Cyclic GMP-AMP synthase (cGAS) is a cytosolic DNA sensor mediating innate antimicrobial immunity. It catalyzes the synthesis of a noncanonical cyclic dinucleotide 2′,5′ cGAMP that binds to STING and mediates the activation of TBK1 and IRF-3. Activated IRF-3 translocates to the nucleus and initiates the transcription of the IFN-β gene. The structure of mouse cGAS bound to an 18 bp dsDNA revealed that cGAS interacts with dsDNA through two binding sites, forming a 2:2 complex. Enzyme assays and IFN-β reporter assays of cGAS mutants demonstrated that interactions at both DNA binding sites are essential for cGAS activation. Mutagenesis and DNA binding studies showed that the two sites bind dsDNA cooperatively and site B plays a critical role in DNA binding. The structure of mouse cGAS bound to dsDNA and 2′,5′ cGAMP provided insight into the catalytic mechanism of cGAS. These results demonstrated that cGAS is activated by dsDNA-induced oligomerization.
SUMMARY Tank-binding kinase 1 (TBK1) is a serine/threonine protein kinase mediating innate antimicrobial immunity. TBK1 is involved in the signaling of TLRs, RLRs, and STING-mediated sensing of cytosolic DNA. Stimulation of these receptors results in the activation of TBK1, which phosphorylates interferon regulatory factor IRF-3. Phosphorylated IRF-3 translocates into the nucleus to initiate the transcription of the IFN-β gene. Here we show that TBK1 is activated by autophosphorylation at residue Ser172. Structures of TBK1 bound to two inhibitors showed that TBK1 has the IκB kinase fold with three distinct domains: the kinase domain, the ubiquitin like domain, and the scaffold and dimerization domain. However, the overall structures of TBK1 monomer and its dimer are different from IKKβ in the arrangements of the three domains and in dimer formation. Phosphorylation of IRF-3 by TBK1 in vitro results in its oligomerization, and phosphorylation of residue Ser386 plays a key role in IRF-3 activation.
Analytical ultracentrifugation (AUC) is a first principles based method to determine absolute sedimentation coefficients and buoyant molar masses of macromolecules and their complexes, reporting on their size and shape in free solution. The purpose of this multi-laboratory study was to establish the precision and accuracy of basic data dimensions in AUC and validate previously proposed calibration techniques. Three kits of AUC cell assemblies containing radial and temperature calibration tools and a bovine serum albumin (BSA) reference sample were shared among 67 laboratories, generating 129 comprehensive data sets. These allowed for an assessment of many parameters of instrument performance, including accuracy of the reported scan time after the start of centrifugation, the accuracy of the temperature calibration, and the accuracy of the radial magnification. The range of sedimentation coefficients obtained for BSA monomer in different instruments and using different optical systems was from 3.655 S to 4.949 S, with a mean and standard deviation of (4.304 ± 0.188) S (4.4%). After the combined application of correction factors derived from the external calibration references for elapsed time, scan velocity, temperature, and radial magnification, the range of s-values was reduced 7-fold with a mean of 4.325 S and a 6-fold reduced standard deviation of ± 0.030 S (0.7%). In addition, the large data set provided an opportunity to determine the instrument-to-instrument variation of the absolute radial positions reported in the scan files, the precision of photometric or refractometric signal magnitudes, and the precision of the calculated apparent molar mass of BSA monomer and the fraction of BSA dimers. These results highlight the necessity and effectiveness of independent calibration of basic AUC data dimensions for reliable quantitative studies.
In ovoid-shaped Gram-positive bacteria, MapZ guides FtsZ-ring positioning at cell equators. The cell wall of the ovococcus Streptococcus mutans contains peptidoglycan decorated with Serotype c Carbohydrates (SCCs). In this study, we identify the major cell separation autolysin AtlA as an SCC binding protein. AtlA binding to SCC is attenuated by the glycerol phosphate (GroP) modification.Using fluorescently-labeled AtlA constructs, we mapped SCC distribution on the streptococcal surface revealing enrichment of GroP-deficient immature SCCs at the cell poles and equators. The immature SCCs co-localize with MapZ at the equatorial rings throughout the cell cycle. In GroP-deficient mutants, AtlA is mislocalized resulting in dysregulated cellular autolysis. These mutants display morphological abnormalities associated with MapZ mislocalization leading to FtsZ-ring misplacement.Altogether, our data support a model in which maturation of a cell wall polysaccharide provides the molecular cues for the recruitment of cell division machinery, ensuring proper daughter cell separation and FtsZ-ring positioning.
The human pathogen Mycobacterium tuberculosis encodes a proteasome that carries out regulated degradation of bacterial proteins. It has been proposed that the proteasome contributes to nitrogen metabolism in M. tuberculosis, although this hypothesis had not been tested. Upon assessing M. tuberculosis growth in several nitrogen sources, we found that a mutant strain lacking the Mycobacterium proteasomal activator Mpa was unable to use nitrate as a sole nitrogen source due to a specific failure in the pathway of nitrate reduction to ammonium. We found that the robust activity of the nitrite reductase complex NirBD depended on expression of the groEL/groES chaperonin genes, which are regulated by the repressor HrcA. We identified HrcA as a likely proteasome substrate, and propose that the degradation of HrcA is required for the full expression of chaperonin genes. Furthermore, our data suggest that degradation of HrcA, along with numerous other proteasome substrates, is enhanced during growth in nitrate to facilitate the derepression of the chaperonin genes. Importantly, growth in nitrate is an example of a specific condition that reduces the steady-state levels of numerous proteasome substrates in M. tuberculosis.
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