DNA is often likened to a blueprint in which specific sequences of A, C, G, and T guide the operation of living things and provide essential information for building life.
But a key detail that this metaphor misses is that our DNA needs to be constantly maintained to maintain its integrity. Without a specialized DNA repair mechanism to regularly fix errors, the information in them can degenerate rapidly.
This repair occurs at cell cycle checkpoints, and they are activated in response to DNA damage. Like quality control experts on the assembly line, proteins involved in DNA damage checkpoints assess whether a cell's DNA is wrong and, if necessary, pause cell division and repair it. When this checkpoint collapses, such as when a genetic mutation is encountered, DNA damage accumulates, and the end result is often cancer.
Although scientists have learned a lot about DNA damage and repair over the past few decades, there are still some important questions that remain unresolved. A particularly vexing problem is a repair protein called 9-1-1 clamp, a type of "first responder" facing DNA damage.
But the details of exactly how it attach itself to broken STRANDs of DNA, thereby activating DNA damage checkpoints, have always been very obscure. In simple terms, scientists know that this attachment is a key step in launching an effective fix, but the specific mechanisms involved are completely unclear.
Now, a team of researchers has figured out a clear picture of how the 9-1-1 clip attaches to the site of DNA damage, and these results challenge previous paradigms. The paper was recently published in Nature, Structural and Molecular Biology.
New modes of opening and closing
This disruptive discovery comes from a collaboration between two laboratories with complementary expertise. One of the labs primarily uses biochemical methods to study dna replication and repair processes. Over the past few years, a major goal of their research has been to reconstruct the entire DNA replication and repair process in a test tube, and several components of the repair mechanism have been purified, including the key 9-1-1 and the proteins that facilitate its binding to DNA.
Another lab specializes in a technique called cryo-electron microscopy (cryo-EM). This technique can study proteins and protein assembly by visualizing the subtle movements of proteins and protein assemblies to reveal extremely high resolutions of individual amino acid locations. Like the gears and levers of a machine, it is the movement of these amino acids that allows proteins to become indispensable working machines in cells.
The two teams hit it off and eventually found surprising results.
The shape of the 9-1-1 clip resembles a ring. To perform its function, it needs to surround the broken DNA at the junction of the exposed end of a strand of double-stranded DNA with the single-stranded DNA. Therefore, this circular structure must be opened so that the single-stranded DNA enters the center of the clip, and then re-closed around the molecule. This does not occur spontaneously, but is promoted by another protein complex, which is called a clip-on complex.
All previous studies had suggested that the clamp would open in a manner similar to a locking washer, a model that has been around for 20 years. The model assumes that the two open ends on the clip rotate out of the plane, creating a narrow gap.
Locking washer. Previous models suggested that the 9-1-1 opened in a similar way to the locking washer. | Image credit: pixabay
However, the new study observed that the 9-1-1 clip opened much larger than expected, and that it was opened entirely within the plane and did not twist like a locking washer.
The new study found a 9-1-1 open pattern. | Image credit: Memorial Sloan Kettering Cancer Center
Another unexpected result was that the 9-1-1 clip-loading complex was observed to bind in the opposite direction to dna from other clip-loading complexes, while those other complexes acted on undamaged DNA during normal DNA replication. This observation explains how 9-1-1 was specifically "recruited" to the site of DNA damage.
From basic research to translation
The researchers believe that this result not only provides new answers to basic biological puzzles, it may eventually lead to better cancer drugs.
Many current chemotherapy drugs work by interfering with DNA replication in cancer cells and causing DNA damage. Because cancer cells have reduced their ability to repair DNA damage, adding DNA-damaging chemotherapy drugs overwhelms the cells' ability to repair DNA, and they die as a result. With new knowledge about how 9-1-1 interacts with other repair proteins and DNA, it is possible for scientists to design drugs that specifically interfere with specific steps in the repair process to make chemotherapy drugs more effective.
#创作团队:
Compile: M ka
Typography: Wenwen
#参考来源:
https://www.mskcc.org/news/ski-scientists-determine-structure-key-piece-dna-repair-machinery
#图片来源:
Cover image: piqsels
首图：Memorial Sloan Kettering Cancer Center