Vickery L. Arcus

OB-fold domains: a snapshot of the evolution of sequence, structure and function

The OB-fold is found in all three kingdoms and is well represented in both sequence and structural databases. The OB-fold is a five-stranded closed beta barrel and the majority of OB-fold proteins use the same face for ligand binding or as an active site. Different OB-fold proteins use this fold-related binding face to, variously, bind oligosaccharides, oligonucleotides, proteins, metal ions and catalytic substrates. Recently, a number of new structures with OB-folds have been reported that augment the variation seen for this set of proteins whilst conserving the characteristic fold and binding face. The conservation of fold and a functional binding face amongst many structures provides a model for investigating the evolutionary trajectory of sequence, structure and function.

Superantigens - powerful modifiers of the immune system

Superantigens are powerful microbial toxins that activate the immune system by binding to class II major histocompatibility complex and T-cell receptor molecules. They cause a number of diseases characterized by fever and shock and are important virulence factors for two human commensal organisms, Staphylococcus aureus and Streptococcus pyogenes, as well as for some viruses. Their mode of action and variation around the common theme of over-stimulating T cells, provides a rich insight into the constant battle between microbes and the immune system.

The TB structural genomics consortium: a resource for Mycobacterium tuberculosis biology

The TB Structural Genomics Consortium is an organization devoted to encouraging, coordinating, and facilitating the determination and analysis of structures of proteins from Mycobacterium tuberculosis. The Consortium members hope to work together with other M. tuberculosis researchers to identify M. tuberculosis proteins for which structural information could provide important biological information, to analyze and interpret structures of M. tuberculosis proteins, and to work collaboratively to test ideas about M. tuberculosis protein function that are suggested by structure or related to structural information. This review describes the TB Structural Genomics Consortium and some of the proteins for which the Consortium is in the progress of determining three-dimensional structures.

The Three-dimensional Structure of a Superantigen-like Protein, SET3, from a Pathogenicity Island of the Staphylococcus aureus Genome

The staphylococcal enterotoxin-like toxins (SETs) are a family of proteins encoded within the Staphylococcus aureus genome that were identified by their similarity to the well described bacterial superantigens. The first crystal structure of a member of the SET family, SET3, has been determined to 1.9 Å (R = 0.205, R free = 0.240) and reveals a fold characteristic of the superantigen family but with significant differences. The SET proteins are secreted at varying levels by staphylococcal isolates, and seroconversion studies of normal individuals indicate that they are strongly antigenic to humans. Recombinant SETs do not exhibit any of the properties expected of superantigens such as major histocompatibility complex class II binding or broad T-cell activation, suggesting they have an entirely different function. The fact that the whole gene family is clustered within the pathogenicity island SaIn2 of theS. aureus genome suggests that they are involved in host/pathogen interactions.

A Flexible And Economical Medium-Throughput Strategy for Protein Production And Crystallization

Large-scale structural genomics centres rely heavily on robotics to ensure that maximum throughput is achieved. However, the size and cost of these approaches is out of the reach of most academic structural biology efforts. A major challenge for such groups is to adapt current high-throughput schemes to a reasonable scale with the resources available. A flexible medium-throughput approach has been developed that is suitable for typical academic research groups. Following nested PCR, targets are routinely cloned into two Gateway expression vectors (pDEST15 for an N-terminal GST tag and pDEST17 for an N-terminal His tag). Expression of soluble recombinant protein in Escherichia coli is rapidly assessed in 96-well format. An eight-probe sonicator is utilized and a six-buffer lysis screen was incorporated to enhance solubility. Robotics is reserved for crystallization, since this is the key bottleneck for crystallography. Screening proteins with a 480-condition protocol using a Cartesian nanolitre-dispensing robot has increased crystallization success markedly, with an overall success rate (structures solved out of proteins screened) of 19%. The methods are robust and economical -- with the exception of the crystallization robot, investment in additional equipment has been minimal at 9000 US dollars. All protocols are designed for individuals so that graduate students and postdoctoral fellows gain expertise in every aspect of the structural pipeline, from cloning to crystallization.

Immunological and Biochemical Characterization of Streptococcal Pyrogenic Exotoxins I and J (SPE-I and SPE-J) from Streptococcus pyogenes

Recently, we described the identification of novel streptococcal superantigens (SAgs) by mining the Streptococcus pyogenes M1 genome database at Oklahoma University. Here, we report the cloning, expression, and functional analysis of streptococcal pyrogenic exotoxin (SPE)-J and another novel SAg (SPE-I). SPE-I is most closely related to SPE-H and staphylococcal enterotoxin I, whereas SPE-J is most closely related to SPE-C. Recombinant forms of SPE-I and SPE-J were mitogenic for PBL, both reaching half maximum responses at 0.1 pg/ml. Evidence from binding studies and cell aggregation assays using a human B-lymphoblastoid cell line (LG-2) suggests that both toxins exclusively bind to the polymorphic MHC class II β-chain in a zinc-dependent mode but not to the generic MHC class II α-chain. The results from analysis by light scattering indicate that SPE-J exists as a dimer in solution above concentrations of 4.0 mg/ml. Moreover, SPE-J induced a rapid homotypic aggregation of LG-2 cells, suggesting that this toxin might cross-link MHC class II molecules on the cell surface by building tetramers of the type HLA-DRβ–SPE-J–SPE-J–HLA-DRβ. SPE-I preferably stimulates T cells bearing the Vβ18.1 TCR, which is not targeted by any other known SAg. SPE-J almost exclusively stimulates Vβ2.1 T cells, a Vβ that is targeted by several other streptococcal SAgs, suggesting a specific role for this T cell subpopulation in immune defense. Despite a primary sequence diversity of 51%, SPE-J is functionally indistinguishable from SPE-C and might play a role in streptococcal disease, which has previously been addressed to SPE-C.

Crystal structure of PAE0151 from Pyrobaculum aerophilum, a PIN-domain (VapC) protein from a toxin-antitoxin operon.

Crystal structure of PAE0151 from Pyrobaculum aerophilum, a PIN-domain (VapC) protein from a toxin-antitoxin operon Richard D. Bunker, Joanna L. McKenzie, Edward N. Baker, and Vickery L. Arcus* 1Maurice Wilkins Centre for Molecular Biodiscovery and School of Biological Sciences, University of Auckland, Private Bag 92019, Auckland, New Zealand 2Department of Biological Sciences, School of Science and Engineering, University of Waikato, Private Bag 3105, Hamilton 3216, New Zealand

Structure of naphthoate synthase (MenB) from Mycobacterium tuberculosis in both native and product-bound forms

Mycobacterium tuberculosis, the cause of tuberculosis, is one of the most devastating human pathogens. New drugs for its control are urgently needed. Menaquinone, also known as vitamin K, is an essential cofactor that is required for electron transfer and the enzymes that synthesize it are therefore potential drug targets. The enzyme naphthoate synthase (MenB) from M. tuberculosis has been expressed in Escherichia coli, purified and crystallized both as the native enzyme and in complex with naphthoyl-CoA. Both structures have been determined by X-ray crystallography: native MenB at 2.15 A resolution (R = 0.203, R(free) = 0.231) and its napthoyl-CoA complex at 2.30 A resolution (R = 0.197, R(free) = 0.225). The protein structure, which has a fold characteristic of the crotonase family of enzymes, is notable for the presence of several highly flexible regions around the active site. The bound naphthoyl-CoA is only visible for one of the three molecules in the asymmetric unit and only partly rigidifies the structure. The C-terminal region of the protein is seen to play a critical role both in completion of the binding pocket and in stabilization of the hexamer, suggesting a link between oligomerization and catalytic activity.

The potential impact of structural genomics on tuberculosis drug discovery

Mycobacterium tuberculosis, the causative agent of tuberculosis (TB) in humans, is a devastating infectious organism that kills approximately two million people annually. The current suite of antibiotics used to treat TB faces two main difficulties: (i) the emergence of multidrug-resistant (MDR) strains of M. tuberculosis, and (ii) the persistent state of the bacterium, which is less susceptible to antibiotics and causes very long antibiotic treatment regimes. The complete genome sequences of a laboratory strain (H37Rv) and a clinical strain (CDC1551) of M. tuberculosis and the concurrent identification of all the open reading frames that encode proteins within this organism, present structural biologists with a wide array of protein targets for structure determination. Comparative genomics of the species that make up the M. tuberculosis complex has also added an array of genomic information to our understanding of these organisms. In response to this, structural genomics consortia have been established for targeting proteins from M. tuberculosis. This review looks at the progress of these major initiatives and the potential impact of large scale structure determination efforts on the development of inhibitors to many proteins. Increasing sophistication in structure-based drug design approaches, in combination with increasing numbers of protein structures and inhibitors for TB proteins, will have a significant impact on the downstream development of TB antibiotics.

Toward More Accurate Ancestral Protein Genotype–Phenotype Reconstructions with the Use of Species Tree-Aware Gene Trees

The resurrection of ancestral proteins provides direct insight into how natural selection has shaped proteins found in nature. By tracing substitutions along a gene phylogeny, ancestral proteins can be reconstructed in silico and subsequently synthesized in vitro. This elegant strategy reveals the complex mechanisms responsible for the evolution of protein functions and structures. However, to date, all protein resurrection studies have used simplistic approaches for ancestral sequence reconstruction (ASR), including the assumption that a single sequence alignment alone is sufficient to accurately reconstruct the history of the gene family. The impact of such shortcuts on conclusions about ancestral functions has not been investigated. Here, we show with simulations that utilizing information on species history using a model that accounts for the duplication, horizontal transfer, and loss (DTL) of genes statistically increases ASR accuracy. This underscores the importance of the tree topology in the inference of putative ancestors. We validate our in silico predictions using in vitro resurrection of the LeuB enzyme for the ancestor of the Firmicutes, a major and ancient bacterial phylum. With this particular protein, our experimental results demonstrate that information on the species phylogeny results in a biochemically more realistic and kinetically more stable ancestral protein. Additional resurrection experiments with different proteins are necessary to statistically quantify the impact of using species tree-aware gene trees on ancestral protein phenotypes. Nonetheless, our results suggest the need for incorporating both sequence and DTL information in future studies of protein resurrections to accurately define the genotype–phenotype space in which proteins diversify.