SLX4 contributes to telomere preservation and regulated processing of telomeric joint molecule intermediates

Abstract

SLX4 assembles a toolkit of endonucleases SLX1, MUS81 and XPF, which is recruited to telomeres via direct interaction of SLX4 with TRF2. Telomeres present an inherent obstacle for DNA replication and repair due to their high propensity to form branched DNA intermediates. Here we provide novel insight into the mechanism and regulation of the SLX4 complex in telomere preservation. SLX4 associates with telomeres throughout the cell cycle, peaking in late S phase and under genotoxic stress. Disruption of SLX4's interaction with TRF2 or SLX1 and SLX1's nuclease activity independently causes telomere fragility, suggesting a requirement of the SLX4 complex for nucleolytic resolution of branched intermediates during telomere replication. Indeed, the SLX1–SLX4 complex processes a variety of telomeric joint molecules in vitro. The nucleolytic activity of SLX1-SLX4 is negatively regulated by telomeric DNA-binding proteins TRF1 and TRF2 and is suppressed by the RecQ helicase BLM in vitro. In vivo, in the presence of functional BLM, telomeric circle formation and telomere sister chromatid exchange, both arising out of nucleolytic processing of telomeric homologous recombination intermediates, are suppressed. We propose that the SLX4-toolkit is a telomere accessory complex that, in conjunction with other telomere maintenance proteins, ensures unhindered, but regulated telomere maintenance.

Complex interactions between the DNA-damage response and mammalian telomeres

“…Unlike mouse SLX4, human SLX4 contains a sequence resembling the Apollo TRF2-binding motif, which drives TRF2 TRFH -dependent permanent recruitment of the SLX4–MUS81–XPF–SLX1 complex to telomeres 46–48 . Using the SLX4–SLX1 complex, TRF2 has been proposed to control telomere-length homeostasis by regulating the trimming mechanism, in which very long telomeres are shortened by t-loop excision.…”

“…50). The basic domain of TRF2 also inhibits resolvase activities at telomeres, thereby promoting telomere replication and stabilizing t loops 48,49 . Rad51 is also recruited to telomeres by BRCA2 during G2, when it has been suggested to play a role in facilitating telomere replication 51 , thus again demonstrating the complex interaction of the HR machinery with telomeres during and after replication.…”

“…TRF2 possesses four distinct domains, the N-terminal basic domain 38,39,48 , the TRFH domain 10,44,46,96–100 , the hinge domain 10,14,93 and the MYB DNA-binding domain 96,97 . Human (H) or mouse (M) amino acid residues that are involved in protein-protein interactions or that disrupt interactions when mutated are specified in the third column.…”

Fanconi anemia proteins in telomere maintenance

“…Thus, ribosylation-mediated control of TRF1 affinity for telomeric DNA may be one possible mechanism where FANCD2 ensures telomere stability. TRF1 also physically blocks the SLX4-nuclease complex from nucleolytically resolving the T-loop [30]. Thus, TRF1 removal from telomeres may promote nucleolytic resolution of telomeric joint molecule intermediates.…”

“…Subsequently, crystallographic, cellular and biochemical studies revealed that SLX4 and its associated nucleases is preferentially recruited to long telomeres via direct interaction between a unique HxLxP (x, any amino acid) motif within the t elomere b inding m otif (TBM) of SLX4 and a docking site at the TRF homology (TRFH) domain of TRF2 (Figure 2B) [38, 47]. The SLX4-nuclease complex is required for multiple aspects of telomere maintenance, including negative regulation of telomere length, regulation of telomere recombination, and prevention of telomere replication defects (manifested as fragile telomeres) [30, 38, 47]. The molecular mechanism behind the role of SLX4 in telomere maintenance likely involves the ability of the SLX4-associated nucleases to process and remove alternate DNA structures such as Holliday Junction (HJ) and T-loop.…”