Isolation and characterization of highly purified streptavidin obtained in a two-step purification procedure from Streptomyces avidinii grown in a synthetic medium

Streptavidin cooperative allosterism upon binding biotin observed by differential changes in intrinsic fluorescence

While the binding of biotin by streptavidin does not appear to be cooperative in the traditional sense of altered binding strength, it has been suggested that it may be cooperative in terms of differential structural changes in the protein. In this work we present intrinsic tryptophan fluorescence data as evidence of a cooperative structural change. The technique involves examination of the differences in fluorescence emission corresponding to distinct tryptophan populations accompanying protein-ligand binding. Specifically we note that the 335 nm emission population (i.e. more hydrophobic) saturates prior to the saturation of the 350 nm emission population commonly used in the standard binding activity assay. We also note that the wavelength of maximum emission, total integrated fluorescence emission and full width at half maximum during the titration of ligand into streptavidin also reach saturation before the expected 4:1 stoichiometric end point. This suggests that the binding of the first 3 biotins effect greater structural changes in the protein than the final ligand.

Preparation and functional evaluation of RGD-modified streptavidin targeting to integrin-expressing melanoma cells

The vertical growth stage is the most dangerous stage of melanoma and is often associated with a poor prognosis. The increased invasiveness and metastasis that is typical for vertically growing melanoma are mediated by the molecules of cell adhesion (particularly, integrins). Integrin αvβ3, which is abundantly expressed on melanoma cells with high metastatic potentials and is characterized by low expression levels in normal melanocytes, is potentially an attractive target for melanoma diagnostics and therapy. Integrin αvβ3 is known to recognize the arginine-glycine-aspartic (RGD) sequence, which has been found in a wide variety of its natural ligands. Here expression vectors bearing the genes of fusion proteins have been constructed for producing these proteins in Escherichia coli. Such fusion proteins consist of a peptidic 'address,' targeting the integrins on melanoma cells, linked to an 'adaptor' for the attachment of a diagnostic or toxic agent. The peptidic 'address' contains the RGD motif, which is stabilized by a disulfide bond to achieve the optimal receptor binding conformation. The 'adaptor' is a tetrameric protein, namely, streptavidin, that is able to achieve high-affinity binding of d-biotin (K(d) = 10(-15) M) and confer avidity to the address peptide. This binding ability facilitates the generation of anti-melanoma diagnostic and therapeutic agents using the appropriate biotin derivatives. These recombinant proteins were purified from the periplasm of E.coli using columns with 2-iminobiotin agarose and demonstrated an ability to adhere to the surface of murine and human melanoma cells.

Design, fabrication and in vitro evaluation of a novel polymer-hydrogel hybrid scaffold for bone tissue engineering

The development of a bone mechanically-compatible and osteoinductive scaffold is important for bone tissue engineering applications, particularly for the repair and regeneration of large area critically-sized bone defects. Although previous studies with weight-bearing scaffolds have shown promising results, there is a clear need to develop better osteoinductive strategies for effective scaffold-based bone regeneration. In this study, we designed and fabricated a novel polymer-hydrogel hybrid scaffold system in which a load-bearing polymer matrix and a peptide hydrogel allowed for the synergistic combination of mechanical strength and great potential for osteoinductivity in a single scaffold. The hybrid scaffold system promoted increased pre-osteoblastic cell proliferation. Further, we biotinylated human recombinant bone morphogenetic protein 2 (rhBMP2), and characterized the biotin addition and its effect on rhBMP2 biological activity. The biotinylated rhBMP2 was tethered to the hybrid scaffold using biotin-streptavidin complexation. Controlled release studies demonstrated increased rhBMP2 retention with the tethered rhBMP2 hybrid scaffold group. In vitro evaluation of the hybrid scaffold was performed with rat bone marrow stromal cells and mouse pre-osteoblast cell line MC3T3-E1 cells. Gene expression of alkaline phosphatase (ALP), collagen I (Col I), osteopontin (OPN), bone sialoprotein (BSP), Runx-2 and osteocalcin (OC) increased in MC3T3-E1 cells seeded on the rhBMP2 tethered hybrid scaffolds over the untethered counterparts, demonstrating osteoinductive potential of the hybrid graft. These findings suggest the possibility of developing a novel polymer-hydrogel hybrid system that is weight bearing and osteoinductive for effective bone tissue engineering.

A simple spectrophotometric streptavidin–biotin binding assay utilizing biotin-4-fluorescein

A new assay for biotin binding capacity of Streptavidin (SA) is presented in this work. The assay is based on the large decrease in the extinction coefficient at 493 nm that accompanies binding of biotin-4-fluorescein (B4F) to SA. This decrease is attributed to formation of a charge transfer complex between the B4F-donor and one or more SA residues. We show that one may observe the stoichiometric binding via monitoring the absorbance at 493 nm using either SA or B4F as the titrant. The sensitivity of the assay is at the lower end of similar fluorimetric and photometric assays. Though the sensitivity is not substantially lower than other comparable techniques, this assay allows one added flexibility in working range and instrumentation, since the same stock solutions may be used for this new photometric assay or the fluorescence assay for which this ligand was first developed.

High-yield production and purification of recombinant T7-tag mature streptavidin in glucose-stressed E. coli

The overexpression of toxic recombinant proteins is often problematic, leading to either low production levels or inclusion bodies. Streptavidin is no exception and thus the highest production level reported to date for streptavidin is 70 mg/L of functional protein. Herein, we report on the production in Escherichia coli and the purification of a recombinant mature streptavidin bearing a T7-tag. Optimization of critical parameters, including the glucose concentration, the pH and the time of induction as well as the use of BL21(DE3)pLysS cell strain, affords up to 120 mg/L functional streptavidin in soluble form. The yield can be further increased by an osmotic stress during the preculture by adding highly concentrated glucose before the inoculation of the culture medium, thus affording reproducibly 230 mg/L of soluble streptavidin. A single denaturing-renaturing step and affinity chromatography afford highly active tetrameric protein with >3.8/4.0 active sites.

Thermal and sodium dodecylsulfate induced transitions of streptavidin

The strong specific binding of streptavidin (SA) to biotin is utilized in numerous biotechnological applications. The SA tetramer is also known to exhibit significant stability, even in the presence of sodium dodecylsulfate (SDS). Despite its importance, relatively little is known about the nature of the thermal denaturation pathway for SA. This work uses a homogeneous SA preparation to expand on the data of previous literature reports, leading to the proposal of a model for temperature induced structural changes in SA. Temperature dependent data were obtained by SDS and native polyacrylamide gel electrophoresis (PAGE), differential scanning calorimetry (DSC), and fluorescence and ultraviolet (UV)-visible spectroscopy in the presence and absence of SDS. In addition to the development of this model, it is found that the major thermal transition of SA in 1% SDS is reversible. Finally, although SA exhibits significant precipitation at elevated temperatures in aqueous solution, inclusion of SDS acts to prevent SA aggregation.

One-step affinity purification of bacterially produced proteins by means of the "Strep tag" and immobilized recombinant core streptavidin

The "Strep tag" is a nine amino acid peptide with intrinsic streptavidin-binding activity. If this sequence is genetically fused to the C-terminus of a polypeptide the recombinant protein can be directly purified by affinity chromatography from the host cell extract on immobilized streptavidin. However, variations were observed in the suitability of different commercial streptavidin-agarose preparations for this purpose. Therefore, the influence of the source of streptavidin, the coupling chemistry, and the nature of the affinity chromatography resin was investigated. A procedure was developed for the production of recombinant core streptavidin in Escherichia coli, followed by its efficient refolding and purification with an overall yield of up to 140 mg functional protein per 11 bacterial culture. When coupled to activated CH-Sepharose 4B this truncated form of streptavidin showed a performance in the affinity chromatography of Strep tag fusion proteins that was superior to all other combinations tested. In contrast to its conventional preparation from Streptomyces strains the recombinant core streptavidin was produced without a proteolytic processing step. Thus, deleterious effects during the streptavidin affinity purification of proteins due to residual proteolytic activity in the immobilized streptavidin were avoided. The versatility of the optimized purification system was demonstrated with five different Strep tag fusion proteins.

Improvement of production, assay and purification of streptavidin

The production of streptavidin by Streptomyces avidinii in several different media was examined at 24, 48 and 72 hours. Flask studies indicated that fermentation media containing either complex or multiple carbon sources resulted in higher yields of streptavidin than media with a single carbon source. Streptavidin could be detected in crude fermentation broths by use of a tritiated biotin binding assay. This assay appears to give useful estimates of streptavidin production. Depending upon the medium employed, streptavidin yields ranged from 0.5 mg/l to 53 mg/l. Production was successfully scaled up to ten liter fermentors. Streptavidin was purified in a one step process from centrifuged, concentrated fermentation broths by binding the protein to an iminobiotin column at pH 11 followed by elution at pH 4.0. Recovery percentages varied depending upon the solubility of the fermentation media ingredients.

Expression of a cloned streptavidin gene in Escherichia coli

We describe the construction of systems for expressing the cloned streptavidin gene in Escherichia coli. Although the streptavidin gene is extremely lethal to the host cells, because of the strong biotin binding of the gene product, the gene was expressed efficiently in E. coli by using T7 RNA polymerase/T7 promoter expression systems. The expressed streptavidin accumulated to more than 35% of the total cell protein. The expressed streptavidin was insoluble in the cell. However, after solubilization by dialysis against 6 M guanidine hydrochloride (pH 1.5) and removal of guanidine hydrochloride by dialysis, the protein became soluble and renatured. This simple procedure yielded streptavidin purified almost to homogeneity. The purified streptavidin bound 3.5-3.9 molecules of biotin per molecule, indicating that it had almost full biotin-binding ability. Some of the purified streptavidin molecules aggregated into oligomers, suggesting that the C-terminal region of the molecule, present in our material but absent in typical preparations, may be responsible for the aggregation.

The immunochemistry of sandwich ELISAs--III. The stoichiometry and efficacy of the protein-avidin-biotin capture (PABC) system

The protein-avidin-biotin capture (PABC) system was developed to decrease the adsorption-induced loss of antigen capture capacity (AgCC) of capture antibodies (CAb) used in sandwich ELISAs. This system involves immobilization of biotinylated CAbs through linkage by streptavidin (SA) to biotinylated carrier proteins adsorbed on polystyrene. Studies reported here describe the stoichiometry of the system and the influence of biotinylation of different carrier proteins and CAbs on the reaction stoichiometry and the AgCC of CAbs. Because of the widespread use of sandwich ELISAs to measure the concn of multivalent protein antigens, the AgCCs of monoclonal and polyclonal CAbs to pig IgG in the PABC system were compared with the AgCCs of these Abs immobilized on the plastic by direct adsorption. Optimal assay conditions for the carrier were obtained when 1 microgram/ml of the biotinylated protein was added to the polystyrene solid phase. An increasing degree of biotin substitution in three carrier proteins was paralleled by an increasing AgCC until a constant maximum was reached. Under conditions of maximal AgCC, 120 ng of the carrier rabbit gamma globulin (RGG; i.e. RGG25biot) was bound to polystyrene, which in turn yielded the maximum amount (i.e. 100 ng) of bound streptavidin (SA; Bdngmax) when 20 micrograms/ml of SA was added. Under conditions giving the Bdngmax for SA, CAb12biot yielded a higher Bdngmax than did CAb25biot or CAb2biot. When the AgCC of equal amounts of differentially biotinylated CAbs were compared, the following order of AgCC was observed: CAb12biot greater than CAb12biot greater than CAb25biot. Hence, while the maximal amount of CAb is immobilized on SA when CAb12biot is used, optimal AgCC is achieved with CAb2biot. The carrier:SA:CAb2biot ratio was 1:2:1 while that for carrier:SA:CAb12biot was 1:2:2. The same ratio was obtained using IgG2biot from four different species. Monoclonal antibodies to swine IgG showed a 5-6-fold increase in Bd%max when immobilized as CAbs using the PABC system versus when adsorbed on polystyrene. Plots of these data suggest that the differences result from a loss of functional affinity. On the contrary, no significant differences in Bd%max and hence functional affinity were observed when a polyclonal antibody to pig IgG was compared using the two assay configurations. Furthermore, when the globulin fraction of the anti-pig polyclonal was adsorbed on plastic, it behaved nearly as well as its affinity-purified counterpart immobilized by the PABC system. The PABC system appears to offer significant advantages for sandwich ELISAs utilizing monoclonal antibodies as the CAb, and may offer some advantages in other s

Production of streptavidin in a synthetic medium

A simple, inexpensive procedure for producing streptavidin has been described. The biotin-binding protein was produced by growing Streptomyces avidinii in a synthetic liquid culture medium containing L-asparagine as the sole nitrogen source. With this procedure, extraneous proteinaceous substances inherently present in culture media prepared with yeast extract or with peptones were not present to interfere with isolation and purification of streptavidin. When harvested after 7-8 days of incubation, the culture fluid was relatively free of contaminating cell breakdown products. Maximal production of streptavidin (100-120 mg/l) was obtained in 8-10 day cultures. For some applications, the culture fluid can be used directly as a source of streptavidin. Under the same conditions used to grow S. avidinii, 11 other actinomycete strains and 134 eumycetes were found to lack the capacity to produce detectable amounts of an extracellular biotin-binding protein.