Functional Coupling between DNA Replication and Sister Chromatid Cohesion Establishment

Abstract

Several lines of evidence suggest the existence in the eukaryotic cells of a tight, yet largely unexplored, connection between DNA replication and sister chromatid cohesion. Tethering of newly duplicated chromatids is mediated by cohesin, an evolutionarily conserved hetero-tetrameric protein complex that has a ring-like structure and is believed to encircle DNA. Cohesin is loaded onto chromatin in telophase/G1 and converted into a cohesive state during the subsequent S phase, a process known as cohesion establishment. Many studies have revealed that down-regulation of a number of DNA replication factors gives rise to chromosomal cohesion defects, suggesting that they play critical roles in cohesion establishment. Conversely, loss of cohesin subunits (and/or regulators) has been found to alter DNA replication fork dynamics. A critical step of the cohesion establishment process consists in cohesin acetylation, a modification accomplished by dedicated acetyltransferases that operate at the replication forks. Defects in cohesion establishment give rise to chromosome mis-segregation and aneuploidy, phenotypes frequently observed in pre-cancerous and cancerous cells. Herein, we will review our present knowledge of the molecular mechanisms underlying the functional link between DNA replication and cohesion establishment, a phenomenon that is unique to the eukaryotic organisms.

Keywords: DNA replication; cell cycle; cohesin; replication proteins; sister chromatid cohesion.

Figure 1

Cohesin cycle in human cells. The cohesin loader, Scc2-Scc4, and unloader, Pds5-Wapl, promote dynamic association of cohesin to chromatin in telophase and G1. During S phase, Smc3 acetylation and Sororin binding stabilize cohesin association to chromatin. Sister chromatids are entrapped by cohesin rings at the replication forks and cohesion is established. In prophase, cohesin bound to chromosome arms is released by Wapl-Pds5 following phosphorylation of Sororin (“prophase pathway”). In contrast, cohesin bound at the centromeres is protected by Shugoshin (Sgo1) and protein phosphatase 2A (PP2A) against the releasing activity of Wapl. At the metaphase-anaphase transition, Shugosin-PP2A dissociates from cohesin. Thus, in anaphase cohesin can be cleaved by Separase to enable chromosome segregation. Cohesin drawing has been inspired by [10] with modifications.

Figure 2

Replication factor C and its alternative forms involved in chromosomal cohesion. Subunit composition and role of each factor are reported. RFC^Ctf18 contains Ctf8 and Dcc1 as two additional subunits.

Figure 3

Regulation of cohesin functions by Esco1 or Esco2 acetyltransferases. Cohesin loading onto chromatin and loop extrusion activity both depend on association with Scc2 factor. Formation of chromatin topologically associating domains (TADs) in interphase cells is due to the loop extrusion activity of the cohesin-Scc2 complex. TAD boundaries are defined by the CCCTC-binding factor (CTCF), a sequence specific DNA binding protein that acts as a chromatin insulator [28]. The activity of Esco1 during interphase, before DNA replication starts, stabilizes TADs to define chromatin spatial organization. Conversely, Esco2 activity during S phase ensures that replicated sister chromatids are stably entrapped within the cohesin rings. Mode of DNA binding of cohesin-Scc2 complex is only indicative.

Figure 4

Recruitment of Esco2 at the replication forks in mammalian cells. (A) Schematic representation of the human Esco2 polypeptide chain. The conserved A, B, C and PIP boxes in the Esco2 N-terminal portion are indicated in different colours. Z stands for zinc finger motif (orange box). (B) Esco2 is recruited at the pre-RC through a direct interaction with the MCM2-7 complex mediated by box A (red). (C) During the elongation phase of DNA replication, Esco2 establishes multiple interactions with PCNA and/or the MCM complex either on the leading or the lagging strand through its conserved N-terminal sequences motifs (red, green, black and blue rectangles). Many DNA replication factors have been omitted for the sake of clarity. See text for details.

Figure 5

A hypothetical model of cohesion establishment at the human DNA replication fork. Some protein factors are omitted for simplicity (MCM10, Sororin, Fen1 and Dna2). Involvement of Pds5-Wapl and Scc2-Scc4 in cohesin conversion and de novo loading mechanism, respectively, has not yet been formally demonstrated in mammalian cells (see text for details). Ac stands for acetylation. Folding back of the lagging strand, proposed to enable DNA polymerase coupling, is not depicted here for simplicity. Relative position of the indicated protein factors is based on the structure of the yeast replisome proposed by Yeeles and colleagues [77].