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Cryo-EM structures of the FANCI-FANCD2 complex bound to DNA reveal that monoubiquitination triggers structural changes that enable the complex to function as a sliding DNA clamp and coordinate the repair of DNA interstrand crosslinks.

Assembly of a catalytic centre formed by HPF1 bound to PARP1 or PARP2 is essential for protein ADP-ribosylation after DNA damage in human cells.

The anti-cancer drug target poly(ADP-ribose) polymerase 1 (PARP1) and its close homologue, PARP2, are early responders to DNA damage in human cells(1,2). After binding to genomic lesions, these enzymes use NAD(+) to modify numerous proteins with mono- and poly(ADP-ribose) signals that are important for the subsequent decompaction of chromatin and the recruitment of repair factors(3,4). These post-translational modifications are predominantly serine-linked and require the accessory factor HPF1, which is specific for the DNA damage response and switches the amino acid specificity of PARP1 and PARP2 from aspartate or glutamate to serine residues(5-10). Here we report a co-structure of HPF1 bound to the catalytic domain of PARP2 that, in combination with NMR and biochemical data, reveals a composite active site formed by residues from HPF1 and PARP1 or PARP2 . The assembly of this catalytic centre is essential for the addition of ADP-ribose moieties after DNA damage in human cells. In response to DNA damage and occupancy of the NAD(+)-binding site, the interaction of HPF1 with PARP1 or PARP2 is enhanced by allosteric networks that operate within the PARP proteins, providing an additional level of regulation in the induction of the DNA damage response. As HPF1 forms a joint active site with PARP1 or PARP2, our data implicate HPF1 as an important determinant of the response to clinical PARP inhibitors.

After activation by an agonist, G-protein-coupled receptors (GPCRs) recruit beta-arrestin, which desensitizes heterotrimeric G-protein signalling and promotes receptor endocytosis(1). Additionally, beta-arrestin directly regulates many cell signalling pathways that can induce cellular responses distinct from that of G proteins(2). In contrast to G proteins, for which there are many high-resolution structures in complex with GPCRs, the molecular mechanisms underlying the interaction of beta-arrestin with GPCRs are much less understood. Here we present a cryo-electron microscopy structure of beta-arrestin 1 (beta arr1) in complex with M2 muscarinic receptor (M2R) reconstituted in lipid nanodiscs. The M2R-beta arr1 complex displays a multimodal network of flexible interactions, including binding of the N domain of beta arr1 to phosphorylated receptor residues and insertion of the finger loop of beta arr1 into the M2R seven-transmembrane bundle, which adopts a conformation similar to that in the M2R-heterotrimeric G(o) protein complex(3). Moreover, the cryo-electron microscopy map reveals that the C-edge of beta arr1 engages the lipid bilayer. Through atomistic simulations and biophysical, biochemical and cellular assays, we show that the C-edge is critical for stable complex formation, beta arr1 recruitment, receptor internalization, and desensitization of G-protein activation. Taken together, these data suggest that the cooperative interactions of beta-arrestin with both the receptor and the phospholipid bilayer contribute to its functional versatility.