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J. Biol. Chem., Vol. 283, Issue 45, 31172-31182, November 7, 2008

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Mcm Subunits Can Assemble into Two Different Active Unwinding Complexes^*

From the Department of Biological Sciences, Vanderbilt University, Nashville, Tennessee 37235

The replication fork helicase in eukaryotes is a large complex that is composed of Mcm2-7, Cdc45, and GINS. The Mcm2-7 proteins form a heterohexameric ring that hydrolyzes ATP and provide the motor function for this unwinding complex. A comprehensive study of how individual Mcm subunit biochemical activities relate to unwinding function has not been accomplished. We studied the mechanism of the Mcm4-Mcm6-Mcm7 complex, a useful model system because this complex has helicase activity in vitro. We separately purified each of three Mcm subunits until they were each nuclease-free, and we then examined the biochemical properties of different combinations of Mcm subunits. We found that Mcm4 and Mcm7 form an active unwinding assembly. The addition of Mcm6 to Mcm4/Mcm7 results in the formation of an active Mcm4/Mcm6/Mcm7 helicase assembly. The Mcm4-Mcm7 complex forms a ringed-shaped hexamer that unwinds DNA with 3' to 5' polarity by a steric exclusion mechanism, similar to Mcm4/Mcm6/Mcm7. The Mcm4-Mcm7 complex has a high level of ATPase activity that is further stimulated by DNA. The ability of different Mcm mixtures to form rings or exhibit DNA stimulation of ATPase activity correlates with the ability of these complexes to unwind DNA. The Mcm4/Mcm7 and Mcm4/Mcm6/Mcm7 assemblies can open to load onto circular DNA to initiate unwinding. We conclude that the Mcm subunits are surprisingly flexible and dynamic in their ability to interact with one another to form active unwinding complexes.

Received for publication, June 19, 2008 , and in revised form, September 8, 2008.

^* This work was supported by Vanderbilt University start-up funds (to D. L. K.), a Vanderbilt University discovery grant (to D. L. K.), a pilot grant from the Vanderbilt Ingram Cancer Center (to D. L. K.), and American Cancer Society Research Scholar Grant SG-08-124-01-CCG (to D. L. K. and I. B.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Table 1.

¹ Both authors contributed equally to this work.

² To whom correspondence should be addressed: VU Station B, Box 35-1634, Vanderbilt University, Nashville, TN 37235. Tel.: 615-322-2072; Fax: 615-343-2707; E-mail: Daniel.Kaplan{at}Vanderbilt.Edu.

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