The structural maintenance of chromosome (SMC) proteins are key elements in controlling chromosome dynamics. In eukaryotic cells, three essential SMC complexes have been defined: cohesin, condensin, and the Smc5/6 complex. The latter is essential for DNA damage responses; in its absence both repair and checkpoint responses fail. In fission yeast, the UV-C and ionizing radiation (IR) sensitivity of a specific hypomorphic allele encoding the Smc6 subunit, rad18-74 (renamed smc6-74), is suppressed by mild overexpression of a six-BRCT-domain protein, Brc1. Deletion of brc1 does not result in a hypersensitivity to UV-C or IR, and thus the function of Brc1 relative to the Smc5/6 complex has remained unclear. Here we show that brc1D cells are hypersensitive to a range of radiomimetic drugs that share the feature of creating lesions that are an impediment to the completion of DNA replication. Through a genetic analysis of brc1D epistasis and by defining genes required for Brc1 to suppress smc6-74, we find that Brc1 functions to promote recombination through a novel postreplication repair pathway and the structure-specific nucleases Slx1 and Mus81. Activation of this pathway through overproduction of Brc1 bypasses a repair defect in smc6-74, reestablishing resolution of lesions by recombination.

Chk1 is a serine/threonine protein kinase that is the effector of the G2 DNA damage checkpoint. Chk1 homologs have a highly conserved N-terminal kinase domain, and a less conserved Cterminal regulatory domain of ~200 residues. In response to a variety of genomic lesions, a number of proteins collaborate to activate Chk1, which in turn ensures that the mitotic cyclin-dependent kinase Cdc2 remains in an inactive state until DNA repair is completed. Chk1 activation requires the phosphorylation of residues in the C-terminal domain, and this is catalyzed by the ATR protein kinase. How phosphorylation of the C-terminal regulatory domain activates the N-terminal kinase domain has not been elucidated, though some studies have suggested that this phosphorylation relieves an inhibitory intramolecular interaction between the N-and C-termini. However, recent studies in the fission yeast Schizosaccharomyces pombe have revealed that there is more to Chk1 regulation than this auto-inhibition model, and we review these findings and their implication to the biology of this genome integrity determinant. Review A little history: control of entry into mitosis and the identification of chk1The cell cycle is an orderly progression driven by the activities of the cyclin-dependent kinases (CDK) that control the transitions from G1 in S-phase, and from G2 into mitosis. The G2/M transition is particularly ancient in origin and is controlled by a universal mechanism common to virtually all eukaryotes [1]. Cdc2 (also known as Cdk1) is the mitotic CDK, and its activity is reliant upon binding to the cyclically expressed A-and B-type cyclins. To ensure that the transition from G2 into mitosis is a rapid switch, Cdc2 molecules that bind to cyclin partners are rapidly inactivated by inhibitory tyrosine phosphorylation on residue 15 (Y15). This inhibitory phosphorylation is catalyzed by the Wee1-family of kinases. During G2, Y15 phosphorylated Cdc2-Cyclin complexes accumulate, and are maintained in this inactive state until conditions appropriate for mitotic entry are completed. Given the highly mechanical and irreversible nature of mitosis, achieving appropriate cell mass, the completion of DNA replication and the absence of genomic lesions are crucial criteria that must be met for the cell to commit to mitotic entry. Once these conditions are met, Cdc2 is rapidly activated by dephosphorylation of Y15, catalyzed by the Cdc25-family of phosphatases [2]. This becomes a "point of no return", and the cells are then committed to pass through mitosis, where proteolysis of the cyclins resets the system for the subsequent cell cycle [3].To ensure cells do not enter mitosis prematurely, checkpoints overlay the core cell cycle machinery to ultimately

The Smc5/6 complex is widely believed to be required for homologous recombination. It is shown that repair defects of Smc5/6 mutants are due to the Nse1-dependent recruitment of dysfunctional complexes to lesions.

T he pairing of sister chromatids in interphase facilitates error-free homologous recombination (HR). Sister chromatids are held together by cohesin, one of three Structural Maintenance of Chromosomes (SMC) complexes. In mitosis, chromosome condensation is controlled by another SMC complex, condensin, and the type II topoisomerase (Top2). In prophase, cohesin is stripped from chromosome arms, but remains at centromeres until anaphase, whereupon it is removed via proteolytic cleavage by separase. The third SMC complex, Smc5/6, is generally described as a regulator of HR-mediated DNA repair. However, cohesin and condensin are also required for DNA repair, and HR genes are not essential for cell viability, but the SMC complexes are. Smc5/6 null mutants die in mitosis, and in fission yeast, Smc5/6 hypomorphs show lethal mitoses following genotoxic stress, or when combined with a Top2 mutant, top2-191. We found these mitotic defects are due to retention of cohesin on chromosome arms. We also show that Top2 functions in the cohesin cycle, and accumulating data suggests this is not related to its decatenation activity. Thus the SMC complexes and Top2 functionally interact, and any DNA repair function ascribed to Smc5/6 is likely a reflection of a more fundamental role in the regulation of chromosome structure.

Structural maintenance of chromosomes (SMC) complexes and DNA topoisomerases are major determinants of chromosome structure and dynamics. The cohesin complex embraces sister chromatids throughout interphase, but during mitosis most cohesin is stripped from chromosome arms by early prophase, while the remaining cohesin at kinetochores is cleaved at anaphase. This two-step removal of cohesin is required for sister chromatids to separate. The cohesin-related Smc5/6 complex has been studied mostly as a determinant of DNA repair via homologous recombination. However, chromosome segregation fails in Smc5/6 null mutants or cells treated with small interfering RNAs. This also occurs in Smc5/6 hypomorphs in the fission yeast Schizosaccharomyces pombe following genotoxic and replication stress, or topoisomerase II dysfunction, and these mitotic defects are due to the postanaphase retention of cohesin on chromosome arms. Here we show that mitotic and repair roles for Smc5/6 are genetically separable in S. pombe. Further, we identified the histone variant H2A.Z as a critical factor to modulate cohesin dynamics, and cells lacking H2A.Z suppress the mitotic defects conferred by Smc5/6 dysfunction. Together, H2A.Z and the SMC complexes ensure genome integrity through accurate chromosome segregation. Chromosomal integrity is essential for normal growth and development. In eukaryotes, there are three essential complexes of proteins that are central to chromosome dynamics. These are cohesin, condensin, and the Smc5/6 complex, known collectively as the structural maintenance of chromosomes (SMC) complex. Each complex shares a related architecture, and central to them are a heterodimer of SMC proteins: Smc1 and -3 in cohesin, Smc2 and -4 in condensin, and Smc5 and -6 in Smc5/6. These SMC proteins are large coiled-coil molecules with globular N and C termini containing Walker A and B ATP-binding motifs. They fold and interact at a flexible hinge, with ATP acting to hold the globular domains together. A kleisin protein bridges each heterodimer to form a putative ring-shaped structure, and each complex has specific additional non-SMC proteins that serve as regulators and effectors of function (1-3).Chromosome condensation and sister chromatid cohesion are the key roles for condensin and cohesin, respectively (1). However, defects in DNA repair have also been described for yeast strains harboring hypomorphic mutant alleles of condensin (4) and cohesin (5-7) subunits. In the case of cohesin, a role in DNA repair could stem from the fact that DNA repair by homologous recombination (HR) requires the sister chromatid to be in close proximity to the damaged chromatid. However, more recently cohesin in mammalian cells has been shown to also act as a transcriptional insulator in collaboration with CTCF (8, 9), and so the DNA repair function of cohesin may be more complex than a simple scaffolding mechanism. As its name suggests, deciphering Smc5/6 function has proved more elusive, though most studies have focused on a role for this complex in...