ADP-ribosylation is a reversible post-translational modification with wide-ranging biological functions in all kingdoms of life. A variety of enzymes use NAD(+) to transfer either single or multiple ADP-ribose (ADPr) moieties onto distinct amino acid substrates, often in response to DNA damage or other stresses. Poly-ADPr-glycohydrolase readily reverses poly-ADP-ribosylation induced by the DNA-damage sensor PARP1 and other enzymes, but it does not remove the most proximal ADPr linked to the target amino acid. Searches for enzymes capable of fully reversing cellular mono-ADP-ribosylation back to the unmodified state have proved elusive, which leaves a gap in the understanding of this modification. Here, we identify a family of macrodomain enzymes present in viruses, yeast and animals that reverse cellular ADP-ribosylation by acting on mono-ADP-ribosylated substrates. Our discoveries establish the complete reversibility of PARP-catalyzed cellular ADP-ribosylation as a regulatory modification.

SummaryCondensin protein complexes coordinate the formation of mitotic chromosomes and thereby ensure the successful segregation of replicated genomes. Insights into how condensin complexes bind to chromosomes and alter their topology are essential for understanding the molecular principles behind the large-scale chromatin rearrangements that take place during cell divisions. Here, we identify a direct DNA-binding site in the eukaryotic condensin complex, which is formed by its Ycg1Cnd3 HEAT-repeat and Brn1Cnd2 kleisin subunits. DNA co-crystal structures reveal a conserved, positively charged groove that accommodates the DNA double helix. A peptide loop of the kleisin subunit encircles the bound DNA and, like a safety belt, prevents its dissociation. Firm closure of the kleisin loop around DNA is essential for the association of condensin complexes with chromosomes and their DNA-stimulated ATPase activity. Our data suggest a sophisticated molecular basis for anchoring condensin complexes to chromosomes that enables the formation of large-sized chromatin loops.

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Poly(ADP-ribose) polymerase I (PARP1) is a primary DNA damage sensor whose (ADP-ribose) polymerase activity is acutely regulated by interaction with DNA breaks. Upon activation at sites of DNA damage, PARP1 modifies itself and other proteins by covalent addition of long branched polymers of ADP-ribose, which in turn recruit downstream DNA repair and chromatin remodelling factors. PARP1 recognizes DNA damage through its N-terminal DNA-binding domain (DBD), which consists of a tandem repeat of an unusual zinc-finger (ZnF) domain. We have now determined the crystal structure of the human PARP1-DBD bound to a DNA break. Along with functional analysis of PARP1 recruitment to sites of DNA damage in vivo, the structure reveals a dimeric assembly whereby ZnF1 and ZnF2 domains from separate PARP1 molecules form a strand-break recognition module that helps activate PARP1 by facilitating its dimerization and consequent trans-automodification.Short-patch repair of DNA single-strand breaks is initiated by poly(ADP-ribose) polymerase-1 (PARP1) -a multi-domain enzyme activated by binding of its N-terminal DNA-binding domain (DBD) to DNA breaks [1][2][3][4][5] . Activated PARP1 utilises NAD + to Correspondence to: Andreas G. Ladurner; Laurence H. Pearl; Antony W. Oliver. AUTHOR CONTRIBUTIONS A.A.E.A. purified the protein, crystallized the complex and collected the X-ray diffraction data; G.T. designed and constructed the PARP1-EGFP constructs and performed the laser DNA damage experiments; M.K. performed the FRAP experiments; P.O.H. engineered the knockdown PARP1 cell line and the wild-type imaging reporter constructs, and performed the in vitro complementation assays; M.H. and R.A.-B. engineered and purified mutant PARP1 constructs; A.G.L. designed the study and analysed the data; L.H.P designed the study, analysed the data and wrote the paper; A.W.O. made the baculovirus constructs, designed the purification protocol, and solved and refined the crystal structure. All authors discussed the results and commented on the manuscript. We have now determined the crystal structure of the DNA-binding domain of PARP1 (PARP1-DBD) encompassing the first two zinc-finger (ZnF) domains, bound to a DNA break. In contrast with structural analysis of the separated domains 22 , we show that DNA binding by both zinc-finger domains is essential to damage recruitment in vivo, and that ZnF1 and ZnF2 domains from separate PARP1 molecules act as a functional unit to generate a dimeric binding module that specifically recognizes the single-strand / double-strand transition at a recessed DNA break. Mutational analysis in vitro and in cells demonstrates the functional requirement for zinc-finger dimerisation and reveals a mechanism for bringing two PARP1 molecules into close proximity at a DNA break as a prerequisite for transmodification. RESULTS Structure of the PARP1-DBD -DNA ComplexAn N-terminal segment of human PARP1 (residues 5-202) was expressed in insect cells and purified by column chromatography. Screening with a range of DNA molecules...