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Genes Cells. 2003 May;8(5):413-21.

Mammalian Mcm2/4/6/7 complex forms a toroidal structure.

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  • 1Department of Molecular Genetics, Research Institute for Microbial Diseases, Osaka University, 3-1 Yamadaoka, Suita, Osaka 565-0871, Japan.



The Mcm proteins are a family of six homologous proteins (Mcm2-7) that play an important role in DNA replication. They form Mcm4/6/7 and Mcm2/4/6/7 complexes, but their structures are not known.


We found that the human Mcm2/4/6/7 tetramer forms a toroidal structure, with a central cavity about 3-4 nm in diameter. Observations were made using electron microscopy, employing the image analysis of single particles. The most predominant averaged image displayed a toroid harbouring four bulges forming corners, one of which was larger than the others. This structure was very similar to the mouse Mcm2/4/6/7 tetramer that was independently prepared and analysed by electron microscopy. These toroidal structures are distinct from that of the Mcm4/6/7 hexamer, which was also examined by electron microscopy. GST(glutathione S-transferase)-pull down and two hybrid experiments suggest that a putative Mcm6-Mcm6 hinge contributes to the formation of the Mcm7/4/6/6/4/7 heterohexamer.


The Mcm2/4/6/7 tetramer forms a toroidal structure that is distinct from that of the Mcm4/6/7 hexamer in size and shape.

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