Biochemistry

DNA Unzipper Found

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Science  07 Jul 2006:
Vol. 313, Issue 5783, pp. 19-21
DOI: 10.1126/science.313.5783.19d

One of the critical steps an organism must perform in order to duplicate its genome is to unzip the two base-paired strands of DNA so that DNA polymerase can read and copy the information. Minichromosome maintenance (Mcm) proteins 2–7 are thought to be involved in unwinding double helical DNA in eukaryotes, but this six-subunit complex does not display helicase activity in vitro. Other proteins—Cdc45 and the four-subunit GINS (Go, Ichi, Nii, and San) complex—have also been implicated, but the precise composition of the active assembly has remained elusive.

Moyer et al. have purified factors that interact with the Drosophila Cdc45 protein and find that they include both the Mcm hexamer and the GINS tetramer. The 11-membered CMG (Cdc45/Mcm2-7/GINS) complex is highly stable and, satisfyingly, exhibits adenosine triphosphate-dependent DNA helicase activity, which is lost or reduced upon immunodepletion of any of its components. Whether the Mcm proteins, which have all the structural features of a bona fide helicase, provide the unzipping activity in the CMG complex is not yet known. Intriguingly, GINS, like Mcm2-7, forms a ring, and the two might stack on top of each other, with DNA passing through the center. — GR

Proc. Natl. Acad. Sci. U.S.A. 103, 10236 (2006).

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