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Molecular and Cellular Biology, May 2000, p. 3667-3676, Vol. 20, No. 10
Department of Cell Biology and Biochemistry,
Texas Tech University Health Sciences Center, Lubbock, Texas 79430
Received 9 November 1999/Returned for modification 21 December
1999/Accepted 14 February 2000
Initiation of DNA replication in eukaryotes requires the assembly
of prereplication complexes (pre-Rcs) at the origins of replication.
The assembly and function of the pre-Rcs appear to be controlled by
phosphorylation events. In this study we report the detailed
characterization of the cell cycle phosphorylation of one component of
the Xenopus pre-Rcs, the Mcm protein complex. We show that
individual Mcm subunits are differentially phosphorylated during the
cell cycle. During mitosis, the Mcm4 subunit is hyperphosphorylated, while the other subunits are not actively phosphorylated. The mitotic
phosphorylation of Mcm4 requires Cdc2-cyclin B and other unknown
kinases. Following exit from mitosis, the Mcm4 subunit of the cytosolic
interphase complex undergoes dephosphorylation, and the Mcm2, Mcm3, or
Mcm6 subunits are then actively phosphorylated by kinase(s) other than
cyclin-dependent kinases (Cdks) or Cdc7. The association of the Mcm
complex with the pre-Rcs correlates with the formation of a transient
interphase complex. This complex contains an intermediately
phosphorylated Mcm4 subunit and is produced by partial
dephosphorylation of the mitotic hyperphosphorylated Mcm4 protein.
Complete dephosphorylation of the Mcm4 subunit inactivates the Mcm
complex and prevents its binding to the chromatin. Once the Mcm complex
is assembled on the chromatin the Mcm4 and the Mcm2 proteins are the
only subunits phosphorylated during the activation of the pre-Rcs.
These chromatin-associated phosphorylations require nuclear transport
and are independent of Cdk2-cyclin E. These results suggest that the
changes in Mcm4 phosphorylation regulate pre-Rc assembly and the
function of the pre-Rcs on the chromatin.
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Distinct Phosphoisoforms of the Xenopus
Mcm4 Protein Regulate the Function of the Mcm Complex
*
Corresponding author. Mailing address: Department of
Cell Biology and Biochemistry, Texas Tech University Health Sciences Center, 3601 4th St., Lubbock, TX 79430. Phone: (806) 743-1558. Fax:
(806) 743-2990. E-mail:
martine.coue{at}ttmc.ttuhsc.edu.
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