Lactose-specific enzyme II of the phosphoenolpyruvate-dependent phosphotransferase system of Staphylococcus aureus. Purification of the histidine-tagged transmembrane component IICBLac and its hydrophilic IIB domain by metal-affinity chromatography, and functional characterization

Bacterial expression, correct membrane targeting and functional folding of the HIV-1 membrane protein Vpu using a periplasmic signal peptide

Viral protein U (Vpu) is a type-III integral membrane protein encoded by Human Immunodeficiency Virus-1 (HIV- 1). It is expressed in infected host cells and plays several roles in viral progeny escape from infected cells, including down-regulation of CD4 receptors. But key structure/function questions remain regarding the mechanisms by which the Vpu protein contributes to HIV-1 pathogenesis. Here we describe expression of Vpu in bacteria, its purification and characterization. We report the successful expression of PelB-Vpu in Escherichia coli using the leader peptide pectate lyase B (PelB) from Erwinia carotovora. The protein was detergent extractable and could be isolated in a very pure form. We demonstrate that the PelB signal peptide successfully targets Vpu to the cell membranes and inserts it as a type I membrane protein. PelB-Vpu was biophysically characterized by circular dichroism and dynamic light scattering experiments and was shown to be an excellent candidate for elucidating structural models.

Membrane docking of an aggregation-prone protein improves its solubilization

We used preS2-S'-beta-galactosidase, a three domain fusion protein that aggregates extensively at 43 degrees C in the cytoplasm of Escherichia coli to search for multicopy suppressors of protein aggregation and inclusion bodies formation, and took advantage of the known differential solubility of preS2-S'-beta-galactosidase at 37 and 43 degrees C to develop a selection procedure for the gene products that would prevent its aggregation in vivo at 43 degrees C. First, we demonstrate that the differential solubility of preS2-S'-beta-galactosidase results in a lactose-positive phenotype at 37 degrees C as opposed to a lactose-negative phenotype at 43 degrees C. We searched for multicopy suppressors of preS2-S'-beta-galactosidase aggregation at 43 degrees C by selecting pink lactose-positive colonies on a background of white lactose-negative colonies after transformation of bacteria with an E. coli gene bank. We found only two multicopy suppressors of preS2-S'-beta-galactosidase aggregation at 43 degrees C, protein isoaspartate methyltransferase (PIMT) and the membrane components ChbBC of the N,N'-diacetylchitobiose phosphotransferase transporter. We have previously shown that PIMT overexpression reduces the level of isoaspartate in preS2-S'-beta-galactosidase, increases its thermal stability and consequently helps in its solubilization at 43 degrees C (Kern et al., J. Bacteriol. 187, 1377-1383). In the present work, we show that ChbBC overexpression targets a fraction of preS2-S'-beta-galactosidase to the membrane, and decreases its amount in inclusion bodies, which results in its decreased thermodenaturation and in a lactose-positive phenotype at 43 degrees C. Cross-linking experiments show that the inner membrane protein ChbC interacts with preS2-S'-beta-galactosidase. Our results suggest that membrane docking of aggregation-prone proteins might be a useful method for their solubilization.

Expression cloned cDNA for 10-deacetylbaccatin III-10-O-acetyltransferase in Escherichia coli: a comparative study of three fusion systems

10-Deacetylbaccatin III-10-O-acetyltransferase (10-DABT) catalyzes the formation of baccatin III, which is an immediate diterpenoid precursor of Taxol. A cDNA encoding 10-DABT was cloned from Taxus baccata by using RT-PCR and screening a cDNA library. A study of its heterologous overexpression in Escherichia coli was carried out. To get high-level expression of recombinant enzyme, three kinds of IPTG inducible fusion expression systems (with glutathione S-transferase (GST), hexahistidine (6x His), and biotinylated tag) were used, and results of expression were compared. Fusion 10-DABT with different tags was expressed with diverse expression levels and solubility in the three systems. Optimum IPTG concentration, temperature, and inducing time for producing recombinant enzymes were found. Under higher IPTG concentration (up to 1 mM), the highest level of expression for fusion protein was obtained in the 6x His fusion system with phage T5 promoter, but expressed products were only partially soluble. With lower IPTG concentration (less than 0.5 mM), the highest expression was detected in the GST fusion system with tac promoter, and the lowest level of expression appeared in the biotinylated fusion system. The expression level in the latter system did not differ dramatically with a range of different inducer concentrations. GST and 6x His fusion proteins were mainly soluble in aqueous solutions and Triton X-100 improved the solubility of biotinylated fusion proteins (inferring this protein is membrane-associated). Fusion proteins could only be partially purified by a single affinity chromatography step for all three systems. Glutathione-coupled matrix and streptavidin-conjugated resin have higher specificity than Ni-NTA resin, and elution conditions were shown to affect enzyme activity. Three kinds of recombinant 10-DABT with different tags showed enzyme activity, but total enzyme activity was lost as a result of the affinity chromatography step. Thrombin and Factor Xa could be used for site-specific cleavage of fusion proteins, but the incubation temperature affected enzyme activity of recombinant enzymes.

Enhancement of heparin cofactor II anticoagulant activity

Heparin cofactor II (HCII) is a serpin whose thrombin inhibition activity is accelerated by glycosaminoglycans. We describe the novel properties of a carboxyl-terminal histidine-tagged recombinant HCII (rHCII-CHis(6)). Thrombin inhibition by rHCII-CHis(6) was increased >2-fold at approximately 5 microgram/ml heparin compared with wild-type recombinant HCII (wt-rHCII) at 50-100 microgram/ml heparin. Enhanced activity of rHCII-CHis(6) was reversed by treatment with carboxypeptidase A. We assessed the role of the HCII acidic domain by constructing amino-terminal deletion mutants (Delta1-52, Delta1-68, and Delta1-75) in wt-rHCII and rHCII-CHis(6). Without glycosaminoglycan, unlike wt-rHCII deletion mutants, the rHCII-CHis(6) deletion mutants were less active compared with full-length rHCII-CHis(6). With glycosaminoglycans, Delta1-68 and Delta1-75 rHCIIs were all less active. We assessed the character of the tag by comparing rHCII-CHis(6), rHCII-CAla(6), and rHCII-CLys(6) to wt-rHCII. Only rHCII-CHis(6) had increased activity with heparin, whereas all three mutants have increased heparin binding. We generated a carboxyl-terminal histidine-tagged recombinant antithrombin III to study the tag on another serpin. Interestingly, this mutant antithrombin III had reduced heparin cofactor activity compared with wild-type protein. In a plasma-based assay, the glycosaminoglycan-dependent inhibition of thrombin by rHCII-CHis(6) was significantly greater compared with wt-rHCII. Thus, HCII variants with increased function, such as rHCII-CHis(6), may offer novel reagents for clinical application.

Staphylococcal phosphoenolpyruvate-dependent phosphotransferase system--two highly similar glucose permeases in Staphylococcus carnosus with different glucoside specificity: protein engineering in vivo?

Previous sequence analysis of the glucose-specific PTS gene locus from Staphylococcus carnosus revealed the unexpected finding of two adjacent, highly similar ORFs, glcA and glcB, each encoding a glucose-specific membrane permease EIICBA(Glc). glcA and glcB show 73% identity at the nucleotide level and glcB is located 131 bp downstream from glcA. Each of the genes is flanked by putative regulatory elements such as a termination stem-loop, promoter and ribosome-binding site, suggesting independent regulation. The finding of putative cis-active operator sequences, CRE (catabolite-responsive elements) suggests additional regulation by carbon catabolite repression. As described previously by the authors, both genes can be expressed in Escherichia coli under control of their own promoters. Two putative promoters are located upstream of glcA, and both were found to initiate transcription in E. coli. Although the two permeases EIICBA(Glc)1 and EIICBA(Glc)2 show 69% identity at the protein level, and despite the common primary substrate glucose, they have different specificities towards glucosides as substrate. EIICBA(Glc)1 phosphorylates glucose in a PEP-dependent reaction with a Km of 12 microM; the reaction can be inhibited by 2-deoxyglucose and methyl beta-D-glucoside. EIICBA(Glc)2 phosphorylates glucose with a Km of 19 microM and this reaction is inhibited by methyl alpha-D-glucoside, methyl beta-D-glucoside, p-nitrophenyl alpha-D-glucoside, o-nitrophenyl beta-D-glucoside and salicin, but unlike other glucose permeases, including EIICBA(Glc)1, not by 2-deoxyglucose. Natural mono- or disaccharides, such as mannose or N-acetylglucosamine, that are transported by other glucose transporters are not phosphorylated by either EIICBA(Glc)1 nor EIICBA(Glc)2, indicating a high specificity for glucose. Together, these findings support the suggestion of evolutionary development of different members of a protein family, by gene duplication and subsequent differentiation. C-terminal fusion of a histidine hexapeptide to both gene products did not affect the activity of the enzymes and allowed their purification by Ni2+-NTA affinity chromatography after expression in a ptsG (EIICB(Glc)) deletion mutant of E. coli. Upstream of glcA, the 3' end of a further ORF encoding 138 amino acid residues of a putative antiterminator of the BglG family was found, as well as a putative target DNA sequence (RAT), which indicates a further regulation by glucose specific antitermination.

Purification and identification of two putative autolytic sites in human calpain 3 (p94) expressed in heterologous systems

Human muscle-specific calpain (CAPN3) was expressed in two heterologous systems: Sf9 insect cells and Escherichia coli cells. Polyclonal antibodies were prepared against peptides whose sequences were taken from the three unique regions of human CAPN3, namely NS, IS1, and IS2, which are not found in other members of the calpain family. Western blot analysis using these antibodies revealed that CAPN3 was well expressed in both systems. However, considerable rapid degradation of the expressed CAPN3 was observed in both Sf9 and E. coli cells. These antibodies were therefore also used to detect CAPN3 and its degradation products in human and rat muscles, as well as to detect the protein throughout the purification of the recombinant His-tagged human CAPN3 by Ni2+ affinity chromatography and by immunopurification over immobilized antibody. An alternative purification procedure was used for purification of all putative CAPN3 immunoreactive fragments by combining SDS-PAGE and hydroxyapatite chromatography. Two fragments of CAPN3 of approximately 55 kDa were purified, and their N-terminal amino acid sequencing demonstrated that cleavage of CANP3 occurred between residues 30-31 and 412-413, thus providing the first evidence for the localization of putative autolytic sites in this enzyme.

Quantitative evaluation of neurotensin receptor purification by immobilized metal affinity chromatography

Immobilized metal affinity chromatography has recently been used for purification of histidine-tagged membrane proteins in the presence of detergents with varying success. Strong binding to the metal resin is essential for purification when expression levels are low. We have investigated the influence of tag length and type of detergent on the purification of a neurotensin receptor fusion protein expressed in Escherichia coli at a level of about 0.1% of membrane protein. Receptors with six C-terminal histidine residues did not bind to nickel resin in the presence of the anionic detergent sodium dodecyl sulfate. In contrast, partial purification assessed by densitometry of Coomassie-stained gels was achieved using the nonionic detergents dodecyl maltoside or Triton X-100 (53% pure), or a detergent mixture containing the zwitterionic detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (46% pure). Linking a highly charged epitope tag to the histidine tail did not affect the nickel-binding properties of receptors. The level of purification was substantially improved (72% pure) by extending the histidine tail to 10 residues because this allowed stringent washes at high imidazole concentration to remove nonspecifically bound contaminants. This strategy not only resulted in efficient purification of receptors from crude membranes, but also worked particularly well for single-step purification from total cell lysates, resulting in 340-fold purification of functional neurotensin receptor.

Purification of a rat neurotensin receptor expressed in Escherichia coli

A truncated rat neurotensin receptor (NTR), expressed in Escherichia coli with the maltose-binding protein fused to its N-terminus and the 13 amino acid Bio tag fused to its C-terminus, was purified to apparent homogeneity in two steps by use of the monomeric avidin system followed by a novel neurotensin column. This purification protocol was developed by engineering a variety of affinity tags on to the C-terminus of NTR. Surprisingly, expression levels varied considerably depending on the C-terminal tag used. Functional expression of NTR was highest (800 receptors/cell) when thioredoxin was placed between the receptor C-terminus and the tag, indicating a stabilizing effect of the thioredoxin moiety. Several affinity chromatography methods were tested for purification. NTR with the in vivo-biotinylated Bio tag was purified with the highest efficiency compared with NTR with the Strep tag or a hexa-histidine tail. Co-expression of biotin ligase improved considerably the in vivo biotinylation of the Bio tag and, therefore, the overall purification yield. Proteolysis of the NTR fusion protein was prevented by removing a protease-sensitive site discovered at the N-terminus of NTR. The ligand binding properties of the purified receptor were similar to those of the membrane-bound protein and the native receptor. The scale-up of this purification scheme, to provide sufficient protein for biophysical studies, is in progress.