Sister chromatid cohesion is mediated by cohesin and is essential for accurate chromosome segregation. The cohesin subunits SMC1, SMC3, and Rad21 form a tripartite ring within which sister chromatids are thought to be entrapped. This event requires the acetylation of SMC3 and the association of sororin with cohesin by the acetyltransferases Esco1 and Esco2 in humans, but the functional mechanisms of these acetyltransferases remain elusive. Here, we showed that Esco1 requires Pds5, a cohesin regulatory subunit bound to Rad21, to form cohesion via SMC3 acetylation and the stabilization of the chromatin association of sororin, whereas Esco2 function was not affected by Pds5 depletion. Consistent with the functional link between Esco1 and Pds5, Pds5 interacted exclusively with Esco1, and this interaction was dependent on a unique and conserved Esco1 domain. Crucially, this interaction was essential for SMC3 acetylation and sister chromatid cohesion. Esco1 localized to cohesin localization sites on chromosomes throughout interphase in a manner that required the Esco1-Pds5 interaction, and it could acetylate SMC3 before and after DNA replication. These results indicate that Esco1 acetylates SMC3 via a mechanism different from that of Esco2. We propose that, by interacting with a unique domain of Esco1, Pds5 recruits Esco1 to chromatin-bound cohesin complexes to form cohesion. Furthermore, Esco1 acetylates SMC3 independently of DNA replication.

Sister chromatid cohesion, mediated by cohesin, is required for accurate chromosome segregation [1, 2]. This process requires acetylation of cohesin subunit SMC3 by evolutionarily conserved cohesin acetyltransferases: Eco1 in budding yeast; XEco1 and XEco2 in Xenopus; and ESCO1 and ESCO2 in human [3-10]. Eco1 is recruited to chromatin through physical interaction with PCNA [11] and is degraded by the Skp1/Cul1/F-box protein complex after DNA replication to prevent ectopic cohesion formation [12]. In contrast, XEco2 recruitment to chromatin requires prereplication complex formation [13] and is degraded by the anaphase-promoting complex (APC) [14]. In human, whereas ESCO1 is expressed throughout the cell cycle, ESCO2 is detectable in S phase and is degraded after DNA replication [6, 15]. Although PDS5, a cohesin regulator, preferentially promotes ESCO1-dependent SMC3 acetylation [16], little is known about the molecular basis of the temporal regulation of ESCO2. Here, we show that ESCO2 is recruited to chromatin before PCNA accumulation. Whereas no interaction between PCNA and ESCO proteins is observed, ESCO2, but not ESCO1, interacts with the MCM complex through a unique ESCO2 domain. Interestingly, the interaction is required to protect ESCO2 from proteasomal degradation and is attenuated in late S phase. We also found that ESCO2 physically interacts with the CUL4-DDB1-VPRBP E3 ubiquitin ligase complex in late S phase and that post-replicative ESCO2 degradation requires the complex as well as APC. Thus, we propose that the MCM complex couples ESCO2 with DNA replication and that the CUL4-DDB1-VPRBP complex promotes post-replicative ESCO2 degradation, presumably to suppress cohesion formation during mitosis.