Checkpoint Mec-tivation comes in many flavors

Pap1 + confers microtubule damage resistance to mut2a, an extragenic suppressor of the rad26:4A allele in S. pombe

The DNA structure checkpoint protein Rad26^ATRIP is also required for an interphase microtubule damage response. This checkpoint delays spindle pole body separation and entry into mitosis following treatment of cells with microtubule poisons. This checkpoint requires cytoplasmic Rad26^ATRIP, which is compromised by the rad26:4A allele that inhibits cytoplasmic accumulation of Rad26^ATRIP following microtubule damage. The rad26::4a allele also disrupts minichromosome stability and cellular morphology, suggesting that the interphase microtubule damage checkpoint pathway operates in an effort to maintain chromosome stability and proper cell shape. To identify other proteins of the Rad26-dependent interphase microtubule damage response, we used ultra violet (UV) radiation to identify extragenic interaction suppressors of the rad26::4A growth defect on microtubule poisons. One suppressor mutation, which we named mut2a, permitted growth of rad26:4A cells on MBC media and conferred sensitivity to a microtubulin poison upon genetic outcross. In an attempt to clone this interaction suppressor using a genomic library complementation strategy, we instead isolated pap1 ⁺ as an extracopy suppressor of the mut2a growth defect. We discuss the mechanism by which pap1 ⁺ overexpression may allow growth of mut2a cells in conditions that destabilize microtubules.

Rad4 mainly functions in Chk1-mediated DNA damage checkpoint pathway as a scaffold protein in the fission yeast Schizosaccharomyces pombe

Rad4/Cut5 is a scaffold protein in the Chk1-mediated DNA damage checkpoint in S. pombe. However, whether it contains a robust ATR-activation domain (AAD) required for checkpoint signaling like its orthologs TopBP1 in humans and Dpb11 in budding yeast has been incompletely clear. To identify the putative AAD in Rad4, we carried out an extensive genetic screen looking for novel mutants with an enhanced sensitivity to replication stress or DNA damage in which the function of the AAD can be eliminated by the mutations. Two new mutations near the N-terminus were identified that caused significantly higher sensitivities to DNA damage or chronic replication stress than all previously reported mutants, suggesting that most of the checkpoint function of the protein is eliminated. However, these mutations did not affect the activation of Rad3 (ATR in humans) yet eliminated the scaffolding function of the protein required for the activation of Chk1. Several mutations were also identified in or near the recently reported AAD in the C-terminus of Rad4. However, all mutations in the C-terminus only slightly sensitized the cells to DNA damage. Interestingly, a mutant lacking the whole C-terminus was found resistant to DNA damage and replication stress almost like the wild type cells. Consistent with the resistance, all known Rad3 dependent phosphorylations of checkpoint proteins remained intact in the C-terminal deletion mutant, indicating that unlike that in Dpb11, the C-terminus of Rad4 does not contain a robust AAD. These results, together with those from the biochemical studies, show that Rad4 mainly functions as a scaffold protein in the Chk1, not the Cds1(CHK2 in humans), checkpoint pathway. It plays a minor role or is functionally redundant with an unknown factor in Rad3 activation.

Four pillars of the S-phase checkpoint

The yeast Mec1 kinase is a key regulator of the DNA damage response (DDR). In this issue of Genes & Development, Kumar and Burgers (pp. 313-321) report that Ddc1, Dpb11, and Dna2 function in concert to activate Mec1 during S phase of the cell cycle. Furthermore, the Tel1 kinase also contributes to the DDR in S phase when Mec1 activation is compromised.

A genome-wide camptothecin sensitivity screen identifies a mammalian MMS22L-NFKBIL2 complex required for genomic stability

Replication stress involving collision of replisomes with camptothecin (CPT)-stabilized DNA-Topoisomerase I adducts activates an ATR-dependent pathway to promote repair by homologous recombination. To identify human genes that protect cells from such replication stress, we performed a genome-wide CPT sensitivity screen. Among numerous candidate genes are two previously unstudied proteins: the ankyrin repeat protein NFKBIL2 and C6ORF167 (MMS22L), distantly related to yeast replication stress regulator Mms22p. MMS22L and NFKBIL2 interact with each other and with FACT (facilitator of chromatin transcription) and MCM (minichromosome maintenance) complexes. Cells depleted of NFKBIL2 or MMS22L are sensitive to DNA-damaging agents, load phosphorylated RPA onto chromatin in a CTIP-dependent manner, activate the ATR/ATRIP-CHK1 and double-strand break repair signaling pathways, and are defective in HR. This study identifies MMS22L-NFKBIL2 as components of the replication stress control pathway and provides a resource for discovery of additional components of this pathway.