POT1-TPP1 telomere length regulation and disease

Abstract

Telomeres are DNA repeats at the ends of linear chromosomes and are replicated by telomerase, a ribonucleoprotein reverse transcriptase. Telomere length regulation and chromosome end capping are essential for genome stability and are mediated primarily by the shelterin and CST complexes. POT1-TPP1, a subunit of shelterin, binds the telomeric overhang, suppresses ATR-dependent DNA damage response, and recruits telomerase to telomeres for DNA replication. POT1 localization to telomeres and chromosome end protection requires its interaction with TPP1. Therefore, the POT1-TPP1 complex is critical to telomere maintenance and full telomerase processivity. The aim of this mini-review is to summarize recent POT1-TPP1 structural studies and discuss how the complex contributes to telomere length regulation. In addition, we review how disruption of POT1-TPP1 function leads to human disease.

Keywords: ATM, Ataxia Telangiectasia Mutated protein; ATR, Ataxia Telangiectasia and Rad3-related Protein; CST, CTC1, Stn1 and Ten1; CTC1, Conserved Telomere Capping Protein 1; POT1; POT1, Protection of telomere 1; RAP1, Repressor/Activator Protein 1; RPA, Replication Protein A; SMCHD1, Structural Maintenance Of Chromosomes Flexible Hinge Domain Containing 1; Shelterin; Stn1, Suppressor of Cdc Thirteen; TERC, Telomerase RNA; TERT, Telomerase Reverse Transcriptase; TIN2, TRF1- and TRF2-Interacting Nuclear Protein 2; TPP1; TPP1 also known as ACD, Adrenocortical Dysplasia Protein Homolog; TRF1, Telomere Repeat binding Factor 1; TRF2, Telomere Repeat binding Factor 2; TSPYL5, Testis-specific Y-encoded-like protein 5; Telomerase; Telomeres; Ten1, Telomere Length Regulation Protein; USP7, ubiquitin-specific-processing protease 7.

Fig. 1

Schematic of shelterin and CST activities at telomeres. A. Shelterin facilitates T-loop formation at the ends of our chromosomes. B. Telomerase is recruited by the POT1-TPP1 processivity factor. C1 and C2. Two possible mechanisms of telomerase-dependent telomere extension: telomerase may have a transient interaction with TPP1 (C1) or remain tethered to TPP1 during telomere replication (C2). D. Inhibition of telomerase dependent telomere replication by the CST complex. E. POT1 contains three OB folds (OB1, OB2, OB3) and one Holliday Junction Resolvase (HJR) domain. While the N-terminal OB1 and OB2 bind ssDNA, OB3 and the HJR domain bind TPP1. F. TPP1 contains an N-terminal OB-fold involved in telomerase recruitment to telomeres, and a POT1- and TIN2-binding domains.

Fig. 2

POT1 structures in complex with TPP1 or DNA. A. POT1 N-terminal 2 OB folds (OB1 and OB2) in complex with single-stranded telomeric DNA (PDB: 1XJV) B. POT1 C-terminal OB fold (OB3) and holliday junction resolvase domain (HJRD) in complex with TPP1 (green). The cysteine cluster is coordinated to a Zn²⁺ ion shown as blue sphere. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Structures of POT1-DNA and POT1-TPP1 showing selected mutations implicated in cancer and telomere syndromes. A. Structure of the POT1-N-terminal OB folds in complex with single-stranded telomeric DNA (PDB ID: 1XJV). The telomere syndrome mutation K90E is shown in stick. Mutations described in detail in the text are denoted by boxes. B. Holliday Junction resolvase domain (HJRD – red) of POT1 in complex with helix a1 of the TPP1 peptide (green). The cancer mutation P446Q and the TPP1 interacting residues H267 and L271 are shown in stick. C. POT1 (OB3) (blue) in complex with TPP1 (green) showing the cancer mutations C591W and Q623H. C591 is buried and contributes to the fold of the protein. In the C591W mutant, the much larger tryptophan side chain would perturb the fold of the OB fold. Q623 makes direct contact with the amide backbone of the TPP1 peptide. D. POT1 (HJRD) (red) in complex with TPP1 (green) showing TPP1 V272 and its interactions with POT1 K422, W424 and F438. E. I322 interacts with the P357 of the POT1 (OB3). F. Overlay of wild type (WT – PDB ID: 2I46) TPP1 and K170 deletion (K170del PDB ID: 5I2Y and 5I2X) of the N-terminal TPP1 OB fold. The region of the K170del in the TPP1 structure is highlighted with a blue dashed circle. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)