The initials BRCA2 may be best known for a gene associated with many cases of breast cancer. The BRCA2 protein is critical to repairing breaks in DNA. Breaks in DNA in breast, ovary, and most body cells are sporadic. Now imagine many breaks taking place simultaneously and on purpose.
In germ cells—the cells that give rise to sperm or eggs—DNA breaks occur in every chromosome before the cells undergo cell-splitting called meiosis. The breaks ensure mixing of genes to create genetic diversity rather than exact copies of the parents. In meiosis each germ cell splits twice so that each egg or sperm ends up with only one copy of each chromosome. Then when egg meets sperm, the embryo has the right number of chromosome pairs. Before the first split occurs, the chromosomes in the germ cell pair up tightly and then each chromosome within a pair breaks and rejoins with pieces from its partner to exchange genes in a process called crossover. Then all these DNA breaks need to be rejoined quickly.
Now U-M scientists have determined the structure of a complex of two proteins—BRCA2 together with MEILB2—that allows repairs to happen efficiently in cells undergoing meiosis. They report their finding in a new article in Nature Structural and Molecular Biology.
Think of a sandwich, explains structural biologist Jayakrishnan Nandakumar, associate professor of molecular, cellular, and developmental biology. The “bun” is composed of four identical copies of a protein called MEILB2 on the top and bottom, with the two BRCA2 proteins between. The MEILB2 protein sandwich carries the BRCA2 protein precisely to the DNA break points.
To determine the structure of this BRCA2 complex, the researchers used x-ray crystallography. In this process, the protein crystal is bombarded with x-rays and the patterns that are generated when the x-rays deflect off the atoms in the crystal allow the researchers to figure out where each atom is located in the 3D structure of the molecule. That would help them figure out how the BRCA2 protein is connected to the MEILB2 protein.
The first step was to grow crystals of the BRCA2 complex. After much trial and error, a Chemical Biology graduate student in the Nandakumar lab, Devon Pendlebury, successfully crystalized the human form of the BRCA2 complex. In a bit of good fortune, the U-M researchers were able to collect data at the Argonne National Laboratory days before all research was shut down in March 2020.
From the x-ray crystallography data and additional experiments by MCDB graduate student Ritvija Agrawal, the team determined the structure of the protein complex and how the two proteins worked together. It was a somewhat unusual protein-interaction, they report.
To validate their findings, they created mutant versions of BRCA2 and MEILB2 based on their structure and showed how these mutants failed to form this complex with each other.
In further validation of the MEILB2-BRCA2 complex structure, collaborators at University of Gothenburg in Sweden introduced equivalent mutant versions in mouse cells undergoing meiosis. Mutant BRCA2 or MEILB2 failed to get to the DNA breaks that needed to be rejoined.
“While we have known BRCA2 was necessary for DNA recombination in meiosis, we didn’t know how it was able to do this critical job efficiently,” says Nandakumar. “The MEILB2 that is part of this repair complex is only supposed to be present in cells that undergo meiosis but MEILB2 has also been found in several cancers. It may be that MEILB2 is very efficiently ‘hijacking’ the BRCA2 in cancer cells, preventing proper repair of the DNA.” Without other factors usually found in meiotic cells, the BRCA2 in these MEILB2-positive cancers might not get to the DNA break points. Having a structure of this complex in hand, researchers may now find new approaches to regain BRCA2 function in MEILB2-positive cancers, Nandakumar suggests.
Nature Structural and Molecular Biology
Structure of a meiosis-specific complex central to BRCA2 localization at recombination sites
Devon F. Pendlebury, Jingjing Zhang, Ritvija Agrawal, Hiroki Shibuya and Jayakrishnan Nandakumar
- Devon F. Pendlebury earned her PhD in the Program in Chemical Biology under JK Nandakumar in the Department of Molecular, Cellular, and Developmental Biology. She is now a postdoctoral fellow at Pharmaceutical Sciences, University of California Irvine, Irvine, CA,
- Jingjing Zhang was a PhD student in the Shibuya lab, Department of Chemistry and Molecular Biology, University of Gothenburg, Gothenburg, Sweden.
- Ritvija Agrawal is a PhD candidate in Molecular, Cellular, and Development Biology in the Nandakumar lab.
- Hiroki Shibuya is an assistant professor in Department of Chemistry and Molecular Biology, University of Gothenburg, Gothenburg, Sweden
- Jayakrishnan Nandakumar is associate professor in the Department of Molecular, Cellular, and Developmental Biology.