Rapid and efficient purification of native histidine-tagged protein expressed by recombinant vaccinia virus

Phosphohistidyl active sites in polyphosphate kinase of Escherichia coli

In the synthesis of inorganic polyphosphate (polyP) from ATP by polyphosphate kinase (PPK; EC 2.7.4.1) of Escherichia coli, an N-P-linked phosphoenzyme was previously identified as the intermediate. The phosphate is presumed to be linked to N3 of the histidine residue because of its chemical stabilities and its resemblance to other enzymes known to contain N3-phosphohistidine. Tryptic digests of [32P]PPK contain a predominant 32P-labeled peptide that includes His-441. Of the 16 histidine residues in PPK of E. coli, 4 are conserved among several bacterial species. Mutagenesis of these 4 histidines shows that two (His-430 and His-598) are unaffected in function when mutated to glutamine, whereas two others (His-441 and His-460) mutated to glutamine or alanine fail to be phosphorylated, show no enzymatic activities, and fail to support polyP accumulation in cells bearing these mutant enzymes.

Treatment of vascular smooth muscle cells with antisense phosphorothioate oligodeoxynucleotides directed against p42 and p44 mitogen-activated protein kinases abolishes DNA synthesis in response to platelet-derived growth factor

We have investigated the requirement for mitogen-activated protein (MAP) kinase in the stimulation of DNA synthesis by platelet-derived growth factor (PDGF) in rat aortic smooth muscle cells using a phosphorothioate-modified oligodeoxy-nucleotide (ODN) to deplete MAP kinase. Treatment for 72 h with MAP kinase antisense ODN directed against both the p42 and p44 isoforms of MAP kinase abolished the expression of MAP kinase and reduced agonist-stimulated MAP kinase activity by approx. 95%. The scrambled control ODN was without effect, but the sense control ODN slightly enhanced the expression of both isoforms. Abolition of MAP kinase activity by antisense ODN treatment prevented angiotensin II- and PDGF-stimulated activation of p90 ribosomal S6 kinase activity, but did not affect activation of MAP kinase kinase. In addition, antisense ODN pretreatment reduced PDGF-stimulated [3H]thymidine incorporation to < 5% of control, and decreased basal incorporation by approx. 90%. In contrast, basal [3H]thymidine incorporation was enhanced approx. 60% by control sense ODN treatment. These results indicate an obligatory role for MAP kinase in the activation of a number of early events in mitogenesis, including DNA synthesis, in vascular smooth muscle cells.

Myosin motors with artificial lever arms

The myosin head consists of a globular catalytic domain and a light chain binding domain (LCBD). The coupling efficiency between ATP hydrolysis and myosin-induced actin movement is known to decline as the LCBD is truncated or destabilized. However, it was not clear whether the observed alteration in the production of force and movement reflects only the mechanical changes to the length of the LCBD or whether these changes also affect the kinetic properties of the catalytic domain. Here we show that replacement of the LCBD with genetically engineered domains of similar rigidity and dimensions produces functional molecular motors with unchanged kinetic properties. The resulting single-chain, single-headed motors were produced in Dictyostelium discoideum and obtained after purification from a standard peptone-based growth medium at levels of up to 12 mg/l. Their actin motility properties are similar or greater than those of native myosin. Rates of 2.5 and 3.3 microm/s were observed for motor domains fused to one or two of these domains, respectively. Their kinetic and functional similarity to the extensively studied myosin subfragment 1 (S1) and their accessibility to molecular genetic approaches makes these simple constructs ideal models for the investigation of chemo-mechanical coupling in the myosin motor.

Purification and in vitro-phospholabeling of secretory envelope proteins E1 and E2 of hepatitis C virus expressed in insect cells

The putative envelope glycoproteins of hepatitis C virus (HCV), E1 and E2, were expressed as recombinant, secretory proteins in Sf9 insect cells through infection with recombinant baculoviruses. The influenza virus hemagglutinin signal sequence (HASS) was inserted upstream of the HCV-cDNAs in order to effect secretion. Furthermore, a hexa-histidine tag for purification on a Ni(2+)-nitrilotriacetic acid (Ni(2+)-NTA) column and a protein kinase A (PKA) recognition sequence for in vitro-phospholabeling were fused upstream of the HCV-cDNA. E1- and E2 proteins lacking their carboxy-terminal, hydrophobic sequence were produced by baculovirus-infected insect cells in bioreactors of 23 1. The medium was concentrated and proteins were purified under native conditions on Ni(2+)-NTA columns. Purified proteins could be phospholabeled in vitro using the catalytic subunit of protein kinase. A isolated from bovine heart and gamma-[32P]ATP. Labeled E1 and E2 proteins expressed in insect cells could be immunoprecipitated with sera from HCV-infected patients. Co-expression of these E1 and E2 proteins led to the formation of E1-E2 complexes within the insect cell and to secretion of these complexes into the medium.

Basal phosphorylation of the PEST domain in the I(kappa)B(beta) regulates its functional interaction with the c-rel proto-oncogene product

The product of the c-rel proto-oncogene (c-Rel) belongs to the NF-kappaB/Rel family of polypeptides and has been implicated in the transcriptional control of cell proliferation and immune function. In human T lymphocytes, c-Rel is sequestered in the cytoplasmic compartment by constitutively phosphorylated inhibitors, including I(kappa)B(alpha) and I(kappa)B(beta). Studies with bacterially expressed forms of these inhibitory proteins revealed that unphosphorylated I(kappa)B(alpha) but not I(kappa)B(beta) assembles with c-Rel and inhibits its DNA binding activity. Furthermore, latent I(kappa)B(beta)-c-Rel complexes derived from mammalian cells were sensitive to phosphatase treatment, whereas I(kappa)B(alpha)-c-Rel complexes were resistant. We have identified a constitutive protein kinase in unstimulated T cells that associates with and phosphorylates I(kappa)B(beta) in vitro. The substrate specificity, electrophoretic mobility, and antigenic properties of this I(kappa)B(beta)-associated kinase (BAK) suggest identity with casein kinase II (CKII), an enzyme known to mediate basal phosphorylation of I(kappa)B(alpha). Phosphorylation of recombinant I(kappa)B(beta) by either BAK or CKII restored the capacity of this inhibitor to antagonize the DNA binding activity of c-Rel. Peptide mapping and mutational analyses localized the bulk of the basal phosphorylation sites in I(kappa)B(beta) to the C-terminal PEST domain, which contains two potential acceptors for CKII-mediated phosphoryl group transfer (Ser-313 and Ser-315). Point mutations introduced into the full-length inhibitor at Ser-313 and Ser-315 led to a significant reduction in the phosphorylation of I(kappa)B(beta) and severely impaired its c-Rel inhibitory function in vivo. Taken together, these findings strongly suggest that basal phosphorylation of the PEST domain of I(kappa)B(beta) at consensus CKII sites is required for the efficient formation of latent I(kappa)B(beta)-c-Rel complexes.

Characterisation of a panel of monoclonal antibodies raised against recombinant HCV core protein

Hepatitis C virus (HCV) has as yet no practical culture system so any antigen or antibody studies must be carried out using recombinant antigens. In this study, HCV core sequence was amplified by PCR, inserted into pRSET, and expressed in E. coli. The resultant core protein was purified using nickel affinity chromatography which bound the six histidine tag attached to the N-terminus of the protein. After elution in imidazole buffer, the core protein was used to immunise Balb/c mice and monoclonal antibodies against HCV core were raised. Six monoclonals were examined in a variety of assays. All of them recognised the p27 kDa protein which they were raised against and 2D2 and 3D7 recognised the core component of an HCV Recombinant Immunoblot Assay (RIBA). None of the antibodies recognised the linear peptides in an Innolia HCV assay. 2D2 showed cytoplasmic granular staining in 1-5% of cells in frozen section of HCV infected livers. Cross-competition assays between themselves and human anti-HCV core positive sera divided the antibodies into two main groups (I and II), with a sub-division of group I into a and b. Group I antibodies were unable to be inhibited by human anti-HCV sera whereas group II antibodies were inhibited by these sera (up to 62%). Epitopes recognised by all the monoclonals were probably conformational with the group I epitope being located within the first 105 amino acids of the core sequence and the group II epitope between amino acids 105 and 160.

Analysis of the recognition mechanism of the alternative pathway of complement by monoclonal anti-factor H antibodies: evidence for multiple interactions between H and surface bound C3b

The ability of the alternative pathway of complement to discriminate targets as either activators or non-activators is mediated by different binding properties of factor H to surface-associated C3b molecules. In the present study we have probed the interaction between H and C3b using five anti-H mAb. The binding sites of the mAb were mapped by Western blotting using both recombinant and trypsin-generated H fragments. Two mAb bound to CCP1 (90X, 196X), two to CCP5 (MRC OX24, 86X) and one to CCP8-15a (131X). At a molar ratio 2:1 of 125I-H:mAb all tested mAb enhanced binding of H to both activator- and non-activator-bound C3b. At higher concentrations two mAb had an inhibitory effect on H binding to surface-associated C3b (OX24, 131X). Thus the mAb 131X inhibits H binding to surface-bound C3b but unlike OX24 it does not bind to the previously described C3b binding site within or near CCP4-5. These results indicate that there is an additional interaction site on factor H for surface-bound C3b.

The Ras GTPase-activating-protein-related human protein IQGAP2 harbors a potential actin binding domain and interacts with calmodulin and Rho family GTPases

We previously described IQGAP1 as a human protein related to a putative Ras GTPase-activating protein (RasGAP) from the fission yeast Schizosaccharomyces pombe. Here we report the identification of a liver-specific human protein that is 62% identical to IQGAP1. Like IQGAP1, the novel IQGAP2 protein harbors an N-terminal calponin homology motif which functions as an F-actin binding domain in members of the spectrin, filamin, and fimbrin families. Both IQGAPs also harbor several copies of a novel 50- to 55-amino-acid repeat, a single WW domain, and four IQ motifs and have 25% sequence identity with almost the entire S. pombe sar1 RasGAP homolog. As predicted by the presence of IQ motifs, IQGAP2 binds calmodulin. However, neither full-length nor truncated IQGAP2 stimulated the GTPase activity of Ras or its close relatives. Instead, IQGAP2 binds Cdc42 and Racl but not RhoA. This interaction involves the C-terminal half of IQGAP2 and appears to be independent of the nucleotide binding status of the GTPases. Although IQGAP2 shows no GAP activity towards Cdc42 and Rac1, the protein did inhibit both the intrinsic and RhoGAP-stimulated GTP hydrolysis rates of Cdc42 and Rac1, suggesting an alternative mechanism via which IQGAPs might modulate signaling by these GTPases. Since IQGAPs harbor a potential actin binding domain, they could play roles in the Cdc42 and Rac1 controlled generation of specific actin structures.

Immobilized metal-ion affinity chromatography: imidazole proton pump and chromatographic sequelae. I. Proton pump

Complexation of imidazole (Im) with an iminodiacetate (IDA) metal chelate [IDA-M(II)] ligand of chelating gel results in an acidification of the mobile phase. The scope of the action of this IDA-M(II)Im 'proton pump' in IMAC is determined by: (a) IDA-M(II) density of the gel; (b) concentration of applied Im; and (c) the buffering capacity of the mobile phase. Application of Im onto a metal chelate column in a gradient rather than in a stepwise manner, mitigates the proton pump's action, as it does an increase of buffer concentration in the mobile phase. However, only an antecedent conversion of the metal chelate gel, IDA-M(II), to its Im derivative, IDA-M(II) Im, can effectively circumscribe the action of the proton pump. The same holds true, as anticipated, when another chelating ligand (nitrilotriacetate) is used.

Immobilized metal-ion affinity chromatography: imidazole proton pump and chromatographic sequelae. II. Chromatographic sequelae

Isocratic elution with imidazole of some protein models from a chelating gel, CSFF-IDA-M(II), resulted in their desorption owing to the low pH(6-->4) of the mobile, phase rather than to the imidazole itself (imidazole-generated fall in pH; proton pump). Gradient elution with imidazole was best accomplished when the chelating gel was initially converted into its imidazole complex, CSFF-IDA-M(II)Im. The exploitation of the imidazole-generated proton pump in the IMAC of proteins may enhance the versatility of this type of chromatography.

Characterization of three potato lipoxygenases with distinct enzymatic activities and different organ-specific and wound-regulated expression patterns

Lipoxygenases are ubiquitous enzymes in eukaryotes. In plants, lipoxygenases are involved in the synthesis of the hormone jasmonic acid that regulates plant responses to wounding and, in addition, is an inducer of tuberization in potato. We have isolated potato lipoxygenase cDNA clones. From their deduced amino acid sequences, three distinct classes are defined (Lox1, Lox2, and Lox3). They are encoded in gene families that display organ-specific expression, lox1 being expressed mostly in tubers and roots, lox2 in leaves, and lox3 in leaves and roots. Consistent with their organ-specific expression pattern, Lox1 expressed in bacteria preferentially uses as substrate linoleic acid, abundant in membrane lipids of tubers, whereas linolenic acid, prevalent in leaves, is the preferred substrate for the other two classes of lipoxygenase. Analyses on reaction products of the enzymes expressed in bacteria reveal that Lox1 primarily produces 9- hydroperoxides. In contrast, the jasmonic acid precursor, 13-hydroperoxylinolenic acid, is the major product of the action of Lox2 and Lox3 on linolenic acid. Upon wounding, the levels of Lox2 and Lox3 transcripts rise markedly in leaves. While Lox3 mRNA accumulation peaks as early as 30 min after wounding, Lox2 shows a steady increase over a 24-h time course, suggesting different roles for these lipoxygenase isoforms in the synthesis of the plant hormone jasmonic acid.

Enhanced metalloadsorption of bacterial cells displaying poly-His peptides

The properties of Escherichia coli cells, acquired by cell surface presentation of one or two hexahistidine (His) clusters carried by the outer membrane LamB protein, have been examined. Strains producing LamB hybrids with the His chains accumulated greater than 11-fold more Cd2+ than E. coli cells expressing the protein without the His insert. Furthermore, the hexa-His chains on the cell surface caused cells to adhere reversibly to a Ni(2+)-containing solid matrix in a metal-dependent fashion. Thus, expression of poly-His peptides enables bacteria to act as a metalloaffinity adsorbent. These results open up the possibility for biosorption of heavy ions using engineered microorganisms.

Heterodimer formation and activity in the human enzyme galactose-1-phosphate uridylyltransferase

One of the fundamental questions concerning expression and function of dimeric enzymes involves the impact of naturally occurring mutations on subunit assembly and heterodimer activity. This question is of particular interest for the human enzyme galactose-l-phosphate uridylyl-transferase (GALT), impairment of which results in the inherited metabolic disorder galactosemia, because many if not most patients studied to date are compound heterozygotes rather than true molecular homozygotes. Furthermore, the broad range of phenotypic severity observed in these patients raises the possibility that allelic combination, not just allelic constitution, may play some role in determining outcome. In the work described herein, we have selected two distinct naturally occurring null mutations of GALT, Q188R and R333W, and asked the questions (i) what are the impacts of these mutations on subunit assembly, and (ii) if heterodimers do form, are they active? To answer these questions, we have established a yeast system for the coexpression of epitope-tagged alleles of human GALT and investigated both the extent of specific GALT subunit interactions and the activity of defined heterodimer pools. We have found that both homodimers and heterodimers do form involving each of the mutant subunits tested and that both heterodimer pools retain substantial enzymatic activity. These results are significant not only in terms of their implications for furthering our understanding of galactosemia and GALT holoenzyme structure-function relationships but also because the system described may serve as a model for similar studies of other complexes composed of multiple subunits.

Cellular mechanisms for the formation of a soluble form derivative of T-cell receptor alpha chain by suppressor T cells

Upon stimulation with anti-CD3, suppressor T-cell (Ts) hybridomas and homologous transfectants of T-cell receptor a (TCRalpha) cDNA in the T-cell hybridoma formed a 55-kDa TCRalpha chain derivative that bound both the monoclonal anti-TCRalpha chain and polyclonal antibodies against glycosylation inhibiting factor (GIF). The peptide is a subunit of antigen-specific suppressor T-cell factor (TsF), and is considered to be a posttranslationally-formed conjugate of TCRalpha chain with GIF peptide. The TCRalpha derivative is synthesized by the transfectant after stimulation with anti-CD3, and not derived from TCR present on the cell surface. Stimulation of the stable homologous transfectants with anti-CD3 induced translocation of the 13-kDa GIF peptide into endoplasmic reticulum (ER). When a helper Ts hybridoma or a stable transfectant of the same TCRalpha cDNA in a helper cell-derived TCRalpha- clone was stimulated with anti-CD3, translocation of GIF peptide was not detected, and these cells failed to secrete a TCRalpha derivative. However, further transfection of a chimeric cDNA encoding a procalcitonin-GIF fusion protein into the helper cell-derived stable transfectant of TCRalpha cDNA resulted in translocation of the GIF protein and formation of bioactive 55-kDa GIF. The results indicated that translocation of GIF peptide through ER is unique for Ts cells, and that this process is essential for the formation/secretion of the soluble form derivative of TCRalpha chain by T cells.

Transactivation by the thyroid hormone receptor is dependent on the spacer sequence in hormone response elements containing directly repeated half-sites

The thyroid hormone receptor (TR) regulates the transcription of its target genes by interacting with specific hormone response elements consisting usually of directly repeated half-sites with the consensus sequence AGGTCA. To investigate the role of the spacer sequences separating the half-sites, heterodimers formed by TRalpha and the retinoid-X receptor (RXR) were used in a PCR based selection and amplification assay. The TRalpha/RXR heterodimer selected for elements with directly repeated half-sites having a spacer of 4 nucleotides (DR4). Preferences for nucleotides in the TR binding half-site motif as well as for the 4 nucleotides separating the two half-sites were found. DNA binding and transfection studies using DR4 elements with different spacer sequences showed the importance of these nucleotides for the activity of the response element: some spacer sequences allowed little or no transactivation from the element, whereas other sequences supported strong transactivation. A pyrimidine nucleotide in position three of the spacer enhanced TRalpha binding and transactivation. Additional experiments showed that heterodimers between RXR and other putative receptors exhibited a similar but distinct specificity for the spacer sequence. Our results thus suggest that the four nucleotides separating the two half-sites in hormone response elements have a major role in determining induction of hormone responsive genes.

A new type of cohesin domain that specifically binds the dockerin domain of the Clostridium thermocellum cellulosome-integrating protein CipA

The cellulosome-integrating protein CipA, which serves as a scaffolding protein for the cellulolytic complex produced by Clostridium thermocellum, comprises a COOH-terminal duplicated segment termed the dockerin domain. This paper reports the cloning and sequencing of a gene, termed sdbA (for scaffoldin dockerin binding), encoding a protein which specifically binds the dockerin domain of CipA. The sequenced fragment comprises an open reading frame of 1,893 nucleotides encoding a 631-amino-acid polypeptide, termed SdbA, with a calculated molecular mass of 68,577 kDa. SAA comprises an NH2-terminal leader peptide followed by three distinct regions. The NH2-terminal region is similar to the NH2-terminal repeats of C. thermocellum OlpB and ORF2p. The central region is rich in lysine and harbors a motif present in Streptococcus M proteins. The COOH-terminal region consists of a triplicated sequence present in several bacterial cell surface proteins. The NH2-terminal region of SdbA and a fusion protein carrying the first NH2-terminal repeat of OlpB were shown to bind the dockerin domain of CipA. Thus, a new type of cohesin domain, which is present in one, two, and four copies in SdbA, ORF2p, and OlpB, respectively, can be defined. Since OlpB and most likely SdbA and ORF2p are located in the cell envelope, the three proteins probably participate in anchoring CipA (and the cellulosome) to the cell surface.

Co-purification and direct interaction of Ras with caveolin, an integral membrane protein of caveolae microdomains. Detergent-free purification of caveolae microdomains

Caveolae are plasma membrane specializations that have been implicated in signal transduction. Caveolin, a 21-24-kDa integral membrane protein, is a principal structural component of caveolae membranes in vivo. G protein alpha subunits are concentrated in purified preparations of caveolae membranes, and caveolin interacts directly with multiple G protein alpha subunits, including G(s), G(o), and G(i2). Mutational or pharmacologic activation of G alpha subunits prevents the interaction of caveolin with G proteins, indicating that inactive G alpha subunits preferentially interact with caveolin. Here, we show that caveolin interacts with another well characterized signal transducer, Ras. Using a detergent-free procedure for purification of caveolin-rich membrane domains and a polyhistidine tagged form of caveolin, we find that Ras and other classes of lipid-modified signaling molecules co-fractionate and co-elute with caveolin. The association of Ras with caveolin was further evaluated using two distinct in vitro binding assays. Wild-type H-Ras interacted with glutathione S-transferase (GST)-caveolin fusion proteins but not with GST alone. Using a battery of GST fusion proteins encoding distinct regions of caveolin, Ras binding activity was localized to a 41-amino acid membrane proximal region of the cytosolic N-terminal domain of caveolin. In addition, reconstituted caveolin-rich membranes (prepared with purified recombinant caveolin and purified lipids) interacted with a soluble form of wild-type H-Ras but failed to interact with mutationally activated soluble H-Ras (G12V). Thus, a single amino acid change (G12V) that constitutively activates Ras prevents or destabilizes this interaction. These results clearly indicate that (i) caveolin is sufficient to recruit soluble Ras onto lipid membranes and (ii) membrane-bound caveolin preferentially interacts with inactive Ras proteins. In direct support of these in vitro studies, we also show that recombinant overexpression of caveolin in intact cells is sufficient to functionally recruit a nonfarnesylated mutant of Ras (C186S) onto membranes, overcoming the normal requirement for lipid modification of Ras. Taken together, these observations suggest that caveolin may function as a scaffolding protein to localize or sequester certain caveolin-interacting proteins, such as wild-type Ras, within caveolin-rich microdomains of the plasma membrane.

Genomic sequence analysis of the bovine male-enhanced antigen-1 (Mea-1) and differential localization of its transcripts and products during spermatogenesis

The male-enhanced antigen-1 (Mea-1) gene was previously isolated from a bovine testicular cDNA library. In the present study, we cloned the full-length bovine genomic Mea-1 gene and compared this with the Mea-1 cDNA. The 1035-nucleotide bovine mRNA for Mea-1 (excluding the poly (A) tail) is encoded in three exons distributed over 3123 base pairs of the genome. Analysis of the 5' flanking sequence by primer extension mapping identified two main transcription start sites and several minor ones. The 5' region contained transcription-related sequences such as TATA/CAAT boxes, GC-rich regions, and several cis elements. When chloramphenicol acetyltransferase (CAT) activities of 5'-deleted clones were measured in CHO, TM4, and BALB/3T3 cells, a critical region for transcription was identified around -249 to -113 bp region from transcription start site. In situ hybridization and immunohistochemistry indicate that transcripts of the Mea-1 gene were localized in primary and secondary spermatocytes, and spermatids, but the protein products were detected only in spermatids. Intensive transcription of Mea-1 gene and specific localization of the gene product suggest that Mea-1 may play a important role in the late stage of spermatogenesis.

Homodimerization of the human U1 snRNP-specific protein C

The U1 snRNP-specific protein C contains an N-terminal zinc finger-like CH motif which is required for the binding of the U1C protein to the U1 snRNP particle. Recently a similar motif was reported to be essential for in vivo homodimerization of the yeast splicing factor PRP9. In the present study we demonstrate that the human U1C protein is able to form homodimers as well. U1C homodimers are found when (i) the human U1C protein is expressed in Escherichia coli, (ii) immunoprecipitations with anti-U1C antibodies are performed on in vitro translated U1C, and when (iii) the yeast two hybrid system is used. Analyses of mutant U1C proteins in an in vitro dimerization assay and the yeast two hybrid system revealed that amino acids within the CH motif, i.e. between positions 22 and 30, are required for homodimerization.

One-step purification of recombinant proteins with the 6xHis tag and Ni-NTA resin

The 6xHis/Ni-NTA system allows rapid and efficient affinity purification of recombinant proteins from virtually any expression system. Protocols and tips for purification under both native and denaturing conditions are provided, as well as a rapid spin procedure for protein minipreps.

Anillin, a contractile ring protein that cycles from the nucleus to the cell cortex

We report the cDNA sequence and localization of a protein first identified by actin filament chromatography of Drosophila embryo extracts as ABP8 (Miller, K. G., C. M. Field, and B. M. Alberts. 1989. J. Cell Biol. 109:2963-2975). The cDNA encodes a 1201-amino acid protein which we name anillin. Anillin migrates at 190 kD on SDS-PAGE. Anillin is expressed throughout Drosophila development and in tissue culture cells. By immunofluorescence, anillin localizes to the nucleus of interphase cells, except in the syncytial embryo where it is always cytoplasmic. During metaphase, it is present in the cytoplasm and cortex, and during anaphase-telophase it becomes highly enriched in the cleavage furrow along with myosin II. In the syncytial embryo, anillin, along with myosin-II, is enriched in cortical areas undergoing cell cycle regulated invagination including metaphase furrows and the cellularization front. In contractile rings, metaphase furrows, and nascent ring canals, anillin remains bound to the invaginated cortex suggesting a stabilizing role. Anillin is not expressed in cells that have left the cell cycle. Anillin isolated from embryo extracts binds directly to actin filaments. The domain responsible for this binding has been mapped to a region of 244 amino acids by expression of protein fragments in bacteria. This domain, which is monomeric in solution, also bundles actin filaments. We speculate that anillin plays a role in organizing and/or stabilizing the cleavage furrow and other cell cycle regulated, contractile domains of the actin cytoskeleton.

Rapid purification of human P-glycoprotein mutants expressed transiently in HEK 293 cells by nickel-chelate chromatography and characterization of their drug-stimulated ATPase activities

P-glycoprotein containing 10 tandem histidine residues at the COOH end of the molecule was transiently expressed in HEK 293 cells and purified by nickel-chelate chromatography. The purified protein had an apparent mass of 170 kDa, and its verapamil-stimulated ATPase activity in the presence of phospholipid was 1.2 mumol/min/mg of P-glycoprotein. We then characterized P-glycoprotein mutants that exhibited altered drug-resistant phenotypes and analyzed the contribution of the two nucleotide binding folds to drug-stimulated ATPase activity. Mutation of residues in either nucleotide binding fold abolished drug-stimulated ATPase activity. The pattern of drug-stimulated ATPase activities of mutants, which conferred increased relative resistance to colchicine (G141V, G185V, G830V) or decreased relative resistance to all drugs (F978A), correlated with their drug-resistant phenotypes. By contrast, the ATPase activity of mutant F335A was significantly higher than that of wild-type enzyme when assayed in the presence of verapamil (3.4-fold), colchicine (9.1-fold), or vinblastine (3.7-fold), even though it conferred little resistance to vinblastine in transfected cells. These results suggest that both nucleotide-binding domains must be intact to couple drug binding to ATPase activity and that the drug-stimulated ATPase activity profile of a mutant does not always correlate with its drug-resistant phenotype.

Production of recombinant bovine enterokinase catalytic subunit in Escherichia coli using the novel secretory fusion partner DsbA

Enterokinase (EK) is a heterodimeric serine protease which plays a key role in initiating the proteolytic digestion cascade in the mammalian duodenum. The enzyme acts by converting trypsinogen to trypsin via a highly specific cleavage following the pentapeptide recognition sequence (Asp)4-Lys. This stringent site specificity gives EK great potential as a fusion protein cleavage reagent. Recently, a cDNA encoding the catalytic (light) chain of bovine enterokinase (EKL) was identified, characterized, and transiently expressed in mammalian COS cells. We report here the production of EKL in Escherichia coli by a novel secretory expression system that utilizes E. coli DsbA protein as an N-terminal fusion partner. The EKL cDNA was fused in-frame to the 3'-end of the coding sequence for DsbA, with the two domains of the fusion protein separated by a linker sequence encoding an enterokinase recognition site. Active, processed recombinant EKL (rEKL) was generated from this fusion protein via an autocatalytic cleavage reaction. The enzymatic properties of the bacterially produced rEKL were indistinguishable from the previously described COS-derived enzyme. Both forms of rEKL were capable of cleaving peptides, polypeptides and trypsinogen with the same specificity exhibited by the native heterodimeric enzyme purified from bovine duodena. Interestingly, rEKL activated trypsinogen poorly relative to the native heterodimeric enzyme, but was superior in its ability to cleave artificial fusion proteins containing the (Asp)4-Lys recognition sequence.

Regulation of phosphoenolpyruvate carboxykinase gene transcription by thyroid hormone involves two distinct binding sites in the promoter

Transcription of the gene for phosphoenolpyruvate carboxy-kinase (PEPCK) is stimulated by thyroid hormone (T3), glucagon (via cyclic AMP) and glucocorticoids. A region of the PEPCK promoter between -332 and -308 mediates the induction of transcription by T3. To characterize this region further, mutations were introduced into this region of the PEPCK promoter and the modified promoters ligated to the chloramphenicol acetyltransferase (CAT) reporter gene. Using these PEPCK-CAT vectors in transient transfections in HepG2 cells, it was found that T3 stimulates PEPCK transcription through two direct repeats of the AGGTCA motif located between nucleotides -330 and -319 [PEPCK-thyroid-hormone-responsive element (TRE)]. The beta form of the T3 receptor (TR beta) bound PEPCK-TRE as a homodimer but bound far more efficiently as a heterodimeric complex with the retinoid X receptor (RXR). An additional region called P3(I) (-250 to -234) is required for T3 responsiveness and binds members of the CCAAT-enhancer-binding protein (C/EBP) family. P3(I) contains an AGGTCA-like motif that can bind the TR beta-RXR heterodimer. Mutagenesis of this motif abolished TR beta-RXR binding without reducing T3 induction. Mutation of the C/EBP-binding site or insertion of a cyclic AMP-responsive-binding-protein site at P3(I) eliminated the T3 response. Our results indicate that T3 stimulation of PEPCK transcription is mediated by TR beta bound to PEPCK-TRE and requires C/EBP to be bound at the P3(I) site.

Low affinity binding of interleukin-1 beta and intracellular signaling via NF-kappa B identify Fit-1 as a distant member of the interleukin-1 receptor family

The fit-1 gene gives rise to two different mRNA isoforms, which code for soluble (Fit-1S) and membrane-bound (Fit-1M) proteins related to the type I interleukin (IL)-1 receptor. To investigate IL-1 binding, we have synthesized and purified histidine-tagged polypeptides corresponding to Fit-1S and the extracellular domain of the type I IL-1 receptor using a vaccinia expression system. Fit-1S is shown to interact with IL-1 beta, but not with IL-1 alpha. However, Fit-1S binds IL-1 beta only with low affinity in contrast to the IL-1 receptor, suggesting that IL-1 beta is not a physiological ligand of Fit-1S. Moreover, expression of the membrane-bound protein Fit-1M in transiently transfected Jurkat cells did not result in activation of the transcription factor NF-kappa B following IL-1 beta treatment. However, a chimeric protein consisting of the extracellular domain of the type I IL-1 receptor and of the transmembrane and intracellular regions of Fit-1M stimulated NF-kappa B-dependent transcription as efficiently as the full-length type I IL-1 receptor. These data indicate that Fit-1M is a signaling molecule belonging to the IL-1 receptor family.

Production of human secretory component with dimeric IgA binding capacity using viral expression systems

The cDNA encoding the NH2-terminal 589 amino acids of the extracellular domain of the human polymeric immunoglobulin receptor was inserted into transfer vectors to generate recombinant baculo- and vaccinia viruses. Following infection of insect and mammalian cells, respectively, the resulting truncated protein corresponding to human secretory component (hSC) was secreted with high efficiency into serum-free culture medium. The Sf9 insect cell/baculovirus system yielded as much as 50 mg of hSC/liter of culture, while the mammalian cells/vaccinia virus system produced up to 10 mg of protein/liter. The M(r) of recombinant hSC varied depending on the cell line in which it was expressed (70,000 in Sf9 cells and 85-95,000 in CV-1, TK- 143B and HeLa). These variations in M(r) resulted from different glycosylation patterns, as evidenced by endoglycosidase digestion. Efficient single-step purification of the recombinant protein was achieved either by concanavalin A affinity chromatography or by Ni(2+)-chelate affinity chromatography, when a 6xHis tag was engineered to the carboxyl terminus of hSC. Recombinant hSC retained the capacity to specifically reassociate with dimeric IgA purified from hybridoma cells.

Protein domains involved in both in vivo and in vitro interactions between human T-cell leukemia virus type I tax and CREB

Gene expression from the human T-cell leukemia virus type I (HTLV-I) long terminal repeat (LTR) is mediated by three cis-acting regulatory elements known as 21-bp repeats and the transactivator protein Tax. The 21-bp repeats can be subdivided into three motifs known as A, B, and C, each of which is important for maximal gene expression in response to Tax. The B motif contains nucleotide sequences known as a cyclic AMP response element (CRE) or tax-response element which binds members of the ATF/CREB family of transcription factors. Though mutations of this element in the HTLV-I LTR eliminate tax activation, Tax will not activate most other promoters containing these CRE sites. In this study, we investigated the mechanism by which Tax activates gene expression in conjunction with members of the ATF/CREB family. We found that Tax enhanced the binding of one member of the ATF/CREB family, CREB 1, to each of the three HTLV-I LTR 21-bp repeats but not another member designated CRE-BP1 or CREB2. Tax enhanced the binding of CREB1 to nonpalindromic CRE binding sites such as those found in the HTLV-I LTR, but Tax did not enhance the binding of CREB1 to palindromic CRE binding sites such as found in the somatostatin promoter. This finding may help explain the failure of Tax to activate promoters containing consensus CRE sites. These studies were extended by use of the mammalian two-hybrid system. Tax was demonstrated to interact directly with CREB1 but not with other bZIP proteins, including CREB2 and Jun. Site-directed mutagenesis of both Tax and CREB1 demonstrated that the amino terminus of Tax and both the basic and the leucine zipper regions of CREB1 were required for direct interactions between these proteins both in vivo and in vitro. This interaction occurred in vivo and thus did not require the presence of the HTLV-I 21-bp repeats, as previously suggested. These results define the domains required for interaction between Tax and CREB that are likely critical for the activation of HTLV-I gene expression.

Overexpression of myosin motor domains in Dictyostelium: screening of transformants and purification of the affinity tagged protein

The eukaryotic organism Dictyostelium discoideum has become one of the organisms of choice for the overexpression of recombinant myosins and myosin fragments. Here, we describe a protocol that facilitates the screening of cells that have been transformed with myosin expression constructs and allows the rapid purification of recombinant myosins. Depletion of cellular ATP is used to recruit most of the endogenous and recombinant myosin into a rigor-like complex with actin. Following cell lysis the insoluble actomyosin complex is precipitated by centrifugation, washed, and Mg(2+)-ATP is added to extract the recombinant protein from the pellet. More than 90% of the protein in the resulting supernatant corresponds to actin, myosin, and the recombinant myosin fragments. Therefore, it is easy to detect any differences in expression level between individual myosin constructs on SDS-polyacrylamide gels. Additionally, the dependence of expression on external factors, such as cell density, can be readily determined. Furthermore, the presence of a band corresponding to the recombinant protein indicates that the overexpressed protein has at least some of the functional properties that are characteristic for a myosin motor. This rapid and selective extraction protocol can also be utilized to facilitate the purification of recombinant myosin motors on a preparative scale and has proved particularly useful in the purification of myosin head fragments, that are tagged with histidine residues, by Ni(2+)-chelate affinity chromatography.

Extracellular matrix binding properties of recombinant fibronectin type II-like modules of human 72-kDa gelatinase/type IV collagenase. High affinity binding to native type I collagen but not native type IV collagen

72-kDa gelatinase/type IV collagenase is an important matrix metalloproteinase in the degradation of basement membranes and denatured collagens (gelatin). These proteolytic processes are required for pathologic tissue destruction and physiologic tissue remodeling. To investigate the molecular determinants of substrate specificity of this enzyme, a 21-kDa domain of 72-kDa gelatinase, consisting of three tandem fibronectin type II-like modules, was expressed in Escherichia coli. Similar to full-length 72-kDa gelatinase and the type II modules in fibronectin, the recombinant (r) fibronectin-like domain of this proteinase bound denatured type I collagen with an apparent Kd in the micromolar range. This domain, designated the collagen-binding domain (rCBD123), possesses at least two collagen-binding sites that can each be simultaneously occupied. rCBD123 also avidly bound elastin and denatured types IV and V collagens, but neither native types IV and V collagens nor fibronectin, all of which are substrates of the enzyme. Although 72-kDa gelatinase is involved in basement membrane degradation, rCBD123 also did not bind reconstituted basement membrane, laminin, or SPARC. Native type I collagen, which is not degraded by 72-kDa gelatinase, competed with gelatin for a shared binding site on rCBD123. rCBD123 also displaced full-length 72-kDa gelatinase bound to native type I collagen, further demonstrating that the collagen binding properties of the recombinant domain closely mimicked those of the full-length enzyme. Since rCBD123 showed reduced binding to pepsin-cleaved type I collagen, either or both of the collagen telopeptide ends contain recognition sites for the 72-kDa gelatinase fibronectin-like domain. This was confirmed by the avid binding of rCBD123 to the alpha 1(I) collagen cyanogen bromide fragment CB2 from the NH2-terminal telopeptide. rCBD123 also bound alpha 1(I)-CB7, which encompasses the fibronectin-binding site, and to alpha 1(I)-CB8, a fragment not bound by fibronectin. Thus, type I collagen contains multiple binding sites for rCBD123 which are partially masked by the triple helical conformation of native collagen and fully exposed upon unfolding of the triple helix. The potential of the fibronectin-like collagen binding domain of 72-kDa gelatinase to bind extracellular matrix proteins may facilitate enzyme localization in connective tissue matrices.

Histidine tagging both allows convenient single-step purification of bovine rhodopsin and exerts ionic strength-dependent effects on its photochemistry

For rapid single-step purification of recombinant rhodopsin, a baculovirus expression vector was constructed containing the bovine opsin coding sequence extended at the 3'-end by a short sequence encoding six histidine residues. Recombinant baculovirus-infected Spodoptera frugiperda cells produce bovine opsin carrying a C-terminal histidine tag (v-opshis6x). The presence of this tag was confirmed by immunoblot analysis. Incubation with 11-cis-retinal produced a photosensitive pigment (v-Rhohis6x) at a level of 15-20 pmol/10(6) cells. The histidine tag was exploited to purify v-Rhohis6x via immobilized metal affinity chromatography. Optimized immobilized metal affinity chromatography yielded a binding capacity of > or = 35 nmol of v-Rhohis6x per ml of resin and purification factors up to 500. Best samples were at least 85% pure, with an average purity of 70% (A280 nm/A500 nm = 2.5 +/- 0.4, n = 7). Remaining contamination was largely removed upon reconstitution into lipids, yielding rhodopsin proteoliposomes with a purity over 95%. Spectral analysis of v-Rhohis6x showed a small but significant red shift (501 +/- 1 nm) compared to wild type rhodopsin (498 +/- 1 nm). The pK alpha of the Meta I<==>Meta II equilibrium in v-Rhohis6x is down-shifted from 7.3 to 6.4 resulting in a significant shift at pH 6.5 toward the Meta I photointermediate. Both effects are reversed upon increasing the ionic strength. FT-IR analysis of the Rho-->Meta II transition shows that the corresponding structural changes are identical in wild type and v-Rhohis6x.

High-level expression of soluble recombinant RNase P protein from Escherichia coli

We have expressed recombinant RNase P protein from Escherichia coli in high yield. A hexahistidine sequence at the amino terminus allowed protein purification in a single step. Mass spectrometry confirmed the molecular weight of the purified protein and indicated a purity of > 95%. Protein functionality was demonstrated by reconstitution of active holoenzyme.

Antigenic differences between European and American isolates of porcine reproductive and respiratory syndrome virus (PRRSV) are encoded by the carboxyterminal portion of viral open reading frame 3

Antigenic differences between European and American isolates of porcine reproductive and respiratory syndrome virus (PRRSV) were revealed by serologic analysis of a recombinant protein derived from PRRSV open reading frame 3 (ORF 3). The hydrophilic carboxyterminal 199 amino acids encoded by the ORF 3 of a European (Lelystad) isolate of PRRSV were expressed as a recombinant fusion protein (BP03-P) in a baculovirus gene expression system. Sera from gnotobiotic swine exposed to prototypic reference European and American isolates of PRRSV and sera from conventionally reared European and American swine convalescing from naturally acquired PRRSV infections were used to characterize the BP03-P protein. Sera from gnotobiotic and conventionally reared swine exposed to European isolates of PRRSV were significantly more reactive (P < 0.01) with BP03-P than were the corresponding American PRRSV antisera using the indirect immunoperoxidase monolayer assay (IPMA). Prototypic European, but not American, PRRSV antisera also recognized BP03-P using western immunoblotting and radioimmunoprecipitation assay (RIPA) procedures. However, gnotobiotically derived antiserum to an atypical American-origin PRRSV was reactive with BP03-P by both IPMA and western immunoblot. Despite a predicted potential for N-linked glycosylation, studies with tunicamycin and peptide-N-glycosidase F (PNGase F) indicated that BP03-P was not N-glycosylated in either insect cell cultures or Trichoplusia ni larvae infected with the recombinant baculovirus. Sera from rabbits inoculated with BP03-P failed to neutralize both the European (Lelystad) and American (ATCC VR-2332) reference isolates of PRRSV and did not react by IPMA with PRRSV-infected cell cultures. Taken together, the data suggest that the carboxyterminal portion of PRRSV ORF 3 encodes a non-neutralizing viral peptide that is partially responsible for the serologic differences noted between European and most American isolates of PRRSV.

Isolation and characterization of a novel dual specific phosphatase, HVH2, which selectively dephosphorylates the mitogen-activated protein kinase

The mitogen-activated protein kinase (MAPK) also known as extracellular signal-regulated kinase (ERK) plays a crucial role in various signal transduction pathways. ERK is activated by its upstream activator, MEK, via threonine and tyrosine phosphorylation. ERK activity in the cell is tightly regulated by phosphorylation and dephosphorylation. Here we report the cloning and characterization of a novel dual specific phosphatase, HVH2, which may function in vivo as a MAP kinase phosphatase. The deduced amino acid sequence of HVH2 shows significant identity to the VH1-related dual specific phosphatase family. In addition, the N-terminal region of HVH2 also displays sequence identity to the cell cycle regulator, Cdc25 phosphatase. Recombinant HVH2 phosphatase exhibited a high substrate specificity toward activated ERK and dephosphorylated both threonine and tyrosine residues of activated ERK1 and ERK2. Immunofluorescence studies with an epitope-tagged HVH2 showed that the enzyme was localized in cell nucleus. Transfection of HVH2 into NIH3T3 cells inhibited the v-src and MEK-induced transcriptional activation of serum-responsive element containing promoter, consistent with the notion that HVH2 promotes the inactivation of MAP kinase. HVH2 mRNA showed an expression pattern distinct from CL100 (human homologue of mouse MKP1) and PAC1, two previously identified MAP kinase phosphatases. Our data suggest a possible role of HVH2 in MAP kinase regulation.

Purification of histidine-tagged ras and its use in the detection of ras binding proteins

Recombinant histidine-tagged v-Ha-ras (his-ras) was purified to homogeneity from extracts of E. coli M15 using a one-step procedure which involved immobilised metal ion chromatography on Ni(2+)-nitriloacetic acid agarose (Ni-NTA). The optimal pH for elution by imidazole was 6.6 and the yield of his-ras protein (greater than 95% pure) was about 4 mg/litre E. coli culture. Chromatography of a mixture of purified his-ras and rat brain cytosol on Ni-NTA together with SDS-PAGE and silver staining of proteins were employed to search for ras-binding proteins present in rat brain cytosol. Chromatography of rat brain cytosol alone on Ni-NTA revealed several protein species which were not readily eluted with imidazole. These are likely to be low-abundance brain metal ion binding proteins. Pre-treatment of rat brain cytosol with Ni-NTA before a second round of chromatography on Ni-NTA removed most of these proteins. Chromatography of a mixture of pre-treated rat brain cytosol and purified his-ras protein revealed four new protein bands with molecular weights of 250, 90, 80 and 70 kDa. These were considered to be candidate ras-binding proteins. It is concluded that the use of his-ras and immobilised metal ion chromatography does provide an approach which can be used to identify ras binding proteins present in cellular extracts.

Cloning and characterization of a TFIIIC2 subunit (TFIIIC beta) whose presence correlates with activation of RNA polymerase III-mediated transcription by adenovirus E1A expression and serum factors

TFIIIC2 is a general factor essential for transcription of 5S RNA, tRNA, and VA RNA genes by mammalian RNA polymerase III and consists of two forms designated TFIIIC2a and TFIIIC2b. TFIIIC2a and TFIIIC2b share common subunits of 220, 102, 90, and 63 kD but differ with respect to transcription activity and the presence of a presumptive 110-kD subunit in the active form (TFIIIC2a). Because both forms can bind the promoter directly, a selective role for the 110-kD subunit in the regulation of RNA polymerase III activity has been suggested. To investigate this possibility, we have cloned and expressed a cDNA encoding the 110-kD subunit (TFIIIC beta). Immunoprecipitation studies with anti-TFIIIC beta antibodies have confirmed that TFIIIC beta is a bona fide subunit present only in TFIIIC2a, that TFIIIC2a and the general factor TFIIIC1 are associated in unfractionated extracts, and that previously undetected polypeptides (potential TFIIIC1 subunits) can be isolated in association with TFIIIC2a. Previous studies have shown that increases in RNA polymerase III activity during infection of cells by adenovirus (with concomitant E1A expression) or during cell growth at high serum concentration results from an increased activity in the TFIIIC fraction. Studies with antibodies to TFIIIC beta have shown that this is strongly correlated with a selective increase in the cellular concentration of the TFIIIC beta 110-kD subunit and a concomitant rise in the ratio of the active-to-inactive forms of TFIIIC2.

The gene for a major exopolyphosphatase of Saccharomyces cerevisiae

The gene encoding a major exopolyphosphatase (scPPX1) in Saccharomyces cerevisiae (H. Wurst and A. Kornberg, J. Biol. Chem. 269:10996-11001, 1994) has been isolated from a genomic library. The gene, located at 57 kbp from the end of the right arm of chromosome VIII, encodes a protein of 396 amino acids. Overexpression in Escherichia coli allowed the ready purification of a recombinant form of the enzyme. Disruption of the gene did not affect the growth rate of S. cerevisiae. Lysates from the mutants displayed considerably lower exopolyphosphatase activity than the wild type. The enzyme is located in the cytosol, whereas the vast accumulation of polyphosphate (polyP) of the yeast is in the vacuole. Disruption of PPX1 in strains with and without deficiencies in vacuolar proteases allowed the identification of exopolyphosphatase activity in the vacuole. This residual activity was strongly reduced in the absence of vacuolar proteases, indicating a dependence on proteolytic activation. A 50-fold-lower protease-independent activity could be distinguished from this protease-dependent activity by different patterns of expression during growth and activation by arginine. With regard to the levels of polyP in various mutants, those deficient in vacuolar ATPase retain less than 1% of the cellular polyP, a loss that is not offset by additional mutations that eliminate the cytosolic exopolyphosphatase and the vacuolar polyphosphatases dependent on vacuolar protease processing.

Bacterial expression and purification of hepatitis C virus capsid proteins of different size

Two capsid sequences of the hepatitis C virus were cloned and expressed in an E. coli system. One sequence (c190) comprised the complete capsid region with 573 nucleotides. The other sequence (c125) spanned 375 5'-nucleotides lacking the hydrophobic 3'-part of the hepatitis C virus capsid gene. A full-length and a truncated construct were chosen, since it is not known whether there is 3'-truncation of the hepatitis C virus capsid during protein maturation similar to the situation in some flaviviridae. The corresponding expression clones 190/4 and 125/4 were constructed by polymerase chain reaction cloning into pQE-vectors. The protein expressed, pc125, which is lacking the hydrophobic carboxyterminus of the full-length capsid protein pc190, showed a stronger signal in western blots using anti-hepatitis C virus/EIAII-positive patient's serum. This could be due to better expression and/or better solubilization of pc125. The truncated protein pc125 displayed the predicted molecular weight of 19 kD, whereas the full-length protein pc190 migrated faster than expected. This could be due to intracellular proteolytic processing, giving rise to a truncated protein or to an atypical mobility in SDS-PAGE gels caused by the hydrophobic nature of the full-length protein. Both proteins were synthesized with an aminoterminal tag of six histidines that could be used for purification by Nickel chelate affinity chromatography. The elution fractions of the two proteins showed additional bands in western blots. Most of these proteins had a mass between 2 and 16 kD and are likely to be degradation products. Protein pc125 could be purified in larger quantities than pc190.(ABSTRACT TRUNCATED AT 250 WORDS)

Heterologous expression systems for P-glycoprotein: E. coli, yeast, and baculovirus

Chemotherapy, though it remains one of the front-line weapons used to treat human cancer, is often ineffective due to drug resistance mechanisms manifest in tumor cells. One common pattern of drug resistance, characterized by simultaneous resistance to multiple amphipathic, but otherwise structurally dissimilar anticancer drugs, is termed multidrug resistance. Multidrug resistance in various model systems, covering the phylogenetic range from bacteria to man, can be conferred by mammalian P-glycoproteins (PGPs), often termed multidrug transporters. PGPs are 170-kD polytopic membrane proteins, predicted to consist of two homologous halves, each with six membrane spanning regions and one ATP binding site. They are members of the ATP-binding cassette (ABC) superfamily of transporters, and are known to function biochemically as energy-dependent drug efflux pumps. However, much remains to be learned about PGP structure-function relationships, membrane topology, posttranslational regulation, and bioenergetics of drug transport. Much of the recent progress in the study of the human and mouse PGPs has come from heterologous expression systems which offer the benefits of ease of genetic selection and manipulation, and/or short generation times of the organism in which PGPs are expressed, and/or high-level expression of recombinant PGP. Here we review recent studies of PGP in E. coli, baculovirus, and yeast systems and evaluate their utility for the study of PGPs, as well as other higher eukaryotic membrane proteins.

Human ribosomal protein L7 inhibits cell-free translation in reticulocyte lysates and affects the expression of nuclear proteins upon stable transfection into Jurkat T-lymphoma cells

Eucaryotic ribosomal protein L7 carries a 'Basic-Region-Leucine-Zipper (BZIP)'-like region which mediates high-affinity binding to mRNA and 28S rRNA and formation of homodimers [P. Hemmerich et al. (1993) Nucleic Acids Res. 21, 223-231). Its biological function is unknown as yet and no direct L7-equivalent in procaryotes has been found. In this report we show that eucaryotic L7 specifically inhibits the cell-free translation of reporter mRNAs. The interaction of L7 with mRNA is an essential step in this reaction which is inhibitable by antibodies directed against the BZIP-like region of L7, and by competitors of mRNA binding. L7-mediated inhibition of cell-free translation of polyA+ RNA from Jurkat T-lymphoma cells is selective in that the synthesis of a major 46 kD protein is suppressed. Upon stable transfection of L7 cDNA into Jurkat lymphoma cells two major proteins disappear, namely one nuclear protein and one which associates with the nucleus. Our data suggest a regulatory role of protein L7 in the eucaryotic translation apparatus.

Chromatography in the downstream processing of biotechnological products

Chromatography techniques are essential for the isolation and purification of most of the high value products of modern biotechnology. The economically sensible and technically satisfactory downstream processing of a therapeutic protein, usually involves a number of chromatographic steps. Its development and optimization require considerable knowledge of the various physico-chemical and engineering aspects of biochemical chromatography. This review addresses the various modes of chromatography and the design of chromatographic separation processes from a biotechnologist's point of view. Strategies for optimizing the structure of the downstream process are outlined and scaling up consideration are discussed. The importance of the different chromatographic methods in research and development is estimated in an analysis of protein purification schemes recently published in the literature. Finally, examples of the application of chromatographic procedures for process scale product purification in the biotechnological industry are given.

Retinoid-dependent in vitro transcription mediated by the RXR/RAR heterodimer

The effects of retinoids on gene regulation are mediated by retinoic acid receptors (RARs) and retinoid X receptors (RXRs). Here, we provide the first biochemical evidence that, in vitro, ligand governs the transcriptional activity of RXR alpha/RAR alpha by inducing conformational changes in the ligand-binding domains. Using limited proteolytic digestion we show that binding of the cognate ligand causes a conformational change in the carboxy-terminal part of the receptor. We also show that recombinant RXR alpha/RAR alpha is partially active in the absence of exogenously added ligand. Trans-activation depends critically on the ligand-dependent transcriptional activation function AF-2 of RAR alpha. Full activation by recombinant RXR alpha/RAR alpha, however, requires the addition of either all-trans RA, 9-cis RA, or other RAR-specific agonists, whereas an RAR alpha-specific antagonist abolishes trans-activation. Intriguingly, the ligand-dependent AF-2 of RXR does not contribute to the level of transcription from the RAR beta 2 promoter in vitro even when the cognate ligand (9-cis RA) is bound. Thus, the major role of RXR in trans-activation of the RAR beta 2 promoter is to serve as an auxiliary factor required for the binding of RAR which, in turn, is directly responsible for transcriptional activity.

Single-step purification of Proteus mirabilis urease accessory protein UreE, a protein with a naturally occurring histidine tail, by nickel chelate affinity chromatography

Proteus mirabilis urease, a nickel metalloenzyme, is essential for the virulence of this species in the urinary tract. Escherichia coli containing cloned structural genes ureA, ureB, and ureC and accessory genes ureD, ureE, ureF, and ureG displays urease activity when cultured in M9 minimal medium. To study the involvement of one of these accessory genes in the synthesis of active urease, deletion mutations were constructed. Cultures of a ureE deletion mutant did not produce an active urease in minimal medium. Urease activity, however, was partially restored by the addition of 5 microM NiCl2 to the medium. The predicted amino acid sequence of UreE, which concludes with seven histidine residues among the last eight C-terminal residues (His-His-His-His-Asp-His-His-His), suggested that UreE may act as a Ni2+ chelator for the urease operon. To exploit this potential metal-binding motif, we attempted to purify UreE from cytoplasmic extracts of E. coli containing cloned urease genes. Soluble protein was loaded onto a nickel-nitrilotriacetic acid column, a metal chelate resin with high affinity for polyhistidine tails, and bound protein was eluted with a 0 to 0.5 M imidazole gradient. A single polypeptide of 20-kDa apparent molecular size, as shown by sodium dodecyl sulfate-10 to 20% polyacrylamide gel electrophoresis, was eluted between 0.25 and 0.4 M imidazole. The N-terminal 10 amino acids of the eluted polypeptide exactly matched the deduced amino acid sequence of P. mirabilis UreE. The molecular size of the native protein was estimated on a Superdex 75 column to be 36 kDa, suggesting that the protein is a dimer. These data suggest that UreE is a Ni(2)+-binding protein that is necessary for synthesis of a catalytically active urease at low Ni(2+) concentrations.

Expression and biochemical characterization of human protein kinase C-theta

In this study, the recently identified human protein kinase C-theta (PKC-theta) isoform has been biochemically characterized in detail. An antiserum raised against the unique V3 domain of PKC-theta identified an 80-kDa protein in all human T-cell lines tested, in erythroleukemia K562 cells and in histiocytic lymphoma U-937 cells, but not in a B-lymphoma line (Raji) or in several melanoma, carcinoma, schwanoma or astrocytoma lines, confirming, at the protein level, its predominant expression in hematopoietic cell lines, in particular T cells. Immunoreactive PKC-theta was detected almost exclusively in the cytosolic compartment of unstimulated Jurkat T cells. Stimulation with phorbol ester, however, caused rapid translocation to the membrane. In order to compare the properties of PKC-theta with a representative member of the Ca(2+)-dependent PKC enzymes, full-length cDNAs encoding PKC-theta or PKC-alpha were transiently expressed in COS-1 cells, and recombinant enzymes were partially purified via a six-histidine peptide tag. The catalytic activity of these PKC enzymes was assayed against distinct substrates in the absence and presence of known PKC cofactors. Significant differences were found with respect to activation requirements and substrate preferences between PKC-theta and PKC-alpha. Both enzymes were stimulated by phospholipid and phorbol ester, and were active towards a PKC-derived substrate peptide corresponding to the pseudosubstrate site of PKC. In contrast to PKC-alpha, however, full activation of PKC-theta did not require Ca2+, and its basal activity towards histone H1 was not stimulated by lipid cofactors. Additionally, a myelin-basic-protein-(MBP)-derived peptide, which was readily phosphorylated by PKC-alpha, was a poor substrate for PKC-theta. Similar to PKC-alpha, transient PKC-theta overexpression in murine EL4 thymoma cells caused an approximately 2.5-fold increase in the phorbol-12-myristate-13-acetate-induced transcriptional activation of an interleukin-2 promoter-reporter gene construct. The unique expression and functional properties of PKC-theta suggest that it may play a specialized role in T-cell signaling pathways.

ATPase and GTPase activities associated with Semliki Forest virus nonstructural protein nsP2

The replication of Semliki Forest virus requires four nonstructural proteins (nsP1 to nsP4), all derived from the same polyprotein. One of these, nsP2, is a multifunctional protein needed in RNA replication and in the processing of the nonstructural polyprotein. On the basis of amino acid sequence homologies, nsP2 was predicted to possess nucleoside triphosphatase and RNA helicase activities. Here, we report the engineered expression in Escherichia coli of nsP2 and of an amino-terminal fragment of it by use of the highly efficient T7 expression system. Both polypeptides were produced as fusion proteins with a histidine tag at the amino terminus and purified by immobilized-metal affinity chromatography. The two recombinant proteins exhibited ATPase and GTPase activities, which were further stimulated by the presence of single-stranded RNA. The activities were not found in similarly prepared fractions from uninduced control cells or cells expressing an unrelated polypeptide. Radiolabeled ribonucleoside triphosphates could be cross-linked to both the full-length and the carboxy-terminally truncated nsP2 protein, and both polypeptides had RNA-binding capacity. We also expressed and purified an nsP2 variant which had a single amino acid substitution in the nucleotide-binding motif (Lys-192-->Asn). No nucleoside triphosphatase activity was associated with this mutant protein.

PCR-based cloning of the full-length Neurospora eukaryotic initiation factor 5A cDNA: polyhistidine-tagging and overexpression for protein affinity binding

Eukaryotic initiation factor 5A (eIF-5A) is the only cellular protein known to contain a hypusine residue that is formed by transferring the aminobutyl moiety from spermidine to a specific lysine residue, followed by hydroxylation at the aminobutyl group. A simple PCR-based strategy was developed to obtain a full-length cDNA of Neurospora crassa eIF-5A. The strategy consists of (i) the design of a pair of key primers (21-mer) based on the highly conserved eIF-5A cDNA domains known in other species, (ii) PCR amplification of Neurospora cDNA using the two key primers to obtain the core sequence for the design of core primers, and (iii) combined use of the key primers, core primers and the universal primers, T3 and T7, to amplify the target sequence in a Neurospora cDNA library. The longest cDNA obtained was cloned into pBlueScript phagemid, and sequence analysis indicated that it encodes a polypeptide of 163 amino acid residues with a codon usage preference characteristic of abundant Neurospora genes. The Neurospora polypeptide showed 59% and 67% identity with human and yeast eIF-5A precursor protein respectively. We subcloned the Neurospora eIF-5A cDNA into pQE-30, which introduces six adjacent histidine residues to the N-terminus of the recombinant protein. The resulting plasmid, pQTy21, was overexpressed in Escherichia coli, and the soluble polyhistidine-tagged protein was purified by metal chelation chromatography. We obtained about 60 mg of purified eIF-5A precursor from 1 litre of culture in a single step using a Ni(II)-nitrilotriacetic acid (NTA)-agarose column. The histidine-tagged eIF-5A precursor protein could be recognized by anti-Neurospora crassa 21 kDa protein serum raised against wild-type eIF-5A precursor and could serve as the substrate protein for deoxyhypusine synthase. Using the histidine-tagged recombinant protein and the Ni(II)-NTA-agarose column, we constructed a protein affinity column and demonstrated an affinity binding between eIF-5A precursor and deoxyhypusine synthase in the presence of NAD+. One-step eIF-5A precursor affinity-column chromatography could lead to a 30-fold purification of deoxyhypusine synthase.

Pausing of the restriction endonuclease EcoRI during linear diffusion on DNA

Linear diffusion is a mechanism to accelerate association rates beyond their three-dimensional diffusional limit. It is employed by the restriction endonuclease EcoRI as well as many other proteins interacting with specific DNA sequences to locate their target sites on the macromolecular substrate. In order to investigate biochemical and biophysical details of the linear diffusion process, we have developed a competitive cleavage assay which allows us to assess with great accuracy the influence of sequence, sequence context, and other structural features on the linear diffusion of EcoRI on DNA. We show here that linear diffusion is not a hopping but a sliding movement in which EcoRI follows the helical pitch of the DNA, because it does not "overlook" any cleavage site. Linear diffusion is slowed when EcoRI encounters sites on the DNA which resemble its recognition site ("star" sites). Pauses of up to 20 s are induced, depending on sequence and orientation of the star site. These data suggest that EcoRI can bind to DNA in two binding modes: one tight, specific, and immobile, leading to DNA cleavage, and another one loose and nonspecific, allowing for linear diffusion. Depending on the similarity between the recognition sequence and the DNA sequence being encountered by EcoRI, there will be a continuous transition between these binding modes. Other proteins bound to the DNA and irregular DNA structures such as bent DNA or a triple helix constitute a barrier that cannot easily be passed by EcoRI.