Isotopically labeled bovine beta-lactoglobulin for NMR studies expressed in Pichia pastoris

Understanding exopolysaccharide byproduct formation in Komagataella phaffii fermentation processes for recombinant protein production

BACKGROUND

Komagataella phaffii (Pichia pastoris) has emerged as a common and robust biotechnological platform organism, to produce recombinant proteins and other bioproducts of commercial interest. Key advantage of K. phaffii is the secretion of recombinant proteins, coupled with a low host protein secretion. This facilitates downstream processing, resulting in high purity of the target protein. However, a significant but often overlooked aspect is the presence of an unknown polysaccharide impurity in the supernatant. Surprisingly, this impurity has received limited attention in the literature, and its presence and quantification are rarely addressed.

RESULTS

This study aims to quantify this exopolysaccharide in high cell density recombinant protein production processes and identify its origin. In stirred tank fed-batch fermentations with a maximal cell dry weight of 155 g/L, the polysaccharide concentration in the supernatant can reach up to 8.7 g/L. This level is similar to the achievable target protein concentration. Importantly, the results demonstrate that exopolysaccharide production is independent of the substrate and the protein production process itself. Instead, it is directly correlated with biomass formation and proportional to cell dry weight. Cell lysis can confidently be ruled out as the source of this exopolysaccharide in the culture medium. Furthermore, the polysaccharide secretion can be linked to a mutation in the HOC1 gene, featured by all derivatives of strain NRRL Y-11430, leading to a characteristic thinner cell wall.

CONCLUSIONS

This research sheds light on a previously disregarded aspect of K. phaffii fermentations, emphasizing the importance of monitoring and addressing the exopolysaccharide impurity in biotechnological applications, independent of the recombinant protein produced.

Food proteins from yeast-based precision fermentation: Simple purification of recombinant β-lactoglobulin using polyphosphate

Proteins produced through precision fermentation are often purified through chromatographic methods. Faster and more cost-effective purification methods are desired for food application. Here, we present a simple method for purification of protein produced from yeast, using β-lactoglobulin secreted from Pichia pastoris as an example. The food-grade salt hexametaphosphate (HMP) was used to precipitate the protein at acidic pH, while the impurities (extracellular polysaccharides; mainly mannan) remained soluble. After re-solubilization of the protein-HMP complex by neutralization, excess HMP was selectively precipitated using calcium chloride. The protein content of the crude sample increased from 26 to 72 wt% (comparable to purification with anion exchange chromatography), containing only residual extracellular polysaccharides (9 wt%) and HMP (1 wt%). The established method had no significant impact on the structural and functional properties (i.e., ability to form emulsions) of the protein. The presented method shows potential for cost-effective purification of recombinant proteins produced through yeast-based expression systems.

Methyl-selective isotope labeling using α-ketoisovalerate for the yeast Pichia pastoris recombinant protein expression system

Methyl-detected NMR spectroscopy is a useful tool for investigating the structures and interactions of large macromolecules such as membrane proteins. The procedures for preparation of methyl-specific isotopically-labeled proteins were established for the Escherichia coli (E. coli) expression system, but typically it is not feasible to express eukaryotic proteins using E. coli. The Pichia pastoris (P. pastoris) expression system is the most common yeast expression system, and is known to be superior to the E. coli system for the expression of mammalian proteins, including secretory and membrane proteins. However, this system has not yet been optimized for methyl-specific isotope labeling, especially for Val/Leu-methyl specific isotope incorporation. To overcome this difficulty, we explored various culture conditions for the yeast cells to efficiently uptake Val/Leu precursors. Among the searched conditions, we found that the cultivation pH has a critical effect on Val/Leu precursor uptake. At an acidic cultivation pH, the uptake of the Val/Leu precursor was increased, and methyl groups of Val and Leu in the synthesized recombinant protein yielded intense ¹H-¹³C correlation signals. Based on these results, we present optimized protocols for the Val/Leu-methyl-selective ¹³C incorporation by the P. pastoris expression system.

Current strategies for protein production and purification enabling membrane protein structural biology

Membrane proteins are still heavily under-represented in the protein data bank (PDB), owing to multiple bottlenecks. The typical low abundance of membrane proteins in their natural hosts makes it necessary to overexpress these proteins either in heterologous systems or through in vitro translation/cell-free expression. Heterologous expression of proteins, in turn, leads to multiple obstacles, owing to the unpredictability of compatibility of the target protein for expression in a given host. The highly hydrophobic and (or) amphipathic nature of membrane proteins also leads to challenges in producing a homogeneous, stable, and pure sample for structural studies. Circumventing these hurdles has become possible through the introduction of novel protein production protocols; efficient protein isolation and sample preparation methods; and, improvement in hardware and software for structural characterization. Combined, these advances have made the past 10-15 years very exciting and eventful for the field of membrane protein structural biology, with an exponential growth in the number of solved membrane protein structures. In this review, we focus on both the advances and diversity of protein production and purification methods that have allowed this growth in structural knowledge of membrane proteins through X-ray crystallography, nuclear magnetic resonance (NMR) spectroscopy, and cryo-electron microscopy (cryo-EM).

Expression, purification, and mass spectrometric analysis of 15N, 13C-labeled RGD-hirudin, expressed in Pichia pastoris, for NMR studies

A novel recombinant hirudin, RGD-hirudin, inhibits the activity of thrombin and the aggregation of platelets. Here, we successfully expressed (15)N, (13)C-labeled RGD-hirudin in Pichia pastoris in a fermenter. The protein was subsequently purified to yield sufficient quantities for structural and functional studies. The purified protein was characterized by HPLC and MALDI-TOF mass spectroscopy. Analysis revealed that the protein was pure and uniformly labeled with (15)N and (13)C. A bioassay showed that the anti-thrombin activity and the anti-platelet aggregation ability of the labeled protein were the same as those of unlabeled RGD-hirudin. Multidimensional heteronuclear NMR spectroscopy has been used to determine almost complete backbone (15)N, (13)C and (1)H resonance assignments of the r-RGD-Hirudin. The (15)N-(1)H HSQC spectrum of uniformly (15)N, (13)C-labeled RGD-hirudin allowed successful assignment of the signals. Examples of the quality of the data are provided for the (15)N-(l)H correlation spectrum, and by selected planes of the CBCA(CO)NH, CBCANH, and HNCO experiments. These results provide a basis for further studies on the structure-function relationship of RGD-hirudin with thrombin and platelets.

Isotopic labeling of heterologous proteins in the yeast Pichia pastoris and Kluyveromyces lactis

Several protein expression systems are available for the preparation of stable isotope-labeled recombinant proteins for NMR studies. Yeast expression systems have several advantages over prokaryotic systems, such as the widely used Escherichia coli expression system. Protein expression using the methylotrophic yeast Pichia pastoris is commonly employed for the preparation of isotope-labeled proteins. Recently, the hemiascomycete yeast Kluyveromyces lactis expression system was reported as being useful for preparing proteins for NMR studies. Since each yeast expression system has different features, their applications have increased in number. In this chapter, we describe procedures for the efficient production of uniformly isotope-labeled proteins using the P. pastoris and the K. lactis yeast expression systems.

The production of soluble and correctly folded recombinant bovine beta-lactoglobulin variants A and B in Escherichia coli for NMR studies

The production of soluble and correctly folded eukaryotic proteins in prokaryotic systems has always been hampered by the difference in or lack of cell machinery responsible for folding, post-translation modification and secretion of the proteins involved. In the case of bovine beta-lactoglobulin (BLG), a major cow's milk allergen and a protein widely used for protein folding studies, a eukaryotic yeast expression system has been the preferred choice of many researchers, particularly for the production of isotopically labeled protein required for NMR studies. Although this system yields high amounts of recombinant protein, the BLG produced is usually associated with extracellular polysaccharides, which is problematic for NMR analysis. In our study we show that when co-expressed with the signal-sequence-less disulfide bond isomerase (Delta ssDsbC) in the dual expression vector, pETDUET-1, both BLG A and BLG B can be reproducibly produced in a soluble form. Expression was carried out in Escherichia coli Origami(DE3), a trxB/gor mutant for thioredoxin- and glutathione reductase, which allows for proper formation of disulfide bonds in the cytoplasm. The protein was purified by anion exchange chromatography followed by salting-out at low pH and size exclusion chromatography. Our expression system is able to consistently produce milligram quantities of correctly folded BLG A and B with no additional amino acid residues at the N-terminus, except for a methionine. (15)N-labeled BLG A and B, prepared and purified using this method, produced HSQC spectra typical of native bovine BLG.

Expression of the Trichoderma reesei tyrosinase 2 in Pichia pastoris: isotopic labeling and physicochemical characterization

Trichoderma reesei tyrosinase TYR2 has been demonstrated to be able to oxidize various phenolic compounds and also peptide and protein bound tyrosine, and thus is of great interest for different biotechnological applications. In order to understand the reaction mechanism of the enzyme it would be essential to solve its three dimensional structure. Pichia pastoris is a suitable expression system for the production of recombinant enzymes for NMR studies and therefore we expressed TYR2 in this host. As a result of extensive optimization, the production yield of active histidine tagged tyrosinase purified from P. pastoris shake flask cultures was increased from 2.5 to 24 mg/L. Correct copper concentration in the growth medium was critical for the expression of this copper containing enzyme. Our analysis showed that TYR2 expressed in P. pastoris is post-translationally modified; the C-terminal domain of 153 amino acids of the protein is proteolytically cleaved off from the catalytic domain and the only potential N-glycosylation site is glycosylated. The activities of TYR2 expressed in P. pastoris and T. reesei on diphenolic L-dopa and monophenolic L-tyrosine were rather similar. The TYR2 expressed in P. pastoris showed the same physicochemical properties in CD and unfolding assays as the native TYR2 enzyme. Uniform isotopic (15)N-labeling of TYR2 was carried out with (15)NH(4)SO(4) in minimal medium to assess the suitability of the expression system for investigation by NMR spectroscopy.

Preparation of amino-acid-type selective isotope labeling of protein expressed in Pichia pastoris

We report the culture conditions for successful amino-acid-type selective (AATS) isotope labeling of protein expressed in Pichia pastoris (P. pastoris). Rhodostomin (Rho), a six disulfide-bonded protein expressed in P. pastoris with the correct fold, was used to optimize the culture conditions. The concentrations of [alpha-15N] selective amino acid, nonlabeled amino acids, and ammonium chloride, as well as induction time, were optimized to avoid scrambling and to increase the incorporation rate and protein yield. The optimized protocol was successfully applied to produce AATS isotope-labeled Rho. The labeling of [alpha-15N]Cys has a 50% incorporation rate, and all 12 cysteine resonances were observed in HSQC spectrum. The labeling of [alpha-15N]Leu, -Lys, and -Met amino acids has an incorporation rate greater than 65%, and the expected number of resonances in the HSQC spectra were observed. In contrast, the labeling of [alpha-15N]Asp and -Gly amino acids has a low incorporation rate and the scrambling problem. In addition, the culture condition was successfully applied to label dendroaspin (Den), a four disulfide-bonded protein expressed in P. pastoris. Therefore, the described condition should be generally applicable to other proteins produced in the P. pastoris expression system. This is the first report to present a protocol for AATS isotope labeling of protein expressed in P. pastoris for NMR study.

Identification and removal of O-linked and non-covalently linked sugars from recombinant protein produced using Pichia pastoris

The use of the methylotrophic yeast Pichia pastoris for large-scale recombinant production of proteins for therapeutic uses and/or biophysical characterisation has been gaining popularity. Here we describe the use of this organism for the production of a von Willebrand factor C domain from procollagen IIA for solution NMR studies. In this research, we specifically identified sites of O-linked glycosylation on the expressed protein, although the native protein is not glycosylated. We demonstrated that it was possible to remove the oligosaccharides by enzymatic digestion, however this approach proved to be prohibitively expensive for the scale of production required for high-resolution structural studies by NMR spectroscopy. After removal of the O-linked glycosylation sites by site-directed mutagenesis, we confirmed that the protein was no longer covalently glycosylated. However, analysis by 1H- and 13C-edited spectroscopy identified the presence of non-covalently associated glycans which were removed by lectin affinity chromatography. We have synthesised methods for the identification and removal of both covalently and non-covalently bound oligosaccharides from heterologous protein expressed in P. pastoris.

Isotopic double-labeling of two honeybee odorant-binding proteins secreted by the methylotrophic yeast Pichia pastoris

Odorant-binding proteins (OBPs) are soluble, low-molecular-weight proteins secreted in the sensillum lymph surrounding the dendrites of olfactory sensilla from a wide range of insect species. These proteins play a role in the solubilization, transport and/or deactivation of pheromones and odorants. In order to study the relationships between the molecular structure in solution and their ligand-binding properties, we have (13)C/(15)N-double-labeled two divergent honeybee OBPs, called ASP1 and ASP2, in sufficient quantities to permit a full determination of the structure and dynamics using heteronuclear NMR spectroscopy. The recombinant labeled proteins produced by the methylotrophic yeast Pichia pastoris have been secreted into a buffered minimal medium using native insect signal peptide. Mass spectrometry and Edman sequencing showed a native-like processing with a labeling efficiency of secreted proteins greater than 98%. After dialysis, the recombinant proteins were purified to homogeneity by one-step reversed-phase liquid chromatography. The final yield after 4-day shake-flask liquid culture was approximately 60 and 100 mg/L for ASP1 and ASP2, respectively. The inexpensive overproduction of labeled recombinant ASP1 and ASP2 should allow NMR studies of the structures and ligand-binding analysis in order to understand the relationships between structure and biological function of these proteins.

Cost-effective and uniform (13)C- and (15)N-labeling of the 24-kDa N-terminal domain of the Escherichia coli gyrase B by overexpression in the photoautotrophic cyanobacterium Anabaena sp. PCC 7120

Structural studies of biomolecules using nuclear magnetic resonance (NMR) rely on the availability of samples enriched in (13)C and (15)N isotopes. While (13)C/(15)N-labeled proteins are generally obtained by overexpression in transformed Escherichia coli cells cultured in the presence of an expensive mixture of labeled precursors, those of the photoautotrophic cyanobacterium Anabaena sp. PCC 7120 can be uniformly labeled by growing them in medium containing Na(15)NO(3) and NaH(13)CO(3) as the sole nitrogen and carbon sources. We report here a novel vector-host system suitable for the efficient preparation of uniformly (13)C/(15)N-labeled proteins in Anabaena sp. PCC 7120. The 24-kDa N-terminal domain of the E. coli gyrase B subunit, used as a test protein, was cloned into the pRL25C shuttle vector under the control of the tac promoter. The transformed Anabaena cells were grown in the presence of the labeled mineral salts and culture conditions were optimized to obtain over 90% of (13)C and (15)N enrichment in the constitutively expressed 24-kDa polypeptide. The yield of purified 24-kDa protein after dual isotope labeling under anaerobic conditions was similar to that obtained with E. coli cells bearing a comparable expression vector and cultured in parallel in a commercially available labeling medium. Furthermore, as probed by NMR spectroscopy and mass spectrometry, the 24-kDa N-terminal domain expressed in Anabaena was identical to the E. coli sample, demonstrating that it was of sufficient quality for 3D-structure determination. Because the Anabaena system was far more advantageous taking into consideration the expense for the labels that were necessary, these results indicate that Anabaena sp. PCC 7120 is an economic alternative for the (13)C/(15)N-labeling of soluble recombinant proteins destined for structural studies.

NMR monitoring of accumulation and folding of 15N-labeled protein overexpressed in Pichia pastoris

Postgenomic studies have led to an increasing demand for isotope-labeled proteins. We present a method for producing large quantities of truly native (15)N-labeled protein. Based on the secretion capabilities of the yeast Pichia pastoris, the recombinant protein is easily purified in a single step as it is secreted. Control of all nitrogen sources permits very high labeling yields. As a result, accumulation and folding of the recombinant protein can be monitored by heteronuclear NMR without purification. Comparison of sample spectra with the spectrum of the purified recombinant protein allows detection of the secreted protein in the culture and monitoring of its folding, from the start of the induction phase. The detection limit for a (15)N-labeled protein is estimated as 20 microM and corresponds, for a 10-kDa protein, to a load of 40 mg/liter in the fermentor. This concentration is reached by most reported preparations in P. pastoris. Further concentration by ultrafiltration would compensate for lower production. This procedure may be useful in many structural genomics and combinatorial chemistry screening projects where most protein productions meet the requirements for this method.

Characterization of glycosylated variants of beta-lactoglobulin expressed in Pichia pastoris

Glycosylated variants of beta-lactoglobulin (BLG) were produced in the methylotrophic yeast Pichia pastoris to mimic the glycosylation pattern of glycodelin, a homologue of BLG found in humans. Glycodelin has three sites for glycosylation, corresponding to amino acids 63-65 (S1), 85-87 (S2) and 28-30 (S3) of BLG. These three sites were engineered into BLG to produce the variants S2, S12 and S123, which carried one, two and three glycosylation sites, respectively. The oligosaccharides on these BLG variants ranged from (mannose)(9)(N-acetylglucosamine)(2) (Man(9)GN(2)) to Man(15)GN(2) and were of the alpha-linked high mannose type. The variant S123 exhibited highest levels of glycosylation, with the range of glycans being Man(9-14)GN(2). Digestion of S123 with alpha-1,2 linkage specific mannosidase resulted in a single product corresponding to Man(6)GN(2). These results indicated a glycosylation pattern consisting of a Man(5)GN(2) structure extended by 4-9 mannose residues attached mainly by alpha-1,2 linkages. The results also indicated extension of the Man(5)GN(2) structure by a single alpha-1,6-linked mannose. The N-linked glycosylation pathway in P.pastoris is significantly different from that in Saccharomyces cerevisiae, with the addition of shorter outer chains to the core and no alpha-1,3 outer extensions.

New developments in isotope labeling strategies for protein solution NMR spectroscopy

The development of novel isotope labeling strategies for proteins has facilitated the study of the structure and dynamics of these molecules. In addition, the recent emergence of alternative methods of bacterial expression for obtaining isotopically labeled proteins permits the study of new classes of important proteins by solution NMR methods.

Recombinant protein expression in Pichia pastoris

The methylotrophic yeast Pichia pastoris is now one of the standard tools used in molecular biology for the generation of recombinant protein. P. pastoris has demonstrated its most powerful success as a large-scale (fermentation) recombinant protein production tool. What began more than 20 years ago as a program to convert abundant methanol to a protein source for animal feed has been developed into what is today two important biological tools: a model eukaryote used in cell biology research and a recombinant protein production system. To date well over 200 heterologous proteins have been expressed in P. pastoris. Significant advances in the development of new strains and vectors, improved techniques, and the commercial availability of these tools coupled with a better understanding of the biology of Pichia species have led to this microbe's value and power in commercial and research labs alike.

Dictyostelium discoideum as expression host: isotopic labeling of a recombinant glycoprotein for NMR studies

The advantages of the organism Dictyostelium discoideum as an expression host for recombinant glycoproteins have been exploited for the production of an isotopically labeled cell surface protein for NMR structure studies. Growth medium containing [(15)N]NH(4)Cl and [(13)C]glycerol was used to generate isotopically labeled Escherichia coli, which was subsequently introduced to D. discoideum cells in simple Mes buffer. A variety of growth conditions were screened to establish minimal amounts of nitrogen and carbon metabolites for a cost-effective protocol. Following single-step purification by anion-exchange chromatography, 8 mg of uniformly (13)C,(15)N-labeled protein secreted by approximately 10(10) D. discoideum cells was isolated from 3.3 liters of supernatant. Mass spectrometry showed the recombinant protein of 16 kDa to have incorporated greater than 99.9% isotopic label. The two-dimensional (1)H-(13)C HSQC spectrum confirms (13)C labeling of both glycan and amino acid residues of the glycoprotein. All heteronuclear NMR spectra showed a good dispersion of cross-peaks essential for high-quality structure determination.

Ligand-binding properties and structural characterization of a novel rat odorant-binding protein variant

After characterization of a novel odorant-binding protein (OBP) variant isolated from the rat nasal mucus, the corresponding cDNA was cloned by RT-PCR. Recombinant OBP-1F, the sequence of which is close to that of previously reported rat OBP-1, has been secreted by the yeast Pichia pastoris at a concentration of 80 mg.L-1 in a form identical to the natural protein as shown by MS, N-terminal sequencing and CD. We observed that, in contrast with porcine OBP-1, purified recombinant OBP-1F is a homodimer exhibiting two disulfide bonds (C44-C48 and C63-C155), a pairing close to that of hamster aphrodisin. OBP-1F interacts with fluorescent probe 1-aminoanthracene (1-AMA) with a dissociation constant of 0.6 +/- 0. 3 microM. Fluorescence experiments revealed that 1-AMA was displaced efficiently by molecules including usual solvents such as EtOH and dimethylsulfoxide. Owing to the large OBP-1F amounts expressed, we set up a novel biomimetic assay (volatile-odorant binding assay) to study the uptake of airborne odorants without radiolabelling and attempted to understand the odorant capture by OBP in the nasal mucus under natural conditions. The assay permitted observations on the binding of airborne odorants of different chemical structures and odors (2-isobutyl-3-methoxypyrazine, linalool, isoamyl acetate, 1-octanal, 1-octanol, dimethyl disulfide and methyl thiobutyrate). Uptake of airborne odorants in nearly physiological conditions strengthens the role of OBP as volatile hydrophobic odorant carriers in the mucus of the olfactory epithelium through the aqueous barrier towards the chemo-sensory cells.

Structural changes accompanying pH-induced dissociation of the beta-lactoglobulin dimer

We have used NMR spectroscopy to determine the three-dimensional (3D) structure, and to characterize the backbone dynamics, of a recombinant version of bovine beta-lactoglobulin (variant A) at pH 2. 6, where the protein is a monomer. The structure of this low-pH form of beta-lactoglobulin is very similar to that of a subunit within the dimer at pH 6.2. The root-mean-square deviation from the pH 6.2 (crystal) structure, calculated for backbone atoms of residues 6-160, is approximately 1.3 A. Differences arise from the orientation, with respect to the calyx, of the A-B and C-D loops, and of the flanking three-turn alpha-helix. The hydrophobic cavity within the calyx is retained at low pH. The E-F loop (residues 85-90), which moves to occlude the opening of the cavity over the pH range 7.2-6.2, is in the "closed" position at pH 2.6, and the side chain of Glu89 is buried. We also carried out measurements of (15)N T(1)s and T(2)s and (1)H-(15)N heteronuclear NOEs at pH 2.6 and 37 degrees C. Although the residues of the E-F loop (residues 86-89) have the highest crystallographic B-factors, the conformation of this loop is reasonably well defined by the NMR data, and its backbone is not especially mobile on the pico- to nanosecond time scale. Several residues (Ser21, Lys60, Ala67, Leu87, and Glu112) exhibit large ratios of T(1) to T(2), consistent with conformational exchange on a micro- to millisecond time scale. The positions of these residues in the 3D structure of beta-lactoglobulin are consistent with a role in modulating access to the hydrophobic cavity.

Heterologous protein expression in the methylotrophic yeast Pichia pastoris

During the past 15 years, the methylotrophic yeast Pichia pastoris has developed into a highly successful system for the production of a variety of heterologous proteins. The increasing popularity of this particular expression system can be attributed to several factors, most importantly: (1) the simplicity of techniques needed for the molecular genetic manipulation of P. pastoris and their similarity to those of Saccharomyces cerevisiae, one of the most well-characterized experimental systems in modern biology; (2) the ability of P. pastoris to produce foreign proteins at high levels, either intracellularly or extracellularly; (3) the capability of performing many eukaryotic post-translational modifications, such as glycosylation, disulfide bond formation and proteolytic processing; and (4) the availability of the expression system as a commercially available kit. In this paper, we review the P. pastoris expression system: how it was developed, how it works, and what proteins have been produced. We also describe new promoters and auxotrophic marker/host strain combinations which extend the usefulness of the system.

Use of Pichia pastoris for the expression, purification, and characterization of rat calretinin "EF-hand" domains

Calretinin (CR) is a calcium-binding, neuronal protein of undefined function. Related proteins either buffer intracellular calcium concentrations or are involved in calcium-signaling pathways. We transformed three CR gene fragment sequences, corresponding to its three complementary domains (I-II, III-IV, and V-VI), into Pichia pastoris. High yields of extracellular expression, of more than 200 mg/liter, were achieved. Simple purification protocols provide high yields of homogenous proteins: dialysis and DEAE-cellulose chromatography for domains I-II and III-IV or ammonium sulfate precipitation and octyl-Sepharose chromatography for domain V-VI. To our knowledge, this is the first report of the expression of an EF-hand protein using P. pastoris. Direct comparison of the purified yields of domain I-II indicates a approximately 20-fold improvement over Escherichia coli. N-terminal amino acid sequencing confirmed our gene products and two anti-calretinin antibodies recognized the appropriate domains. All three CR domains bind (45)Ca and the domain containing EF-hands V and VI seems to have a lower calcium capacity than the other domains. Circular dichroism indicates a high helix content for each of the domains. Calcium-induced structural changes in the first two domains, followed by tryptophan fluorescence, correspond with previous studies, while tyrosine emission fluorescence indicates calcium-induced structural changes also occur in domain V-VI. The methods and expression levels achieved are suitable for future NMR labeling of the proteins, with (15)N and (13)C, and structure-function studies that will help to further understand CR function.

Cloning and expression of a queen pheromone-binding protein in the honeybee: an olfactory-specific, developmentally regulated protein

Odorant-binding proteins (OBPs) are small abundant extracellular proteins thought to participate in perireceptor events of odor-pheromone detection by carrying, deactivating, and/or selecting odor stimuli. The honeybee queen pheromone is known to play a crucial role in colony organization, in addition to drone sex attraction. We identified, for the first time in a social insect, a binding protein called antennal-specific protein 1 (ASP1), which binds at least one of the major queen pheromone components. ASP1 was characterized by cDNA cloning, expression in Pichia pastoris, and pheromone binding. In situ hybridization showed that it is specifically expressed in the auxiliary cell layer of the antennal olfactory sensilla. The ASP1 sequence revealed it as a divergent member of the insect OBP family. The recombinant protein presented the exact characteristics of the native protein, as shown by mass spectrometry, and N-terminal sequencing and exclusion-diffusion chromatography showed that recombinant ASP1 is dimeric. ASP1 interacts with queen pheromone major components, opposite to another putative honeybee OBP, called ASP2. ASP1 biosynthetic accumulation, followed by nondenaturing electrophoresis during development, starts at day 1 before emergence, in concomitance with the functional maturation of olfactory neurons. The isobar ASP1b isoform appears simultaneously to ASP1a in workers, but only at approximately 2 weeks after emergence in drones. Comparison of in vivo and heterologous expressions suggests that the difference between ASP1 isoforms might be because of dimerization, which might play a physiological role in relation with mate attraction.