The APC/C inhibitor, Emi1, is essential for prevention of rereplication

Running on a treadmill: dynamic inhibition of APC/C by the spindle checkpoint

During mitosis, the genome duplicated during S-phase is synchronously and accurately segregated to the two daughter cells. The spindle checkpoint prevents premature sister-chromatid separation and mitotic exit. The anaphase-promoting complex/cyclosome (APC/C) is a key target of the spindle checkpoint. Upon checkpoint activation, the mitotic checkpoint complex (MCC) containing Mad2, Bub3, Mad3/BubR1 and Cdc20 inhibits APC/C. Two independent studies in budding yeast have now shed light on the mechanism by which MCC inhibits APC/C. These studies indicate that Mad3 binds to the mitotic activator of APC/C Cdc20 using peptide motifs commonly found in APC/C substrates and thus competes with APC/C substrates for APC/C^Cdc20 binding. In addition, Mad3 binding to APC/C^Cdc20 induces Cdc20 ubiquitination by APC/C, leading to the dissociation of MCC. Meanwhile, two other studies have shown that a deubiquitinating enzyme is required for the spindle checkpoint whereas APC/C-dependent ubiquitination is needed for checkpoint inactivation. Collectively, these studies suggest a dynamic model for APC/C^Cdc20 regulation by MCC in which APC/C- and Mad3-dependent ubiquitination of Cdc20 constitutes a self-regulated switch that rapidly inactivates the spindle checkpoint upon correct chromosome attachment.

Cyclin B2 suppresses mitotic failure and DNA re-replication in human somatic cells knocked down for both cyclins B1 and B2

Cyclin-dependent kinase 1 (CDK1) plays a crucial role in establishing metaphase and has also been shown to prevent DNA re-replication. Cyclins B1 and B2 are two known activators of CDK1 operating during mitosis in human cells. Little is known about the specific roles of each of these cyclins in CDK1 activation, but cyclin B2 is thought to play a minor role and to be unable to replace cyclin B1 for mitosis completion. In our study, we found that severe reduction by separate RNA interference of either cyclin B1 or cyclin B2 protein levels results in little or no alteration of the cell cycle and, more specifically, of mitosis progression. In contrast, simultaneous depletion of both B-type cyclins leads to massive accumulation of 4N cells, mitotic failure, premature mitosis exit and DNA re-replication. These defects can be corrected by the ectopic expression of a cyclin B2 resistant to the short hairpin RNA. Altogether, these data show that, in cycling human cells, cyclin B2 can compensate for the downregulation of cyclin B1 during mitosis. They also clearly implicate cyclins B1 and B2 as crucial activators of CDK1 in its biological function of DNA re-replication prevention.

C. elegans CUL-4 prevents rereplication by promoting the nuclear export of CDC-6 via a CKI-1-dependent pathway

Genome stability requires that genomic DNA is replicated only once per cell cycle. The replication-licensing system ensures that the formation of prereplicative complexes is temporally separated from the initiation of DNA replication [1-4]. The replication-licensing factors Cdc6 and Cdt1 are required for the assembly of prereplicative complexes during G1 phase. During S phase, metazoan Cdt1 is targeted for degradation by the CUL4 ubiquitin ligase [5-8], and vertebrate Cdc6 is translocated from the nucleus to the cytoplasm [9, 10]. However, because residual vertebrate Cdc6 remains in the nucleus throughout S phase [10-13], it has been unclear whether Cdc6 translocation to the cytoplasm prevents rereplication [1, 2, 14]. The inactivation of C. elegans CUL-4 is associated with dramatic levels of DNA rereplication [5]. Here, we show that C. elegans CDC-6 is exported from the nucleus during S phase in response to the phosphorylation of multiple CDK sites. CUL-4 promotes the phosphorylation and subsequent translocation of CDC-6 via negative regulation of the CDK-inhibitor CKI-1. Rereplication can be induced by coexpression of nonexportable CDC-6 with nondegradable CDT-1, indicating that redundant regulation of CDC-6 and CDT-1 prevents rereplication. This demonstrates that CDC-6 translocation is critical for preventing rereplication and that CUL-4 independently controls both replication-licensing factors.

Strength in numbers: preventing rereplication via multiple mechanisms in eukaryotic cells

In eukaryotic cells, prereplication complexes (pre-RCs) are assembled on chromatin in the G1 phase, rendering origins of DNA replication competent to initiate DNA synthesis. When DNA replication commences in S phase, pre-RCs are disassembled, and multiple initiations from the same origin do not occur because new rounds of pre-RC assembly are inhibited. In most experimental organisms, multiple mechanisms that prevent pre-RC assembly have now been identified, and rereplication within the same cell cycle can be induced through defined perturbations of these mechanisms. This review summarizes the diverse array of inhibitory pathways used by different organisms to prevent pre-RC assembly, and focuses on the challenge of understanding how in any one cell type, various mechanisms cooperate to strictly enforce once per cell cycle regulation of DNA replication.

APC/C--the master controller of origin licensing?

DNA replication must be tightly controlled to prevent initiation of a second round of replication until mitosis is complete. So far, components of the pre-replicative complex (Cdt1, Cdc6 and geminin) were considered key players in this regulation. In a new study, Machida and Dutta have shown that depletion of Emi1 caused cells to replicate their DNA more than once per cell cycle [1]. This effect was dependent on the ability of Emi1 to inhibit the APC/C. In addition to its role in regulating entry into mitosis, oscillation of APC/C activity regulates pre-RC formation: high APC/C activity in late M/G1 allows pre-RC formation and low APC/C activity in S/G2 prevents pre-RC formation for a second time thereby preventing rereplication. Each redundant pathway to prevent rereplication is dependent on regulating one of the pre-RC components, and all of the pathways are co-regulated by Emi1 through the APC/C. In this commentary we discuss how this new role of Emi1 adds to our understanding of the regulation of replication initiation. We also review the literature to analyze whether APC/C has a role in regulating endoreduplication (a normal state of polyploidy in some differentiated cells). Similarly a role of premature APC/C activation in genomic instability of tumors is discussed.