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DNA Repair Changes with the Flip of a Switch

The DNA blueprint in each human cell undergoes about 100,000 damaging events every day. Because a cell’s survival depends on the repair of these damaged molecules, each injury signals a team of proteins to work together to fix the mutated DNA.

In pursuit of a better fundamental understanding of DNA repair, researchers from the National Institute of Environmental Health Sciences use NSLS to image a large, multi-part molecule called a scaffolding protein. The molecule, XRCC1, orchestrates DNA repair by holding the other repair proteins together in a multi-molecule complex.

A technique called SAXS, for small-angle x-ray scattering, allows the researchers to take a low-resolution picture of the molecule, obtaining a macroscopic image rather than the details of its fine structure. In SAXS, a protein in solution (the state in which it exists in the body) can be targeted with x-rays, which then scatter and hit a detector, allowing researchers to reconstruct an image of the protein’s surface.

Using SAXS, the researchers studied a subsection of XRCC1 called the N-terminal domain. The N-terminal domain interacts with DNA polymerase b, a protein that actively repairs damaged DNA. Their study shows that this interaction can change with the flick of a biological switch.

The N-terminal domain contains a “disulfide switch,” a potential bond whose formation changes the molecule’s secondary structure. When the bond forms, the switch is flipped on, transforming the molecule from a reduced state to an oxidized state. In the reduced state, one of the amino acids that make up the N-terminal domain has electrons to spare. This makes the amino acid very susceptible to oxidation, a process in which it shares these electrons with another amino acid, forming a disulfide bond between the electron sharer and receiver and transforming the N-terminal domain into its oxidized state.

In this case, the oxidized surface binds more strongly to DNA polymerase b than does the surface of the reduced form.

Discovering the disulfide switch’s role would help improve scientists’ understanding of how DNA repair works, knowledge that could lead to better treatments for disease.

M.J. Cuneo, R.E. London, “Oxidation State of the XRCC1 N-terminal Domain Regulates DNA Polymerase β Binding Affinity,” PNAS, 107 (15), 6805 (2010).

Top: Model of the complex formed by XRCC1’s N-terminal domain (in purple), damaged DNA (in orange), and DNA polymerase b (with component parts thumb, lyase, fingers, and palm labeled).

Bottom: Matthew Cuneo (left) and Robert London (photo courtesy of Steven R. McCaw, Image Associates)

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