The goal of structural biology is to reveal details of the molecular structure of proteins in order to understand their function and mechanism. X-ray crystallography and NMR are the two best methods for atomic level structure determination. However, these methods require milligram quantities of proteins. In this chapter a reproducible methodology for large-scale protein production applicable to a diverse set of proteins is described. The approach is based on protein expression in E. coli as a fusion with a cleavable affinity tag that was tested on over 20,000 proteins. Specifically, a protocol for fermentation of large quantities of native proteins in disposable culture vessels is presented. A modified protocol that allows for the production of selenium-labeled proteins in defined media is also offered. Finally, a method for the purification of His6-tagged proteins on immobilized metal affinity chromatography columns that generates high-purity material is described in detail.
Protein expression; Protein purification; Disposable vessel fermentation; Selenomethionine-labeling; IMAC; His-tag; High-throughput
Carrier proteins (CPs) play a critical role in the biosynthesis of various natural products, especially in nonribosomal peptide synthetase (NRPS) and polyketide synthase (PKS) enzymology, where the CPs are referred to as peptidyl-carrier proteins (PCPs) or acyl-carrier proteins (ACPs), respectively. CPs can either be a domain in large multifunctional polypeptides or standalone proteins, termed Type I and Type II, respectively. There have been many biochemical studies of the Type I PKS and NRPS CPs, and of Type II ACPs. However, recently a number of Type II PCPs have been found and biochemically characterized. In order to understand the possible interaction surfaces for combinatorial biosynthetic efforts we crystallized the first characterized and representative Type II PCP member, BlmI, from the bleomycin biosynthetic pathway from Streptomyces verticillus ATCC 15003. The structure is similar to CPs in general but most closely resembles PCPs. Comparisons with previously determined PCP structures in complex with catalytic domains reveals a common interaction surface. This surface is highly variable in charge and shape, which likely confers specificity for interactions. Previous nuclear magnetic resonance (NMR) analysis of a prototypical Type I PCP excised from the multimodular context revealed three conformational states. Comparison of the states with the structure of BlmI and other PCPs reveals that only one of the NMR states is found in other studies, suggesting the other two states may not be relevant. The state represented by the BlmI crystal structure can therefore serve as a model for both Type I and Type II PCPs.
protein–protein interaction; natural product; biosynthesis; phylogenetics; structural genomics; reductive methylation
The growth of diffraction-quality single crystals is of primary importance in protein X-ray crystallography. Chemical modification of proteins can alter their surface properties and crystallization behavior. The Midwest Center for Structural Genomics (MCSG) has previously reported how reductive methylation of lysine residues in proteins can improve crystallization of unique proteins that initially failed to produce diffraction-quality crystals. Recently, this approach has been expanded to include ethylation and isopropylation in the MCSG protein crystallization pipeline. Applying standard methods, 180 unique proteins were alkylated and screened using standard crystallization procedures. Crystal structures of 12 new proteins were determined, including the first ethylated and the first isopropylated protein structures. In a few cases, the structures of native and methylated or ethylated states were obtained and the impact of reductive alkylation of lysine residues was assessed. Reductive methylation tends to be more efficient and produces the most alkylated protein structures. Structures of methylated proteins typically have higher resolution limits. A number of well-ordered alkylated lysine residues have been identified, which make both intermolecular and intramolecular contacts. The previous report is updated and complemented with the following new data; a description of a detailed alkylation protocol with results, structural features, and roles of alkylated lysine residues in protein crystals. These contribute to improved crystallization properties of some proteins.
Chemical modification; Lysine reductive alkylation; Methylation; Ethylation; Isopropylation; Protein crystallization
Phage viruses that infect prokaryotes integrate their genome into the host chromosome; thus, microbial genomes typically contain genetic remnants of both recent and ancient phage infections. Often phage genes occur in clusters of atypical G+C content that reflect integration of the foreign DNA. However, some phage genes occur in isolation without other phage gene neighbors, probably resulting from horizontal gene transfer. In these cases, the phage gene product is unlikely to function as a component of a mature phage particle, and instead may have been co-opted by the host for its own benefit. The product of one such gene from Salmonella enterica serovar Typhimurium, STM3605, encodes a protein with modest sequence similarity to phage-like lysozyme (N-acetylmuramidase) but appears to lack essential catalytic residues that are strictly conserved in all lysozymes. Close homologs in other bacteria share this characteristic. The structure of the STM3605 protein was characterized by X-ray crystallography, and functional assays showed that it is a stable, folded protein whose structure closely resembles lysozyme. However, this protein is unlikely to hydrolyze peptidoglycan. Instead, STM3605 is presumed to have evolved an alternative function because it shows some lytic activity and partitions to micelles.
Crystal structure; mutagenesis; oligomeric state; phage-like lysozyme; Salmonella
Anaeromyxobacter dehalogenans is a δ-proteobacterium found in diverse soils and sediments. It is of interest in bioremediation efforts due to its dechlorination and metal-reducing capabilities. To gain an understanding on A. dehalogenans' abilities to adapt to diverse environments we analyzed its signal transduction proteins. The A. dehalogenans genome codes for a large number of sensor histidine kinases (HK) and methyl-accepting chemotaxis proteins (MCP); among these 23 HK and 11 MCP proteins have a sensor domain in the periplasm. These proteins most likely contribute to adaptation to the organism's surroundings. We predicted their three-dimensional folds and determined the structures of two of the periplasmic sensor domains by X-ray diffraction. Most of the domains are predicted to have either PAS-like or helical bundle structures, with two predicted to have solute-binding protein fold, and another predicted to have a 6-phosphogluconolactonase like fold. Atomic structures of two sensor domains confirmed the respective fold predictions. The Adeh_2942 sensor (HK) was found to have a helical bundle structure, and the Adeh_3718 sensor (MCP) has a PAS-like structure. Interestingly, the Adeh_3718 sensor has an acetate moiety bound in a binding site typical for PAS-like domains. Future work is needed to determine whether Adeh_3718 is involved in acetate sensing by A. dehalogenans.
Acetate; chemotaxis; helical bundle; PAS-like; periplasmic sensor domains; sensor histidine kinase
Microcin C (McC) is heptapeptide-adenylate antibiotic produced by Escherichia coli strains carrying the mccABCDEF gene cluster encoding enzymes, in addition to the heptapeptide structural gene mccA, necessary for McC biosynthesis and self-immunity of the producing cell. The heptapeptide facilitates McC transport into susceptible cells, where it is processed releasing a non-hydrolyzable aminoacyl adenylate that inhibits an essential aminoacyl-tRNA synthetase. The self-immunity gene mccF encodes a specialized serine-peptidase that cleaves an amide bond connecting the peptidyl or aminoacyl moieties of, respectively, intact and processed McC with the nucleotidyl moiety. Most mccF orthologs from organisms other than E. coli are not linked to the McC biosynthesis gene cluster. Here, we show that a protein product of one such gene, MccF from Bacillus anthracis (BaMccF), is able to cleave intact and processed McC and we present a series of structures of this protein. Structural analysis of apo-BaMccF and its AMP-complex reveal specific features of MccF-like peptidases that allow them to interact with substrates containing nucleotidyl moieties. Sequence analyses and phylogenetic reconstructions suggest that several distinct subfamilies form the MccF clade of the large S66 family of bacterial serine peptidases. We show that various representatives of the MccF clade can specifically detoxify non-hydrolyzable aminoacyl adenylates differing in their aminoacyl moieties. We hypothesize that bacterial mccF genes serve as a source of bacterial antibiotic resistance.
MccF; serine peptidase; nucleophilic elbow; catalytic triad (Ser-His-Glu); substrate binding loop
The ultimate goal of structural biology is to understand the structural basis of proteins in cellular processes. In structural biology, the most critical issue is the availability of high-quality samples. “Structural biology-grade” proteins must be generated in the quantity and quality suitable for structure determination using X-ray crystallography or nuclear magnetic resonance (NMR) spectroscopy. The purification procedures must reproducibly yield homogeneous proteins or their derivatives containing marker atom(s) in milligram quantities. The choice of protein purification and handling procedures plays a critical role in obtaining high-quality protein samples. With structural genomics emphasizing a genome-based approach in understanding protein structure and function, a number of unique structures covering most of the protein folding space have been determined and new technologies with high efficiency have been developed. At the Midwest Center for Structural Genomics (MCSG), we have developed semi-automated protocols for high-throughput parallel protein expression and purification. A protein, expressed as a fusion with a cleavable affinity tag, is purified in two consecutive immobilized metal affinity chromatography (IMAC) steps: (i) the first step is an IMAC coupled with buffer-exchange, or size exclusion chromatography (IMAC-I), followed by the cleavage of the affinity tag using the highly specific Tobacco Etch Virus (TEV) protease;  the second step is IMAC and buffer exchange (IMAC-II) to remove the cleaved tag and tagged TEV protease. These protocols have been implemented on multidimensional chromatography workstations and, as we have shown, many proteins can be successfully produced in large-scale. All methods and protocols used for purification, some developed by MCSG, others adopted and integrated into the MCSG purification pipeline and more recently the Center for Structural Genomics of Infectious Diseases (CSGID) purification pipeline, are discussed in this chapter.
domain design; expression vectors; gene cloning; protein purification; crystallization screening; quality assessment
The crystal structures of two 6-P-β-glucosidases from the GH1 family were determined in the apo form and in the presence of a 6′-P-salicin substrate, of the reaction product 6-P-β-glucose and of glucose corresponding to the aglycon molecule. The presence of natural ligands enabled the definition of the structural elements responsible for the recognition and hydrolysis of 6′-P-β-glucosides.
In lactic acid bacteria and other bacteria, carbohydrate uptake is mostly governed by phosphoenolpyruvate-dependent phosphotransferase systems (PTSs). PTS-dependent translocation through the cell membrane is coupled with phosphorylation of the incoming sugar. After translocation through the bacterial membrane, the β-glycosidic bond in 6′-P-β-glucoside is cleaved, releasing 6-P-β-glucose and the respective aglycon. This reaction is catalyzed by 6-P-β-glucosidases, which belong to two glycoside hydrolase (GH) families: GH1 and GH4. Here, the high-resolution crystal structures of GH1 6-P-β-glucosidases from Lactobacillus plantarum (LpPbg1) and Streptococcus mutans (SmBgl) and their complexes with ligands are reported. Both enzymes show hydrolytic activity towards 6′-P-β-glucosides. The LpPbg1 structure has been determined in an apo form as well as in a complex with phosphate and a glucose molecule corresponding to the aglycon molecule. The S. mutans homolog contains a sulfate ion in the phosphate-dedicated subcavity. SmBgl was also crystallized in the presence of the reaction product 6-P-β-glucose. For a mutated variant of the S. mutans enzyme (E375Q), the structure of a 6′-P-salicin complex has also been determined. The presence of natural ligands enabled the definition of the structural elements that are responsible for substrate recognition during catalysis.
6-P-β-glucosidases; glycoside hydrolases; GH1; cellobiose; gentiobiose; salicin
The Human Proteome Organisation’s Proteomics Standards Initiative (HUPO-PSI) has developed the GelML data exchange format for representing gel electrophoresis experiments performed in proteomics investigations. The format closely follows the reporting guidelines for gel electrophoresis, which are part of the Minimum Information About a Proteomics Experiment (MIAPE) set of modules. GelML supports the capture of metadata (such as experimental protocols) and data (such as gel images) resulting from gel electrophoresis so that laboratories can be compliant with the MIAPE Gel Electrophoresis guidelines, while allowing such data sets to be exchanged or downloaded from public repositories. The format is sufficiently flexible to capture data from a broad range of experimental processes, and complements other PSI formats for mass spectrometry data and the results of protein and peptide identifications to capture entire gel-based proteome workflows. GelML has resulted from the open standardisation process of PSI consisting of both public consultation and anonymous review of the specifications.
data standard; gel electrophoresis; database; ontology
The New Delhi Metallo-β-lactamase (NDM-1) gene makes multiple pathogenic microorganisms resistant to all known β-lactam antibiotics. The rapid emergence of NDM-1 has been linked to mobile plasmids that move between different strains resulting in world-wide dissemination. Biochemical studies revealed that NDM-1 is capable of efficiently hydrolyzing a wide range of β-lactams, including many carbapenems considered as “last resort” antibiotics. The crystal structures of metal-free apo- and monozinc forms of NDM-1 presented here revealed an enlarged and flexible active site of class B1 metallo-β-lactamase. This site is capable of accommodating many β-lactam substrates by having many of the catalytic residues on flexible loops, which explains the observed extended spectrum activity of this zinc dependent β-lactamase. Indeed, five loops contribute “keg” residues in the active site including side chains involved in metal binding. Loop 1 in particular, shows conformational flexibility, apparently related to the acceptance and positioning of substrates for cleavage by a zinc-activated water molecule.
We present the crystal structure of the extracytoplasmic domain of the Bacillus subtilis PhoR sensor histidine kinase, part of a two-component system involved in adaptation to low environmental phosphate concentrations. In addition to the PhoR structure, we predict that the majority of the extracytoplasmic domains of B. subtilis sensor kinases will adopt a fold similar to the ubiquitous PAS domain.