title: Identification and characterization of a Xenopus homolog of Dbf4, a regulatory subunit of the Cdc7 protein kinase required for the initiation of DNA replication.
abstract: Dbf4 is a regulatory subunit for the Cdc7 protein kinase that is required for the initiation of eukaryotic DNA replication, but the precise roles of Dbf4-Cdc7 remain to be determined. Here we identified a Xenopus homolog of Dbf4 (XDbf4) and characterized XDbf4 and Xenopus Cdc7 (XCdc7) in Xenopus egg extracts. XDbf4 formed a complex with XCdc7 in egg extracts and activated XCdc7 kinase activity in vitro. In contrast with Dbf4 in yeast and mammalian cultured cells, the XDbf4 levels in egg extracts did not change during the cell cycle progression. XDbf4 was a phosphoprotein in interphase extracts, and was apparently hyperphosphorylated in cytostatic factor (CSF)-mediated, metaphase-arrested extracts and in mitotic extracts. However, the hyperphosphorylation of XDbf4 did not seem to affect the level of kinase activation, or chromatin binding of the XDbf4-XCdc7 complex. Upon release from CSF-arrest, XDbf4 was partially dephosphorylated and bound to chromatin. Interestingly, XDbf4 was loaded onto chromatin before XCdc7 during DNA replication in egg extracts. These results suggest that the function of XDbf4-XCdc7 during the early embryonic cell cycle is regulated in a manner distinct from that during the somatic cell cycle.
authors.affiliation: Research Institute for Microbial Diseases, and Graduate School of Science, Osaka University, Suita, Osaka 565-0871.
authors.forename: Asako
authors.initials: A
authors.lastname: Furukohri
authors.forename: Noriko
authors.initials: N
authors.lastname: Sato
authors.forename: Hisao
authors.initials: H
authors.lastname: Masai
authors.forename: Ken-ichi
authors.initials: K
authors.lastname: Arai
authors.forename: Akio
authors.initials: A
authors.lastname: Sugino
authors.forename: Shou
authors.initials: S
authors.lastname: Waga
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D018797:Cell Cycle Proteins / Q000235:genetics / Q000378:metabolism*; D017124:Conserved Sequence; D004261:DNA Replication; D005786:Gene Expression Regulation; D008969:Molecular Sequence Data; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism*; D014981:Xenopus / Q000235:genetics* / Q000378:metabolism; D029867:Xenopus Proteins
chemical_list: D018797:Cell Cycle Proteins; C088096:Dbf4 protein, S cerevisiae; D011994:Recombinant Proteins; D029701:Saccharomyces cerevisiae Proteins; D029867:Xenopus Proteins; C489404:CDC7 protein, S cerevisiae; C489405:CDC7 protein, Xenopus; D017346:Protein Serine-Threonine Kinases