Functions and regulation of the MRX complex at DNA double-strand breaks

Abstract

DNA double-strand breaks (DSBs) pose a serious threat to genome stability and cell survival. Cells possess mechanisms that recognize DSBs and promote their repair through either homologous recombination (HR) or non-homologous end joining (NHEJ). The evolutionarily conserved Mre11-Rad50-Xrs2 (MRX) complex plays a central role in the cellular response to DSBs, as it is implicated in controlling end resection and in maintaining the DSB ends tethered to each other. Furthermore, it is responsible for DSB signaling by activating the checkpoint kinase Tel1 that, in turn, supports MRX function in a positive feedback loop. The present review focuses mainly on recent works in the budding yeast Saccharomyces cerevisiae to highlight structure and regulation of MRX as well as its interplays with Tel1.

Keywords: MRX; Rif2; Sae2; Tel1; double-strand break; nucleases; resection.

Figure 1. FIGURE 1: Structural organization of the MRX complex.

The ATP-bound state of Rad50 negatively regulates MRX nuclease activity by masking the Mre11 nuclease sites. ATP hydrolysis by Rad50 causes conformational changes of both Rad50 and Mre11, resulting in disengagement of Rad50 dimer and exposure of the Mre11 active sites that can access DNA to initiate DSB resection. The Mre11 nuclease sites are indicated by yellow stars. ATP and ADP are indicated by purple and pink dots, respectively.

Figure 2. FIGURE 2: Model for DSB resection.

The MRX complex and Sae2 are recruited to DNA ends. In the ATP-bound state, Rad50 blocks the Mre11 nuclease and MRX promotes DNA tethering. After ATP hydrolysis by Rad50, the Mre11 nuclease sites are exposed and can catalyze an endonucleolytic cleavage of the 5’ strand. Rif2 can promote the ATP hydrolysis activity of Rad50. MRX-mediated incision requires Sae2 phosphorylation by Cdk1-Clb and allows bidirectional processing by Exo1 and Sgs1-Dna2 in the 5’-3’ direction from the nick and by MRX in the 3’ to 5’ direction toward the DSB ends. ATP and ADP are indicated by purple and pink dots, respectively.

Figure 3. FIGURE 3: Crosstalk between MRX and Tel1.

The MRX complex is required to recruit and activate Tel1, which initiates DSB signaling. Tel1, once loaded to the DSB ends by MRX, supports MRX function by promoting its association to the DSBs ends. Rif2 counteracts Tel1 recruitment to DSBs by competing with Tel1 for binding to MRX and stimulates Rad50 ATPase activity. Initiation of DSB resection by MRX-Sae2, Exo1 and Sgs1-Dna2 generate 3’-ended ssDNA tails that promotes a switch from a dsDNA-Tel1 to a ssDNA-Mec1 signaling activity.