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Mammalian Mcm2/4/6/7 complex forms a toroidal structure
Norikazu Yabuta1, Naoko Kajimura, Kouta Mayanagi
1Department of Molecular Genetics, Research Institute for Microbial Diseases, Osaka University, 3-1 Yamadaoka, Suita, Osaka 565-0871, Japan.
Background:
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.
Results:
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.
Conclusions:
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.
Insights
The Mcm2-7 protein complex, crucial for DNA replication, forms a toroidal structure. This distinct toroidal shape differs from the Mcm4/6/7 hexamer, providing new insights into replication machinery.
Area of Science:
- Molecular Biology
- Structural Biology
- Biochemistry
Background:
- The Mcm2-7 protein family is essential for DNA replication.
- Mcm proteins assemble into Mcm4/6/7 and Mcm2/4/6/7 complexes.
- The precise structures of these complexes were previously unknown.
Purpose of the Study:
- To determine the structural organization of the human Mcm2/4/6/7 tetramer.
- To compare the structure of the Mcm2/4/6/7 tetramer with the Mcm4/6/7 hexamer.
Main Methods:
- Single-particle electron microscopy and image analysis were employed.
- Comparative structural analysis with mouse Mcm2/4/6/7 tetramer.
- Biochemical assays including GST-pull down and two-hybrid experiments.
Main Results:
- The human Mcm2/4/6/7 tetramer adopts a toroidal structure with a central cavity (3-4 nm diameter).
- A predominant averaged image revealed a toroid with four bulges, one larger than the others.
- This structure closely resembles the mouse Mcm2/4/6/7 tetramer and differs from the Mcm4/6/7 hexamer.
Conclusions:
- The Mcm2/4/6/7 tetramer forms a distinct toroidal structure.
- This structure is differentiated in size and shape from the Mcm4/6/7 hexamer.
- A Mcm6-Mcm6 hinge is suggested to be involved in Mcm7/4/6/6/4/7 heterohexamer formation.