The structure of the complex between avidin and the dye, 2-(4'-hydroxyazobenzene) benzoic acid (HABA)

The crystal structure of the complex formed between the egg-white biotin-binding protein, avidin, and the dye, 2-(4'-hydroxyazobenzene) benzoic acid (HABA), was determined to a resolution of 2.5 A. The interaction of avidin with the benzoate ring of HABA is essentially identical to that of the complex formed between HABA and streptavidin (the bacterial analogue of the egg-white protein). This interaction emulates the definitive high-affinity interaction of both proteins with the ureido moiety of biotin. The major difference between the avidin- and streptavidin-HABA complexes lies in their interaction with the hydroxyphenyl ring of the dye molecule; in avidin, two adjacent amino acid residues (Phe72 and Ser73), which are not present in streptavidin, form additional interactions with this ring. These are suggested to account for the higher affinity of avidin for HABA. The characteristic red shift, which accompanies the interaction of both proteins with the dye, was traced to a proposed charge-transfer complex formed between the hydroxyphenyl ring of HABA and the indole ring of Trp70 in avidin (Trp79 in streptavidin). Comparison of binding site residues of two such similar proteins versus their markedly different affinities for two such different substrates should eventually contribute to a better design of biomimetic reagents and drugs.

The cellulose paradox: pollutant par excellence and/or a reclaimable natural resource?

The various aspects of cellulose as a pollutant are considered in view of its lack of toxicity on the one hand and its recalcitrant durable nature on the other. The microbial degradation of cellulosics is discussed, and the contrast between its success in handling natural cellulosic wastes versus its failure to cope with man-made refuse is described. Research carried out in the past decade has demonstrated that cellulolytic organisms are provided with cell surface multifunctional multienzyme conglomerates, called cellulosomes, which are capable of solubilizing solid cellulosic substrates. The intriguing properties of such complexes include their cohesive nature, their many enzymatic components, and a characteristic glycosylated cellulose-binding, 'scaffolding' component. The latter appears to serve as a substrate-targeting carrier, which delivers the other (hydrolytic) components to the cellulose. Progress in establishing efficient model systems for in vitro solubilization of purified cellulose or natural cellulosic substrates has been achieved using purified cellulosome preparations, fortified with beta-glucosidase and pectinase. The latter enzymes were required in order to alleviate the phenomenon of product inhibition which reduces the efficiency of the free cellulosome. Such combined enzyme systems are proposed as examples of future tailor-made cellulolytic systems for the degradation of natural cellulosics.

Three-dimensional structures of avidin and the avidin-biotin complex

The crystal structures of a deglycosylated form of the egg-white glycoprotein avidin and of its complex with biotin have been determined to 2.6 and 3.0 A, respectively. The structures reveal the amino acid residues critical for stabilization of the tetrameric assembly and for the exceptionally tight binding of biotin. Each monomer is an eight-stranded antiparallel beta-barrel, remarkably similar to that of the genetically distinct bacterial analog streptavidin. As in streptavidin, binding of biotin involves a highly stabilized network of polar and hydrophobic interactions. There are, however, some differences. The presence of additional hydrophobic and hydrophilic groups in the binding site of avidin (which are missing in streptavidin) may account for its higher affinity constant. Two amino acid substitutions are proposed to be responsible for its susceptibility to denaturation relative to streptavidin. Unexpectedly, a residual N-acetylglucosamine moiety was detected in the deglycosylated avidin monomer by difference Fourier synthesis.

Monoclonal anti-biotin antibodies simulate avidin in the recognition of biotin

The sequence of the VH gene of a monoclonal anti-biotin antibody was determined. Biotin-binding motifs, similar to those in avidin and streptavidin, were identified in complementary determining regions 2 and 3, suggesting that natural selection of functional motifs may occur in unrelated protein types.

Streptavidin blocks immune reactions mediated by fibronectin-VLA-5 recognition through an Arg-Gly-Asp mimicking site

Streptavidin is a biotin-binding analogue of egg-white avidin which is secreted by the bacterium Streptomyces avidinii. We have recently reported that streptavidin contains an Arg-Tyr-Asp-Ser (RYDS) sequence which exhibits structural homology to the Arg-Gly-Asp-Ser (RGDS) cell adhesion domain of fibronectin and other matrix-associated glycoproteins. Competition studies with RGD peptides indicated that streptavidin binds to cells via this site and that the binding is independent of biotin recognition. Since the RGD-containing peptide has been shown to play a key role in integrin-mediated cell adhesion, we assumed that streptavidin may utilize the RYDS site to bind to immune cells and thereby abrogate their adhesion-dependent functions. We now report that streptavidin modulates several matrix-dependent interactions of immune cells. In this context, immobilized streptavidin was found to support activated human CD4+ T cell adhesion in an RGD-specific, alpha 5 beta 1-dependent manner. In addition, soluble streptavidin (the commercially available or biotin-blocked forms) inhibited T cell adhesion to fibronectin and interfered with its co-stimulatory effect on tumor necrosis factor-alpha secretion by co-cultures of CD4+ T cells and macrophages. These results suggest that streptavidin is a novel example of a bacterial protein which utilizes RGD mimicry to interfere with integrin-mediated immune responses.

Natural antibodies to avidin in human serum

Human serum was found to contain natural antibodies to the egg-white glycoprotein avidin. Of 270 samples tested, all contained antibodies to different extents, mainly of the IgG and IgM classes. Anti-avidin antibodies could be isolated by affinity chromatography.

Cell adhesion to streptavidin via RGD-dependent integrins

Biotin-blocked streptavidin binds specifically (Kd approximately 3 x 10(-8) M) to cell surfaces, presumably via an RYD-containing sequence. This site is distinct from the biotin-binding cleft of the protein and bears high homology to the RGD-containing cell-binding domain of fibronectin. We show here that various cell types adhere to immobilized streptavidin and that the soluble protein interferes specifically with cell adhesion to fibronectin substrata (with an IC50 of about 1 x 10(-7) M) but less so to other adhesive glycoproteins (e.g., collagen type I, vitronectin). Immunochemical evidence combined with peptide competition studies demonstrated that cells bind to streptavidin primarily via the major fibronectin receptor (the alpha 5 beta 1 integrin). The results suggest that streptavidin acts as a relatively strict fibronectin mimetic, thus reflecting the great similarity in their respective RYD/RGD sequence and the immediate flanking regions. The bacterial protein emulates and competes with fibronectin and other extracellular matrix adhesive proteins in the initial recognition and binding to cell surfaces, but appears not to induce subsequent processes (e.g., anchorage and spreading). Streptavidin may thus represent a novel example of bacterial protein mimicry of a key adhesion motif.

Identification of the cellulose-binding domain of the cellulosome subunit S1 from Clostridium thermocellum YS

The 3' region of a gene designated cipB, which shows strong homology with cipA that encodes the cellulosome SL subunit of Clostridium thermocellum ATCC 27405, was isolated from a gene library of C. thermocellum strain YS. The truncated S1 protein encoded by the cipB derivative bound tightly to cellulose. The cellulose-binding domain in this polypeptide consisted of a C-terminal proximal 167 residue sequence which showed complete identity with residues 337-503 of mature SL from C. thermocellum strain ATCC 27405. The cellulose-binding domain interacted with both crystalline and amorphous cellulose, but not with xylan.

Cell-adhesive properties of streptavidin are mediated by the exposure of an RGD-like RYD site

The interaction of streptavidin with various cell systems was studied using fluorescent derivatives of the protein. The native unprocessed form of streptavidin bound to cells at low levels and in a nonspecific manner. In contrast, both the truncated "core" streptavidin (the commercially available form) and the biotin-blocked unprocessed protein bound to cells in enhanced levels and in a specific, saturable manner. This suggests that the binding of biotin or cleavage of the terminal portion(s) of the native protein molecule causes conformational changes which lead to the exposure of sites which presumably interact with cell surface receptors. Peptide inhibition studies demonstrated that the majority of binding to cells appears to be dependent on RGD-like specificity, suggesting that the GRYDS sequence of the streptavidin molecule may exhibit such specificity. Indirect immunofluorescence assays revealed that the protein is associated mainly with the cell surface. Moreover, streptavidin was demonstrated to compete with specific monoclonal antibodies to the RGD-binding site on the GpIIbIIIa integrin of activated platelets, thus suggesting that streptavidin may facilitate binding to ubiquitous cell-surface adhesion receptors via RGD mimicry.

Novel oligosaccharide constituents of the cellulase complex of Bacteroides cellulosolvens

The multiple cellulase-containing protein complex, isolated from the cellulolytic bacterium Bacteroides cellulosolvens, contains oligosaccharides which are O-linked mainly to a 230-kDa subunit. The oligosaccharide chains were liberated by alkaline-borohydride treatment and fractionated as oligosaccharide alditols via gel-permeation chromatography and HPLC. The fractions were investigated by one- and two-dimensional (correlation, homonuclear Hartmann-Hahn, rotating-frame nuclear Overhauser enhancement) 500-MHz 1H-NMR spectroscopy in combination with monosaccharide and methylation analyses and with fast-atom-bombardment mass spectrometry. The following carbohydrate structures could be established: [formula: see text] The results indicate an interesting similarity between the oligosaccharide moieties of the cellulase complex of B. cellulosolvens and of Clostridium thermocellum [Gerwig, G. J., Kamerling, J. P., Vliegenthart, J. F. G., Morag (Morgenstern), E., Lamed, R. & Bayer, E. A. (1991) Eur. J. Biochem. 196, 115-122], having 3, 5 and 6 as common elements. The furanose form of a terminal alpha-D-galactose residue demonstrated an inhibitory effect on the interaction of Griffonia simplicifolia I isolectin B4 with the cellulosome-like entity of B. cellulosolvens.

Immunochemical applications of avidin-biotin technology

The major advantage in the use of avidin-biotin technology for isolation purposes is that an improved capacity for purification of an antigen often results (1,2). The antibody, bound to the column via an avidin-biotin bridge, is less affected by the chemistry of immobilization or by physical interactions (hydrophobic, electrostatic, salting-out effects, precipitation, and so on) with the solid support. Such potential deleterious effects are borne by avidin, which is a highly stable protein, and thus serves as a physicochemical buffer for the antibody.

Studies on the biotin-binding site of avidin. Minimized fragments that bind biotin

The object of this study was to define minimized biotin-binding fragments, or 'prorecognition sites', of either the egg-white glycoprotein avidin or its bacterial analogue streptavidin. Because of the extreme stability to enzymic hydrolysis, fragments of avidin were prepared by chemical means and examined for their individual biotin-binding capacity. Treatment of avidin with hydroxylamine was shown to result in new cleavage sites in addition to the known Asn-Gly cleavage site (position 88-89 in avidin). Notably, the Asn-Glu and Asp-Lys peptide bonds (positions 42-43 and 57-58 respectively) were readily cleaved; in addition, lesser levels of hydrolysis of the Gln-Pro (61-62) and Asn-Asp (12-13 and 104-105) bonds could be detected. The smallest biotin-binding peptide fragment, derived from hydroxylamine cleavage of either native or non-glycosylated avidin, was identified to comprise residues 1-42. CNBr cleavage resulted in a 78-amino acid-residue fragment (residues 19-96) that still retained activity. The data ascribe an important biotin-binding function to the overlapping region (residues 19-42) of avidin, which bears the single tyrosine moiety. This contention was corroborated by synthesizing a tridecapeptide corresponding to residues 26-38 of avidin; this peptide was shown to recognize biotin. Streptavidin was not susceptible to either enzymic or chemical cleavage methods used in this work. The approach taken in this study enabled the experimental distinction between the chemical and structural elements of the binding site. The capacity to assign biotin-binding activity to the tyrosine-containing domain of avidin underscores its primary chemical contribution to the binding of biotin by avidin.

The quaternary structure of streptavidin in urea

We report on the interactions of urea and guanidinium salts with streptavidin. Gel filtration chromatography in 0, 4, 6, and 7 M urea indicates that the streptavidin tetramer remains intact in urea. Biotin alters the electrophoretic mobility of streptavidin whether or not 6 M urea is present. The intrinsic fluorescence of streptavidin is increased and blue-shifted in 6 M urea. The fluorescence changes indicate the absence of unfolding. A conformational response to urea is possible, but much of the fluorescence change is due to urea binding as a weak biotin analog (Ka approximately 1.3 M-1). The resistance to structural perturbation by urea reflects the structural stability of streptavidin's anti-parallel beta-barrel motif. Unfolding is sluggish in 6 M guanidinium hydrochloride (half-time, approximately 50 days). After guanidinium thiocyanate unfolding, streptavidin can be refolded, but the unfolding and refolding transitions are centered at different concentrations of perturbant. Slow unfolding, with a 15th power dependence on guanidinium thiocyanate concentration, may be partially responsible for the noncoincidence of the unfolding and refolding processes. Nonequilibrium behavior is also seen in 6 M urea, as native streptavidin does not unfold and guanidinium thiocyanate unfolded streptavidin does not refold. Refolding does occur at lower concentrations of urea. Guanidinium thiocyanate only slowly unfolds the biotin-streptavidin complex. In the presence of biotin, unfolded streptavidin does not refold in 6 M guanidinium thiocyanate or in 6 M urea.

Anomalous dissociative behavior of the major glycosylated component of the cellulosome of Clostridium thermocellum

The cellulosome of Clostridium thermocellum is a highly cohesive multienzyme complex that is capable of completely solubilizing insoluble cellulose. One of the major cellulosomal components, the glycosylated S1 subunit, is believed to play an important structural role and normally migrates in sodium dodecyl sulfate-polyacrylamide gel electrophoresis with an Mr of 210,000. It is shown here that by simply altering the conditions (pH or ionic strength) of the environment prior to electrophoresis, a different migratory profile for S1 emerges, yielding a collection of bands, all of which migrate faster than the parent band. The original electrophoretic behavior of S1 can be reproduced on restoration of the original pH and ionic strength. These results may bear important significance for the physiological role of the S1 subunit in facilitating the observed synergistic action of the other (cellulolytic) components of the cellulosome.

Specific interchain cross-linking of antibodies using bismaleimides. Repression of ligand leakage in immunoaffinity chromatography

The extensive use of antibody-containing affinity columns in the purification of biologically active compounds (e.g., genetically engineered proteins) is severely hampered by the leaching of antibody (or portions thereof) from the immunoaffinity resin during elution of the target antigen. One of the major problems in this context is the combined use of reducing (i.e., thiols) and chaotropic (e.g., detergents and denaturants) agents in the elution step, which causes the disassociation of heavy and/or light chains from the immobilized antibody, thereby contaminating the resultant product. In order to overcome this problem, we have cross-linked the four antibody chains at their sites of disulfide interlinkage, thus producing a single antibody chain. To accomplish this, interchain disulfide bonds were reduced, and the resultant thiol groups were cross-linked by using bifunctional SH-specific reagents (particularly bismaleimides). Cross-linking of up to 95% of the available SH groups produced was achieved with concomitant retention of antigen-binding activity. The cross-linked antibody was immobilized onto CNBr-activated Sepharose, and the resultant column was found to be substantially more stable to harsh elution conditions than similar columns which contain the un-cross-linked antibody.

Isolation and properties of a major cellobiohydrolase from the cellulosome of Clostridium thermocellum

In the anaerobic, thermophilic, cellulolytic bacterium Clostridium thermocellum, efficient solubilization of the insoluble cellulose substrate is accomplished largely through the action of a cellulose-binding multienzyme complex, the cellulosome. A major cellobiohydrolase activity from the cellulosome has been traced to its Mr 75,000 S8 subunit, and an active fragment of this subunit was prepared by a novel procedure involving limited proteolytic cleavage. The truncated Mr 68,000 fragment, termed S8-tr, was purified by gel filtration and high-performance ion-exchange chromatography. The purified protein adsorbed weakly to amorphous cellulose, and its enzymatic action yielded cellobiose as the major end product from both amorphous and crystalline cellulose preparations. The high ratio of exo- to endo-beta-glucanase activities was supported by viscosimetric measurements. The use of model substrates showed that the smallest cellodextrin to be degraded was cellotetraose, but cellopentaose was degraded at a much greater rate. Cellobiose dramatically inhibited the cellulolytic activities. In the absence of calcium or other bivalent metal ions, both the truncated cellobiohydrolase activity of S8-tr and the true cellulase activity of the parent cellulosome were relatively unstable at temperatures above 50 degrees C. Cysteine further enhanced the stabilizing effect of calcium. This is the first report of a defined cellobiohydrolase in C. thermocellum. Its association with the cellulosome and the correspondence of several of their major distinctive properties suggest that this cellobiohydrolase plays a key role in the solubilization of cellulose by the intact cellulosomal complex.

Primary structure of O-linked carbohydrate chains in the cellulosome of different Clostridium thermocellum strains

The cell-free forms of the multiple cellulase-containing protein complex (cellulosome), isolated from the cellulolytic bacterium Clostridium thermocellum strains YS, ATCC 27405 and LQRI, have a total carbohydrate content of 5-7% (by mass), consisting of O-linked oligosaccharide chains. The carbohydrate chains were liberated by alkaline-borohydride treatment and fractionated as oligosaccharide alditols via gel-permeation chromatography and HPLC. The fractions were investigated by 500-MHz 1H-NMR spectroscopy in combination with monosaccharide and methylation analysis and with fast-atom-bombardment mass spectrometry (FAB-MS). In addition to the previously described major oligosaccharide, (formula; see text) [Gerwig, G. J., de Waard, P., Kamerling, J. P., Vliegenthart, J. F. G., Morgenstern, E., Lamed, R. & Bayer, E. A. (1989) J. Biol. Chem. 264, 1027-1035], the following partial structures of this compound could be established: (formula; see text). Cell-free and cell-associated forms of the cellulosome of C. thermocellum, as determined for strain YS, have the same oligosaccharide pattern. Based on the oligosaccharide structures, a biosynthetic pathway is suggested.

A coupled enzyme assay for measurement of sialidase activity

A multi-coupled enzyme assay system for determining sialidase activity is described. Enzymes, substrates and chromogens are reacted in situ and determined spectrophotometrically in ELISA microtiter plates. Sialidase is assayed by the extent of desialylated galactose on an appropriate sialoglycoconjugate (fetuin), which is otherwise unavailable for oxidation by galactose oxidase. The oxidation is monitored by the coupling of H2O2 released to a third enzyme, peroxidase. The rate of change of absorbance at 405 nm, resulting from the oxidized chromogen is a measure of the reaction rate of the coupled enzyme system. A similar system can be used for determining galactose oxidase in solution, or on blots using galactose as substrate. Due to the small-scale single-step measurement, the described assay is a sensitive, convenient, and inexpensive alternative to the classic colorimetric determination.

Affinity cleavage and targeted catalysis of proteins using the avidin-biotin system

The avidin-biotin system was used in order to target enzymes to their substrates in complex mixtures of proteins in solution. The approach described here thus mimics natural systems in which enzymes usually act in selective fashion, due, perhaps, to proximity effects. For affinity cleavage studies, biotinyl transferrin was used as a model target substrate. Avidin or streptavidin was then employed to bridge between the biotinylated target protein and a biotinyl protease. Bovine serum albumin was included in the reaction mixtures to assess the level of nonspecific cleavage. In the case of an unbiotinylated target protein, avidin could be used to inhibit the hydrolytic action of the biotinyl protease. In some systems, a biotinyl antibody could be used to direct the avidin-bridged biotinyl protease to an unbiotinylated target antigen. The data support the contention that preferential cleavage reflects two separate phenomena: (i) avidin confers a conformational alteration of the biotinylated target protein, and (ii) the biotinyl protease is targeted (via the avidin bridge) to the proximity of the biotinylated target protein, thereby promoting cleavage of the conformationally altered molecule. This is the first report in which a proteolytic enzyme could be selectively targeted to specifically hydrolyze a defined protein substrate in solutions containing a complex mixture of other proteins. The approach appears to be a general phenomenon for "targeted catalysis", appropriate for other applications, particularly for affinity cleavage and targeted catalysis of cell-based macromolecules.

Biotin binding changes the conformation and decreases tryptophan accessibility of streptavidin

Biotin binding reduces the tryptophan fluorescence emissions of streptavidin by 39%, blue shifts the emission peak from 333 to 329 nm, and reduces the bandwidth at half height from 53 to 46 nm. The biotin-induced emission difference spectrum resembles that of a moderately polar tryptophan. Streptavidin fluorescence can be described by two lifetime classes: 2.6 nsec (34%) and 1.3 nsec (66%). With biotin bound, lifetimes are 1.3 nsec (26%) and 0.8 nsec (74%). Biotin binding reduces the average fluorescence lifetime from 1.54 to 0.88 nsec. Biotin does not quench the fluorescence of indoles. The fluorescence changes are consistent with biotin binding causing a conformational change which moves tryptophans into proximity to portions of streptavidin which reduce the quantum yield and lifetimes. Fluorescence quenching by acrylamide revealed two classes of fluorophores. Analysis indicated a shielded component comprising 20-28% of the initial fluorescence with (KSV + V) less than or equal to 0.55 M-1. The more accessible component has a predominance of static quenching. Measurements of fluorescence lifetimes at different acrylamide concentrations confirmed the strong static quenching. Since static quenching could be due to acrylamide binding to streptavidin, a dye displacement assay for acrylamide binding was constructed. Acrylamide does bind to streptavidin (Ka = 5 M-1), and probably binds within the biotin-binding site. In the absence of biotin, none of streptavidin's fluorescence is particularly accessible to iodide. In the presence of biotin, iodide neither quenches fluorescence nor alters emission spectra, and acrylamide access is dramatically reduced. We propose that the three tryptophans which always line the biotin site are sufficiently close to the surface of the binding site to be quenched by bound acrylamide. These tryptophans are shielded from iodide, most probably due to steric or ionic hindrances against diffusion into the binding site. Most of the shielding conferred by biotin binding can be attributed to the direct shielding of these residues and of a fourth tryptophan which moves into the binding site when biotin binds, as shown by X-ray studies (Weber et al., 1989).

Relationship of cellulosomal and noncellulosomal xylanases of Clostridium thermocellum to cellulose-degrading enzymes

Xylanase activity of Clostridium thermocellum, an anaerobic thermophilic cellulolytic bacterium, was characterized. The activity was localized both in the cellulosome (the principal multienzyme, cellulose-solubilizing protein complex) and in noncellulosomal fractions. Each of these fractions contained at least four major polypeptide bands which contributed to the xylanolytic activity. In both cases, pH and temperature optima, product pattern, and other features of the xylanase activity were almost identical. The main difference was in the average molecular weights of the respective polypeptides which appeared responsible for the activity. In the noncellulosomal fraction, xylanases with Mrs ranging from 30,000 to 65,000 were detected. Distinct from these were the cellulosomal xylanases, which exhibited much larger Mrs (up to 170,000). The cellulosome-associated xylanases corresponded to known cellulosomal subunits, some of which also exhibited endoglucanase activity, and others which coincided with subunits which appeared to express exoglucanaselike activity. In contrast, the noncellulosomal xylanases hydrolyzed xylan exclusively. beta-Glucosidase and beta-xylosidase activities were shown to be the action of different enzymes; both were associated exclusively with the cell and were not components of the cellulosome. Despite the lack of growth on and utilization of xylan or its degradation products, C. thermocellum produces a highly developed xylanolytic apparatus which is interlinked with its cellulase system.

Streptavidin contains an RYD sequence which mimics the RGD receptor domain of fibronectin

Streptavidin binds at low levels and high affinity to cell surfaces, the cause of which can be traced to the occurrence of a sequence containing RYD (Arg-Tyr-Asp) in the protein molecule. This binding is enhanced in the presence of biotin. Cell-bound streptavidin can be displaced by fibronectin, as well as by RGD- and RYD-containing peptides. In addition, streptavidin can displace fibronectin from cell surfaces. The RYD sequence of streptavidin thus mimics RGD (Arg-Gly-Asp), the universal recognition domain present in fibronectin and other adhesion-related molecules. The observed adhesion to cells has no relevance to biotin-binding since the RYD sequence is not part of the biotin-binding site of streptavidin. Since the use of streptavidin in avidin-biotin technology is based on its biotin-binding properties, researchers are hereby warned against its indiscriminate use in histochemical and cytochemical studies.

Studies on the biotin-binding sites of avidin and streptavidin. Tyrosine residues are involved in the binding site

The involvement of tyrosine in the biotin-binding sites of the egg-white glycoprotein avidin and the bacterial protein streptavidin was examined by using the tyrosine-specific reagent p-nitrobenzenesulphonyl fluoride (Nbs-F). Modification of an average of about 0.5 mol of tyrosine residue/mol of avidin subunit caused the complete loss of biotin binding. This indicates that the single tyrosine residue (Tyr-33) in the avidin subunit is directly involved in the biotin-binding site and that its modification by Nbs also abolishes the binding properties of a neighbouring subunit. This suggests that the tyrosine residues of the egg-white protein may also contribute to the stabilization of the native protein structure. In streptavidin, however, the modification of an average of 3 mol of tyrosine residue/mol of subunit was required to inactivate completely the biotin-binding activity of the protein, but only 1 mol (average) of tyrosine residue/mol of subunit was protected in the presence of biotin. The difference between the h.p.l.c. elution profiles of the enzymic digests of Nbs-modified streptavidin and the Nbs-modified streptavidin-biotin complex revealed two additional fractions in the unprotected protein that contain Nbs-modified tyrosine residues. These residues, Tyr-43 (major fraction) and Tyr-54 (minor fraction), appear to contribute to the biotin-binding site in streptavidin.

Application of avidin-biotin technology to affinity-based separations

During the last decade, avidin-biotin technology has become a commercially viable tool for research, medical and industrial applications. From the beginning, mediation via the avidin-biotin complex was proposed for affinity-based separations. This particular application, however, has been slow in gaining acceptance. One of the reasons is that the strength of binding between avidin and biotin is sometimes inappropriate for the desired affinity system. Another problem involves certain "undesirable" structural properties in the avidin molecule which may lead to high levels of "non-specific" binding. Recent progress in understanding the molecular requirements for binding biotin may eventually lead to the design of avidin-like proteins which will exhibit preferred recognition properties according to the desired application.

Avidin column as a highly efficient and stable alternative for immobilization of ligands for affinity chromatography

The avidin/biotin system was applied as a general mediator in the adsorption/desorption or immobilization of biologically active macromolecules to solid supports. In this context, model biotinylated proteins (lectins and antibodies) were attached to avidin-coupled Sepharose. As examples for affinity chromatography, peanut agglutinin and anti-transferrin antibody were used to isolate asialofetuin and transferrin, respectively. The capacity and product yields were significantly better than those achieved with conventional affinity chromatography on CNBr-activated Sepharose columns containing the same lectin or antibody. Moreover, the columns were characterized by improved stability properties exhibiting remarkably low levels of leakage.

Shielding of tryptophan residues of avidin by the binding of biotin

The binding of biotin to tetrameric avidin changes the environment of tryptophan residues. Binding reduces the total tryptophan fluorescence by 34%, shifts the emission peak from 337 to 324 nm, and reduces the fluorescence bandwidth from 61 to 46 nm. These changes are consistent with the movement of tryptophans to a nonpolar, internal environment. In the absence of biotin, iodide readily quenches the fluorescence of 20-29% of the initial fluorescence, which likely corresponds to one tryptophan located in a positively charged environment. Iodide may have weak access to additional fluorescence, corresponding to perhaps one additional tryptophan. Acrylamide, in the absence of biotin, has good access to three-fourths or more of the fluorescence, but the remainder, due to one or two tryptophans, is well shielded. The binding of biotin completely prevents iodide quenching and decreases acrylamide access dramatically. The data indicate that biotin binding shifts two or three tryptophans to an internal, hydrophobic, shielded environment.

Avidin-biotin technology ten years on: has it lived up to its expectations?

Following the practical success attributed to avidin-biotin technology, researchers are now returning to the basics to try to understand the structural and functional requirements for the high-affinity avidin-biotin interaction.

Postsecretory modifications of streptavidin

Streptavidin, an extracellular biotin-binding protein from Streptomyces avidinii, exhibits a multiplicity in its electrophoretic mobility pattern which depends both upon the conditions for growth of the bacterium and upon the protocol used in the purification of the protein. The observed structural heterogeneity appears to reflect the action of two types of postsecretory molecular events: proteolytic digestion of the intact Mr-18,000 subunit to a minimal molecular size (approx. Mr 14,000), and aggregation of the native tetramer into higher-order oligomeric forms. The extent of subunit degradation and/or tetrameric aggregation affects the capacity of a given streptavidin preparation to interact with biotin-conjugated proteins in different assay systems.

Studies on the biotin-binding sites of avidin and streptavidin. A chemically induced dynamic nuclear polarization investigation of the status of tyrosine residues

We applied the protein photochemically induced dynamic nuclear polarization (photo-c.i.d.n.p.) method to explore the conformation of the side chains of tyrosine, tryptophan and histidine residues in three biotin-binding proteins. The c.i.d.n.p. spectra of avidin, streptavidin and 'core' streptavidin were compared with those of their complexes with biotin and its derivatives. The data indicate that the single tyrosine residue (Tyr-33) of avidin is clearly inaccessible to the triplet flavin photo-c.i.d.n.p. probe. The same holds for all tryptophan and histidine side chains. Although the analogous Tyr-43 residue of streptavidin is also buried, at least three of the other tyrosine residues of this protein are exposed. The same conclusions apply to the truncated form of the protein, core streptavidin. As judged by the photo-c.i.d.n.p. results, complexing of avidin and streptavidin with biotin, N-epsilon-biotinyl-L-lysine (biocytin) or biotinyltyrosine has little or no effect on tyrosine accessibility in these proteins. Biotinyltyrosine can be used to probe the depth of the corresponding binding site. The accessibility of the tyrosine side chain of biotinyltyrosine in the complex demonstrates the exquisite fit of the biotin-binding cleft of avidin: only the biotin moiety appears to be accommodated, leaving the tyrosine side chain exposed.

Novel O-linked carbohydrate chains in the cellulase complex (cellulosome) of Clostridium thermocellum. 3-O-Methyl-N-acetylglucosamine as a constituent of a glycoprotein

Alkaline borohydride treatment of the cellulosome of Clostridium thermocellum yielded two major oligosaccharide-alditols, namely D-Galp-beta(1----4)-D-GalOH and (formula; see text) The compounds, isolated via gel permeation chromatography and high performance liquid chromatography, were analyzed by monosaccharide analysis, methylation analysis, gas-liquid chromatography/mass spectrometry, fast atom bombardment/mass spectrometry, and one- and two-dimensional 500-MHz (COSY, HOHAHA, ROESY) 1H NMR spectroscopy. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis combined with blotting technology indicated that the tetrasaccharide is mainly associated with one of the cellulosome subunits.

Studies on the biotin-binding site of streptavidin. Tryptophan residues involved in the active site

Streptavidin, the non-glycosylated bacterial analogue of the egg-white glycoprotein avidin, was modified with the tryptophan-specific reagent 2-hydroxy-5-nitrobenzyl (Hnb) bromide. As with avidin, complete loss of biotin-binding activity was achieved upon modification of an average of one tryptophan residue per streptavidin subunit. Tryptic peptides obtained from an Hnb-modified streptavidin preparation were fractionated by reversed-phase h.p.l.c., and three major Hnb-containing peptide fractions were isolated. Amino acid and N-terminal sequence analysis revealed that tryptophan residues 92, 108 and 120 are modified and probably comprise part of the biotin-binding site of the streptavidin molecule. Unlike avidin, the modification of lysine residues in streptavidin failed to result in complete loss of biotin-binding activity. The data imply subtle differences in the fine structure of the respective biotin-binding sites of the two proteins.

Biocytin hydrazide--a selective label for sialic acids, galactose, and other sugars in glycoconjugates using avidin-biotin technology

Biocytin hydrazide (BCHZ), a new, water-soluble, long-chained, biotin-containing hydrazide, was synthesized and used for the selective nonradioactive detection of glycoconjugates. Procedures were developed for labeling glycoconjugates on blots. The method involves either chemical (periodate-induced) or enzymatic (via galactose oxidase) oxidation of glycoconjugates, the resultant aldehyde groups are then labeled with biocytin hydrazide, followed by interaction with an avidin-based enzyme probe. Since the biotin-containing reagent is a relatively small, charged molecule, the primary labeling step may be carried out on intact cells and on membrane preparations as well as on blotted samples. On blots, the labeling pattern was similar for both periodate- and galactose oxidase-induced biotinylation procedures. In contrast, periodate-induced labeling of either erythrocyte membranes or cells (prior to blotting) produced an altered labeling pattern. Combined enzyme-induced biotinylation of membranes or cells resulted in a pattern similar to that observed for the direct staining of blots. Using galactose oxidase on human erythrocyte membranes, the procedure was sensitive enough to selectively label the Band 3 lactosaminoglycoprotein.

Studies on the biotin-binding site of avidin. Tryptophan residues involved in the active site

Egg-white avidin was modified with the tryptophan-specific reagent 2-hydroxy-5-nitrobenzyl bromide. The complete loss of biotin-binding activity was achieved upon modification of an average of one tryptophan residue per avidin subunit. The identity of the modified residues was determined by isolating the relevant tryptic and chymotryptic peptides from CNBr-cleaved avidin fragments. The results demonstrate that Trp-70 and Trp-110 are modified in approximately equivalent proportions. It is believed that these residues are located in the active site of avidin and take part in the binding of biotin.

Biotin binding to avidin. Oligosaccharide side chain not required for ligand association

A commercially available, purified preparation of avidin was found to comprise two polypeptide bands (Mr 18,000 and Mr 15,500 respectively). Both bands bound biotin as assessed by biotin overlays of protein blots. The Mr 15,500 polypeptide was found to differ from the Mr 18,000 polypeptide only in its sugar content. When the commercial preparation was applied to a concanavalin A affinity column, the glycosylated forms were retarded as expected, and homotypic nonglycosylated avidin tetramers which failed to bind selectively to the column were collected in the effluent. The biotin-binding properties of the nonglycosylated avidin were equivalent to those obtained for the native (glycosylated) avidin molecule, indicating that the oligosaccharide moiety is not essential for the binding activity.

A simple two-step labeling procedure for ultrastructural localization of cell surface anionic sites

We propose a new method for ultrastructural localization of cell surface anionic sites. The method consists of sequential interaction of aldehyde-fixed cells with a polycationic reagent, poly-L-lysine (PL), followed by secondary interaction with a negatively charged marker, ferritin. By use of PL of low molecular weight (4000) on aldehyde-pre-fixed red blood cells and macrophages, the reaction resulted in binding of ferritin particles to cell surface anionic sites with a density distribution resembling that of cationized ferritin (CF). The density of the attached ferritin molecules increased in direct correlation with the MW of PL used. The primary PL interaction can be carried out at low pH (less than 2), thus restricting the labeling mainly to membrane-bound sialyl residues.