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In selecting a method to produce a recombinant protein, a researcher is faced with a bewildering array of choices as to where to start. To facilitate decision-making, we describe a consensus 'what to try first' strategy based on our collective analysis of the expression and purification of over 10,000 different proteins. This review presents methods that could be applied at the outset of any project, a prioritized list of alternate strategies and a list of pitfalls that trip many new investigators.

IMP dehydrogenase (IMPDH) is an essential enzyme that catalyzes the first step unique to GTP synthesis. To provide a basis for the evaluation of IMPDH inhibitors as antimicrobial agents, we have expressed and characterized IMPDH from the pathogenic bacterium Streptococcus pyogenes. Our results show that the biochemical and kinetic characteristics of S. pyogenes IMPDH are similar to other bacterial IMPDH enzymes. However, the lack of sensitivity to mycophenolic acid and the Km for NAD (1180 microM) exemplify some of the differences between the bacterial and mammalian IMPDH enzymes, making it an attractive target for antimicrobial agents. To evaluate the basis for these differences, we determined the crystal structure of the bacterial enzyme at 1.9 A with substrate bound in the catalytic site. The structure was determined using selenomethionine-substituted protein and multiwavelength anomalous (MAD) analysis of data obtained with synchrotron radiation from the undulator beamline (19ID) of the Structural Biology Center at Argonne's Advanced Photon Source. S. pyogenes IMPDH is a tetramer with its four subunits related by a crystallographic 4-fold axis. The protein is composed of two domains: a TIM barrel domain that embodies the catalytic framework and a cystathione beta-synthase (CBS) dimer domain of so far unknown function. Using information provided by sequence alignments and the crystal structure, we prepared several site-specific mutants to examine the role of various active site regions in catalysis. These variants implicate the active site flap as an essential catalytic element and indicate there are significant differences in the catalytic environment of bacterial and mammalian IMPDH enzymes. Comparison of the structure of bacterial IMPDH with the known partial structures from eukaryotic organisms will provide an explanation of their distinct properties and contribute to the design of specific bacterial IMPDH inhibitors.

The ability to use lactate as a sole source of carbon and energy is one of the key metabolic signatures of Shewanellae, a diverse group of dissimilatory metal-reducing bacteria commonly found in aquatic and sedimentary environments. Nonetheless, homology searches failed to recognize orthologs of previously described bacterial D-or L-lactate oxidizing enzymes (Escherichia coli genes dld and lldD) in any of the 13 analyzed genomes of Shewanella spp. By using comparative genomic techniques, we identified a conserved chromosomal gene cluster in Shewanella oneidensis MR-1 (locus tag: SO1522-SO1518) containing lactate permease and candidate genes for both D-and L-lactate dehydrogenase enzymes. The predicted D-LDH gene (dld-II, SO1521) is a distant homolog of FAD-dependent lactate dehydrogenase from yeast, whereas the predicted L-LDH is encoded by 3 genes with previously unknown functions (lldEGF, SO1520 -SO1518). Through a combination of genetic and biochemical techniques, we experimentally confirmed the predicted physiological role of these novel genes in S. oneidensis MR-1 and carried out successful functional validation studies in Escherichia coli and Bacillus subtilis. We conclusively showed that dld-II and lldEFG encode fully functional D-and L-LDH enzymes, which catalyze the oxidation of the respective lactate stereoisomers to pyruvate. Notably, the S. oneidensis MR-1 LldEFG enzyme is a previously uncharacterized example of a multisubunit lactate oxidase. Comparative analysis of >400 bacterial species revealed the presence of LldEFG and Dld-II in a broad range of diverse species accentuating the potential importance of these previously unknown proteins in microbial metabolism.central carbon metabolism ͉ genome context analysis ͉ lactate dehydrogenase M any aerobic and anaerobic bacteria are able to grow by using D-and/or L-lactate as a sole source of carbon and energy (1-4). Although lactate is a common product of carbohydrate fermentation (5, 6), it is rarely detected in environmental samples (7,8), suggesting that it is either a minor metabolic product or that its conversion rates are very high. In support of the latter possibility, Finke et al. (9) recently reported constant production and consumption of lactate in marine sediments, linking its high turnover rates with microbiological reduction of sulfate and metals.Among microorganisms actively coupling lactate oxidation to the reduction of multiple electron acceptors is a diverse and ubiquitous group of dissimilatory metal-reducing bacteria, which belong to the genus Shewanella (10). Shewanellae are commonly found in complex microbial communities within aquatic and sedimentary systems, many of which are subject to spatial and temporal variations in the type and concentration of organic and inorganic substrates that reflect redox gradients (10). The versatile flexibility of energygenerating pathways, which enables respiration of various electron acceptors including O 2 , Fe(III), Mn(IV), thiosulfate, elemental sulfur, and nitrate, contributes to the ability...

Methods for chemical modifications of proteins have been crucial for the advancement of proteomics. In particular, site-specific covalent labeling of proteins with fluorophores and other moieties has permitted the development of a multitude of assays for proteome analysis. A common approach for such a modification is solvent-accessible cysteine labeling using thiol-reactive dyes. Cysteine is very attractive for site-specific conjugation due to its relative rarity throughout the proteome and the ease of its introduction into a specific site along the protein's amino acid chain. This is achieved by site-directed mutagenesis, most often without perturbing the protein's function. Bottlenecks in this reaction, however, include the maintenance of reactive thiol groups without oxidation before the reaction, and the effective removal of unreacted molecules prior to fluorescence studies. Here, we describe an efficient, specific, and rapid procedure for cysteine labeling starting from well-reduced proteins in the solid state. The efficacy and specificity of the improved procedure are estimated using a variety of single-cysteine proteins and thiol-reactive dyes. Based on UV/vis absorbance spectra, coupling efficiencies are typically in the range 70-90%, and specificities are better than ~95%. The labeled proteins are evaluated using fluorescence assays, proving that the covalent modification does not alter their function. In addition to maleimide-based conjugation, this improved procedure may be used for other thiol-reactive conjugations such as haloacetyl, alkyl halide, and disulfide interchange derivatives. This facile and rapid procedure is well suited for high throughput proteome analysis.

Production of milligram quantities of numerous proteins for structural and functional studies requires an efficient purification pipeline. We found that the dual tag, his 6 -tag-maltose-binding protein (MBP), intended to facilitate purification and enhance proteins' solubility, disrupted such a pipeline, requiring additional screening and purification steps. Not all proteins rendered soluble by fusion to MBP remained soluble after its proteolytic removal, and in those cases where the protein remained soluble, standard purification protocols failed to remove completely the stoichiometric amount of his 6 -tagged MBP generated by proteolysis. Both liabilities were alleviated by construction of a vector that produces fusion proteins in which MBP, the his 6 -tag and the target protein are separated by highly specific protease cleavage sites in the configuration MBP-site-his 6 -site-protein. In vivo cleavage at the first site by co-expressed protease generated untagged MBP and his 6 -tagged target protein.Proteins not truly rendered soluble by transient association with MBP precipitated, and untagged MBP was easily separated from the his-tagged target protein by conventional protocols. The second protease cleavage site allowed removal of the his 6 -tag. KeywordsHigh-throughput; Structural genomics; Maltose-binding protein; TVMV protease; Ligationindependent cloningThe burgeoning genomic information now available makes vast numbers of proteins accessible for structural and functional studies, and many large-scale projects have developed automated protocols for amplifying, cloning, and expressing genes, and for screening proteins for desirable properties [1][2][3][4][5]. Similar strides have been made in streamlining protein purification, but production of sufficient material for detailed structural and functional characterization remains labor-intensive and time-consuming [3,4,6,7]. Typically, purification is facilitated by fusing proteins to affinity tags, most commonly a his-tag, which allows purification by immobilized metal-ion affinity chromatography (IMAC,[8]). Additional tags are often attached to improve proteins' solubility, such as maltose-binding protein (MBP) [2][3][4]9,10]. In typical protein production pipelines, the resulting fusion proteins are first screened for ☆ This manuscript has been created by the University of Chicago as operator of Argonne National Laboratory under Contract No.W-31-109-ENG-38 with the US Department of Energy. The US government retains for itself, and others acting on its behalf, a paid-up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the government. The US Government's right to retain a nonexclusive royaltyfree license in and to the copyright covering this paper, for governmental purposes, is acknowledged. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript solubility, then purified by semi-robotic p...

Cyclic di-GMP (c-di-GMP) is a ubiquitous bacterial second messenger that is involved in the regulation of cell surface-associated traits and the persistence of infections. Omnipresent GGDEF and EAL domains, which occur in various combinations with regulatory domains, catalyze c-di-GMP synthesis and degradation, respectively. The crystal structure of full-length YkuI from Bacillus subtilis, composed of an EAL domain and a C-terminal PAS-like domain, has been determined in its native form and in complex with c-di-GMP and Ca 2؉. The EAL domain exhibits a triose-phosphate isomerase-barrel fold with one antiparallel ␤-strand. The complex with c-di-GMP-Ca 2؉ defines the active site of the putative phosphodiesterase located at the C-terminal end of the ␤-barrel. The EAL motif is part of the active site with Glu-33 of the motif being involved in cation coordination. The structure of the complex allows the proposal of a phosphodiesterase mechanism, in which the divalent cation and the general base Glu-209 activate a catalytic water molecule for nucleophilic in-line attack on the phosphorus. The C-terminal domain closely resembles the PASfold. Its pocket-like structure could accommodate a yet unknown ligand. YkuI forms a tight dimer via EAL-EAL and trans EAL-PASlike domain association. The possible regulatory significance of the EAL-EAL interface and a mechanism for signal transduction between sensory and catalytic domains of c-di-GMP-specific phosphodiesterases are discussed.

GlvA, a 6-phospho-alpha-glucosidase from Bacillus subtilis, catalyzes the hydrolysis of maltose-6'-phosphate and belongs to glycoside hydrolase family GH4. GH4 enzymes are unique in their requirement for NAD(H) and a divalent metal for activity. We have determined the crystal structure of GlvA in complex with its ligands to 2.05 A resolution. Analyses of the active site architecture, in conjunction with mechanistic studies and precedent from the nucleotide diphosphate hexose dehydratases and other systems, suggest a novel mechanism of glycoside hydrolysis by GlvA that involves both the NAD(H) and the metal.