The SMC Loader Scc2 Promotes ncRNA Biogenesis and Translational Fidelity

Mediator recruits the cohesin loader Scc2 to RNA Pol II-transcribed genes and promotes sister chromatid cohesion

The ring-like cohesin complex plays an essential role in chromosome segregation, organization, and double-strand break repair through its ability to bring two DNA double helices together. Scc2 (NIPBL in humans) together with Scc4 functions as the loader of cohesin onto chromosomes. Chromatin adapters such as the RSC complex facilitate the localization of the Scc2-Scc4 cohesin loader. Here, we identify a broad range of Scc2-chromatin protein interactions that are evolutionarily conserved and reveal a role for one complex, Mediator, in the recruitment of the cohesin loader. We identified budding yeast Med14, a subunit of the Mediator complex, as a high copy suppressor of poor growth in Scc2 mutant strains. Physical and genetic interactions between Scc2 and Mediator are functionally substantiated in direct recruitment and cohesion assays. Depletion of Med14 results in defective sister chromatid cohesion and the decreased binding of Scc2 at RNA Pol II-transcribed genes. Previous work has suggested that Mediator, Nipbl, and cohesin connect enhancers and promoters of active mammalian genes. Our studies suggest an evolutionarily conserved fundamental role for Mediator in the direct recruitment of Scc2 to RNA Pol II-transcribed genes.

Chl1 DNA helicase and Scc2 function in chromosome condensation through cohesin deposition

Chl1 DNA helicase promotes sister chromatid cohesion and associates with both the cohesion establishment acetyltransferase Eco1/Ctf7 and the DNA polymerase processivity factor PCNA that supports Eco1/Ctf7 function. Mutation in CHL1 results in precocious sister chromatid separation and cell aneuploidy, defects that arise through reduced levels of chromatin-bound cohesins which normally tether together sister chromatids (trans tethering). Mutation of Chl1 family members (BACH1/BRIP/FANCJ and DDX11/ChlR1) also exhibit genotoxic sensitivities, consistent with a role for Chl1 in trans tethering which is required for efficient DNA repair. Chl1 promotes the recruitment of Scc2 to DNA which is required for cohesin deposition onto DNA. There is limited evidence, however, that Scc2 also directs the deposition onto DNA of condensins which promote tethering in cis (intramolecular DNA links). Here, we test the ability of Chl1 to promote cis tethering and the role of both Chl1 and Scc2 to promote condensin recruitment to DNA. The results reveal that chl1 mutant cells exhibit significant condensation defects both within the rDNA locus and genome-wide. Importantly, chl1 mutant cell condensation defects do not result from reduced chromatin binding of condensin, but instead through reduced chromatin binding of cohesin. We tested scc2-4 mutant cells and similarly found no evidence of reduced condensin recruitment to chromatin. Consistent with a role for Scc2 specifically in cohesin deposition, scc2-4 mutant cell condensation defects are irreversible. We thus term Chl1 a novel regulator of both chromatin condensation and sister chromatid cohesion through cohesin-based mechanisms. These results reveal an exciting interface between DNA structure and the highly conserved cohesin complex.

E3 ubiquitin ligase Bre1 couples sister chromatid cohesion establishment to DNA replication in Saccharomyces cerevisiae

Bre1, a conserved E3 ubiquitin ligase in Saccharomyces cerevisiae, together with its interacting partner Lge1, are responsible for histone H2B monoubiquitination, which regulates transcription, DNA replication, and DNA damage response and repair, ensuring the structural integrity of the genome. Deletion of BRE1 or LGE1 also results in whole chromosome instability. We discovered a novel role for Bre1, Lge1 and H2Bub1 in chromosome segregation and sister chromatid cohesion. Bre1’s function in G1 and S phases contributes to cohesion establishment, but it is not required for cohesion maintenance in G2 phase. Bre1 is dispensable for the loading of cohesin complex to chromatin in G1, but regulates the localization of replication factor Mcm10 and cohesion establishment factors Ctf4, Ctf18 and Eco1 to early replication origins in G1 and S phases, and promotes cohesin subunit Smc3 acetylation for cohesion stabilization. H2Bub1 epigenetically marks the origins, potentially signaling the coupling of DNA replication and cohesion establishment.

Most of the DNA in a cell is stored in structures called chromosomes. During every cell cycle, each cell needs to replicate its chromosomes, hold the two chromosome copies (also known as “sister chromatids”) together before cell division, and distribute them equally to the two new cells. Each step must be executed accurately otherwise the new cells will have extra or missing chromosomes – a condition that is seen in many cancer cells and that can cause embryos to die. Since these processes are so essential to life, they are highly similar in a range of species, from single-celled organisms such as yeast to multicellular organisms like humans. However, it was not clear when and how sister chromatids first join together, or how this process is linked to DNA replication.

The DNA in the sister chromatids is wrapped around proteins called histones to form a structure known as chromatin. An enzyme called Bre1 plays roles in gene transcription and DNA replication and repair by adding ubiquitin molecules to a histone called H2B. Now, by using genetic, molecular and cell biological approaches to study baker and brewer yeast cells, Zhang et al. show that the activity of Bre1 helps to hold sister chromatids together. Specifically, Bre1 recruits proteins to the chromatin before and during DNA replication, which help to initiate replication and to establish cohesion between the sister chromatids. The ubiquitin molecule attached to H2B by Bre1 is also essential for establishing cohesion, acting as a mark that helps to link the two processes.

In the future it will be worthwhile to investigate whether genetic mutations that prevent sister chromatids adhering to each other is a major cause of the chromosome abnormalities seen in cancer cells. This knowledge may be useful for diagnosing cancers. Drugs that prevent the activity of Bre1 and other proteins involved in holding together sister chromatids could also be developed as potential cancer treatments that kill cancer cells by causing instability in their number of chromosomes.

E3 ubiquitin ligase Bre1 couples sister chromatid cohesion establishment to DNA replication in Saccharomyces cerevisiae

Bre1, a conserved E3 ubiquitin ligase in Saccharomyces cerevisiae, together with its interacting partner Lge1, are responsible for histone H2B monoubiquitination, which regulates transcription, DNA replication, and DNA damage response and repair, ensuring the structural integrity of the genome. Deletion of BRE1 or LGE1 also results in whole chromosome instability. We discovered a novel role for Bre1, Lge1 and H2Bub1 in chromosome segregation and sister chromatid cohesion. Bre1's function in G1 and S phases contributes to cohesion establishment, but it is not required for cohesion maintenance in G2 phase. Bre1 is dispensable for the loading of cohesin complex to chromatin in G1, but regulates the localization of replication factor Mcm10 and cohesion establishment factors Ctf4, Ctf18 and Eco1 to early replication origins in G1 and S phases, and promotes cohesin subunit Smc3 acetylation for cohesion stabilization. H2Bub1 epigenetically marks the origins, potentially signaling the coupling of DNA replication and cohesion establishment.

A Ribosomopathy Reveals Decoding Defective Ribosomes Driving Human Dysmorphism

Ribosomal protein (RP) gene mutations, mostly associated with inherited or acquired bone marrow failure, are believed to drive disease by slowing the rate of protein synthesis. Here de novo missense mutations in the RPS23 gene, which codes for uS12, are reported in two unrelated individuals with microcephaly, hearing loss, and overlapping dysmorphic features. One individual additionally presents with intellectual disability and autism spectrum disorder. The amino acid substitutions lie in two highly conserved loop regions of uS12 with known roles in maintaining the accuracy of mRNA codon translation. Primary cells revealed one substitution severely impaired OGFOD1-dependent hydroxylation of a neighboring proline residue resulting in 40S ribosomal subunits that were blocked from polysome formation. The other disrupted a predicted pi-pi stacking interaction between two phenylalanine residues leading to a destabilized uS12 that was poorly tolerated in 40S subunit biogenesis. Despite no evidence of a reduction in the rate of mRNA translation, these uS12 variants impaired the accuracy of mRNA translation and rendered cells highly sensitive to oxidative stress. These discoveries describe a ribosomopathy linked to uS12 and reveal mechanistic distinctions between RP gene mutations driving hematopoietic disease and those resulting in developmental disorders.

Structure of the cohesin loader Scc2

The functions of cohesin are central to genome integrity, chromosome organization and transcription regulation through its prevention of premature sister-chromatid separation and the formation of DNA loops. The loading of cohesin onto chromatin depends on the Scc2-Scc4 complex; however, little is known about how it stimulates the cohesion-loading activity. Here we determine the large 'hook' structure of Scc2 responsible for catalysing cohesin loading. We identify key Scc2 surfaces that are crucial for cohesin loading in vivo. With the aid of previously determined structures and homology modelling, we derive a pseudo-atomic structure of the full-length Scc2-Scc4 complex. Finally, using recombinantly purified Scc2-Scc4 and cohesin, we performed crosslinking mass spectrometry and interaction assays that suggest Scc2-Scc4 uses its modular structure to make multiple contacts with cohesin.

NIPBL Controls RNA Biogenesis to Prevent Activation of the Stress Kinase PKR

NIPBL, a cohesin loader, has been implicated in transcriptional control and genome organization. Mutations in NIPBL, cohesin, and its deacetylase HDAC8 result in Cornelia de Lange syndrome. We report activation of the RNA-sensing kinase PKR in human lymphoblastoid cell lines carrying NIPBL or HDAC8 mutations, but not SMC1A or SMC3 mutations. PKR activation can be triggered by unmodified RNAs. Gene expression profiles in NIPBL-deficient lymphoblastoid cells and mouse embryonic stem cells reveal lower expression of genes involved in RNA processing and modification. NIPBL mutant lymphoblastoid cells show reduced proliferation and protein synthesis with increased apoptosis, all of which are partially reversed by a PKR inhibitor. Non-coding RNAs from an NIPBL mutant line had less m⁶A modification and activated PKR activity in vitro. This study provides insight into the molecular pathology of Cornelia de Lange syndrome by establishing a relationship between NIPBL and HDAC8 mutations and PKR activation.

Phosphorylation of the Scc2 cohesin deposition complex subunit regulates chromosome condensation through cohesin integrity

The cohesion of replicated sister chromatids promotes chromosome biorientation, gene regulation, DNA repair, and chromosome condensation. Cohesion is mediated by cohesin, which is deposited on chromosomes by a separate conserved loading complex composed of Scc2 and Scc4 in Saccharomyces cerevisiae. Although it is known to be required, the role of Scc2/Scc4 in cohesin deposition remains enigmatic. Scc2 is a phosphoprotein, although the functions of phosphorylation in deposition are unknown. We identified 11 phosphorylated residues in Scc2 by mass spectrometry. Mutants of SCC2 with substitutions that mimic constitutive phosphorylation retain normal Scc2-Scc4 interactions and chromatin association but exhibit decreased viability, sensitivity to genotoxic agents, and decreased stability of the Mcd1 cohesin subunit in mitotic cells. Cohesin association on chromosome arms, but not pericentromeric regions, is reduced in the phosphomimetic mutants but remains above a key threshold, as cohesion is only modestly perturbed. However, these scc2 phosphomimetic mutants exhibit dramatic chromosome condensation defects that are likely responsible for their high inviability. From these data, we conclude that normal Scc2 function requires modulation of its phosphorylation state and suggest that scc2 phosphomimetic mutants cause an increased incidence of abortive cohesin deposition events that result in compromised cohesin complex integrity and Mcd1 turnover.