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bioRxiv · 10.1101/2020.10.05.326181

TopBP1 assembles nuclear condensates to switch on ATR signalling.

Frattini, C.·Promonet, A.·Alghoul, E.·Vidal Eychenie, S.·Lamarque, M.·Blanchard, M.-P.·Urbach, S.·Basbous, J.·Constantinou, A.
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Abstract

ATR checkpoint signalling is crucial for cellular responses to DNA replication impediments. Using an optogenetic platform, we show that TopBP1, the main activator of ATR, self-assembles extensively to yield micron-sized condensates. These opto-TopBP1 condensates are functional entities organized in tightly packed clusters of spherical nano-particles. TopBP1 condensates are reversible, occasionally fuse and co-localise with TopBP1 partner proteins. We provide evidence that TopBP1 condensation is a molecular switch that amplifies ATR activity to phosphorylate checkpoint kinase 1 (Chk1) and slowdown replication forks. Single amino acid substitutions of key residues in the intrinsically disordered ATR-activation domain disrupt TopBP1 condensation and, consequently, ATR/Chk1 signalling. In physiologic salt concentration and pH, purified TopBP1 undergoes liquid-liquid phase separation in vitro. We propose that the actuation mechanism of ATR signalling is the assembly of TopBP1 condensates driven by highly regulated multivalent and cooperative interactions.

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Frattini, C., Promonet, A., Alghoul, E., Vidal Eychenie, S., Lamarque, M., Blanchard, M.-P., Urbach, S., Basbous, J., Constantinou, A.. 2020-10-05. TopBP1 assembles nuclear condensates to switch on ATR signalling.. https://doi.org/10.1101/2020.10.05.326181

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