Genome guardians stop and reel in DNA to correct replication errors


Date:
July 16, 2020
Source:
North Carolina State University
Summary:
New research shows how proofreading proteins prevent DNA replication
errors by creating an immobile structure that calls more proteins
to the site to repair the error. This structure could also prevent
the mismatched region from being ''packed'' back into the cell
during division.



FULL STORY ==========================================================================
On the DNA assembly line, two proofreading proteins work together as an emergency stop button to prevent replication errors. New research from
North Carolina State University and the University of North Carolina at
Chapel Hill shows how these proteins -- MutL and MutS -- prevent DNA replication errors by creating an immobile structure that calls more
proteins to the site to repair the error. This structure could also
prevent the mismatched region from being "packed" back into the cell
during division.


==========================================================================
When a cell prepares to divide, the DNA splits, with the double helix "unzipping" into two separate backbones. New nucleotides -- adenine,
cytosine, guanine or thymine -- are filled into the gaps on the other side
of the backbone, pairing with their counterparts (adenine with thymine
and cytosine with guanine) and replicating the DNA to make a copy for
both the old and the new cells. The nucleotides are a correct match
most of the time, but occasionally -- about one time in 10 million --
there is a mismatch.

"Although mismatches are rare, the human genome contains approximately six billion nucleotides in every cell, resulting in approximately 600 errors
per cell, and the human body consists of more than 37 trillion cells,"
says Dorothy Erie, chemistry professor at UNC-Chapel Hill, member of
UNC's Lineberger Comprehensive Cancer Center and co-corresponding author
of the work.

"Consequently, if these errors go unchecked they can result in a vast
array of mutations, which in turn can result in a variety of cancers, collectively known as Lynch Syndrome." A pair of proteins known as MutS
and MutL work together to initiate repair of these mismatches. MutS slides along the newly created side of the DNA strand after it's replicated, proofreading it. When it finds a mismatch, it locks into place at the
site of the error and recruits MutL to come and join it. MutL marks the
newly formed DNA strand as defective and signals a different protein to
gobble up the portion of the DNA containing the error. Then the nucleotide matching starts over, filling the gap again. The entire process reduces replication errors around a thousand-fold, serving as one of our body's
best defenses against genetic mutations that can lead to cancer.

"We know that MutS and MutL find, bind, and recruit repair proteins to
DNA," says biophysicist Keith Weninger, university faculty scholar at NC
State and co-corresponding author of the work. "But one question remained
-- do MutS and MutL move from the mismatch during the repair recruiting process, or stay where they are?" In two separate papers appearing in Proceedings of the National Academy of Sciences, Weninger and Erie looked
at both human and bacterial DNA to gain a clearer temporal and structural picture of what happens when MutS and MutL engage in mismatch repair.

Using both fluorescent and non-fluorescent imaging techniques, including
atomic force microscopy, optical spectroscopy and tethered particle
motion, the researchers found that MutL "freezes" MutS in place at
the site of the mismatch, forming a stable complex that stays in that
vicinity until repair can take place. The complex appears to reel in the
DNA around the mismatch as well, marking and protecting the DNA region
until repair can occur.

"Due to the mobility of these proteins, current thinking envisioned
MutS and MutL sliding freely along the mismatched strand, rather than stopping," Weninger says. "This work demonstrates that the process is
different than previously thought.

"Additionally, the complex's interaction with the strand effectively
stops any other processes until repair takes place. So the defective
DNA strand cannot be repacked into a chromosome and then carried forward through cell division."

========================================================================== Story Source: Materials provided by North_Carolina_State_University. Note: Content may be edited for style and length.


========================================================================== Journal References:
1. Pengyu Hao, Sharonda J. LeBlanc, Brandon C. Case, Timothy C. Elston,
Manju M. Hingorani, Dorothy A. Erie, Keith R. Weninger. Recurrent
mismatch binding by MutS mobile clamps on DNA localizes repair
complexes nearby. Proceedings of the National Academy of Sciences,
2020; 201918517 DOI: 10.1073/pnas.1918517117
2. Kira C. Bradford, Hunter Wilkins, Pengyu Hao, Zimeng M. Li, Bangchen
Wang, Dan Burke, Dong Wu, Austin E. Smith, Logan Spaller, Chunwei
Du, Jacob W. Gauer, Edward Chan, Peggy Hsieh, Keith R. Weninger,
Dorothy A.

Erie. Dynamic human MutSa-MutLa complexes compact mismatched DNA.

Proceedings of the National Academy of Sciences, 2020; 117 (28):
16302 DOI: 10.1073/pnas.1918519117 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200716123002.htm

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