DNA replication is very important for living organisms to faithfully provide genetic information from stem cells to children's cells. Many proteins gather in the parent DNA to work as a replication machine. Among them, cell proliferation nuclear antigen (PCNA) is a key replication protein. This ring-structured molecule surrounds the chromosome, a thread-like structure in which DNA molecules are packed, during DNA replication. Just like a ring on a string, PCNA-rings are closely related to DNA. After landing on DNA, PCNA recruits another protein to efficiently copy parental DNA. The stable connection of PCNA to DNA makes it an important platform for many replication machines.
However, this is not the end of the story. When DNA synthesis is complete, PCNA must be removed from DNA to ensure the complete end of the cycle. Otherwise, protein replication makes unstable genomic DNA that can cause genetic mutations. Then how is the replication machine cleaned from DNA after DNA replication? Disassembly of PCNA from DNA clears the replication machine from DNA after DNA replication. Although the demolition of PCNA is very important for maintaining genome stability, it is still unclear how PCNA was lowered during cessation of replication.
Led by director Myung Kyungjae, professor Kim Hajin and Dr. Kang Sukhyun at the Center for Genomic Integrity at the Institute for Basic Science (IBS) at the National Institute of Science and Technology (UNIST) Ulsan, South Korea reported that they revealed a novel molecular mechanism for regulating the PCNA cycle during DNA replication. They found that a specific protein complex called ATAD5-RFC-Like-Complex (ATAD5-RLC) was a disassembly of PCNA. Dr. group Myung has built an in vitro recovery system where loading and unloading PCNA can be monitored. They proved that ATAD5-RLC opened the PCNA ring to be removed from DNA as a breaker of bona fide PCNA.
This finding actually comes from a previous study. Another protein complex, Replication-Factor-C (RFC) is known to open the PCNA ring and load it into linear DNA molecules before DNA synthesis. Until now, scientists in the field of DNA replication believed that RFCs also functioned to reduce PCNA because RFC bacterial homologues carried out loading and unloading reactions. Dr. group Myung suggested that ATAD5-RLC, which has a structure similar to RFC, could be a candidate for PCNA unloaders. This study proves biochemically that ATAD5-RLC is a bona fide breaker of PCNA. The scientists also revealed that ATAD5-RLC can also reduce modified PCNA. PCNA is modified at replication pressure to deal with DNA damage. This finding highlights the role of ATAD5-RLC as a universal PCNA sweeper for cessation during DNA replication and repair.
The scientists also identified the main motive in ATAD5 which made complex proteins as disassembly of PCNA. They revealed the mechanistic differences between PCNA loading and unloading processes. The scientists found a mutation point in the main motive of ATAD5 from melanoma patients as well. Dr. Sukhyun Kang, one of the appropriate authors of the study, said, "It is important to dissect ATAD5 functionally and purify active ATAD5-RLC. The formation of an in vitro reconstitution system allows us to study mechanistic for PCNA disassembly."
This study provides a complete understanding of the PCNA cycle in DNA replication. It has been a controversial complex responsible for the removal of PCNA after DNA replication and repair. Scientists show that two structurally related complexes play different roles in the relationship of PCNA-DNA. RFC loads PCNA to initiate DNA synthesis and ATAD5-RLC disassembles PCNA to end DNA replication / repair. The different mechanisms between loading and unpacking the PCNA revealed by this study give us conceptual progress in understanding DNA replication. Because PCNA disassembly is closely related to chromatin assembly, this study will be the basis of future studies for the relationship between the termination of replication and maintenance of epigenetic information.
"This is a big advance to understand the regulatory mechanisms for the cessation of replication. Because uncontrolled dismantling of the replication machine causes genome instability that can lead to cellular transformation, our research will be useful for developing strategies for cancer treatment," director Myung explained.
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