The Cdc45/Mcm2-7/GINS (CMG) helicase separates DNA strands during replication in eukaryotes. How the CMG is assembled and engages DNA substrates remains unclear. Using electron microscopy, we have determined the structure of the CMG in the presence of ATPγS and a DNA duplex bearing a 3′ single-stranded tail. The structure shows that the MCM subunits of the CMG bind preferentially to single-stranded DNA, establishes the polarity by which DNA enters into the Mcm2-7 pore, and explains how Cdc45 helps prevent DNA from dissociating from the helicase. The Mcm2-7 subcomplex forms a cracked-ring, right-handed spiral when DNA and nucleotide are bound, revealing unexpected congruencies between the CMG and both bacterial DnaB helicases and the AAA+ motor of the eukaryotic proteasome. The existence of a subpopulation of dimeric CMGs establishes the subunit register of Mcm2-7 double hexamers and together with the spiral form highlights how Mcm2-7 transitions through different conformational and assembly states as it matures into a functional helicase.DOI: http://dx.doi.org/10.7554/eLife.03273.001

Monoubiquitination and deubiquitination of FANCD2:FANCI heterodimer is central to DNA repair in a pathway that is defective in the cancer predisposition syndrome Fanconi anemia (FA). The "FA core complex" contains the RING-E3 ligase FANCL and seven other essential proteins that are mutated in various FA subtypes. Here, we purified recombinant FA core complex to reveal the function of these other proteins. The complex contains two spatially separate FANCL molecules that are dimerized by FANCB and FAAP100. FANCC and FANCE act as substrate receptors and restrict monoubiquitination to the FANCD2:FANCI heterodimer in only a DNA-bound form. FANCA and FANCG are dispensable for maximal in vitro ubiquitination. Finally, we show that the reversal of this reaction by the USP1:UAF1 deubiquitinase only occurs when DNA is disengaged. Our work reveals the mechanistic basis for temporal and spatial control of FANCD2:FANCI monoubiquitination that is critical for chemotherapy responses and prevention of Fanconi anemia.

Retroviral integrase (IN) functions within the intasome nucleoprotein complex to catalyze insertion of viral DNA into cellular chromatin. Using cryo-electron microscopy, we now visualize the functional maedi-visna lentivirus intasome at 4.9 Å resolution. The intasome comprises a homo-hexadecamer of IN with a tetramer-of-tetramers architecture featuring eight structurally distinct types of IN protomers supporting two catalytically competent subunits. The conserved intasomal core, previously observed in simpler retroviral systems, is formed between two IN tetramers, with a pair of C-terminal domains from flanking tetramers completing the synaptic interface. Our results explain how HIV-1 IN, which self-associates into higher order multimers, can form a functional intasome, reconcile the bulk of early HIV-1 IN biochemical and structural data, and provide a lentiviral platform for design of HIV-1 IN inhibitors.

Systemic light chain amyloidosis (AL)  is a life-threatening disease caused by aggregation and deposition of monoclonal immunoglobulin light chains (LC) in target organs. Severity of heart involvement is the most important factor determining prognosis. Here, we report the 4.0 Å resolution cryo-electron microscopy map and molecular model of amyloid fibrils extracted from the heart of an AL amyloidosis patient with severe amyloid cardiomyopathy. The helical fibrils are composed of a single protofilament, showing typical 4.9 Å stacking and cross-β architecture. Two distinct polypeptide stretches (total of 77 residues) from the LC variable domain (Vl) fit the fibril density. Despite Vl high sequence variability, residues stabilizing the fibril core are conserved through different cardiotoxic Vl, highlighting structural motifs that may be common to misfolding-prone LCs. Our data shed light on the architecture of LC amyloids, correlate amino acid sequences with fibril assembly, providing the grounds for development of innovative medicines.

Systemic light chain (AL) amyloidosis is a life-threatening disease caused by aggregation and deposition of monoclonal immunoglobulin light chains (LC) in target organs. Severity of heart involvement is the most important factor determining prognosis. Here, we report the 4.0 Å resolution cryo-electron microscopy (cryo-EM) map and structural model of amyloid fibrils extracted from the heart of an AL patient affected by severe amyloid cardiomyopathy. The fibrils are composed of one asymmetric protofilament, showing typical 4.9 Å stacking and parallel cross-β architecture. Two distinct polypeptide stretches belonging to the LC variable domain (Vl) could be modelled in the density (total of 77 residues), stressing the role of the Vl domain in fibril assembly and LC aggregation. Despite high levels of Vl sequence variability, residues stabilising the observed fibril core are conserved through several Vl domains, highlighting structural motifs that may be common to misfolded LCs. Our data shed first light on the architecture of life-threatening LC amyloid deposits, and provide a rationale for correlating LC amino acid sequences and fibril structures.

RecQ helicases are a widely conserved family of ATP-dependent motors with diverse roles in nearly every aspect of bacterial and eukaryotic genome maintenance. However, the physical mechanisms by which RecQ helicases recognize and process specific DNA replication and repair intermediates are largely unknown. Here, we solved crystal structures of the human RECQ1 helicase in complexes with tailed-duplex DNA and ssDNA. The structures map the interactions of the ssDNA tail and the branch point along the helicase and Zn-binding domains, which, together with reported structures of other helicases, define the catalytic stages of helicase action. We also identify a strand-separating pin, which (uniquely in RECQ1) is buttressed by the protein dimer interface. A duplex DNA-binding surface on the C-terminal domain is shown to play a role in DNA unwinding, strand annealing, and Holliday junction (HJ) branch migration. We have combined EM and analytical ultracentrifugation approaches to show that RECQ1 can form what appears to be a flat, homotetrameric complex and propose that RECQ1 tetramers are involved in HJ recognition. This tetrameric arrangement suggests a platform for coordinated activity at the advancing and receding duplexes of an HJ during branch migration.DNA helicases | RecQ | genome stability | Holliday junction | fork reversal R ecQ helicases are a family of ATP-dependent motor proteins that play central roles in maintaining genome stability. Defects in three of the five human RecQ homologs give rise to distinct genetic disorders associated with genomic instability, cancer predisposition, and premature aging (1-5). The unique clinical features of these disorders support the notion that the different RecQ helicases have nonoverlapping functions, but the molecular basis for their different enzymatic activities remains unclear. RecQ helicases catalyze ATP-dependent DNA unwinding in the 3′-5′ direction. Additionally, members of this helicase family have been shown to tackle an unparalleled breath of noncanonical DNA structures, such as fork DNA, G-quadruplexes, D-loops, and Holliday junction (HJ) structures (6-8). However, our understanding of the physical mechanisms by which RecQ helicases recognize and process their physiological substrates remains remarkably limited.RECQ1 is the shortest of the human RecQ-family helicases, comprising the bipartite ATPase domain common to all superfamily 2 (SF2) helicases, the RecQ-specific C-terminal domain (RQC), and short extensions on the N and C termini. We recently discovered a specific function of RECQ1 in branch migration and restart of reversed DNA replication forks upon DNA topoisomerase I inhibition that is not shared by other human RecQ helicases, such as Werner (WRN) or Bloom (BLM) syndrome proteins (9). On the other hand, BLM is the sole human RecQ helicase member specifically able to resolve double-HJ junction structures in conjunction with DNA topoisomerase III alpha and the RMI1 and RMI2 accessory proteins (10-12). These findings lead us to hypothesize that the sp...

Highlights d HCN4 structure is shown in ligand-free and ligandbound state d Pore domain is shown in closed and in open configuration d Permeability and selectivity mechanisms of HCN channels are uncovered d A metal ion coordination site functionally couples cytoplasmic and transmembrane domains

SummaryActivation of the main DNA interstrand crosslink repair pathway in higher eukaryotes requires mono-ubiquitination of FANCI and FANCD2 by FANCL, the E3 ligase subunit of the Fanconi anemia core complex. FANCI and FANCD2 form a stable complex; however, the molecular basis of their ubiquitination is ill defined. FANCD2 mono-ubiquitination by FANCL is stimulated by the presence of the FANCB and FAAP100 core complex components, through an unknown mechanism. How FANCI mono-ubiquitination is achieved remains unclear. Here, we use structural electron microscopy, combined with crosslink-coupled mass spectrometry, to find that FANCB, FANCL, and FAAP100 form a dimer of trimers, containing two FANCL molecules that are ideally poised to target both FANCI and FANCD2 for mono-ubiquitination. The FANCC-FANCE-FANCF subunits bridge between FANCB-FANCL-FAAP100 and the FANCI-FANCD2 substrate. A transient interaction with FANCC-FANCE-FANCF alters the FANCI-FANCD2 configuration, stabilizing the dimerization interface. Our data provide a model to explain how equivalent mono-ubiquitination of FANCI and FANCD2 occurs.