CST in maintaining genome stability: Beyond telomeres

Abstract

The human CST (CTC1-STN1-TEN1) complex is an RPA-like single-stranded DNA binding protein complex. While its telomeric functions have been well investigated, numerous studies have revealed that hCST also plays important roles in maintaining genome stability beyond telomeres. Here, we review and discuss recent discoveries on CST in various global genome maintenance pathways, including findings on the CST supercomplex structure, its functions in unperturbed DNA replication, stalled replication, double-strand break repair, and the ATR-CHK1 activation pathway. By summarizing these recent discoveries, we hope to offer new insights into genome maintenance mechanisms and the pathogenesis of CST mutation-associated diseases.

Keywords: CTC1-STN1-TEN1; DNA replication; DSB repair; Fork stalling; Genome stability; Replication stress.

Figure 1.

Structures of the human CST complex. (A) Domain organizations of CST and RPA based on the published crystal or cryo-EM structures. STN1 has an extra C-terminal wHTH domain compared to RPA32. DNA binding sites are indicated in blue bars, subunit interactions in shaded parallelograms, and domains for interacting with other proteins in orange bars/arrows (in CST) or red bars (in RPA). (B) Structure of monomeric CST derived from PDB ID:6W6W. (C) Human STN1n and TEN1 subunits share structural similarity with RPA32 and RPA14, respectively. STN1n and TEN1 are colored in green, and RPA32 and RPA14 are colored in yellow. Structures are derived from Protein Data Bank with structure codes 4JOI and IQUQ.

Figure 2.

Models illustrating the roles of CST in genome stability maintenance. (A) CST regulates telomere stability via the telomeric synthesis processes. In the ds telomeric region, CST resolves G4 structures and relieves replication stress in the telomeric region. At telomere ends, CST binds to ss G-overhangs to inhibit the access of telomerase to telomeres. CST also mediates C-strand fill-in to replenish C-strands. (B) CST regulates DNA replication initiation under normal replication condition. In the G1 phase, CST blocks origin licensing by interacting with MCM to prevent CDT1 interacting with MCM. In the S phase, CST interacts with AND-1 and POLα and then facilitates replisome assembly and subsequent initiation of DNA synthesis. (C) CST protects reversed fork stability under replication stress. Top: CST directly binds to the regressed arms of reversed forks to inhibit unscheduled MRE11 degradation of nascent strand DNA. In addition, CST indirectly protects reversed forks via recruiting RAD51 (bottom). (D) CST regulates the ATR-CHK1 pathway under replication stress. CST prevents the degradation of the ATR activator TOPBP1. CST deficiency decreases TOPBP1 protein level, therefore suppressing CHK1 phosphorylation following replication stress. (E) CST controls end-resection during DSB repair and favors c-NHEJ. At DSB sites, 53BP1-RIF1 recruits the Shieldin complex (SHLD1-SHLD2-SHLD3-REV7), which interacts with and recruits CST to DSB ends to counteract end resection.