Structure Of DNA-Protein Complex Revealed

Using cryo-electron microscopy, a team of scientists in Japan have visualized the shape of tightly packed bundles of DNA and proteins inside the nuclei of cells.

AsianScientist (Jan. 16, 2018) – In a study published in the journal Molecular Cell, a research group in Japan has obtained the high resolution structure of DNA tightly wrapped around proteins, also known as heterochromatin.

Wireless headphones, two yo-yos connected by a string, earmuffs: all these items could be used to describe heterochromatin, a type of chromatin consisting of tightly packed DNA and proteins. For decades, scientists could only speculate about its shape.

In this study, researchers from the Okinawa Institute of Science and Technology Graduate University (OIST) and Waseda University in Japan used cryo-electron microscopy techniques to define the structure of heterochromatin in high resolution.

At Waseda University, the researchers first purified the heterochromatin. OIST researchers then imaged these samples in glass-like amorphous ice under a cryo-electron microscope. Using a computer algorithm to classify individual heterochromatin particles by type, the scientists ‘cut out’ those particles facing in the same direction. Then, they stacked these digital cutouts atop one another, combining hundreds of images to create a clearer picture.

The researchers demonstrated that heterochromatin may be less densely packed than previously thought. Made up of nucleosomes—roll-shaped bundles of DNA and protein, the heterochromatin is held together by the velcro-like heterochromatin protein 1 (HP1). This fundamental feature allows the body to ‘lock down’ genes so that they are not transcribed.

“Life as we know it relies on these principles,” said Assistant Professor Matthias Wolf, one of the authors of the paper and head of the Molecular Cryo-Electron Microscopy Unit at OIST. “We were surprised at how the heterochromatin looked, but this could be consistent with other functions, like the binding of other proteins to exposed DNA.”

In the future, the researchers hope to use this knowledge to understand higher order structures, like entire strings of nucleosomes.


The article can be found at: Machida et al. (2018) Structural Basis of Heterochromatin Formation by Human HP1.

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Source: Okinawa Institute of Science and Technology Graduate University; Photo: Yoshima Takizawa.
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