TIAR marks nuclear G2/M transition granules and restricts CDK 1 activity under replication stress

Abstract

The G2/M checkpoint coordinates DNA replication with mitosis and thereby prevents chromosome segregation in the presence of unreplicated or damaged DNA. Here, we show that the RNA‐binding protein TIAR is essential for the G2/M checkpoint and that TIAR accumulates in nuclear foci in late G2 and prophase in cells suffering from replication stress. These foci, which we named G2/M transition granules (GMGs), occur at low levels in normally cycling cells and are strongly induced by replication stress. In addition to replication stress response proteins, GMGs contain factors involved in RNA metabolism as well as CDK1. Depletion of TIAR accelerates mitotic entry and leads to chromosomal instability in response to replication stress, in a manner that can be alleviated by the concomitant depletion of Cdc25B or inhibition of CDK1. Since TIAR retains CDK1 in GMGs and attenuates CDK1 activity, we propose that the assembly of GMGs may represent a so far unrecognized mechanism that contributes to the activation of the G2/M checkpoint in mammalian cells.

Cells take a break when they are TIAR ed

“…Taken together, the discovery by Lafarga et al on TIAR's role in restraining CDK1 activity and preventing premature mitotic entry extends recent work on intrinsic and replication stress‐associated checkpoint functions , and adds an intriguing layer of spatio‐temporal confinement of CDK activity on top, which may well turn out to have broader applicability.…”

“…In this issue of EMBO Reports, Lafarga et al uncover a new mechanism of G2/M checkpoint control and also identify a novel membraneless compartment associated with G2/M checkpoint modulation. Their scientific journey begins by studying the role of TIAR, an RNA‐binding protein with both nuclear and cytoplasmic functions , in cell cycle progression.…”

“…In order to address how TIAR modulates the G2/M checkpoint, Lafarga et al examined TIAR subcellular localization and noticed a transient clustering of TIAR into discrete nuclear foci, specifically at the transition from late G2 phase to prophase in mitosis. TIAR contains, besides three RNA recognition motifs (RRMs), a glutamine‐rich prion‐related domain (QRD/PRD), and it self‐assembles upon various stresses into cytoplasmic stress granules (SGs), phase‐separated membraneless compartments .…”

“… Cell cycle progression relies on controlled transition from one phase of the cell cycle to the next, and sensing whether or not a certain cell cycle phase has been completed before the next phase is allowed to start is crucial for genome integrity and cell survival. In this issue of EMBO Reports , Lafarga et al report an original mechanism for spatio‐temporal control of CDK 1 activity, which depends on a nuclear function of the RNA ‐binding protein TIAR . The role of TIAR to restrain mitotic onset is linked to a newly discovered membraneless compartment, termed G2/M transition granule ( GMG ), which transiently sequesters CyclinB‐ CDK 1 in response to replication stress to curb CDK 1 kinase activity and thereby tune G2 duration and mitotic commitment. …”

Working on Genomic Stability: From the S-Phase to Mitosis

“…In addition, new data show that in the presence of replication stress (replication fork slow down, pausing, or instability caused by endogenous or exogenous sources), G2/M transition granules (GMGs), mainly formed by the RNA-binding protein TIAR, accumulate in the nucleus of late G2 and prophase cells. GMGs also contain proteins associated with stalled replication forks and RNA metabolism factors and retain Cdk1 to attenuate its activity and prevent mitotic entry [6]. Subsequent to the S-phase, the G2 phase is characterized by increasing CDK activity and is becoming more important, not only because it allows the precise timing of mitosis through activation of specific cell cycle checkpoints but also because late DNA replicating regions are still able to be replicated during this G2 period in many cells [1,7].…”