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			PubMed Journals: Proc Natl Acad Sci U S A

  Source:		PMID: 10339564


    		Proc Natl Acad Sci U S A. 1999 May 25;96(11):6193-8.
     
			Multistep regulation of DNA replication
			by Cdk phosphorylation of HsCdc6.

			Jiang W(1), Wells NJ, Hunter T.

			Author Information
			(1) Molecular Biology and Virology Laboratory,
			The Salk Institute, 10010 North Torrey Pines
			Road, La Jolla, CA 92037, USA. wjiang@salk.edu

			We have characterized HsCdc6, a human protein
			homologous to the budding yeast Cdc6p that
			is essential for DNA replication. We show
			that, unlike Cdc6p, the levels of HsCdc6
			protein remain constant throughout the cell
			cycle in human cells. However, phosphorylation
			of HsCdc6 is regulated during the cell cycle.
			HsCdc6 is an excellent substrate for Cdk2
			in vitro and is phosphorylated in vivo at
			three sites (Ser-54, Ser-74, and Ser-106)
			that are phosphorylated by Cdk2 in vitro,
			strongly suggesting that HsCdc6 is an in
			vivo Cdk substrate. HsCdc6 is nuclear in
			G1, but translocates to the cytoplasm at
			the start of S phase via Crm1-dependent
			export. An HsCdc6A1A2A3 mutant, which mimics
			unphosphorylated HsCdc6, is exclusively
			nuclear, and its expression inhibits initiation
			of DNA replication. An HsCdc6E1E2E3 mutant,
			which mimics phosphorylated HsCdc6, is exclusively
			cytoplasmic and is not associated with the
			chromatin/nuclear matrix fraction. Based
			on these results, we propose that phosphorylation
			of HsCdc6 by Cdks regulates DNA replication
			of at least two steps: first, by promoting
			initiation of DNA replication and, second,
			through nuclear exclusion preventing DNA
			rereplication.

			PMCID: PMC26858 PMID: 10339564 [Indexed
			for MEDLINE]

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