Cohesin Releases DNA through Asymmetric ATPase-Driven Ring Opening

Cohesin stably holds together the sister chromatids from S phase until mitosis. To do so, cohesin must be protected against its cellular antagonist Wapl. Eco1 acetylates cohesin's Smc3 subunit, which locks together the sister DNAs. We used yeast genetics to dissect how Wapl drives cohesin from chromatin and identified mutants of cohesin that are impaired in ATPase activity but remarkably confer robust cohesion that bypasses the need for the cohesin protectors Eco1 in yeast and Sororin in human cells. We uncover a functional asymmetry within the heart of cohesin's highly conserved ABC-like ATPase machinery and find that both ATPase sites contribute to DNA loading, whereas DNA release is controlled specifically by one site. We propose that Smc3 acetylation locks cohesin rings around the sister chromatids by counteracting an activity associated with one of cohesin's two ATPase sites. Tight regulation of DNA entrapment and release by the cohesin complex is crucial for its multiple cellular functions. Elbatsh et al. find that cohesin's release from DNA requires an activity associated with one of its ATPase sites, whereas both sites control cohesin's loading onto DNA

Intramolecular coordination systems at platinum and palladium

This thesis describes studies into the synthesis, characterisation and reactivity of organometallic complexes of the nickel triad elements containing intramolecular coordination systems based on N-heterocyclic donor groups. Oxidative addition of the bromoarene 2,6-bis{(3,5-dimethylpyrazol-1 yl)methyl}bromobenzene to [PtMez(SEt)2 ]2 gives the platinum(IV) complex PtBrMe2{2,6 (3,5-Me2pzCH2)2C6H3} in which the NCN ligand coordinates in a facial manner. The reagent reacts differently with [Pt(p-tolyl)z(SEt)2 ]2 to give the platinum(II) complex PtBr{2,6-(3,5 Me2pzCH2)2C6H3} and 4,4'-bitolyl. The method used to obtain the platinum(II) complex represents a new synthetic route to the platinum(II) NCN kernel and is shown to be applicable to other NCN ligand systems. Activation of both C(sp2)-H and C(sp3)-H bonds by palladium(II) acetate is used to generate aryl- and alkylpalladium(II) NCN complexes. Arylpalladium complexes are prepared with pyrazol-1-yl based donor groups and alkylpalladium complexes with pyrazol-1-yl, pyridin-2-yl and N-methylimidazol-2-yl donor groups. A hexametallic complex bearing six palladated NCN units at its periphery is obtained on palladation of the ligand hexakis{3,5 bis[(pyrazol-1-yl)methyl]phenyl} benzene. Oxidative addition of the bromoarene reagents 2,6-bis{ (pyrazol-1 yl)methyl} bromobenzene and 2,6-bis{ (3,5-dimethylpyrazol-1-yl)methyl} bromobenzene to the nickel(O) reagents affords nickel(II) NCN complexes. Cationic platinum, palladium and nickel NCN aqua complexes of the type [M(OH2){2,6 (3,5-Me2pzCH2)2C6H3} ][BF4], are found to catalyse the Michael addition of α-cyano carboxylates to methyl vinyl ketone. The palladium and nickel complexes are found to be more active catalysts than the platinum complex. Platinum(IV) and palladium(IV) complexes containing CN intramolecular coordination systems have been prepared. Ionic complexes of the type [PdR1R2{(pz)2BH2} ]K {(R1, R2) = (Me, Me), (Me, Ph), (Me, p-tolyl), (CH2CMe3, p-tolyl)} react with 8-(bromomethyl)quinoline to give the first isolable palladium(IV) complexes containing an intramolecular coordination system, Pd(CH2C9H6N-C,N)R1Rz{(pz)2BH2}. Structural studies of the complexes Pd(CH2C9H6N-C,N)Me2{(pz)2BH2} and Pd(CH2C9H6N-C,N)MePh{(pz)2BH2} reveal C-N intramolecular coordination of the 8-methylquinolinyl groups and include the first structural analysis of an arylpalladium(IV) complex. The palladium(IV) complexes show high thermal stability in solution but decompose in refluxing acetone by reductive elimination, with low selectivity in C-C coupling. The platinum complex Pt(CH2C9H6N-C,N)Me2{(pz)2BH2} has been prepared and found to be isostructural with Pd(CH2C9H6N-C,N)Me2{(pz)2BH2}, with the M-N bonds significantly longer for the palladium complex but the M-C bonds slightly shorter. Complexes containing bis(pyrazol-1-yl)borate in the absence of an intramolecular coordination system have also been studied, and include the structural characterisation of PtMe3 { (pz)2BH2} (py) and the detection of PdMe3 { (pz)2BH2} (PMePh2), the first example of a neutral palladium(IV) complex containing a phosphorus donor ligand

REV7 counteracts DNA double-strand break resection and affects PARP inhibition

Error-free repair of DNA double-strand breaks (DSBs) is achieved by homologous recombination (HR), and BRCA1 is an important factor for this repair pathway. In the absence of BRCA1-mediated HR, the administration of PARP inhibitors induces synthetic lethality of tumour cells of patients with breast or ovarian cancers. Despite the benefit of this tailored therapy, drug resistance can occur by HR restoration. Genetic reversion of BRCA1-inactivating mutations can be the underlying mechanism of drug resistance, but this does not explain resistance in all cases. In particular, little is known about BRCA1-independent restoration of HR. Here we show that loss of REV7 (also known as MAD2L2) in mouse and human cell lines re-establishes CTIP-dependent end resection of DSBs in BRCA1-deficient cells, leading to HR restoration and PARP inhibitor resistance, which is reversed by ATM kinase inhibition. REV7 is recruited to DSBs in a manner dependent on the H2AX-MDC1-RNF8-RNF168-53BP1 chromatin pathway, and seems to block HR and promote end joining in addition to its regulatory role in DNA damage tolerance. Finally, we establish that REV7 blocks DSB resection to promote non-homologous end-joining during immunoglobulin class switch recombination. Our results reveal an unexpected crucial function of REV7 downstream of 53BP1 in coordinating pathological DSB repair pathway choices in BRCA1-deficient cells