Cohesin acetylation speeds the replication fork

Abstract

Cohesin rings encircle pairs of sister DNA molecules during cell division to allow their proper segregation. These rings inhibit progression of the transcriptional apparatus, but do not prevent the replication machinery from duplicating the genome in the S phase of the cell cycle. Single-molecule analysis now shows that a replication complex (called replication factor C–CTF18 clamp loader) acetylates cohesin, thereby alleviating cohesin's association with regulatory factors and facilitating replication fork progression. Loss of acetylation, as observed in cells from patients with Roberts syndrome, causes defects in fork progression and the accumulation of DNA damage. Cohesin inhibits the transcriptional machinery's interaction with and movement along chromatin, but does not prevent replication forks from duplicating the genome in S phase. Using single-molecule analysis, a replication complex is now found to affect acetylation of a subunit of cohesin, and this acetylation appears to be a central determinant of fork processivity. Loss of this regulatory mechanism leads to the spontaneous accrual of DNA damage. Cohesin not only links sister chromatids but also inhibits the transcriptional machinery’s interaction with and movement along chromatin1,2,3,4,5,6. In contrast, replication forks must traverse such cohesin-associated obstructions to duplicate the entire genome in S phase. How this occurs is unknown. Through single-molecule analysis, we demonstrate that the replication factor C (RFC)–CTF18 clamp loader (RFCCTF18)1,7 controls the velocity, spacing and restart activity of replication forks in human cells and is required for robust acetylation of cohesin’s SMC3 subunit and sister chromatid cohesion. Unexpectedly, we discovered that cohesin acetylation itself is a central determinant of fork processivity, as slow-moving replication forks were found in cells lacking the Eco1-related acetyltransferases ESCO1 or ESCO2 (refs 8–10) (including those derived from Roberts’ syndrome patients, in whom ESCO2 is biallelically mutated11) and in cells expressing a form of SMC3 that cannot be acetylated. This defect was a consequence of cohesin’s hyperstable interaction with two regulatory cofactors, WAPL and PDS5A (refs 12, 13); removal of either cofactor allowed forks to progress rapidly without ESCO1, ESCO2, or RFCCTF18. Our results show a novel mechanism for clamp-loader-dependent fork progression, mediated by the post-translational modification and structural remodelling of the cohesin ring. Loss of this regulatory mechanism leads to the spontaneous accrual of DNA damage and may contribute to the abnormalities of the Roberts’ syndrome cohesinopathy.