous structural and functional features within the TR and TERT essential for activity. A number of accessory proteins have also been identified with various functions in enzyme biogenesis, localization, and regulation. Understanding the molecular mechanism of telomerase function has significance for the development of therapies for telomere-mediated disorders and cancer. Here we review telomerase structural and functional features, and the techniques for assessing telomerase dysfunction.

Fig. 1

A model of the human telomerase reaction cycle. The telomerase reaction is divided between nucleotide addition (left) which is common to all polymerases and template translocation (right, pale-blue box), a unique property of telomerase. After assembly of the telomerase catalytic core, composed of TERT (grey) and TR (green), with the DNA primer (blue), six nucleotides (violet) are sequential added in a template-dependent manner (dark-grey arrows).

Fig. 2

TR and TERT motifs and domain organization. Top, the TR is composed of three functional domains. The template/pseudoknot (pale-green box) and CR4/5 (pale-violet box) domains bind to TERT and are essential for enzymatic activity. While the H/ACA domain (grey box) is dispensable for activity, it is essential for in vivo biogenesis, accumulation, and RNP assembly. Additionally there is the RHAU RNA helicase binding to the G-quadruplex structure (orange) at the 5′ end and the TBE composed of helix P1b (blue) located upstream of the template. Bottom, the TERT protein is composed of 4 independently folding domains. The TEN domain (green) and TRBD (violet) are shaded in colors corresponds to the template/pseudoknot and CR4/5 domains. Important motifs within each domain are colored similarly to the domain.

Fig. 3

A model of telomerase RNP biogenesis. Active telomerase localizes to the telomere through a complex pathway. TERT protein expression follows the canonical eukaryotic transcription, RNA maturation, and nuclear export to the cytoplasm (pale-red) for translation. The TERT protein (grey) is then imported back into the nucleus and localizes to the nucleolus (pale-violet) prior to assembly with the TR (black). The TR precursor, synthesized by RNA polymerase II and TMG capped, is bound by two copies of the protein complex formed by dyskerin (red), NOP10 (violet), NHP2 (green), and GAR1 (yellow) for 3′ end processing and internal modifications. The RHAU RNA helicase (light-blue) resolves the G-quadruplex at the 5′ end. TCAB1 (dark-blue) binds to the CAB box for localization of the mature TR to Cajal bodies (pale-blue). TERT localizes to Cajal bodies for assembly with the TR, aided by the chaperone proteins hsp90 (orange) and p23 (cyan). The now fully assembled active telomerase complex localizes to the telomere (blue) for telomeric DNA synthesis.