Keeping the integrity of our DNA is essential to remain healthy. However, cellular by-products like reactive oxygen species or external insults such as UV light or alcohol constantly damage DNA. DNA replication, an essential cellular process in each proliferative cell, is especially sensitive to DNA damage. During DNA replication, each strand of a double-stranded DNA molecule is copied so that the two daughter cells each get a complete copy. The duplication of DNA is not straightforward. As the copying machinery moves along the double strand, it usually encounters blockages. These blockages come in various forms, but one very common issue is proteins covalently linked to the DNA by naturally produced linking agents, especially aldehydes. With a protein bonded to DNA, the DNA replication machinery will get jammed, and if unblocked, the genome instability would rise. In fact, people unable to solve this molecular issue correctly age faster and develop early onset cancers.
Previously, researchers have known that a protein called SPRTN was necessary to remove proteins bonded to DNA thanks to its protease activity, and that it was important for faithful DNA replication. However, they didn’t know for sure how SPRTN regulates DNA replication and protects us from accelerated ageing and cancer.
Kristijan Ramadan, Swagata Halder, Ignacio Torrecilla and other co-authors have filled this gap in our knowledge. Their Nature Communications paper published on Wed 17 July (https://www.nature.com/articles/s41467-019-11095-y#Sec1 ) describes how SPRTN activates appropriate checkpoint signals to sustain a smooth DNA replication while removing DNA-bound proteins. It does so by a pathway not envisaged previously. According to current understanding, most defects in DNA replication and damage unfold with the persistent presence of single strands of DNA, which would activate a master protein called ATR, which talks to a middle-management protein called CHK1, which in turn arrests the cell cycle and coordinates the resolution of the damaged or stalled replication fork. However, obstructive DNA-bound proteins cannot activate ATR because there is no single strand formation, so the process of halting replication and repairing the DNA must be initiated by another means. In the paper, the authors describe how SPRTN and CHK1 undertake a mutual activation dance that keeps both proteins working to clear blockages before they cause a problem: SPRTN, a protease able to cut other proteins, cleaves CHK1 to release its active part and trigger its function; CHK1, in turn, adds phosphate groups on SPRTN and stimulates its activity.
This work fills in a huge gap in our knowledge of DNA replication and raises an important set of new questions, one of which is how to use this knowledge to prevent accelerated ageing and to treat cancer. Kristijan Ramandan and Ignacio Torrecilla have already started looking into this and we look forward to reading about their new findings.