Closing the DNA Replication Cycle

DNA replication is essential for living organisms to faithfully deliver genetic information from parental cells to daughter cells. Many proteins are assembled on the parental DNA to work as replication machinery. Among them is a ring-shaped molecule called PCNA (proliferating cell nuclear antigen), which encircles chromosomes during DNA replication. After it is loaded onto DNA, PCNA recruits other proteins to copy the parental DNA. PCNA’s stable connection to DNA makes it an essential platform for many replication machineries.

When DNA synthesis is complete, PCNA must be removed from the DNA. Although PCNA unloading is crucial for maintaining genomic stability, the mechanism has remained unclear. The study, published today in Nature Communications, presents a mechanism for the regulation of PCNA cycling during DNA replication.

It was already known that a protein complex called RFC (Replication-Factor-C) opens the PCNA-ring and loads it onto the linear DNA molecule before DNA synthesis. Until now, scientists have believed that RFC also functions to unload PCNA. However, the researchers suggested that another protein complex called ATAD5-RLC (ATAD5-RFC-Like-Complex), having a similar structure to RFC, could be a PCNA-unloading candidate. The study findings show that ATAD5-RLC is, in fact, a bona fide PCNA unloader—even when that PCNA has been modified due to replication stress and DNA damage.

The scientists also identified key motifs in ATAD5 that make the protein complex a PCNA unloader, and they revealed the mechanistic differences between the PCNA loading and unloading processes. “It was crucial to functionally dissect ATAD5 and purify active ATAD5-RLC,” says corresponding author Sukhyun Kang of the Institute for Basic Science (IBS). “Establishment of an in vitro reconstitution system allowed us mechanistic studies for PCNA unloading.”

In this study, the scientists show that two structurally related complexes play distinct roles in PCNA–DNA association. RFC loads PCNA to initiate DNA synthesis and ATAD5-RLC unloads PCNA to terminate DNA replication and repair. Since PCNA unloading is closely linked to chromatin assembly, this study will be the basis of future studies on the relationship between replication termination and maintenance of epigenetic information.

Senior author Kyungjae Myung of IBS believes this study is a major step toward understanding the regulatory mechanism for replication termination. “Since uncontrolled disassembly of replication machineries causes genomic instability that may result in cellular transformation, our study will be beneficial to develop strategies for cancer treatment,” Myung suggests.

Image: PCNA loading by RFC marks initiation of DNA synthesis. ATAD5-RLC functions as a universal PCNA sweeper that is essential for the termination of DNA replication and repair. Image courtesy of IBS.