Development of a recombinant tetravalent dengue virus vaccine: immunogenicity and efficacy studies in mice and monkeys

Truncated recombinant dengue virus envelope protein subunits (80E) are efficiently expressed using the Drosophila Schneider-2 (S2) cell expression system. Binding of conformationally sensitive antibodies as well as X-ray crystal structural studies indicate that the recombinant 80E subunits are properly folded native-like proteins. Combining the 80E subunits from each of the four dengue serotypes with ISCOMATRIX adjuvant, an adjuvant selected from a set of adjuvants tested for maximal and long lasting immune responses, results in high titer virus neutralizing antibody responses. Immunization of mice with a mixture of all four 80E subunits and ISCOMATRIX adjuvant resulted in potent virus neutralizing antibody responses to each of the four serotypes. The responses to the components of the tetravalent mixture were equivalent to the responses to each of the subunits administered individually. In an effort to evaluate the potential protective efficacy of the Drosophila expressed 80E, the dengue serotype 2 (DEN2-80E) subunit was tested in both the mouse and monkey challenge models. In both models protection against viral challenge was achieved with low doses of antigen in the vaccine formulation. In non-human primates, low doses of the tetravalent formulation induced good virus neutralizing antibody titers to all four serotypes and protection against challenge with the two dengue virus serotypes tested. In contrast to previous reports, where subunit vaccine candidates have generally failed to induce potent, protective responses, native-like soluble 80E proteins expressed in the Drosophila S2 cells and administered with appropriate adjuvants are highly immunogenic and capable of eliciting protective responses in both mice and monkeys. These results support the development of a dengue virus tetravalent vaccine based on the four 80E subunits produced in the Drosophila S2 cell expression system.

Evidence for a three-state model of protein folding from kinetic analysis of ubiquitin variants with altered core residues

To elucidate the kinetic importance of structural intermediates in single-domain proteins, we measured the effect of solution conditions and amino-acid changes at a central core residue of ubiquitin (Val 26) on the kinetics of folding and unfolding. Kinetic analysis in terms of a sequential three-state mechanism provides insight into the contribution of specific interactions within the ubiquitin core to the structural stability of the native and intermediate states. The observations that disruptive mutations and/or addition of denaturants result in an apparent two-state folding process with slower rates is explained by the destabilization of a partially folded intermediate, which is in rapid equilibrium with unfolded states. The model predicts that under sufficiently stabilizing conditions kinetic intermediates may become populated even for proteins showing apparent two-state kinetics.

Folding and stability of a tryptophan-containing mutant of ubiquitin

To provide a fluorescence probe for equilibrium and kinetic folding studies on ubiquitin, cassette mutagenesis in an Escherichia coli expression plasmid was used to replace the largely buried Phe 45 by a tryptophan. Under native conditions, the tryptophan fluorescence spectrum of this F45W mutant exhibits a blue-shifted emission maximum at 336 nm indicative of a largely solvent-shielded tryptophan environment. In contrast, the unfolded protein in 6 M guanidine hydrochloride (GuHCl) shows a 4-fold more intense emission band at 353 nm matching that of free tryptophan. The two-dimensional 1H NMR spectrum of F45W ubiquitin was assigned by comparison with published assignments of the wild type. The mutation results in only limited chemical shift changes for residues in the immediate vicinity of residue 45. The structural similarity of F45W with wild-type ubiquitin was confirmed by a preliminary analysis of the nuclear Overhauser spectrum. NMR and circular dichroism measurements of the reversible GuHCl-induced unfolding transition show that the F45W mutation lowers the stability of the folded ubiquitin structure by less than 0.4 kcal/mol. The biological activity of the mutant was found to be indistinguishable from that of wild-type in terms of its reaction with the ubiquitin activating enzyme E1 and an in vitro assay of ATP-dependent protein degradation. The kinetics of folding and unfolding of F45W ubiquitin was studied at two temperatures (8 and 25 degrees C) in a series of fluorescence-detected stopped-flow measurements over a wide range of GuHCl concentrations (0.5-6 M). The measurements at 25 degrees C are consistent with a two-state model with strongly denaturant-dependent folding and unfolding rates above about 2 M GuHCl. However, at lower denaturant concentrations, the rate of the major folding phase becomes GuHCl-independent, and up to 60% of the total fluorescence change occurs during the 2-ms dead time of the stopped-flow measurement. These observations provide clear evidence for the formation of an early folding intermediate during the first few milliseconds of refolding with a partially developed hydrophobic core involving Trp 45. The sigmoid denaturant dependence of the initial amplitude with an apparent midpoint of 1.3 M GuHCl suggests the presence of a discrete state that is destabilized at higher denaturant concentrations. In contrast, there is no evidence for an early intermediate in the folding kinetics at 8 degrees C. The destabilization of the intermediate at low temperature is consistent with a collapsed state stabilized primarily by hydrophobic interactions.

Isolation and characterization of an antifreeze protein precursor from transgenic Drosophila: evidence for partial processing

Transgenic Drosophila melanogaster that contain a winter flounder antifreeze protein (AFP) gene fused to the transcriptional and translational control sequences of the host heat shock protein 70 gene express the transgene under heat shock conditions. They secrete into the hemolymph small quantities of a protein that reacts with antisera to AFP and is of a similar size to the proAFP precursor. To facilitate purification and characterization of this precursor, transformed fly lines homozygous for inserts on the 2nd, 3rd and X chromosomes were crossed together to generate a line with five and six AFP genes present in males and females, respectively. AFP production in the multi-gene line was approximately equal to the sum of that observed in the three starting lines and was just sufficient to perturb the growth of ice crystals. The AFP component was purified from heat-denatured hemolymph of this line by cation- and anion-exchange chromatography, followed by reverse-phase HPLC. Edman degradation sequencing of the purified protein showed that its N-terminus began two amino acids in from the predicted signal peptide cleavage point. An additional amino acid sequence was present that began two amino acids further into the 'pro' sequence. These AFP products are consistent with processing of the proAFP in Drosophila by a type IV dipeptidyl aminopeptidase, as has been suggested for processing in flounder.

Wolffish Antifreeze Protein from Transgenic Drosophila

We have expressed two antifreeze protein genes from the Atlantic wolffish, Anarhichas lupus, in Drosophila melanogaster by placing them under the divergent transcriptional control of the host yolk polypeptide (1 and 2) gene promoters. Both genes were joined to the central promoter region by fusion within the DNA encoding the signal polypeptides. The chimeric genes were introduced into flightless mutant Drosophila through P-element transformation. Transformed adult females from individual lines accumulated 1.5-5 mg/ml of antifreeze protein in their hemolymph. The protein was purified to homogeneity from hemolymph following thermal denaturation, step elution from SP-Sephadex, and reverse-phase HPLC. It was recovered in high yield and retained full biological activity even though one of the two gene fusions gave rise to a seven amino acid N-terminal extension on its antifreeze protein product.

Biosynthesis of winter flounder antifreeze proprotein in E.coli

A semisynthetic winter flounder antifreeze proprotein (proAFP) coding region was constructed and inserted into a lacZ expression vector. ProAFP was produced from the vector in Escherichia coli as a C-terminal fusion to the first 289 amino acids of beta-galactosidase (beta-gal). The proAFP and beta-gal domains of the beta-gal-proAFP fusion protein were separated by the recognition signal for the blood coagulation protease, factor Xa. Upon induction with isopropylthio-beta-D-galactoside the fusion protein accumulated to levels of 15% of the total protein. The beta-gal-proAFP fusion protein was partially purified by differential centrifugation, but required solubilization prior to factor Xa digestion. The solubilized fusion protein was efficiently and correctly cleaved by factor Xa, after which the proAFP was purified by gel permeation. Bacterial proAFP was indistinguishable from natural proAFP by the criteria of antifreeze activity, amino-terminal sequence (15 cycles), reverse-phase HPLC and SDS-polyacrylamide gel electrophoresis. Circular dichroism measurements showed that proAFP is a composite of random coil and alpha-helical secondary structure, with an alpha-helix content of 44% at 0 degrees C. It seems probable that the C-terminal region of proAFP, which corresponds to the mature AFP protein, is mainly alpha-helical, and that the N-terminal pro-segment is random coiled.