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Death domain (DD)-fold assemblies play a crucial role in regulating the signaling to cell survival or death. Here we report the crystal structure of the caspase recruitment domain (CARD)-CARD disk of the human apoptosome. The structure surprisingly reveals that three 1:1 Apaf-1:procaspase-9 CARD protomers form a novel helical DD-fold assembly on the heptameric wheel-like platform of the apoptosome. The small-angle X-ray scattering and multi-angle light scattering data also support that three protomers could form an oligomeric complex similar to the crystal structure. Interestingly, the quasi-equivalent environment of CARDs could generate different quaternary CARD assemblies. We also found that the type II interaction is conserved in all DD-fold complexes, whereas the type I interaction is found only in the helical DD-fold assemblies. This study provides crucial insights into the caspase activation mechanism, which is tightly controlled by a sophisticated and highly evolved CARD assembly on the apoptosome, and also enables better understanding of the intricate DD-fold assembly.

The innate immune response is the first defence system against pathogenic microorganisms, and cytosolic detection of pathogen-derived DNA is believed to be one of the major mechanisms of interferon production. Recently, the mammalian ER membrane protein STING (stimulator of IFN genes; also known as MITA, ERIS, MPYS and TMEM173) has been found to be the master regulator linking the detection of cytosolic DNA to TANK-binding kinase 1 (TBK1) and its downstream transcription factor IFN regulatory factor 3 (IRF3). In addition, STING itself was soon discovered to be a direct sensor of bacterial cyclic dinucleotides such as c-di-GMP or c-di-AMP. However, structural studies of apo STING and its complexes with these cyclic dinucleotides and with other cognate binding proteins are essential in order to fully understand the roles played by STING in these crucial signalling pathways. In this manuscript, the successful crystallization of the C-terminal domain of murine STING (STING-CTD; residues 138-344) is reported. Native and SeMet-labelled crystals were obtained and diffracted to moderate resolutions of 2.39 and 2.2 Å, respectively. Published version

XC6422 is a conserved hypothetical protein from Xanthomonas campestris pathovar campestris (Xcc), a Gram-negative yellow-pigmented pathogenic bacterium that causes black rot, one of the major worldwide diseases of cruciferous crops. The protein consists of 220 amino acids and its structure has been determined to 1.6 A resolution using the multi-wavelength anomalous dispersion (MAD) method. Although it has very low sequence identity to protein sequences in the PDB (less than 20%), the determined structure nevertheless shows that it belongs to the superfamily of serine alpha/beta-hydrolases, with an active site that is fully accessible to solvent owing to the absence of a lid domain. Modelling studies with the serine esterase inhibitor E600 indicate that XC6422 adopts a conserved Ser-His-Asp catalytic triad common to this superfamily and has a preformed oxyanion hole for catalytic activation. These structural features suggest that XC6422 is most likely to be a hydrolase active on a soluble ester or a small lipid. An extra strand preceding the first beta-strand in the canonical alpha/beta-hydrolase fold leads to extensive subunit interactions between XC6422 monomers, which may explain why XC6422 crystals of good diffraction quality can grow to dimensions of up to 1.5 mm in a few days.

Cyclic diguanosine monophosphate (c-di-GMP) is a key signalling molecule involved in regulating many important biological functions in bacteria. The synthesis of c-di-GMP is catalyzed by the GGDEF-domain-containing diguanylate cyclase (DGC), the activity of which is regulated by the binding of product at the allosteric inhibitory (I) site. However, a significant number of GGDEF domains lack the RxxD motif characteristic of the allosteric I site. Here, the structure of XCC4471(GGDEF), the GGDEF domain of a DGC from Xanthomonas campestris, in complex with c-di-GMP has been solved. Unexpectedly, the structure of the complex revealed a GGDEF-domain dimer cross-linked by two molecules of c-di-GMP at the strongly conserved active sites. In the complex (c-di-GMP)(2) adopts a novel partially intercalated form, with the peripheral guanine bases bound to the guanine-binding pockets and the two central bases stacked upon each other. Alteration of the residues involved in specific binding to c-di-GMP led to dramatically reduced K(d) values between XCC4471(GGDEF) and c-di-GMP. In addition, these key residues are strongly conserved among the many thousands of GGDEF-domain sequences identified to date. These results indicate a new product-bound form for GGDEF-domain-containing proteins obtained via (c-di-GMP)(2) binding at the active site. This novel XCC4471(GGDEF)-c-di-GMP complex structure may serve as a general model for the design of lead compounds to block the DGC activity of GGDEF-domain-containing proteins in X. campestris or other microorganisms that contain multiple GGDEF-domain proteins.

Considerable insights into the oxidoreduction activity of the Xanthomonas campestris bacterioferritin comigratory protein (XcBCP) have been obtained from trapped intermediate/ligand complex structures determined by X-ray crystallography. Multiple sequence alignment and enzyme assay indicate that XcBCP belongs to a subfamily of atypical 2-Cys peroxiredoxins (Prxs), containing a strictly conserved peroxidatic cysteine (C(P)48) and an unconserved resolving cysteine (C(R)84). Crystals at different states, i.e. Free_SH state, Intra_SS state, and Inter_SS state, were obtained by screening the XcBCP proteins from a double C48S/C84S mutant, a wild type, and a C48A mutant, respectively. A formate or an alkyl analog with two water molecules that mimic an alkyl peroxide substrate was found close to the active site of the Free_SH or Inter_SS state, respectively. Their global structures were found to contain a novel substrate-binding pocket capable of accommodating an alkyl chain of no less than 16 carbons. In addition, in the Intra_SS or Inter_SS state, substantial local unfolding or complete unfolding of the C(R)-helix was detected, with the C(P)-helix remaining essentially unchanged. This is in contrast to the earlier observation that the C(P)-helix exhibits local unfolding during disulfide bond formation in typical 2-Cys Prxs. These rich experimental data have enabled us to propose a pathway by which XcBCP carries out its oxidoreduction activity through the alternate opening and closing of the substrate entry channel and the disulfide-bond pocket.

Xanthomonas campestris pv. campestris is a Gram-negative yellow-pigmented pathogenic bacterium that causes black rot, one of the major worldwide diseases of cruciferous crops. Its genome contains approximately 4500 genes, roughly one third of which have no known structure and/or function. However, some genes of unknown function are highly conserved among several different bacterial genuses. XC6422 is one such conserved hypothetical protein and has been overexpressed in Escherichia coli, purified and crystallized in a variety of forms using the hanging-drop vapour-diffusion method. Crystals grew to approximately 2 x 1.5 x 0.4 mm in size after one week and diffracted to at least 1.6 A resolution. They belong to the monoclinic space group C2, with one molecule per asymmetric unit and unit-cell parameters a = 75.8, b = 79.3, c = 38.2 A, beta = 109.4 degrees. Determination of this structure may provide insights into the protein's function.