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...

Short-patch repair of DNA single-strand breaks and gaps (SSB) is coordinated by XRCC1, a scaffold protein that recruits the DNA polymerase and DNA ligase required for filling and sealing the damaged strand. XRCC1 can also recruit end-processing enzymes, such as PNK (polynucleotide kinase 3′-phosphatase), Aprataxin and APLF (aprataxin/PNK-like factor), which ensure the availability of a free 3′-hydroxyl on one side of the gap, and a 5′-phosphate group on the other, for the polymerase and ligase reactions respectively. PNK binds to a phosphorylated segment of XRCC1 (between its two C-terminal BRCT domains) via its Forkhead-associated (FHA) domain. We show here, contrary to previous studies, that the FHA domain of PNK binds specifically, and with high affinity to a multiply phosphorylated motif in XRCC1 containing a pSer-pThr dipeptide, and forms a 2:1 PNK:XRCC1 complex. The high-resolution crystal structure of a PNK–FHA–XRCC1 phosphopeptide complex reveals the basis for this unusual bis-phosphopeptide recognition, which is probably a common feature of the known XRCC1-associating end-processing enzymes.

The orexin system, which consists of the two G protein-coupled receptors OX1 and OX2, activated by the neuropeptides OX-A and OX-B, is firmly established as a key regulator of behavioral arousal, sleep, and wakefulness and has been an area of intense research effort over the past two decades. X-ray structures of the receptors in complex with 10 new antagonist ligands from diverse chemotypes are presented, which complement the existing structural information for the system and highlight the critical importance of lipophilic hotspots and water molecules for these peptidergic GPCR targets. Learnings from the structural information regarding the utility of pharmacophore models and how selectivity between OX1 and OX2 can be achieved are discussed.

Exploitation of potential new targets for drug and vaccine development has an absolute requirement for multimilligram quantities of soluble protein. While recombinant expression of full-length proteins is frequently problematic, high-yield soluble expression of functional subconstructs is an effective alternative, so long as appropriate termini can be identified. Bioinformatics localizes domains, but doesn't predict boundaries with sufficient accuracy, so that subconstructs are typically found by trial and error. Combinatorial Domain Hunting (CDH) is a technology for discovering soluble, highly expressed constructs of target proteins. CDH combines unbiased, finely sampled gene-fragment libraries, with a screening protocol that provides ''holistic'' readout of solubility and yield for thousands of protein fragments. CDH is free of the ''passenger solubilization'' and out-of-frame translational start artifacts of fusion-protein systems, and hits are ready for scale-up expression. As a proof of principle, we applied CDH to p85a, successfully identifying soluble and highly expressed constructs encapsulating all the known globular domains, and immediately suitable for downstream applications.