My watch list


DNA repair: Opening the hatch to heal the break

L. Käshammer

MR Complex.

05-Sep-2019: LMU researchers have determined the structure of a key enzyme complex that is involved in DNA repair, and traced the cycle of conformational changes that it undergoes while performing its biochemical function.

Various types of DNA damage can have serious repercussions, both for the individual cells in which they occur and for the organism as a whole. Instances of simultaneous breakage of both strands of the DNA helix are particularly harmful. Such double-strand breaks (DSBs) can be induced by radiation, as well as by environmental toxins, and can lead to cell death. However, cells possess efficient mechanisms for detecting and repairing DSBs. A molecular machine known as the MR complex plays a central role in this process. It recognizes and binds to DSBs, and initiates repair of the broken double helix. A team of scientists led by Professor Karl-Peter Hopfner, who holds the Chair of Structural Molecular Biology at LMU’s Gene Center, has now deciphered the complete three-dimensional structure of the MR complex and determined how it works.

Hopfner’s group had previously shown that the MR complex is made up of four proteins. Two of these are nucleases, which themselves cut DNA strands. The others are ATPases – enzymes that remove a phosphate group from the molecule ATP, and in so doing release chemical energy. The complex itself has an open architecture. But when it encounters a DSB, it binds to the DNA and brings the ends together, acting as the molecular equivalent of a hook-and-loop fastener. The complex then detaches any chemical adducts that block the ends of the break and removes further subunits from the DNA. These operations then provide a ‘clean break’ in preparation for the repair process itself. “Because of the intricate architecture of the complex, the conformational changes involved in this process had not been elucidated,” says Lisa Käshammer, lead author of the paper. The ATPase enzymes contain prominent filamentous structural elements that are referred to as coiled-coils, which are made up of closely apposed and helically ordered amino-acid sequences. These filaments are long and flexible, and their mobility had made it impossible to discern their precise courses using conventional X-ray crystallographic methods.

The team turned to cryo-electron microscopy to get around this problem, and succeeded in determining the complete structure of the MR complex found in the bacterium Escherichia coli. The bacterial complex is somewhat less complicated than its counterpart in higher organisms, but in their overall structure the two forms are similar to each other. The new structure now makes it possible to discern how the nucleases actually bind to the DNA – a question that had remained open up to now. “In the previously published structures, the DNA was either positioned relatively far from the active center of the nuclease, or bound in such a way that the active center was inaccessible,” Käshammer explains. The new structure reveals that binding to the DNA causes the nuclease to execute a 120-degree turn, which opens up a channel that enables the DNA to bind to the active center of the enzyme.

The study also clarifies the function of the coiled-coil regions. As the new structural model demonstrates, these segments form a brace that stabilizes the DNA, and are actively involved in the binding and processing of the DNA ends. “Our findings constitute an important advance in the quest for a deeper understanding of the complex mechanisms that make it possible for these enzymes to repair double-strand breaks in DNA,” says Hopfner. New insights into the function of the complex are also of potential medical significance, as DSBs play a significant role in the pathogenesis of many types of cancer.

Original publication:
"Mechanism of DNA end sensing and processing by the Mre11-Rad50 complex"; Lisa Käshammer, Jan-Hinnerk Saathoff, Fabian Gut, Joseph Bartho, Aaron Alt, Brigitte Kessler, Katja Lammens, Karl-Peter Hopfner; Molecular Cell; 2019

Facts, background information, dossiers

  • DNA repair
  • DNA damage
  • cryo-electron microscopy

More about LMU

  • News

    Alzheimer’s disease: Protective immune response in the brain?

    LMU researchers Christian Haass and Michael Ewers have identified a factor that might possibly delay the emergence and slow the progression of Alzheimer’s disease. Researchers at the German Center for Neurodegenerative Diseases (DZNE) and the Institute for Stroke and Dementia Research (ISD) ... more

    One transistor for all purposes

    In mobiles, fridges, planes – transistors are everywhere. But they often operate only within a restricted current range. LMU physicists have now developed an organic transistor that functions perfectly under both low and high currents. Transistors are semiconductor devices that control volt ... more

    Tiny “blinkers” reveal molecules inside cells

    LMU physicists led by Ralf Jungmann introduce an entirely new approach to super-resolution microscopy: Tiny ‘blinkers’ enable simultaneous imaging of multiple biomolecules. In everyday life, blinking lights can send signals – for example, that a car is going to turn. Now, researchers have e ... more

  • Authors

    Prof. Dr. Thomas Carell

    Thomas Carell graduated in chemistry, completing his doctorate at the Max Planck Institute for Medical Research under the tutelage of Prof. Dr Dr H. A. Staab. Following a research position in the USA, he accepted a position at ETH Zurich, setting up his own research group in the Laboratory ... more

q&more – the networking platform for quality excellence in lab and process

The q&more concept is to increase the visibility of recent research and innovative solutions, and support the exchange of knowledge. In the broad spectrum of subjects covered, the focus is on achieving maximum quality in highly innovative sectors. As a modern knowledge platform, q&more offers market participants one-of-a-kind networking opportunities. Cutting-edge research is presented by authors of international repute. Attractively presented in a high-quality context, and published in German and English, the original articles introduce new concepts and highlight unconventional solution strategies.

> more about q&more

q&more is supported by:


Your browser is not current. Microsoft Internet Explorer 6.0 does not support some functions on Chemie.DE