Stimulation of Th1-polarizing cytokines,C-C chemokines,maturation of dendritic cells,and adjuvant function by the peptide binding fragment of heat shock protein 70

The peptide binding C-terminal portion of heat shock protein (HSP)70 (aa 359-610) stimulates human monocytes to produce IL-12, TNF-alpha, NO, and C-C chemokines. The N-terminal, ATPase portion (HSP70(1-358)) failed to stimulate any of these cytokines or chemokines. Both native and the truncated HSP70(359-610) stimulation of chemokine production is mediated by the CD40 costimulatory molecule. Maturation of dendritic cells was induced by stimulation with native HSP70, was not seen with the N-terminal HSP70(1-358), but was enhanced with HSP70(359-610), as demonstrated by up-regulation of CD83, CCR7, CD86, CD80, and HLA class II. In vivo studies in macaques showed that immunization with HSP70(359-610) enhances the production of IL-12 and RANTES. Immunization with peptide-bound HSP70(359-610) in mice induced higher serum IgG2a and IgG3 Abs than the native HSP70-bound peptide. This study suggests that the C-terminal, peptide-binding portion of HSP70 is responsible for stimulating Th1-polarizing cytokines, C-C chemokines, and an adjuvant function.     

Lysine biotinylation and methionine oxidation in the heat shock protein HSP60 synergize in the elimination of reactive oxygen species in human cell cultures

Previous studies suggest that the number of proteins containing covalently bound biotin is larger than previously thought. Here, we report the identity of some of these proteins. Using mass spectrometry, we discovered 108 novel biotinylation sites in the human embryonic kidney HEK293 cell proteome; members of the heat shock protein (HSP) superfamily were overrepresented among the novel biotinylated proteins. About half of the biotinylated proteins also displayed various degrees of methionine oxidation, which is known to play an important role in the defense against reactive oxygen species; for biotinylated HSPs, the percent of methionine sulfoxidation approached 100%. Protein structure analysis suggests that methionine sulfoxides localize in close physical proximity to the biotinylated lysines on the protein surface. Mass spectrometric analysis revealed that between one and five of the methionine residues in the C-terminal KEEKDPGMGAMGGMGGGMGGGMF motif are oxidized in HSP60. The likelihood of methionine sulfoxidation is higher if one of the adjacent lysine residues is biotinylated. Knockdown of HSP60 caused a 60% increase in the level of reactive oxygen species in fibroblasts cultured in biotin-sufficient medium. When HEK293 cells were transferred from biotin-sufficient medium to biotin-free medium, the level of reactive oxygen species increased by >9 times compared with baseline controls and a time-response relationship was evident. High levels of methionine sulfoxidation coincided with cell cycle arrest in the G0/G1 and S phases in biotin-depleted cells. We conclude that biotinylation of lysines synergizes with sulfoxidation of methionines in heat shock proteins such as HSP60 in the defense against reactive oxygen species.     

Affinity thermoprecipitation and recovery of biotinylated biomolecules via a mutant streptavidin-smart polymer conjugate

A system has been developed for reversibly binding and thermoprecipitating biotinylated macromolecules. A high off-rate Ser45Ala (S45A) streptavidin mutant has been covalently conjugated to poly(N-isopropylacrylamide) (PNIPAAm), a temperature-responsive polymer. The resulting conjugate is shown to coprecipitate biotinylated immunoglobulin G (IgG) and a biotinylated oligonucleotide in response to a thermal stimulus. Thermally precipitated biotinylated macromolecules can be released from the S45A-PNIPAAm conjugate by simple treatment with excess free biotin. This release step has been shown to be unique to the mutant streptavidin conjugate-a conjugate of wild type (WT) streptavidin and PNIPAAm does not release bound biotinylated molecules upon treatment with excess free biotin. The capture efficiency (fraction of target molecule precipitated from solution) of the S45A-PNIPAAm conjugate is similar to that of the WT-PNIPAAm conjugate for the biotinylated IgG target molecule (near 100%), but significantly smaller for the biotinylated oligonucleotide target (approximately 60% for the S45A-PNIPAAm conjugate compared to 80% for the WT-PNIPAAm conjugate). The release efficiency (fraction of originally precipitated target molecule released after treatment with free biotin) of the S45A-PNIPAAm conjugate is 70-80% for the biotinylated IgG target and nears 100% for the biotinylated oligonucleotide target. This system demonstrates the use of a high off-rate streptavidin mutant to add reversibility to a system based on smart-polymer-streptavidin conjugates.     

Ligand-modified aminobisphosphonate for linking proteins to hydroxyapatite and bone surface

An increase in bone resorption is one of the main symptoms of osteoporosis, a disease that affects more and more individuals every day. Bisphosphonates are known to inhibit bone resorption and thus are being used as a treatment for osteoporosis. Aminobisphosphonates present a functionality that can be easily used for conjugation to other molecules, such as peptides, proteins, and ligands for protein recognition. In this study, an aminobisphosphonate conjugated with biotin was used as a model linker for protein attachment to bone. With this system, the interaction of biotinylated aminobisphosphonate with hydroxyapatite, a major mineral component of bone, was investigated. Quantification of the binding of aminobisphosphonate to hydroxyapatite was performed using a fluorescently labeled antibody for biotin. Additionally, the interaction of the biotinylated aminobisphosphonate with multiple treatments of cortical bone from the midshaft of a cow femur was studied. It was demonstrated that modified aminobisphosphonate reagents can bind hydroxyapatite and bone at high levels, while the biotin functionality is free to be recognized by the fluorescently labeled antibiotin antibody, suggesting that modified aminobisphosphonates could be used to link other peptides or proteins to the bone surface.     

Easily reversible desthiobiotin binding to streptavidin,avidin, and other biotin-binding proteins: uses for protein labeling,detection, and isolation

The high-affinity binding of biotin to avidin, streptavidin, and related proteins has been exploited for decades. However, a disadvantage of the biotin/biotin-binding protein interaction is that it is essentially irreversible under physiological conditions. Desthiobiotin is a biotin analogue that binds less tightly to biotin-binding proteins and is easily displaced by biotin. We synthesized an amine-reactive desthiobiotin derivative for labeling proteins and a desthiobiotin-agarose affinity matrix. Conjugates labeled with desthiobiotin are equivalent to their biotinylated counterparts in cell-staining and antigen-labeling applications. They also bind to streptavidin and other biotin-binding protein-based affinity columns and are recognized by anti-biotin antibodies. Fluorescent streptavidin conjugates saturated with desthiobiotin, but not biotin, bind to a cell-bound biotinylated target without further processing. Streptavidin-based ligands can be gently stripped from desthiobiotin-labeled targets with buffered biotin solutions. Thus, repeated probing with fluorescent streptavidin conjugates followed by enzyme-based detection is possible. In all applications, the desthiobiotin/biotin-binding protein complex is easily dissociated under physiological conditions by either biotin or desthiobiotin. Thus, our desthiobiotin-based reagents and techniques provide some distinct advantages over traditional 2-iminobiotin, monomeric avidin, or other affinity-based techniques.     

Functional characterization of recombinant human HSP70 domains and interdomain interactions

ATPase and peptide-binding activity of recombinant human heat shock proteins HSP70_A1B and HSC70 and two hybrid proteins derived from them was investigated. UV-spectral recorded data were used to characterize conformational rearrangements induced by domain replacement or HSP70-peptide interaction. It was shown that the N-terminal domain dramatically affects the substrate specificity of the C-terminal peptide-binding domain, which puts forward a new hypothesis for HSP70 chaperone machinery. On the other hand, the peptide-binding domain affected the ATPase activity of the recombinant proteins. There was a linear relationship between the ATPase activity and the peptide complex percentage. This connection can be used for quantification of HSP70 complexes with unlabeled peptides.     

Global profiling of the cell surface proteome of cancer cells uncovers an abundance of proteins with chaperone function

There is currently limited data available pertaining to the global characterization of the cell surface proteome. We have implemented a strategy for the comprehensive profiling and identification of surface membrane proteins. This strategy has been applied to cancer cells, including the SH-SY5Y neuroblastoma, the A549 lung adenocarcinoma, the LoVo colon adenocarcinoma, and the Sup-B15 acute lymphoblastic leukemia (B cell) cell lines and ovarian tumor cells. Surface membrane proteins of viable, intact cells were subjected to biotinylation then affinity-captured and purified on monomeric avidin columns. The biotinylated proteins were eluted from the monomeric avidin columns as intact proteins and were subsequently separated by two-dimensional PAGE, transferred to polyvinylidene difluoride membranes, and visualized by hybridization with streptavidin-horseradish peroxidase. Highly reproducible, but distinct, two-dimensional patterns consisting of several hundred biotinylated proteins were obtained for the different cell populations analyzed. Identification of a subset of biotinylated proteins among the different cell populations analyzed using matrix-assisted laser desorption ionization and tandem mass spectrometry uncovered proteins with a restricted expression pattern in some cell line(s), such as CD87 and the activin receptor type IIB. We also identified more widely expressed proteins, such as CD98, and a sushi repeat-containing protein, a member of the selectin family. Remarkably, a set of proteins identified as chaperone proteins were found to be highly abundant on the cell surface, including GRP78, GRP75, HSP70, HSP60, HSP54, HSP27, and protein disulfide isomerase. Comprehensive profiling of the cell surface proteome provides an effective approach for the identification of commonly occurring proteins as well as proteins with restricted expression patterns in this compartment.     

Biotin-assisted folding of streptavidin on the yeast surface

Yeast surface display allows heterologously expressed proteins to be targeted to the exterior of the cell wall and thus has a potential as a biotechnology platform. In this study, we report the successful display of functional streptavidin on the yeast surface. Streptavidin binds the small molecule biotin with high affinity (K(d) ≈ 10(-14)M) and is used widely in applications that require stable noncovalent interaction, including immobilization of biotinylated compounds on a solid surface. As such, engineering functional streptavidin on the yeast surface may find novel uses in future biotechnology applications. Although the molecule does not require any post-translational modification, streptavidin is difficult to fold in bacteria. We show that Saccharomyces cerevisiae can fold the protein correctly if induced at 20°C. Contrary to a previous report, coexpression of anchored and soluble streptavidin subunits is not necessary, as expressing the anchored subunit alone is sufficient to form a functional complex. For unstable monomer mutants, however, addition of free biotin during protein induction is necessary to display a functional molecule, suggesting that biotin helps the monomer fold. To show that surface displayed streptavidin can be used to immobilize other biomolecules, we used it to capture biotinylated antibody, which is then used to immunoprecipitate a protein target.     

Characterization of recombinant human HSP70's domains functions and interdomain interactions

We investigated ATP-ase and peptide-binding activity of recombinant human heat shock protein HSP70(A1B), HSC70, and two hybrid proteins derived from those. The UV-spectral recorded data was used to characterize conformational rearrangements, which were induced by domain replacement or HSP70-peptide interaction. We have shown that N-terminal domain dramatically affect substrate specificity of C-terminal peptide-binding domain. This proposes new hypothesis for HSP70 chaperone machinery. The linear dependence between ATP-ase activity and peptide complex ratio was found. This relationship could be used for unlabeled peptide-HSP70 complex quantification.     

Intracellular production of a soluble and functional monomeric streptavidin in Escherichia coli and its application for affinity purification of biotinylated proteins

Monomeric forms of avidin and streptavidin [(strept)avidin] have many potential applications. However, generation of monomeric (strept)avidin in sufficient quantity is a major limiting factor. We report the successful intracellular production of an improved version of monomeric streptavidin (M4) in a soluble and functional state at a level of approximately 70 mg/L of an Escherichia coli shake flask culture. It could be affinity purified in one step using biotin agarose with 70% recovery. BIAcore biosensor analysis using biotinylated bovine serum albumin confirmed its desirable kinetic properties. Two biotinylated proteins with different degrees of biotinylation (5.5 and 1 biotin per protein) pre-mixed with cellular extracts from Bacillus subtilis were used to examine the use of M4-agarose in affinity purification of protein. Both biotinylated proteins could be purified in high purity with 75-80% recovery. With the mild elution and matrix regeneration conditions, the M4-agarose had been reused four times without any detectable loss of binding capability. The relatively high-level overproduction and easy purification of M4, excellent kinetic properties with biotinylated proteins and mild procedure for protein purification make vital advancements in cost-effective preparation of monomeric streptavidin affinity matrix with desirable properties for purification of biotinylated molecules.     

Endogenous biotin‐binding proteins: an overlooked factor causing false positives in streptavidin‐based protein detection

Biotinylation is widely used in DNA, RNA and protein probing assays as this molecule has generally no impact on the biological activity of its substrate. During the streptavidin‐based detection of glycoproteins in Lactobacillus rhamnosus GG with biotinylated lectin probes, a strong positive band of approximately 125 kDa was observed, present in different cellular fractions. This potential glycoprotein reacted heavily with concanavalin A (ConA), a lectin that specifically binds glucose and mannose residues. Surprisingly, this protein of 125 kDa could not be purified using a ConA affinity column. Edman degradation of the protein, isolated via cation and anion exchange chromatography, lead to the identification of the band as pyruvate carboxylase, an enzyme of 125 kDa that binds biotin as a cofactor. Detection using only the streptavidin conjugate resulted in more false positive signals of proteins, also in extracellular fractions, indicating biotin‐associated proteins. Indeed, biotin is a known cofactor of numerous carboxylases. The potential occurence of false positive bands with biotinylated protein probes should thus be considered when using streptavidin‐based detection, e.g. by developing a blot using only the streptavidin conjugate. To circumvent these false positives, alternative approaches like detection based on digoxigenin labelling can also be used.     

Site-specific photobiotinylation of antibodies, light chains, and immunoglobulin fragments

The high affinity of biotin for avidin has been exploited for many antibody-based assays. This requires that biotin is covalently conjugated to the antibody molecule. Several chemically reactive biotinylation reagents are commercially available. Except for the attachment via sulfhydryl groups in the immunoglobulin (Ig) molecule, these reagents attach biotin randomly to various amino acid side chains. Although non-site-specific modification of antibodies does not interfere in most immunoassays, specific application and sensitive antibodies would benefit from site-specific biotinylation. Here we describe an affinity biotinylation technique based on a photoreactive biotin reagent. The design of this reaction was possible from the discovery of a conserved binding site in the variable Ig domain for nucleotides and nucleosides. The described photoaffinity biotinylation offers the advantages of ease, convenience, and production of a reproducible and defined biotinylated antibody preparation.     

Protease-Induced Formation of the Sperm Acrosomal Filament

Filament extension during the sperm acrosome reaction in Sicyonia ingentis is triggered by an egg trypsin-like protease whose action can be mimicked using trypsin. Using biotinylated trypsin and either a fluorescently-labeled or colloidal gold-labeled antibody to biotin, trypsin binding was localized to the anterior granule of the sperm which is exposed upon acrosomal exocytosis. The binding was to proteinaceous material at the base of the granule juxtaposed to the inner acrosomal membrane. Other labeled proteins also bound in the same pattern but only in the presence of unlabeled trypsin; non-proteolytic proteins did not induce filament formation. Binding of all proteins tested occurred slowly over a period of about 30 min. A minimum of 30 min of trypsin exposure was required in order to trigger filament formation, and increasing trypsin concentration did not reduce this time requirement. These results indicate that the protease slowly uncovers a binding site for itself (or other proteins), and then its proteolytic activity is again required to induce filament formation. The protease kallikrein appeared to be a more potent inducer than trypsin, while thrombin and clostripain had no apparent inducing activity.     

Exogenous HSP70 becomes cell associated,but not internalized,by stressed arterial smooth muscle cells

Summary Cell death within atherosclerotic plaques leads to necrosis and rupture, resulting in vascular occlusion. We have previously demonstrated that addition of exogenous 70 kDa heat shock protein (HSP70) to arterial smooth muscle cells (aSMCs) in vitro can protect against toxins that may initiate necrosis. To determine whether exogenous HSP70 enters aSMCs or acts from outside cells to preserve viability, cultured rabbit aSMCs were stressed by serum deprivation and treated with fluorescently labeled (7-aminomethyl-4-coumarin-3-acetate) or¹²⁵I-radiolabeled HSP70. Cell-associated HSP70 was analyzed using Western blotting, fluoresence spectroscopy, and gamma counting/autoradiogarphy. Surface binding of HSP70 to aSMCs was differentiated from uptake by using trypsin treatment to degrade non-internalized HSP70. Specificity of HSP70 binding was tested by inhibiting uptake of¹²⁵I-HSP70 with excess unlabeled HSP70 or bovine serum albumin (BSA). The effect of unlabeled exogenous HSP70 on endogenous HSP synthesis was also tested. Exogenous HSP70 increased total cell-associated HSP70 2.9- to 3.6-fold over levels present in unstressed aSMCs. However, <5% of the exogenous HSP70 was trypsin-insensitive, indicating that bound HSP70 was not internalized. Binding of¹²⁵I-HSP70 was inhibited by both unlabeled HSP70 and BSA, implying a non-specific interaction with the plasmalemma. Exogenous HSP70 significantly lowered overall protein synthesis by serum-deprived aSMCs, but it did not specifically inhibit synthesis of endogenous HSPs after heat shock. The results indicate that exogenous HSP70 protects viability of stressed aSMCs through interactions with the cell surface rather than via internalization.     

Biotin IgM Antibodies in Human Blood: A Previously Unknown Factor Eliciting False Results in Biotinylation-Based Immunoassays

Biotin is an essential vitamin that binds streptavidin or avidin with high affinity and specificity. As biotin is a small molecule that can be linked to proteins without affecting their biological activity, biotinylation is applied widely in biochemical assays. In our laboratory, IgM enzyme immuno assays (EIAs) of μ-capture format have been set up against many viruses, using as antigen biotinylated virus like particles (VLPs) detected by horseradish peroxidase-conjugated streptavidin. We recently encountered one serum sample reacting with the biotinylated VLP but not with the unbiotinylated one, suggesting in human sera the occurrence of biotin-reactive antibodies. In the present study, we search the general population (612 serum samples from adults and 678 from children) for IgM antibodies reactive with biotin and develop an indirect EIA for quantification of their levels and assessment of their seroprevalence. These IgM antibodies were present in 3% adults regardless of age, but were rarely found in children. The adverse effects of the biotin IgM on biotinylation-based immunoassays were assessed, including four inhouse and one commercial virus IgM EIAs, showing that biotin IgM do cause false positivities. The biotin can not bind IgM and streptavidin or avidin simultaneously, suggesting that these biotin-interactive compounds compete for the common binding site. In competitive inhibition assays, the affinities of biotin IgM antibodies ranged from 2.1×10(-3) to 1.7×10(-4 )mol/L. This is the first report on biotin antibodies found in humans, providing new information on biotinylation-based immunoassays as well as new insights into the biomedical effects of vitamins.     

Synthesis of biotinylated glycoconjugates and their use in a novel ELISA for direct comparison of HIV-1 Gp120 recognition of GalCer and related carbohydrate analogues

As part of our program directed toward the design and synthesis of high-affinity ligands for the GalCer-binding site on the HIV cell surface glycoprotein, gp120, we required a reliable method for qualitatively assessing relative binding affinities for related analogues. Due to the hydrophilic nature of these synthetic conjugates, difficulties were encountered with typical ELISA methods, which rely upon hydrophobic interactions to anchor the ligand to a microtiter plate. Other types of assays were also problematic due to nonspecific binding of gp120. Therefore, we developed a general method for plating water-soluble ligands on microtiter plates using biotin/NeutrAvidin recognition for adhesion. A water-soluble GalCer analogue was prepared by conjugating psychosine to biotin using a novel tetraethylene glycol linker. In a similar manner, LacCer and GlcCer analogues were prepared and these conjugates were plated into microtiter wells containing NeutrAvidin. Unoccupied sites were blocked using biotin functionalized as a primary amide. Gp120 binding to galactosyl sphingosine, GalSph (19), GlcSph (22), and LacSph (23) conjugates was assessed through incubation with recombinant HRP-gp120. It was determined that LacSph has the strongest interaction with gp120. The binding affinities of GalSph and GlcSph were similar to each other and less strong than LacSph. These data contradict earlier studies where HPTLC showed that LacCer and GlcCer do not significantly bind gp120. They also contradict liposome-based assays that reported psychosine is not recognized by gp120. The extent of plating for each biotinylated molecule was quantified using HRP-biotin, allowing direct comparison of ligand plating efficiencies for the first time. Several other synthetic biotin conjugates were prepared and tested, demonstrating the feasibility of performing ELISA on water-soluble ligands.     

Protein engineering of streptavidin for in vivo assembly of streptavidin beads

Escherichia coli was engineered to intracellularly manufacture streptavidin beads. Variants of streptavidin (monomeric, core and mature full length streptavidin) were C-terminally fused to PhaC, the polyester granule forming enzyme of Cupriavidus necator. All streptavidin fusion proteins mediated formation of the respective granules in E. coli and were overproduced at the granule surface. The monomeric streptavidin showed biotin binding (0.7 ng biotin/microg bead protein) only when fused as single-chain dimer. Core streptavidin and the corresponding single-chain dimer mediated a biotin binding of about 3.9 and 1.5 ng biotin/mug bead protein, respectively. However, biotin binding of about 61 ng biotin/mug bead protein with an equilibrium dissociation constant (KD) of about 4 x 10(-8)M was obtained when mature full length streptavidin was used. Beads displaying mature full length streptavidin were characterized in detail using ELISA, competitive ELISA and FACS. Immobilisation of biotinylated enzymes or antibodies to the beads as well as the purification of biotinylated DNA was used to demonstrate the applicability of these novel streptavidin beads. This study proposes a novel method for the cheap and efficient one-step production of versatile streptavidin beads by using engineered E. coli as cell factory.     

Identification of the tRNA-binding protein Arc1p as a novel target of in vivo biotinylation in Saccharomyces cerevisiae

Biotin is an essential cofactor of cell metabolism serving as a protein-bound coenzyme in ATP-dependent carboxylation, in transcarboxylation, and certain decarboxylation reactions. The involvement of biotinylated proteins in other cellular functions has been suggested occasionally, but available data on this are limited. In the present study, a Saccharomyces cerevisiae protein was identified that reacts with streptavidin on Western blots and is not identical to one of the known biotinylated yeast proteins. After affinity purification on monomeric avidin, the biotinylated protein was identified as Arc1p. Using 14C-labeled biotin, the cofactor was shown to be incorporated into Arc1p by covalent and alkali-stable linkage. Similar to the known carboxylases, Arc1p biotinylation is mediated by the yeast biotin:protein ligase, Bpl1p. Mutational studies revealed that biotinylation occurs at lysine 86 within the N-terminal domain of Arc1p. In contrast to the known carboxylases, however, in vitro biotinylation of Arc1p is incomplete and increases with BPL1 overexpression. In accordance to this fact, Arc1p lacks the canonical consensus sequence of known biotin binding domains, and the bacterial biotin:protein ligase, BirA, is unable to use Arc1p as a substrate. Arc1p was shown previously to organize the association of MetRS and GluRS tRNA synthetases with their cognate tRNAs thereby increasing the substrate affinity and catalytic efficiency of these enzymes. Remarkably, not only biotinylated but also the biotin-free Arc1p obtained by replacement of lysine 86 with arginine were capable of restoring Arc1p function in both arc1Delta and arc1Deltalos1Delta mutants, indicating that biotinylation of Arc1p is not essential for activity.     

Identification and solution structures of a single domain biotin/lipoyl attachment protein from Bacillus subtilis

Protein biotinylation and lipoylation are post-translational modifications, in which biotin or lipoic acid is covalently attached to specific proteins containing biotin/lipoyl attachment domains. All the currently reported natural proteins containing biotin/lipoyl attachment domains are multidomain proteins and can only be modified by either biotin or lipoic acid in vivo. We have identified a single domain protein with 73 amino acid residues from Bacillus subtilis strain 168, and it can be both biotinylated and lipoylated in Escherichia coli. The protein is therefore named as biotin/lipoyl attachment protein (BLAP). This is the first report that a natural single domain protein exists as both a biotin and lipoic acid receptor. The solution structure of apo-BLAP showed that it adopts a typical fold of biotin/lipoyl attachment domain. The structure of biotinylated BLAP revealed that the biotin moiety is covalently attached to the side chain of Lys(35), and the bicyclic ring of biotin is folded back and immobilized on the protein surface. The biotin moiety immobilization is mainly due to an interaction between the biotin ureido ring and the indole ring of Trp(12). NMR study also indicated that the lipoyl group of the lipoylated BLAP is also immobilized on the protein surface in a similar fashion as the biotin moiety in the biotinylated protein.     

Artifactual detection of biotin on histones by streptavidin

Biotinylation is a recent addition to the list of reported posttranslational modifications made to histones. Holocarboxylase synthetase (HCS) and biotinidase have been implicated as biotinylating enzymes. However, the details of the mechanism and the regulation of biotin transfer on and off histones remains unclear. Here we report that in a cell culture system low biotin availability reduces biotinylation of carboxylases, yet apparent biotinylation of histones is unaffected. This is despite biotin depletion having detrimental effects on cell viability and proliferation. Further analysis of the widely used method for detecting biotin on histones, streptavidin blotting, revealed that streptavidin interacts with histones independently of biotin binding. Preincubation of streptavidin with free biotin reduced binding to biotinylated carboxylases but did not block binding to histones. To investigate biotinylation of histones using an alternative detection method independent of streptavidin, incorporation of 14C biotin into biotinylated proteins was analyzed. Radiolabeled biotin was readily detectable on carboxylases but not on histones, implying very low levels of biotin in the nucleus attached to histone proteins (< 0.03% biotinylation). In conclusion, we would caution against the use of streptavidin for investigating histone biotinylation.