Structural Features of Nucleoprotein CST/Shelterin Complex Involved in the Telomere Maintenance and Its Association with Disease Mutations

Abstract

Telomere comprises the ends of eukaryotic linear chromosomes and is composed of G-rich (TTAGGG) tandem repeats which play an important role in maintaining genome stability, premature aging and onsets of many diseases. Majority of the telomere are replicated by conventional DNA replication, and only the last bit of the lagging strand is synthesized by telomerase (a reverse transcriptase). In addition to replication, telomere maintenance is principally carried out by two key complexes known as shelterin (TRF1, TRF2, TIN2, RAP1, POT1, and TPP1) and CST (CDC13/CTC1, STN1, and TEN1). Shelterin protects the telomere from DNA damage response (DDR) and regulates telomere length by telomerase; while, CST govern the extension of telomere by telomerase and C strand fill-in synthesis. We have investigated both structural and biochemical features of shelterin and CST complexes to get a clear understanding of their importance in the telomere maintenance. Further, we have analyzed ~115 clinically important mutations in both of the complexes. Association of such mutations with specific cellular fault unveils the importance of shelterin and CST complexes in the maintenance of genome stability. A possibility of targeting shelterin and CST by small molecule inhibitors is further investigated towards the therapeutic management of associated diseases. Overall, this review provides a possible direction to understand the mechanisms of telomere borne diseases, and their therapeutic intervention.

Keywords: CST complex; OB-fold proteins; Telomere replication; cancer; mutations; shelterin complex; small molecule inhibitors; structural genomics; telomere shortening.

Figure 1

Schematic representation and key molecular function of shelterin and CST complexes at the telomere. The key functions of each component of CST and shelterin complexes were briefly discussed. Shelterin complex proteins are mainly involved in suppressing DNA damage response; however, CST complex is primarily regulating the length of telomere. Figure was adapted and modified from [7,8,9].

Figure 2

Domain organization and tertiary structure of the shelterin complex. (A) telomere repeat factor 1 (TRF1) (PDB ID: 3BQO; depicts the cartoon representation of TRF homology (TRFH) domain of TRF1 in red. 1W0T; the DNA-binding domain of TRF1 shown in green in complex with telomeric DNA which is shown as light orange). (B) TRF2 (PDB ID: 4M7C; represents TRFH domain of TRF2. 3K6G; represents the repressor/activator protein 1 (RAP1)–binding motif of TRF2 in red. 1W0U; represents the DNA-binding motif of TRF2). (C) RAP1 (PDB ID: 1FEX; represent the Myb domain shown in green and 3K6G; shows RCT domain in red). (D) TIN2 (PDB ID: 5XYF; cartoon representation in green depicts TRFH domain and 3BQO; TRF1-interacting nuclear factor 2 (TIN2) peptide shown in red complexed with TRFH domain of TRF1). (E) TPP1 (PDB ID: 2I46; depicts the OB-fold domain in purple and 5I2X; represents the POT1 interacting domain of TPP1 (cyan) with C-terminal domain of POT1 (gray)). (F) POT1 (PDB ID: 1XJV; depicts the C-terminal domain of POT1 in purple in complex with telomeric DNA (light orange) 5H65; represents the POT1 interacting domain of TPP1 (red) with C-terminal domain of POT1 (purple).

Figure 3

Domain organizations and tertiary structures of CST (CTC1-STN1) complex. (A) CDC13 (PDB ID: 3NWT; OB1 domain shown in purple, 4HCE; represents the dimer of OB2 domain, 1S40; depicts the DNA-binding domain of CDC13 in complex with telomeric DNA). (B) CTC1 (PDB ID: 5W2L; represent the OB-fold domain of CTC1 in purple. (C) STN1 (PDB ID: 4JOI; cartoon representation of N-terminal OB-fold domain of STN1and 4JQF depicts the N-terminal domain of STN1 in salmon color). (D) TEN1 (PDB ID: 4JOI; depicts the OB-fold domain of TEN1 in chocolate color).

Figure 4

Molecular target directly affecting the integrity of telomere. (A) G4-ligands bind with telomeric end to stabilize or promote the formation of G4 structure which abrogates the telomeric end extension. Telomerase inhibitors target either hTERT or hTR and block telomere extension, consequently disrupting telomere integrity. The CST complex is crucial for C-strand synthesis and defect in this complex dysregulate telomerase action and telomere loss. (B) Targeting components of the shelterin complex involved in the protection of telomeric end. (C) Use of T-oligos technology to block the lengthening of telomere. Figure is adapted and modified from the papers [192,206,207].

Figure 5

Structures of small molecule inhibitors that target different molecular interactions responsible for the telomere integrity. The chemical structures compounds depicted here have binding affinity against protection of telomeres 1 (POT1).

Figure 6

Proposed key target site in CST and shelterin complex that can be targeted for therapeutic applications. Some of the key interacting partners of these complexes have been considering as an emerging targets for telomere borne diseases, while others are being investigation for therapeutic targets. The dashes line in black indicates the interaction between components of both complexes and bar-headed lines showing possible inhibition site in red.