Joint study on activation of eukaryotic replication origins

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Cdc7-related kinases are conserved from yeasts to human. We have also shown that kinase activity of these catalytic subunits are regulated by regulatory subunits as has been reported in S.cerevisae. We have shown that the function of H37, which binds and activates huCdc7 kinase, is required for G1 to S transition in mammalian cells by antibody microinjection experiments. Expression of H37 is both growth- and cell cycle-regulated, being low in quiescent as well as in G1 phase, increasing at late G1 and being kept high during S phase. MCM2 and MCM3 are among substrates of huCdc7/H37 kinase complex. Disruption of both alleles of the muCdc7 (mouse homologue of Cdc7) gene resulted in early embryonic lethality, indicating the requirement of Cdc7 function for growth and/or early development of mammals.Genetic and biochemical analyses of fission yeast Cdc7-related kinase complex (Hsk1/Him1) revealed two novel functions of Cdc7-related kinases, namely requirement for premeiotic DNA replication and for response to HU-induced replication fork blocks and DNA damages. Comparison of the structures of the regulatory subunits revealed the presence of two conserved motifs. Motif-C is essential for mitotic functions, while motif-N, dispensable for normal mitotic growth, may be essential for survival after replication fork blocks or recovery from DNA damages. Him1 protein undergoes hyperphosphorylation upon S phase arrest by HU.We have ldentified three serine/threonine residues conserved in the motif-N of Him1 protein that appear to be critical for cellular responses to replication fork blocks and to DNA damages. In consistent with these results, him1 is identical to previously isolated rad35^+.