INO80 and SWR1 complexes: the non-identical twins of chromatin remodelling

The INO80 family of chromatin remodellers are multisubunit complexes that perform a variety of tasks on nucleosomes. Family members are built around a heterohexamer of RuvB-like protein, an ATP-dependent DNA translocase,nuclear actin and actin-related proteins, and a few complex-specific subunits. They modify chromatin in a number of ways including nucleosome sliding and exchange of variant histones. Recent structural information on INO80 and SWR1 complexes has revealed similarities in the basic architecture of the complexes. However, structural and biochemical data on the complexes bound to nucleosomes reveal these similarities to be somewhat superficial and their biochemical activities and mechanisms are very different. Consequently, the INO80 family displays a surprising diversity of function that is based upon a similar structural framework

The type 2 diabetes gene product STARD10 is a phosphoinositide binding protein that controls insulin secretory granule biogenesis

ObjectiveRisk alleles for type 2 diabetes at the STARD10 locus are associated with lowered STARD10 expression in the β-cell, impaired glucose-induced insulin secretion and decreased circulating proinsulin:insulin ratios. Although likely to serve as a mediator of intracellular lipid transfer, the identity of the transported lipids, and thus the pathways through which STARD10 regulates β-cell function, are not understood. The aim of this study was to identify the lipids transported and affected by STARD10 in the β-cell and and the role of the protein in controlling proinsulin processing and insulin granule biogenesis and maturation.MethodsWe used isolated islets from mice deleted selectively in the β-cell for Stard10 (βStarD10KO) and performed electron microscopy, pulse-chase, RNA sequencing and lipidomic analyses. Proteomic analysis of STARD10 binding partners was executed in INS1 (832/13) cell line. X-ray crystallography followed by molecular docking and lipid overlay assay were performed on purified STARD10 protein.ResultsβStard10KO islets had a sharply altered dense core granule appearance, with a dramatic increase in the number of “rod-like” dense cores. Correspondingly, basal secretion of proinsulin was increased versus wild-type islets. Solution of the crystal structure of STARD10 to 2.3 Å resolution revealed a binding pocket capable of accommodating polyphosphoinositides, and STARD10 was shown to bind to inositides phosphorylated at the 3’ position. Lipidomic analysis of βStard10KO islets demonstrated changes in phosphatidyl inositol levels, and the inositol lipid kinase PIP4K2C was identified as a STARD10 binding partner. Also consistent with roles for STARD10 in phosphoinositide signalling, the phosphoinositide binding proteins Pirt and Synaptotagmin 1 were amongst the differentially expressed genes in βStarD10KO islets.ConclusionOur data indicate that STARD10 binds to, and may transport, phosphatidylinositides, influencing membrane lipid composition, insulin granule biosynthesis and insulin processing

The Impact of the C-Terminal Domain on the Interaction of Human DNA Topoisomerase II α and β with DNA

<b>Background</b> Type II DNA topoisomerases are essential, ubiquitous enzymes that act to relieve topological problems arising in DNA from normal cellular activity. Their mechanism of action involves the ATP-dependent transport of one DNA duplex through a transient break in a second DNA duplex; metal ions are essential for strand passage. Humans have two isoforms, topoisomerase IIα and topoisomerase IIβ, that have distinct roles in the cell. The C-terminal domain has been linked to isoform specific differences in activity and DNA interaction. <b>Methodology/Principal Findings</b> We have investigated the role of the C-terminal domain in the binding of human topoisomerase IIα and topoisomerase IIβ to DNA in fluorescence anisotropy assays using full length and C-terminally truncated enzymes. We find that the C-terminal domain of topoisomerase IIβ but not topoisomerase IIα affects the binding of the enzyme to the DNA. The presence of metal ions has no effect on DNA binding. Additionally, we have examined strand passage of the full length and truncated enzymes in the presence of a number of supporting metal ions and find that there is no difference in relative decatenation between isoforms. We find that calcium and manganese, in addition to magnesium, can support strand passage by the human topoisomerase II enzymes. <b>Conclusions/Significance</b> The C-terminal domain of topoisomerase IIβ, but not that of topoisomerase IIα, alters the enzyme's KD for DNA binding. This is consistent with previous data and may be related to the differential modes of action of the two isoforms in vivo. We also show strand passage with different supporting metal ions for human topoisomerase IIα or topoisomerase IIβ, either full length or C-terminally truncated. They all show the same preferences, whereby Mg > Ca > Mn

Break dosage, cell cycle stage and DNA replication influence DNA double strand break response

DNA double strand breaks (DSBs) can be repaired by non-homologous end joining (NHEJ) or homology-directed repair (HR). HR requires nucleolytic degradation of 5′ DNA ends to generate tracts of single-stranded DNA (ssDNA), which are also important for the activation of DNA damage checkpoints. Here we describe a quantitative analysis of DSB processing in the budding yeast Saccharomyces cerevisiae. We show that resection of an HO endonuclease-induced DSB is less extensive than previously estimated and provide evidence for significant instability of the 3′ ssDNA tails. We show that both DSB resection and checkpoint activation are dose-dependent, especially during the G1 phase of the cell cycle. During G1, processing near the break is inhibited by competition with NHEJ, but extensive resection is regulated by an NHEJ-independent mechanism. DSB processing and checkpoint activation are more efficient in G2/M than in G1 phase, but are most efficient at breaks encountered by DNA replication forks during S phase. Our findings identify unexpected complexity of DSB processing and its regulation, and provide a framework for further mechanistic insights

Exploiting bacterial DNA gyrase as a drug target: current state and perspectives

DNA gyrase is a type II topoisomerase that can introduce negative supercoils into DNA at the expense of ATP hydrolysis. It is essential in all bacteria but absent from higher eukaryotes, making it an attractive target for antibacterials. The fluoroquinolones are examples of very successful gyrase-targeted drugs, but the rise in bacterial resistance to these agents means that we not only need to seek new compounds, but also new modes of inhibition of this enzyme. We review known gyrase-specific drugs and toxins and assess the prospects for developing new antibacterials targeted to this enzyme

A Genome-Wide Association Study of Diabetic Kidney Disease in Subjects With Type 2 Diabetes

Identification of sequence variants robustly associated with predisposition to diabetic kidney disease (DKD) has the potential to provide insights into the pathophysiological mechanisms responsible. We conducted a genome-wide association study (GWAS) of DKD in type 2 diabetes (T2D) using eight complementary dichotomous and quantitative DKD phenotypes: the principal dichotomous analysis involved 5,717 T2D subjects, 3,345 with DKD. Promising association signals were evaluated in up to 26,827 subjects with T2D (12,710 with DKD). A combined (T1D+T2D) GWAS was performed using complementary data available for subjects with T1D, which, with replication samples, involved up to 40,340 diabetic subjects (and 18,582 DKD cases). Analysis of specific DKD phenotypes identified a novel signal near GABRR1 (rs9942471, p=4.5×10-8) associated with 'microalbuminuria' in European T2D cases. However, no replication of this signal was observed in Asian subjects with T2D, or in the equivalent T1D analysis. There was only limited support, in this substantially enlarged analysis, for association at previously-reported DKD signals, except for those at UMOD and PRKAG2, both associated with 'EGFR'. We conclude that, despite challenges in addressing phenotypic heterogeneity, access to increased sample sizes will continue to provide more robust inference regarding risk-variant discovery for DKD.</p

Crystal structure of an Hsp90-nucleotide-p23/Sba1 closed chaperone complex

Hsp90 (heat shock protein of 90 kDa) is a ubiquitous molecular chaperone responsible for the assembly and regulation of many eukaryotic signalling systems and is an emerging target for rational chemotherapy of many cancers. Although the structures of isolated domains of Hsp90 have been determined, the arrangement and ATP-dependent dynamics of these in the full Hsp90 dimer have been elusive and contentious. Here we present the crystal structure of full-length yeast Hsp90 in complex with an ATP analogue and the co-chaperone p23/Sba1. The structure reveals the complex architecture of the ‘closed’ state of the Hsp90 chaperone, the extensive interactions between domains and between protein chains, the detailed conformational changes in the amino-terminal domain that accompany ATP binding, and the structural basis for stabilization of the closed state by p23/Sba1. Contrary to expectations, the closed Hsp90 would not enclose its client proteins but provides a bipartite binding surface whose formation and disruption are coupled to the chaperone ATPase cycle

Nucleosome–Chd1 structure and implications for chromatin remodelling

Chromatin-remodelling factors change nucleosome positioning and facilitate DNA transcription, replication, and repair. The conserved remodelling factor chromodomain-helicase-DNA binding protein 1(Chd1) can shift nucleosomes and induce regular nucleosome spacing. Chd1 is required for the passage of RNA polymerase IIthrough nucleosomes and for cellular pluripotency. Chd1 contains the DNA-binding domains SANT and SLIDE, a bilobal motor domain that hydrolyses ATP, and a regulatory double chromodomain. Here we report the cryo-electron microscopy structure of Chd1 from the yeast Saccharomyces cerevisiae bound to a nucleosome at a resolution of 4.8 Å. Chd1 detaches two turns of DNA from the histone octamer and binds between the two DNA gyres in a state poised for catalysis. The SANT and SLIDE domains contact detached DNA around superhelical location (SHL) -7 of the first DNA gyre. The ATPase motor binds the second DNA gyre at SHL +2 and is anchored to the N-terminal tail of histone H4, as seen in a recent nucleosome-Snf2 ATPase structure. Comparisons with published results reveal that the double chromodomain swings towards nucleosomal DNA at SHL +1, resulting in ATPase closure. The ATPase can then promote translocation of DNA towards the nucleosome dyad, thereby loosening the first DNA gyre and remodelling the nucleosome. Translocation may involve ratcheting of the two lobes of the ATPase, which is trapped in a pre- or post-translocation state in the absence or presence, respectively, of transition state-mimicking compounds