Cdc7 is an essential kinase required for the initiation of eukaryotic DNA replication. Previous studies in many species showed that the minichromosome maintenance complex is a major physiological target of this kinase. In this study, we have mapped the sites in human Mcm2 protein that are phosphorylated by Cdc7. The in vitro phosphorylation of several Mcm2 truncated proteins and peptides revealed that Mcm2 contains two major ( 5 S and 53 S) and at least three minor phosphorylation sites ( 4 S, 7 S, and 59 T) located at the N-terminal region. Alanine substitution experiments with Mcm2 peptides showed that the phosphorylation of 5 S and 53 S by Cdc7 required the presence of an acidic amino acid adjacent to a serine residue. Furthermore, although Cdc7 was unable to phosphorylate a Mcm2 peptide (spanning amino acids 19 -30 and containing 26 S and 27 S), it phosphorylated 26 S efficiently when this peptide contained a chemically synthesized phospho-27 S modification. Hence, additional Cdc7 phosphorylation sites could be generated in Mcm2 by its prior phosphorylation by a cyclin-dependent kinase. This finding may explain why the sequential action of cyclindependent and Cdc7 kinases is essential for the initiation of DNA replication.replication ͉ prereplicative complex ͉ origin activation C dc7 plays an essential role in the initiation of eukaryotic DNA replication (1-3). Cdc7 encodes a serine͞threonine kinase that is highly conserved from yeast to human (4). The activity of this kinase fluctuates during the cell cycle and depends completely on the regulatory subunit, Dbf4. Dbf4 accumulates during the S and G 2 phases of the cell cycle and is degraded rapidly during the G 1 phase by the anaphase promoting complex (5, 6). In Saccharomyces cerevisiae, Dbf4 binds to chromatin at the G 1 ͞S transition and remains on chromatin during the S phase (7). In the Xenopus egg extract cell-free system, Cdc7 was found to bind to chromatin during S phase, and this association required the minichromosome maintenance (MCM) complex (8).Both genetic and biochemical studies in S. cerevisiae indicate that Cdc7 kinase activity is required throughout S phase to activate replication origins on chromosomes (9). Cdc7 kinase acts directly on individual replication origins, presumably by phosphorylating components of the prereplicative complex (pre-RC), which may lead to the remodeling of the pre-RC for the unwinding of replication origins and subsequent recruitment of the replication fork machinery. Although the mechanism-oforigin activation by Cdc7 kinase remains unclear, the MCM complex appears to be the primary physiological target of this kinase activity (2).The MCM complex is composed of six distinct subunits (Mcm2, Mcm3, Mcm4, Mcm5, Mcm6, and Mcm7), which are all related structurally and highly conserved in eukaryotes (10). All six proteins are essential for the initiation of DNA replication and contain sequence motifs required for DNA helicase activity. In keeping with this notion, the MCM subcomplex containing Mcm4, Mcm6, and Mcm7 posses...

In 16 mature New Zealand white rabbits mesenchymal stem cells were aspirated from the bone marrow, cultured in monolayer and implanted on to a full-thickness osteochondral defect artificially made on the patellar groove of the same rabbit. A further 13 rabbits served as a control group. The rabbits were killed after 14 weeks. Healing of the defect was investigated histologically using haematoxylin and eosin and Safranin-O staining and with immunohistochemical staining for type-II collagen. We also used a reverse transcription-polymerase chain reaction (RT-PCR) to detect mRNA of type-I and type-II collagen. The semiquantitative histological scores were significantly higher in the experimental group than in the control group (p < 0.05). In the experimental group immunohistochemical staining on newly formed cartilage was more intense for type-II collagen in the matrix and RT-PCR from regenerated cartilage detected mRNA for type-II collagen in mature chondrocytes. These findings suggest that repair of cartilage defects can be enhanced by the implantation of cultured mesenchymal stem cells.

Tim (Timeless) and Tipin (Tim-interacting protein) form a stable heterodimeric complex that influences checkpoint responses and replication fork progression. We report that the Tim-Tipin complex interacts with essential replication fork proteins and affects their biochemical properties. The Tim-Tipin complex, reconstituted and purified using the baculovirus expression system, interacts directly with Mcm complexes and inhibits the single-stranded DNA-dependent ATPase activities of the Mcm2-7 and Mcm4/6/7 complexes, the DNA unwinding activity of the Mcm4/6/7 complex, and the DNA unwinding and ATPase activity of Cdc45-Mcm2-7-GINS complex, the presumed replicative DNA helicase in eukaryotes. Although stable interactions between Tim-Tipin and DNA polymerases (pols) were not observed in immunoprecipitation experiments with purified proteins, Tim was shown to interact with DNA pols α, δ, and e in cells. Furthermore, the Tim-Tipin complex significantly stimulated the pol activities of DNA pols α, δ, and e in vitro. The effects of Tim-Tipin on the catalytic activities of the Mcm complexes and DNA pols are mediated by the Tim protein alone, and distinct regions of the Tim protein are responsible for the inhibition of Mcm complex activities and stimulation of DNA pols. These results suggest that the Tim-Tipin complex might play a role in coupling DNA unwinding and DNA synthesis by directly affecting the catalytic activities of replication fork proteins.replication checkpoint | fork movement | genome integrity

Though RecQL4 was shown to be essential for the initiation of DNA replication in mammalian cells, its role in initiation is poorly understood. Here, we show that RecQL4 is required for the origin binding of Mcm10 and Ctf4, and their physical interactions and association with replication origins are controlled by the concerted action of both CDK and DDK activities. Although RecQL4-dependent binding of Mcm10 and Ctf4 to chromatin can occur in the absence of pre-replicative complex, their association with replication origins requires the presence of the pre-replicative complex and CDK and DDK activities. Their association with replication origins and physical interactions are also targets of the DNA damage checkpoint pathways which prevent initiation of DNA replication at replication origins. Taken together, the RecQL4-dependent association of Mcm10 and Ctf4 with replication origins appears to be the first important step controlled by S phase promoting kinases and checkpoint pathways for the initiation of DNA replication in human cells.