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J. Biol. Chem., Vol. 282, Issue 46, 33795-33804, November 16, 2007

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Differences in the Single-stranded DNA Binding Activities of MCM2-7 and MCM467

MCM2 AND MCM5 DEFINE A SLOW ATP-DEPENDENT STEP^*

From the Department of Biological Sciences, University of Pittsburgh, Pittsburgh, Pennsylvania 15260

The MCM2-7 complex, a hexamer containing six distinct and essential subunits, is postulated to be the eukaryotic replicative DNA helicase. Although all six subunits function at the replication fork, only a specific subcomplex consisting of the MCM4, 6, and 7 subunits (MCM467) and not the MCM2-7 complex exhibits DNA helicase activity in vitro. To understand why MCM2-7 lacks helicase activity and to address the possible function of the MCM2, 3, and 5 subunits, we have compared the biochemical properties of the Saccharomyces cerevisiae MCM2-7 and MCM467 complexes. We demonstrate that both complexes are toroidal and possess a similar ATP-dependent single-stranded DNA (ssDNA) binding activity, indicating that the lack of helicase activity by MCM2-7 is not due to ineffective ssDNA binding. We identify two important differences between them. MCM467 binds dsDNA better than MCM2-7. In addition, we find that the rate of MCM2-7/ssDNA association is slow compared with MCM467; the association rate can be dramatically increased either by preincubation with ATP or by inclusion of mutations that ablate the MCM2/5 active site. We propose that the DNA binding differences between MCM2-7 and MCM467 correspond to a conformational change at the MCM2/5 active site with putative regulatory significance.

Received for publication, May 9, 2007 , and in revised form, August 29, 2007.

^* This work was supported by Grant RSG-05-113-01-CCG from the American Cancer Society (to A. S.). 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 Tables S1 and S2 and supplemental Figs. S1-S3.

¹ To whom correspondence should be addressed: 4249 Fifth Ave., 560 Crawford Hall, Pittsburgh, PA 15260. Tel.: 412-624-4307; Fax: 412-624-4759; E-mail: schwacha{at}pitt.edu.

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