chiffon

EVOLUTIONARY HOMOLOGS part 1/2 | part 2/2

Multiple functions of Chiffon homologs

The initiation of DNA synthesis is an important cell cycle event that defines the beginning of S phase. This critical event involves the participation of proteins whose functions are regulated by cyclin dependent protein kinases (Cdks). The Mcm2-7 (see Drosophila Disc proliferation abnormal) proteins comprise a family of six conserved proteins that are essential for the initiation of DNA synthesis in all eukaryotes. In Saccharomyces cerevisiae, members of the Mcm2-7 family undergo cell cycle-specific phosphorylation. Phosphorylation of Mcm proteins at the beginning of S phase coincides with the removal of these proteins from chromatin and the onset of DNA synthesis. In this study, DBF4, which encodes the regulatory subunit of a Cdk-like protein kinase Cdc7-Dbf4, has been identified in a screen for second site suppressors of mcm2-1. The dbf4 suppressor mutation restores competence to initiate DNA synthesis in the mcm2-1 mutant. Cdc7-Dbf4 interacts physically with Mcm2 and phosphorylates Mcm2 and three other members of the Mcm2-7 family in vitro. Blocking the kinase activity of Cdc7-Dbf4 at the G1-to-S phase transition also blocks the phosphorylation of Mcm2 at this defined point of the cell cycle. Taken together, these data suggest that at the G1-to-S phase transition, phosphorylation by Cdc7-Dbf4 of Mcm2 and probably other members of the Mcm2-7 proteins, is a critical step in the initiation of DNA synthesis at replication origins (Lei, 1997).

Eukaryotic cells use multiple replication origins to replicate their large genomes. Some origins fire early during S phase whereas others fire late. In Saccharomyces cerevisiae, initiator sequences (ARSs) are bound by the origin recognition complex (ORC). Cdc6p synthesized at the end of mitosis joins ORC and facilitates recruitment of Mcm proteins, which renders origins competent to fire. However, origins fire at the G1/S boundary only upon the subsequent activation of S phase cyclin-dependent kinases (S-CDKs) and Dbf4/Cdc7. A chromatin immunoprecipitation assay has been used to measure the association with ARS sequences of DNA primase and the single-stranded DNA binding replication protein A (RPA) when fork movement is inhibited by hydroxyurea (HU). RPA's association with origins requires S-CDKs, Dbf4/Cdc7 kinase and an Mcm protein. The recruitment of DNA primase depends on RPA. Furthermore, early- and late-firing origins differ not in the timing of their recruitment of an Mcm protein, but in the timing of RPA's recruitment. RPA is recruited to early but not to late origins in HU. Rad53 kinase is required to prevent RPA association with a late origin in HU. These data suggest that the origin unwinding accompanied by RPA association is a key step, regulated by S-CDKs, Dbf4/Cdc7 and Rad53p. Thus, in the presence of active S-CDKs and Dbf4/Cdc7, Mcms may open origins and thereby facilitate the loading of RPA (Tanaka, 1998).

Eukaryotic cells coordinate chromosome duplication by assembly of protein complexes at origins of DNA replication and by activation of cyclin-dependent kinase and Cdc7p-Dbf4p kinase. In Saccharomyces cerevisiae it has been shown that although Cdc7p levels are constant during the cell division cycle, Dbf4p and Cdc7p-Dbf4p kinase activities fluctuate. Dbf4p binds to chromatin near the G(1)/S-phase boundary well after binding of the minichromosome maintenance (Mcm) proteins, and it is stabilized at the non-permissive temperature in mutants of the anaphase-promoting complex, suggesting that Dbf4p is targeted for destruction by ubiquitin-mediated proteolysis. Arresting cells with hydroxyurea (HU) or with mutations in genes encoding DNA replication proteins results in a more stable, hyper-phosphorylated form of Dbf4p and an attenuated kinase activity. The Dbf4p phosphorylation in response to HU is RAD53 dependent. This suggests that an S-phase checkpoint function regulates Cdc7p-Dbf4p kinase activity. Cdc7p may also play a role in adapting from the checkpoint response since deletion of CDC7 results in HU hypersensitivity. Recombinant Cdc7p-Dbf4p kinase has been purified and both subunits are autophosphorylated. Cdc7p-Dbf4p efficiently phosphorylates several proteins that are required for the initiation of DNA replication, including five of the six Mcm proteins and the p180 subunit of DNA polymerase alpha-primase (Weinreich, 1999).

Chiffon homologs and checkpoint function

Saccharomyces cerevisiae CDC7 encodes a serine/threonine kinase required for G(1)/S transition, and its related kinases are present in fission yeast as well as in higher eukaryotes, including humans. Kinase activity of Cdc7 protein depends on the regulatory subunit, Dbf4, which also interacts with replication origins. him1(+) has been identified from two-hybrid screening with Hsk1, a fission yeast homolog of Cdc7 kinase; it has been shown to encode a regulatory subunit of Hsk1. Him1, identical to Dfp1, previously identified as an associated molecule of Hsk1, binds to Hsk1 and stimulates its kinase activity, which phosphorylates both catalytic and regulatory subunits as well as recombinant MCM2 protein in vitro. him1(+) is essential for DNA replication in fission yeast cells, and its transcription is cell cycle regulated, increasing at middle M to late G(1). The protein level is low at START in G(1), increases at the G(1)/S boundary, and is maintained at a high level throughout S phase. Him1 protein is hyperphosphorylated at G(1)/S through S during the cell cycle as well as in response to early S-phase arrest induced by nucleotide deprivation. Deletion of one of the motifs conserved in regulatory subunits for Cdc7-related kinases as well as alanine substitution of three serine and threonine residues present in the same motif results in a defect in checkpoint regulation normally induced by hydroxyurea treatment. The alanine mutant also shows growth retardation after UV irradiation and the addition of methylmethane sulfonate. In keeping with this result, a database search indicates that him1(+) is identical to rad35(+). These results reveal a novel function for the Cdc7/Dbf4-related kinase complex in S-phase checkpoint control as well as in growth recovery from DNA damage, in addition to its predicted essential function in S-phase initiation (Takeda, 1999).

The Cdc7p and Dbf4p proteins form an active kinase complex in Saccharomyces cerevisiae that is essential for the initiation of DNA replication. A genetic screen for mutations that are lethal in combination with cdc7-1 led to the isolation of seven lsd (lethal with seven defect) complementation groups. The lsd7 complementation group contains two temperature-sensitive dbf4 alleles. The lsd1 complementation group contains a new allele of RAD53, which was designated rad53-31. RAD53 encodes an essential protein kinase that is required for the activation of DNA damage and DNA replication checkpoint pathways; it is implicated as a positive regulator of S phase. Unlike other RAD53 alleles, it has been demonstrated that the rad53-31 allele retains an intact checkpoint function. Thus, the checkpoint function and the DNA replication function of RAD53 can be functionally separated. The activation of DNA replication through RAD53 most likely occurs through DBF4. Two-hybrid analysis indicates that the Rad53p protein binds to Dbf4p. Furthermore, the steady-state level of DBF4 message and Dbf4p protein is reduced in several rad53 mutant strains, indicating that RAD53 positively regulates DBF4. These results suggest that two different functions of the cell cycle, initiation of DNA replication and the checkpoint function, can be coordinately regulated through the common intermediate RAD53 (Dohrmann, 1999).

Chiffon homologs and entry into mitosis

In Saccharomyces cerevisiae, the heteromeric kinase complex Cdc7p-Dbf4p plays a pivotal role at replication origins in triggering the initiation of DNA replication during the S phase. The kinase activity of endogenous levels of Cdc7p kinase has been examined by using Mcm2p, a likely physiological target as a substrate. Using this assay, it has been confirmed that Cdc7p kinase activity fluctuates during the cell cycle; it is low in the G1 phase, rises as cells enter the S phase, and remains high until cells complete mitosis. These changes in kinase activity cannot be accounted for by changes in the levels of the catalytic subunit Cdc7p, since these levels are constant during the cell cycle. However, the fluctuations in kinase activity do correlate with levels of the regulatory subunit Dbf4p. The regulation of Dbf4p levels can be attributed in part to increased degradation of the protein in G1 cells. This G1-phase instability is cdc16 dependent, suggesting a role of the anaphase-promoting complex in the turnover of Dbf4p. Overexpression of Dbf4p in the G1 phase can partially overcome this elevated turnover and lead to an increase in Cdc7p kinase activity. Thus, the regulation of Dbf4p levels through the control of Dbf4p degradation has an important role in the regulation of Cdc7p kinase activity during the cell cycle (Oshiro, 1999).

The precise duplication of eukaryotic genetic material takes place once and only once per cell cycle and is dependent on the completion of the previous mitosis. Two evolutionarily conserved kinases, the cyclin B (Clb)/cyclin-dependent kinase (Cdk/Cdc28p) and Cdc7p (along with its interacting factor Dbf4p) are required late in G1 to initiate DNA replication. The levels of Dbf4p are cell cycle regulated. Dbf4p levels increase as cells begin S phase and remain high through late mitosis, after which they decline dramatically as cells begin the next cell cycle. Dbf4p levels are sensitive to mutations in key components of the anaphase-promoting complex (APC). In addition, Dbf4p is modified in response to DNA damage, and this modification is dependent upon the DNA damage response pathway. Dbf4p interacts with the M phase polo-like kinase Cdc5p, a key regulator of the APC late in mitosis. These results further link the actions of the initiator protein, Dbf4p, to the completion of mitosis and suggest possible roles for Dbf4p during progression through mitosis (Cheng, 1999).

chiffon Evolutionary homologs part 1/2

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