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Knotted peptides present a wealth of structurally diverse, biologically active molecules, with the inhibitor cystine knot/knottin class among the most ecologically common ones. Many of these natural products interact with extracellular targets such as voltage-gated ion channels with exquisite selectivity and potency, making them intriguing therapeutic modalities. Such compounds are often produced in low concentrations by intractable organisms, making structural and biological characterization challenging, which is frequently overcome by various expression strategies. Here, we sought to test a biosynthetic route for the expression and study of knotted peptides. We screened expression constructs for a biosynthesized knotted peptide to determine the most influential parameters for successful disulfide folding and used NMR spectroscopic fingerprinting to validate topological structures. We performed pharmacokinetic characterization, which indicated that the interlocking disulfide structure minimizes liabilities of linear peptide sequences, and propose a mechanism by which knotted peptides are cleared. We then developed an assay to monitor solution folding in real time, providing a strategy for studying the folding process during maturation, which provided direct evidence for the importance of backbone organization as the driving force for topology formation.

The transcription factor Rv0081 of Mycobacterium tuberculosis controls hypoxic gene expression and acts as a regulatory hub in the latent phase of tuberculosis (TB) infection. We report here the crystal structure of Rv0081 at 2.9 Å resolution revealing that it belongs to the well-known ArsR/SmtB family proteins. However, unlike other members in this family, Rv0081 has neither a metal-binding domain nor does it possess Cys residues, suggesting an alternate mechanism of gene regulation. Our structural and biochemical analyses suggest the molecular basis for the recognition of self-regulatory DNA sequences and a plausible mechanism of regulation of Rv0081 in the latent phase of TB infection. DATABASE: Structural data are available in the Protein Data Bank under the accession number - 6JMI.

The transcription factor Rv0081 ofM. tuberculosiscontrols the hypoxic gene expression and acts as a regulatory hub in the latent phase of tuberculosis infection. We report here the crystal structure of Rv0081 at 3.3 Å resolution revealing that it belongs to the well-known ArsR/SmtB family proteins. ArsR/SmtB family transcriptional repressors exert gene regulation by reversible metal binding. Hypoxia in general is sensed by bacterial transcriptional regulators via metals or Cys-mediated thiol switches. Oxygen sensing typically leads to transcriptional repressor changing its conformational state with altered DNA-binding property under different oxygen levels. Surprisingly Rv0081 neither has a metal binding domain nor does it possess Cys residues suggesting an alternate mechanism of gene regulation. Our structural analysis identified Ser 48, Ser 49, Ser 52 and Gln 53 as potential residues of Rv0081 involved in DNA binding. We probed DNA-binding of Rv0081 with electrophoretic mobility shift assay (EMSA) as well as surface plasmon resonance (SPR), where the Alanine mutants of these residues showed diminished DNA binding. Similarly, Aspartate mutants of these Ser residues was shown to fail to bind to DNA. Since, phosphorylation of various regulatory proteins is one of the important controlling mechanisms, we expected the role of Ser-phosphorylation of Rv0081 in hypoxic condition. Probing Rv0081 with anti-phosphoserine antibodies inM. tuberculosiscell lysate showed marked enhancement in the phosphorylation of Rv0081 protein under hypoxia. Overall, our structural and biochemical analysis provides the molecular basis for the regulation of Rv0081 in the latent phase of tuberculosis infection.IMPORTANCETuberculosis is one of the deadliest infectious diseases caused by the bacteriumMycobacterium tuberculosis. In about 90% of the infected people,M. tuberculosisexists in a dormant or a latent stage which can be reactivated in favorable conditions. Hypoxia (low oxygen pressure) is one of causes of dormancy. Understanding hypoxic gene regulation inM. tuberculosisis therefore an important step to understand latency. Rv0081 is a transcriptional regulator of genes expressed during hypoxia. In order to understand the mechanism by which Rv00081 regulates gene expression during hypoxia, we have solved the crystal structure of Rv0081 and identified amino acid residues which are critical in its transcriptional regulator activity. The crystal structure is suggestive of mechanism of gene regulation under hypoxia.

Fast inhibitory neurotransmission in the mammalian nervous system is largely mediated by GABAA receptors, chloride-selective members of the superfamily of pentameric Cys-loop receptors. Native GABAA receptors are heteromeric assemblies sensitive to many important drugs, from sedatives to anesthetics and anticonvulsant agents, with mutant forms of GABAA receptors implicated in multiple neurological diseases. Despite the profound importance of heteromeric GABAA receptors in neuroscience and medicine, they have proven recalcitrant to structure determination. Here we present the structure of a tri-heteromeric α1β1γ2SEM GABAA receptor in complex with GABA, determined by single particle cryo-EM at 3.1–3.8 Å resolution, elucidating molecular principles of receptor assembly and agonist binding. Remarkable N-linked glycosylation on the α1 subunit occludes the extracellular vestibule of the ion channel and is poised to modulate receptor assembly and perhaps ion channel gating. Our work provides a pathway to structural studies of heteromeric GABAA receptors and a framework for rational design of novel therapeutic agents.

Fast inhibitory neurotransmission in the mammalian nervous system is largely mediated by GABAAreceptors, chloride-selective members of the superfamily of pentameric Cys-loop receptors. Native GABAAreceptors are heteromeric assemblies sensitive to many important drugs, from sedatives to anesthetics and anticonvulsive agents, with mutant forms of GABAAreceptors implicated in multiple neurological diseases, including epilepsy. Despite the profound importance of heteromeric GABAAreceptors in neuroscience and medicine, they have proven recalcitrant to structure determination. Here we present the structure of the triheteromeric α1β1γ2EMGABAAreceptor in complex with GABA, determined by single particle cryo-EM at 3.1-3.8 Å resolution, elucidating the molecular principles of receptor assembly and agonist binding. Remarkable N-linked glycosylation on the α1 subunit occludes the extracellular vestibule of the ion channel and is poised to modulate receptor assembly and perhaps ion channel gating. Our work provides a pathway to structural studies of heteromeric GABAAreceptors and a framework for the rational design of novel therapeutic agents.

Poxviruses display species tropism—variola virus is a human-specific virus, while vaccinia virus causes repeated outbreaks in dairy cattle. Consistent with this, variola virus complement regulator SPICE (smallpox inhibitor of complement enzymes) exhibits selectivity in inhibiting the human alternative complement pathway and vaccinia virus complement regulator VCP (vaccinia virus complement control protein) displays selectivity in inhibiting the bovine alternative complement pathway. In the present study, we examined the species specificity of VCP and SPICE for the classical pathway (CP). We observed that VCP is ∼43-fold superior to SPICE in inhibiting bovine CP. Further, functional assays revealed that increased inhibitory activity of VCP for bovine CP is solely due to its enhanced cofactor activity, with no effect on decay of bovine CP C3-convertase. To probe the structural basis of this specificity, we utilized single- and multi-amino-acid substitution mutants wherein 1 or more of the 11 variant VCP residues were substituted in the SPICE template. Examination of these mutants for their ability to inhibit bovine CP revealed that E108, E120, and E144 are primarily responsible for imparting the specificity and contribute to the enhanced cofactor activity of VCP. Binding and functional assays suggested that these residues interact with bovine factor I but not with bovine C4(H 2 O) (a moiety conformationally similar to C4b). Mapping of these residues onto the modeled structure of bovine C4b-VCP-bovine factor I supported the mutagenesis data. Taken together, our data help explain why the vaccine strain of vaccinia virus was able to gain a foothold in domesticated animals. IMPORTANCE Vaccinia virus was used for smallpox vaccination. The vaccine-derived virus is now circulating and causing outbreaks in dairy cattle in India and Brazil. However, the reason for this tropism is unknown. It is well recognized that the virus is susceptible to neutralization by the complement classical pathway (CP). Because the virus encodes a soluble complement regulator, VCP, we examined whether this protein displays selectivity in targeting bovine CP. Our data show that it does exhibit selectivity in inhibiting the bovine CP and that this is primarily determined by its amino acids E108, E120, and E144, which interact with bovine serine protease factor I to inactivate bovine C4b—one of the two subunits of CP C3-convertase. Of note, the variola complement regulator SPICE contains positively charged residues at these positions. Thus, these variant residues in VCP help enhance its potency against the bovine CP and thereby the fitness of the virus in cattle.

Members of the NrdH family of redox proteins, which consists of small glutaredoxin-like proteins with thioredoxin-like activity, serve as the reducing partners of class Ib ribonucleotide reductases. Here, we report the crystal structure of NrdH from Mycobacterium tuberculosis, refined to a crystallographic R factor of 14.02% (Rfree = 15.53%) at 0.87 Å resolution. The tertiary structure of M. tuberculosis NrdH has a typical thioredoxin fold as expected. The extremely high resolution of the structure allows us to dissect the functionality of the protein in great depth. Structural superimposition of M. tuberculosis NrdH and thioredoxin reductase over the Escherichia coli thioredoxin reductase-thioredoxin complex suggests the ability of NrdH to accept electrons from M. tuberculosis thioredoxin reductase. This raises the important question of why glutaredoxins are unable to accept electrons from thioredoxin reductases and why thioredoxins are unable to reduce ribonucleotide reductases. Furthermore, forms of NrdH from other organisms have been shown to be a specific reductant of class Ib ribonucleotide reductases. We attempt to explain this substrate specificity by modeling the C-terminal peptide of a ribunucleotide subunit, NrdE, in the active site of NrdH using the already available Grx-NrdA-Cter-peptide structure. Statistical coupling analysis of NrdH, glutaredoxins, and thioredoxins reveals different sets of co-evolving contiguous clusters of amino acid residues, which might explain the differences in the biochemical properties of these structurally similar yet functionally distinct subclasses of proteins.

Carotenoids represent a diverse group of pigments derived from the common isoprenoid precursors and fulfill a variety of critical functions in plants and animals. Phytoene synthase (PSY), a transferase enzyme that catalyzes the first specific step in carotenoid biosynthesis plays a central role in the regulation of a number of essential functions mediated via carotenoids. PSYs have been deeply investigated in plants, bacteria and algae however in apicomplexans it is poorly studied. In an effort to characterize PSY in apicomplexans especially the malaria parasite Plasmodium falciparum (P. falciparum), a detailed bioinformatics analysis is undertaken. We have analysed the Phylogenetic relationship of PSY also referred to as octaprenyl pyrophosphate synthase (OPPS) in P. falciparum with other taxonomic groups. Further, we in silico characterized the secondary and tertiary structures of P. falciparum PSY/OPPS and compared the tertiary structures with crystal structure of Thermotoga maritima (T. maritima) OPPS. Our results evidenced the resemblance of P. falciparum PSY with the active site of T. maritima OPPS. Interestingly, the comparative structural analysis revealed an unconserved unique loop in P. falciparum OPPS/PSY. Such structural insights might contribute novel accessory functions to the protein thus, offering potential drug targets.