The CRL5-SPSB3 ubiquitin ligase targets nuclear cGAS for degradation

Cyclic GMP-AMP synthase (cGAS) senses aberrant DNA during infection, cancer and inflammatory disease, and initiates potent innate immune responses through the synthesis of 2'3'-cyclic GMP-AMP (cGAMP)1-7. The indiscriminate activity of cGAS towards DNA demands tight regulatory mechanisms that are necessary to maintain cell and tissue homeostasis under normal conditions. Inside the cell nucleus, anchoring to nucleosomes and competition with chromatin architectural proteins jointly prohibit cGAS activation by genomic DNA8-15. However, the fate of nuclear cGAS and its role in cell physiology remains unclear. Here we show that the ubiquitin proteasomal system (UPS) degrades nuclear cGAS in cycling cells. We identify SPSB3 as the cGAS-targeting substrate receptor that associates with the cullin-RING ubiquitin ligase 5 (CRL5) complex to ligate ubiquitin onto nuclear cGAS. A cryo-electron microscopy structure of nucleosome-bound cGAS in a complex with SPSB3 reveals a highly conserved Asn-Asn (NN) minimal degron motif at the C terminus of cGAS that directs SPSB3 recruitment, ubiquitylation and cGAS protein stability. Interference with SPSB3-regulated nuclear cGAS degradation primes cells for type I interferon signalling, conferring heightened protection against infection by DNA viruses. Our research defines protein degradation as a determinant of cGAS regulation in the nucleus and provides structural insights into an element of cGAS that is amenable to therapeutic exploitation.

Structure of a complex between a cap analogue and mRNA guanylyl transferase demonstrates the structural chemistry of RNA capping

Paramecium bursaria Chlorella virus PBCV-1 mRNA guanylyl transferase (capping enzyme) has been complexed with an mRNA cap analogue G[5']ppp[5']G and crystallized. The crystals belong to space group C2221 with unit cell dimensions a = 78.4 A, b = 164.1 A, c = 103.3 A, and diffraction data to 3.1 A has been collected by using synchrotron radiation. The structure has been solved by molecular replacement by using each of the two domains in the previously determined structure of the enzyme in complex with GTP. The conformation is open with respect to the active site cleft, and all contacts between enzyme and ligand are mediated by domain 1. One of the guanine bases is bound in the same pocket that is utilized by GTP. The conformation of the ligand positions the beta phosphate and the active site lysine on opposite sides of the alpha phosphate. This geometry is optimal for nucleophilic substitution reactions and has previously been found for GTP in the closed conformational form of the capping enzyme, where the lysine can be guanylylated upon treatment with excess manganese(II) ions. The remainder of the cap analogue runs along the conserved active site Lys82 Thr83 Asp84 Gly85 Ile86 Arg87 motif, and the second guanine, corresponding to the 5' RNA base, is stacked against the hydrophobic Ile86. The ligand displays approximate 2-fold symmetry with intramolecular hydrogen bonding between the 2' and 3' hydroxyls of the two ribose rings.

Localization of a C-terminal region of lambda2 protein in reovirus cores

The 144-kDa lambda2 protein is a structural component of mammalian reovirus particles and contains the guanylyltransferase activity involved in adding 5' caps to reovirus mRNAs. After incubation of reovirus T3D core particles at 52 degrees C, the lambda2 protein became sensitive to partial protease degradation. Sequential treatments with heat and chymotrypsin caused degradation of a C-terminal portion of lambda2, leaving a 120K core-associated fragment. The four other proteins in cores--lambda1, lambda3, mu2, and sigma2--were not affected by the treatment. Purified cores with cleaved lambda2 were subjected to transmission cryoelectron microscopy and image reconstruction. Reconstruction analysis demonstrated that a distinctive outer region of lambda2 was missing from the modified cores. The degraded region of lambda2 corresponded to the one that contacts the base of the sigma1 protein fiber in reovirus virions and infectious subvirion particles, suggesting that the sigma1-binding region of lambda2 is near its C terminus. Cores with cleaved lambda2 were shown to retain all activities required to transcribe and cap reovirus mRNAs, indicating that the C-terminal region of lambda2 is dispensable for those functions.

Protein-protein interactions directing resolvase site-specific recombination: a structure-function analysis

Recombination catalyzed by the gamma delta resolvase requires assembly of a nucleo-protein complex, the synaptosome, whose structure is determined by resolvase-res and resolvase-resolvase interactions. In crystals of the resolvase catalytic domain, monomers of resolvase were closely associated with one another across three different dyad axes; one of these subunit contacts was shown to be an essential inter-dimer interaction. To investigate the relevance of the remaining two interfaces, we have made site-directed mutations at positions suggested by the structure. Cysteine substitutions were designed to link the interfaces covalently, mutations to arginine were used to disrupt intersubunit contacts, and mutations to tryptophan were used to study the hydrophobicity and solvent accessibility of potential interfaces by fluorescence quenching. Characterization of the mutant proteins has allowed us to identify the dimer interface of resolvase and to assign a structural role to a second intersubunit contact. The data presented here, together with our previous results, suggest that all three of the dyad-related intersubunit interactions observed in the crystal play specific roles in synapsis and recombination.

DNA-protein complexes during attachment-site synapsis in Mu DNA transposition

Initial events in Mu DNA transposition involve specific recognition of Mu DNA ends (att sites) and an internal enhancer site by the Mu transposase (A protein). This interaction between A protein and Mu DNA sequences present on a supercoiled DNA substrate leads to the formation of a stable synaptic complex in which the att ends are nicked, prior to DNA strand transfer. This study examines the properties of a synaptic complex proficient for DNA transposition. We show that the A protein binds as a monomer to its binding sites, and causes the DNA to bend through approximately 90 degrees at each site. All six att binding sites (three at each Mu end) are occupied by A within the synaptic complex. Three of these sites are loosely held and can be emptied of A upon challenge with heparin. A synaptic complex with only three sites occupied is stable and is fully competent in the subsequent strand-transfer step of transposition.