Structural Studies Reveal the Functional Modularity of the Scc2-Scc4 Cohesin Loader

Abstract

The remarkable accuracy of eukaryotic cell division is partly maintained by the cohesin complex acting as a molecular glue to prevent premature sister chromatid separation. The loading of cohesin onto chromosomes is catalyzed by the Scc2-Scc4 loader complex. Here, we report the crystal structure of Scc4 bound to the N terminus of Scc2 and show that Scc4 is a tetratricopeptide repeat (TPR) superhelix. The Scc2 N terminus adopts an extended conformation and is entrapped by the core of the Scc4 superhelix. Electron microscopy (EM) analysis reveals that the Scc2-Scc4 loader complex comprises three domains: a head, body, and hook. Deletion studies unambiguously assign the Scc2N-Scc4 as the globular head domain, whereas in vitro cohesin loading assays show that the central body and the hook domains are sufficient to catalyze cohesin loading onto circular DNA, but not chromatinized DNA in vivo, suggesting a possible role for Scc4 as a chromatin adaptor.

The Arabidopsis homolog of Scc4/MAU2 is essential for embryogenesis

“…The interacting interface comprises Scc4 TPR repeats, tandemly arranged short helices forming a superhelix that engulfs the unstructured N-terminus of Scc2 (Chao et al, 2015;Hinshaw et al, 2015). We demonstrate that AtSCC4 interacts with the N-terminus of AtSCC2 in yeast and in plants (Fig.…”

“…Yeast Scc2 interacts with the cohesin ring and facilitates its loading on DNA in vitro in the absence of Scc4 (Chao et al, 2015), while Scc4 is required for loading cohesin rings at specific sites on chromosomes (Hinshaw et al, 2015). Immobilization of AtSCC4 in the nuclei is most likely a result of its association with chromatin and is indicative that AtSCC4 might serve as an adapter between the AtSCC2-cohesin complex and the sites of cohesin loading on chromatin.…”

“…While Scc2 is required for closing the cohesin ring around DNA, Scc4 is necessary for delivering the complex to DNA in vivo (Chao et al, 2015;Hinshaw et al, 2015). A highly conserved region on the surface of Scc4 is required for targeting the Scc2-Scc4 complex to centromeric regions of chromosomes (Hinshaw et al, 2015).…”

“…The Scc4 subunit of the cohesin-loading complex contains multiple tetratricopeptide repeats (TPRs) forming a barrel-like superhelix essential for interaction with the N-terminus of Scc2 (Chao et al, 2015;Hinshaw et al, 2015). While Scc2 is required for closing the cohesin ring around DNA, Scc4 is necessary for delivering the complex to DNA in vivo (Chao et al, 2015;Hinshaw et al, 2015).…”

The Arabidopsis homolog of Scc4/MAU2 is essential for embryogenesis

“…The cohesin-loading complex defines both the timing and initial location of cohesin rings on chromatin (Ciosk et al, 2000). Scc2 physically interacts with cohesin and its C-terminus is sufficient to entrap DNA in a cohesin ring in vitro (Chao et al, 2015).…”