Daniel J. Ericsson

Structural Basis for Assembly and Function of a Heterodimeric Plant Immune Receptor

Universal Immune Function Certain pathogen effectors are detected in plants by cytoplasmic receptors. First solving the crystal structures of Arabidopsis receptors, Williams et al. (p. 299; see the Perspective by Nishimura and Dangl) discovered that in the resting state, the structures form a heterodimer that readies the complex for effector binding and keeps the signaling domains from firing too early. Once the pathogen effector binds, the structure of the complex shifts such that the signaling domains can form a homodimer to initiate downstream signaling. Similarities between these plant-pathogen receptors and Toll-like receptors in animals suggest the molecular mechanisms may translate broadly. A heterodimer stands at the ready; a homodimer responds with action. [Also see Perspective by Nishimura and Dangl] Cytoplasmic plant immune receptors recognize specific pathogen effector proteins and initiate effector-triggered immunity. In Arabidopsis, the immune receptors RPS4 and RRS1 are both required to activate defense to three different pathogens. We show that RPS4 and RRS1 physically associate. Crystal structures of the N-terminal Toll–interleukin-1 receptor/resistance (TIR) domains of RPS4 and RRS1, individually and as a heterodimeric complex (respectively at 2.05, 1.75, and 2.65 angstrom resolution), reveal a conserved TIR/TIR interaction interface. We show that TIR domain heterodimerization is required to form a functional RRS1/RPS4 effector recognition complex. The RPS4 TIR domain activates effector-independent defense, which is inhibited by the RRS1 TIR domain through the heterodimerization interface. Thus, RPS4 and RRS1 function as a receptor complex in which the two components play distinct roles in recognition and signaling.

Structure-Informed Design of an Enzymatically Inactive Vaccine Component for Group A Streptococcus

ABSTRACT Streptococcus pyogenes (group A Streptococcus [GAS]) causes ~700 million human infections/year, resulting in >500,000 deaths. There is no commercial GAS vaccine available. The GAS surface protein arginine deiminase (ADI) protects mice against a lethal challenge. ADI is an enzyme that converts arginine to citrulline and ammonia. Administration of a GAS vaccine preparation containing wild-type ADI, a protein with inherent enzymatic activity, may present a safety risk. In an approach intended to maximize the vaccine safety of GAS ADI, X-ray crystallography and structural immunogenic epitope mapping were used to inform vaccine design. This study aimed to knock out ADI enzyme activity without disrupting the three-dimensional structure or the recognition of immunogenic epitopes. We determined the crystal structure of ADI at 2.5 Å resolution and used it to select a number of amino acid residues for mutagenesis to alanine (D166, E220, H275, D277, and C401). Each mutant protein displayed abrogated activity, and three of the mutant proteins (those with the D166A, H275A, and D277A mutations) possessed a secondary structure and oligomerization state equivalent to those of the wild type, produced high-titer antisera, and avoided disruption of B-cell epitopes of ADI. In addition, antisera raised against the D166A and D277A mutant proteins bound to the GAS cell surface. The inactivated D166A and D277A mutant ADIs are ideal for inclusion in a GAS vaccine preparation. There is no human ortholog of ADI, and we confirm that despite limited structural similarity in the active-site region to human peptidyl ADI 4 (PAD4), ADI does not functionally mimic PAD4 and antiserum raised against GAS ADI does not recognize human PAD4. IMPORTANCE We present an example of structural biology informing human vaccine design. We previously showed that the administration of the enzyme arginine deiminase (ADI) to mice protected the mice against infection with multiple GAS serotypes. In this study, we determined the structure of GAS ADI and used this information to improve the vaccine safety of GAS ADI. Catalytically inactive mutant forms of ADI retained structure, recognition by antisera, and immunogenic epitopes, rendering them ideal for inclusion in GAS vaccine preparations. This example of structural biology informing vaccine design may underpin the formulation of a safe and efficacious GAS vaccine. We present an example of structural biology informing human vaccine design. We previously showed that the administration of the enzyme arginine deiminase (ADI) to mice protected the mice against infection with multiple GAS serotypes. In this study, we determined the structure of GAS ADI and used this information to improve the vaccine safety of GAS ADI. Catalytically inactive mutant forms of ADI retained structure, recognition by antisera, and immunogenic epitopes, rendering them ideal for inclusion in GAS vaccine preparations. This example of structural biology informing vaccine design may underpin the formulation of a safe and efficacious GAS vaccine.

Competitive Inhibitors of Mycobacterium tuberculosis Ribose-5-phosphate Isomerase B Reveal New Information about the Reaction Mechanism

Ribose-5-phosphate isomerase (Rpi), an important enzyme in the pentose phosphate pathway, catalyzes the interconversion of ribulose 5-phosphate and ribose 5-phosphate. Two unrelated isomerases have been identified, RpiA and RpiB, with different structures and active site residues. The reaction catalyzed by both enzymes is thought to proceed via a high energy enediolate intermediate, by analogy to other carbohydrate isomerases. Here we present studies of RpiB from Mycobacterium tuberculosis together with small molecules designed to resemble the enediolate intermediate. The relative affinities of these inhibitors for RpiB have a different pattern than that observed previously for the RpiA from spinach. X-ray structures of RpiB in complex with the inhibitors 4-phospho-d-erythronohydroxamic acid (Km 57 μm) and 4-phospho-d-erythronate (Ki 1.7 mm) refined to resolutions of 2.1 and 2.2 Å, respectively, allowed us to assign roles for most active site residues. These results, combined with docking of the substrates in the position of the most effective inhibitor, now allow us to outline the reaction mechanism for RpiBs. Both enzymes have residues that can catalyze opening of the furanose ring of the ribose 5-phosphate and so can improve the efficiency of the reaction. Both enzymes also have an acidic residue that acts as a base in the isomerization step. A lysine residue in RpiAs provides for more efficient stabilization of the intermediate than the corresponding uncharged groups of RpiBs; this same feature lies behind the more efficient binding of RpiA to 4-phospho-d-erythronate.

Mechanism of Bacterial Interference with TLR4 Signaling by Brucella Toll/Interleukin-1 Receptor Domain-containing Protein TcpB

Background: TcpB is a TIR domain-containing protein form Brucella. Results: TcpB interacts with the host Toll-like receptor and adaptors, and its structure reveals a dimer essential for activity. Conclusion: TcpB forms a nonfunctional complex with host molecules, thus suppressing signaling. Significance: The work explains the structural and functional basis of immune suppression by the protein TcpB from a pathogenic bacterium. Upon activation of Toll-like receptors (TLRs), cytoplasmic Toll/interleukin-1 receptor (TIR) domains of the receptors undergo homo- or heterodimerization. This in turn leads to the recruitment of adaptor proteins, activation of transcription factors, and the secretion of pro-inflammatory cytokines. Recent studies have described the TIR domain-containing protein from Brucella melitensis, TcpB (BtpA/Btp1), to be involved in virulence and suppression of host innate immune responses. TcpB interferes with TLR4 and TLR2 signaling pathways by a mechanism that remains controversial. In this study, we show using co-immunoprecipitation analyses that TcpB interacts with MAL, MyD88, and TLR4 but interferes only with the MAL-TLR4 interaction. We present the crystal structure of the TcpB TIR domain, which reveals significant structural differences in the loop regions compared with other TIR domain structures. We demonstrate that TcpB forms a dimer in solution, and the crystal structure reveals the dimerization interface, which we validate by mutagenesis and biophysical studies. Our study advances the understanding of the molecular mechanisms of host immunosuppression by bacterial pathogens.

Wzy-Dependent Bacterial Capsules as Potential Drug Targets

The bacterial capsule is a recognized virulence factor in pathogenic bacteria. It likely works as an antiphagocytic barrier by minimizing complement deposition on the bacterial surface. With the continual rise of bacterial pathogens resistant to multiple antibiotics, there is an increasing need for novel drugs. In the Wzy-dependent pathway, the biosynthesis of capsular polysaccharide (CPS) is regulated by a phosphoregulatory system, whose main components consist of bacterial-tyrosine kinases (BY-kinases) and their cognate phosphatases. The ability to regulate capsule biosynthesis has been shown to be vital for pathogenicity, because different stages of infection require a shift in capsule thickness, making the phosphoregulatory proteins suitable as drug targets. Here, we review the role of regulatory proteins focusing on Streptococcus pneumoniae, Staphylococcus aureus, and Escherichia coli and discuss their suitability as targets in structure-based drug design.

Novel pseudopeptides incorporating a benzodiazepine-based turn mimetic--targeting Mycobacterium tuberculosis ribonucleotide reductase.

Peptides mimicking the C-terminus of the small subunit (R2) of Mycobacterium tuberculosis ribonucleotide reductase (RNR) can compete for binding to the large subunit (R1) and thus inhibit RNR activity. Moreover, it has been suggested that the binding of the R2 C-terminus is very similar in M. tuberculosis and Salmonella typhimurium. Based on modeling studies of a crystal structure of the holocomplex of the S. typhimurium enzyme, a benzodiazepine-based turn mimetic was identified and a set of novel compounds incorporating the benzodiazepine scaffold was synthesized. The compounds were evaluated in a competitive fluorescence polarization assay and in an RNR activity assay. These studies revealed that the compounds incorporating the benzodiazepine scaffold have the ability to compete for the M. tuberculosis R2 binding site with low-micromolar affinity.

Identification of small peptides mimicking the R2 C-terminus of Mycobacterium tuberculosis ribonucleotide reductase

Ribonucleotide reductase (RNR) is a viable target for new drugs against the causative agent of tuberculosis, Mycobacterium tuberculosis. Previous work has shown that an N‐acetylated heptapeptide based on the C‐terminal sequence of the smaller RNR subunit can disrupt the formation of the holoenzyme sufficiently to inhibit its function. Here the synthesis and binding affinity, evaluated by competitive fluorescence polarization, of several truncated and N‐protected peptides are described. The protected single‐amino acid Fmoc‐Trp shows binding affinity comparable to the N‐acetylated heptapeptide, making it an attractive candidate for further development of non‐peptidic RNR inhibitors. Copyright

Structural and Biochemical Analysis of a Single Amino-Acid Mutant of WzzBSF That Alters Lipopolysaccharide O-Antigen Chain Length in Shigella flexneri.

Lipopolysaccharide (LPS), a surface polymer of Gram-negative bacteria, helps bacteria survive in different environments and acts as a virulence determinant of host infection. The O-antigen (Oag) component of LPS exhibits a modal chain-length distribution that is controlled by polysaccharide co-polymerases (PCPs). The molecular basis of the regulation of Oag chain-lengths remains unclear, despite extensive mutagenesis and structural studies of PCPs from Escherichia coli and Shigella. Here, we identified a single mutation (A107P) of the Shigella flexneri WzzBSF, by a random mutagenesis approach, that causes a shortened Oag chain-length distribution in bacteria. We determined the crystal structures of the periplasmic domains of wild-type WzzBSF and the A107P mutant. Both structures form a highly similar open trimeric assembly in the crystals, and show a similar tendency to self-associate in solution. Binding studies by bio-layer interferometry reveal cooperative binding of very short (VS)-core-plus-O-antigen polysaccharide (COPS) to the periplasmic domains of both proteins, but with decreased affinity for the A107P mutant. Our studies reveal that subtle and localized structural differences in PCPs can have dramatic effects on LPS chain-length distribution in bacteria, for example by altering the affinity for the substrate, which supports the role of the structure of the growing Oag polymer in this process.

Stability of the Octameric Structure Affects Plasminogen-Binding Capacity of Streptococcal Enolase

Group A Streptococcus (GAS) is a human pathogen that has the potential to cause invasive disease by binding and activating human plasmin(ogen). Streptococcal surface enolase (SEN) is an octameric α-enolase that is localized at the GAS cell surface. In addition to its glycolytic role inside the cell, SEN functions as a receptor for plasmin(ogen) on the bacterial surface, but the understanding of the molecular basis of plasmin(ogen) binding is limited. In this study, we determined the crystal and solution structures of GAS SEN and characterized the increased plasminogen binding by two SEN mutants. The plasminogen binding ability of SENK312A and SENK362A is ~2- and ~3.4-fold greater than for the wild-type protein. A combination of thermal stability assays, native mass spectrometry and X-ray crystallography approaches shows that increased plasminogen binding ability correlates with decreased stability of the octamer. We propose that decreased stability of the octameric structure facilitates the access of plasmin(ogen) to its binding sites, leading to more efficient plasmin(ogen) binding and activation.

Structural and Functional Characterization of a Cross-Reactive Dengue Virus Neutralizing Antibody that Recognizes a Cryptic Epitope

Understanding the molecular basis of the neutralizing antibody response to dengue virus (DENV) is an essential component in the design and development of effective vaccines and immunotherapeutics. Here we present the structure of a cross-reactive, neutralizing antibody, 3E31, in complex with domain III (DIII) of the DENV envelope (E) protein and reveal a conserved, temperature-sensitive, cryptic epitope on DIII that is not available in any of the known conformations of E on the dengue virion. We observed that 3E31 inhibits E-mediated membrane fusion, suggesting that the antibody is able to neutralize virus through binding an as-yet uncharacterized intermediate conformation of DENV E and sterically block trimer formation. Finally, we show that, unlike cross-reactive fusion peptide-specific antibodies, 3E31 does not promote antibody-dependent enhancement of infection at sub-neutralizing concentrations. Our results highlight the 3E31 epitope on the E protein DIII as a promising target for immunotherapeutics or vaccine design.