Researchers solve structure of BRCA2 protein complex important in DNA
repair

Date:
August 13, 2021
Source:
University of Michigan
Summary:
The initials BRCA2 may be best known for a gene associated with
many cases of breast cancer, and the protein encoded by the BRCA2
gene is critical to repairing breaks in DNA. The breakdown of this
interaction is a hallmark of many cancers. Now scientists have
determined the structure of a complex of two proteins -- BRCA2
together with MEILB2 -- that allows repairs to happen efficiently
in cells undergoing cell-splitting, called meiosis. Their results
have major implications for cancer and infertility.



FULL STORY ==========================================================================
The initials BRCA2 may be best known for a gene associated with many
cases of breast cancer, and the protein encoded by the BRCA2 gene is
critical to repairing breaks in DNA.


==========================================================================
The breakdown of this interaction is a hallmark of many cancers. 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 cell-splitting, called meiosis. Their results, reported
in Nature Structural and Molecular Biology, have major implications for
cancer and infertility.

"We know how the literature is rich with examples of BRCA2 mutations in
cancer, but our findings now suggest that the MEILB2-binding region
of BRCA2 might be a hotspot for discovering mutations related to
infertility," said study author and U-M structural biologist Jayakrishnan Nandakumar, associate professor of molecular, cellular, and developmental biology.

In germ cells -- the cells that give rise to sperm or eggs -- DNA breaks
occur in every chromosome before the cells undergo 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.

Think of a sandwich, Nandakumar explains. 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, Devon Pendlebury, a chemical biology graduate student
in the Nandakumar lab, successfully crystallized 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 the 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,"
Nandakumar said. "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 breakpoints. Having a structure of this complex in hand, researchers
may now find new approaches to regain BRCA2 function in MEILB2-positive cancers, Nandakumar suggests.

Co-authors include Hiroki Shibuya, an assistant professor in the
Department of Chemistry and Molecular Biology at the University of
Gothenburg.

========================================================================== Story Source: Materials provided by University_of_Michigan. Original
written by Suzanne Tainter. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Devon F. Pendlebury, Jingjing Zhang, Ritvija Agrawal, Hiroki
Shibuya,
Jayakrishnan Nandakumar. Structure of a meiosis-specific complex
central to BRCA2 localization at recombination sites. Nature
Structural & Molecular Biology, 2021; 28 (8): 671 DOI:
10.1038/s41594-021-00635-0 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210813120006.htm

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