Identification of surface proteins on bovine leukocytes by a biotin-avidin protein blotting technique

Decreased solubility and increased adsorptivity of a biotinylated humanized anti-cocaine mAb

Biotinylation of proteins, including antibodies, is a very useful and important modification for a variety of biochemical characterizations, including anti-drug antibody (ADA) assays used to detect antibodies raised against therapeutic antibodies. We assessed different degrees of biotin labeling of an anti-cocaine mAb currently under development for treating cocaine use disorder. We noted that higher levels of biotin labeling dramatically decreased mAb solubility, and increased the tendency to bind to surfaces, complicating characterization of the biotinylated antibody. Specifically, in phosphate buffered saline, labeling stoichiometries of more than about 3 biotin/mAb resulted in decreased recoveries due to increased binding to surfaces and decreased mAb solubility. Native gel agarose electrophoresis, differential scanning fluorimetry, and isothermal titration calorimetry all demonstrated changes in the mAb which became more pronounced above a labeling ratio of 3 biotin/mAb. At 3.0 biotin/mAb, there were minimal changes in solubility, adsorptivity, exposure of hydrophobic dye-binding sites, heat stability, and cocaine binding, in stark contrast to labeling with 5.6 biotin/mAb. Thus, the degree of biotinylation should be kept at about 3 biotin/mAb to maintain antigen binding and general structure, solubility, and stability of this mAb, a finding which may be important for other similar mAbs currently in use or under development.

SLAPSHOT reveals rapid dynamics of extracellularly exposed proteome in response to calcium-activated plasma membrane phospholipid scrambling

To facilitate our understanding of proteome dynamics during signaling events, robust workflows affording fast time resolution without confounding factors are essential. We present Surface-exposed protein Labeling using PeroxidaSe, H2O2, and Tyramide-derivative (SLAPSHOT) to label extracellularly exposed proteins in a rapid, specific, and sensitive manner. Simple and flexible SLAPSHOT utilizes recombinant soluble APEX2 protein applied to cells, thus circumventing the engineering of tools and cells, biological perturbations, and labeling biases. We applied SLAPSHOT and quantitative proteomics to examine the TMEM16F-dependent plasma membrane remodeling in WT and TMEM16F KO cells. Time-course data ranging from 1 to 30 min of calcium stimulation revealed co-regulation of known protein families, including the integrin and ICAM families, and identified proteins known to reside in intracellular organelles as occupants of the freshly deposited extracellularly exposed membrane. Our data provide the first accounts of the immediate consequences of calcium signaling on the extracellularly exposed proteome.

Detecting PTP Protein-Protein Interactions by Fluorescent Immunoprecipitation Analysis (FIPA)

Identifying protein-protein interactions is crucial for revealing protein functions and characterizing cellular processes. Manipulating PPIs has become widespread in treating human diseases such as cancer, autoimmunity, and infections. It has been recently applied to the regulation of protein tyrosine phosphatases (PTPs) previously considered undruggable. A broad panel of methods is available for studying PPIs. To complement the existing toolkit, we developed a simple method called fluorescent immunoprecipitation analysis (FIPA). This method is based on coimmunoprecipitation followed by protein gel electrophoresis and fluorescent imaging to visualize components of a protein complex simultaneously on a gel. The FIPA allows the detection of proteins expressed under native conditions and is compatible with mass spectrometry identification of protein bands.

Microfluidics enabled multi-omics triple-shot mass spectrometry for cell-based therapies

In recent years, cell-based therapies have transformed medical treatment. These therapies present a multitude of challenges associated with identifying the mechanism of action, developing accurate safety and potency assays, and achieving low-cost product manufacturing at scale. The complexity of the problem can be attributed to the intricate composition of the therapeutic products: living cells with complex biochemical compositions. Identifying and measuring critical quality attributes (CQAs) that impact therapy success is crucial for both the therapy development and its manufacturing. Unfortunately, current analytical methods and tools for identifying and measuring CQAs are limited in both scope and speed. This Perspective explores the potential for microfluidic-enabled mass spectrometry (MS) systems to comprehensively characterize CQAs for cell-based therapies, focusing on secretome, intracellular metabolome, and surfaceome biomarkers. Powerful microfluidic sampling and processing platforms have been recently presented for the secretome and intracellular metabolome, which could be implemented with MS for fast, locally sampled screening of the cell culture. However, surfaceome analysis remains limited by the lack of rapid isolation and enrichment methods. Developing innovative microfluidic approaches for surface marker analysis and integrating them with secretome and metabolome measurements using a common analytical platform hold the promise of enhancing our understanding of CQAs across all "omes," potentially revolutionizing cell-based therapy development and manufacturing for improved efficacy and patient accessibility.

Pyridinium-Based Strategy for a Bioorthogonal Conjugation-Assisted Purification Method for Profiling Cell Surface Proteome

Cell surface proteins (CSPs) are valuable targets for therapeutic agents, but achieving highly selective CSP enrichment in cellular physiology remains a technical challenge. To address this challenge, we propose a newly developed sulfo-pyridinium ester (SPE) cross-linking probe, followed by two-step imaging and enrichment. The SPE probe showed higher efficiency in labeling proteins than similar NHS esters at the level of cell lysates and demonstrated specificity for Lys in competitive experiments. More importantly, this probe could selectively label the cell membranes in cell imaging with only negligible labeling of the intracellular compartment. Moreover, we successfully performed this strategy on MCF-7 live cells to label 425 unique CSPs from 1162 labeled proteins. Finally, we employed our probe to label the CSPs of insulin-cultured MCF-7, revealing several cell surface targets of key functional biomarkers and insulin-associated pathogenesis. The above results demonstrate that the SPE method provides a promising tool for the selective labeling of cell surface proteins and monitoring transient cell surface events.

SLAPSHOT reveals rapid dynamics of extracellularly exposed proteome in response to calcium-activated plasma membrane phospholipid scrambling

To facilitate our understanding of the often rapid and nuanced dynamics of extracellularly exposed proteomes during signaling events, it is important to devise robust workflows affording fast time resolution without biases and confounding factors. Here, we present Surface-exposed protein Labeling using PeroxidaSe, H2O2, and Tyramide-derivative (SLAPSHOT), to label extracellularly exposed proteins in a rapid, sensitive, and specific manner, while preserving cellular integrity. This experimentally simple and flexible method utilizes recombinant soluble APEX2 peroxidase that is applied to cells, thus circumventing biological perturbations, tedious engineering of tools and cells, and labeling biases. APEX2 neither requires metal cations for activity nor contains disulfide bonds, conferring versatility for a wide spectrum of experimental setups. We applied SLAPSHOT followed by quantitative mass spectrometry-based proteomics analysis to examine the immediate and extensive cell surface expansion and ensuing restorative membrane shedding upon the activation of Scott syndrome-linked TMEM16F, a ubiquitously expressed calcium-dependent phospholipid scramblase and ion channel. Time-course data ranging from one to thirty minutes of calcium stimulation using wild-type and TMEM16F deficient cells revealed intricate co-regulation of known protein families, including those in the integrin and ICAM families. Crucially, we identified proteins that are known to reside in intracellular organelles, including ER, as occupants of the freshly deposited membrane, and mitovesicles as an abundant component and contributor to the extracellularly exposed proteome. Our study not only provides the first accounts of the immediate consequences of calcium signaling on the extracellularly exposed proteome, but also presents a blueprint for the application of SLAPSHOT as a general approach for monitoring extracellularly exposed protein dynamics.

Advances in experimental and computational methodologies for the study of microbial-surface interactions at different omics levels

The study of the biological response of microbial cells interacting with natural and synthetic interfaces has acquired a new dimension with the development and constant progress of advanced omics technologies. New methods allow the isolation and analysis of nucleic acids, proteins and metabolites from complex samples, of interest in diverse research areas, such as materials sciences, biomedical sciences, forensic sciences, biotechnology and archeology, among others. The study of the bacterial recognition and response to surface contact or the diagnosis and evolution of ancient pathogens contained in archeological tissues require, in many cases, the availability of specialized methods and tools. The current review describes advances in in vitro and in silico approaches to tackle existing challenges (e.g., low-quality sample, low amount, presence of inhibitors, chelators, etc.) in the isolation of high-quality samples and in the analysis of microbial cells at genomic, transcriptomic, proteomic and metabolomic levels, when present in complex interfaces. From the experimental point of view, tailored manual and automatized methodologies, commercial and in-house developed protocols, are described. The computational level focuses on the discussion of novel tools and approaches designed to solve associated issues, such as sample contamination, low quality reads, low coverage, etc. Finally, approaches to obtain a systems level understanding of these complex interactions by integrating multi omics datasets are presented.

The Precise Detection of HER-2 Expression in Breast Cancer Cell via Au25 Probes

Triple-negative breast cancer (TNBC) accounts for nearly one-quarter of all breast cancer cases, but effective targeted therapies for this disease remain elusive because TNBC cells lack the expression of the most common three receptors seen in other subtypes of breast cancers. The medium-term diagnosis of breast cancers is essential for development and prognosis. According to reports, patients with TNBC may be converted to a positive epidermal growth factor receptor 2(HER-2) after chemotherapy, and trastuzumab treatment will have a better prognosis. Therefore, it is important to accurately quantify the expression of HER-2 in breast cancer cells. Herein, we design a red fluorescent Au25 probe synthesized with BSA-biotin as the ligand, which is accurately quantified by HER-2 primary antibody-biotin using the avidin system. The quantitative detection of the expression of HER-2 in breast cancers is helpful for the companion diagnostic of breast cancer treatment and provides follow-up treatment.

Rapid Enzyme-Mediated Biotinylation for Cell Surface Proteome Profiling

Cell surface is the primary site for sensing extracellular stimuli. The knowledge of the transient changes on the surfaceome upon a perturbation is very important as the initial changed proteins could be driving molecules for some phenotype. In this study, we report a fast cell surface labeling strategy based on peroxidase-mediated oxidative tyrosine coupling strategy, enabling efficient and selective cell surface labeling within seconds. With a labeling time of 1 min, 2684 proteins, including 1370 (51%) cell surface-annotated proteins (cell surface/plasma membrane/extracellular), 732 transmembrane proteins, and 81 cluster of differentiation antigens, were identified from HeLa cells. By comparison with the negative control experiment using quantitative proteomics, 500 (68%) out of the 731 significantly enriched proteins (p-value < 0.05, ≥2-fold) in positive experimental samples were cell surface-annotated proteins. Finally, this technology was applied to track the dynamic changes of the surfaceome upon insulin stimulation at two time points (5 min and 2 h) in HepG2 cells. Thirty-two proteins, including INSR, CTNNB1, TFRC, IGF2R, and SORT1, were found to be significantly regulated (p-value < 0.01, ≥1.5-fold) after insulin exposure by different mechanisms. We envision that this technique could be a powerful tool to analyze the transient changes of the surfaceome with a good time resolution and to delineate the temporal and spatial regulation of cellular signaling.

An overview on enrichment methods for cell surface proteome profiling

Cell surface proteins are essential for many important biological processes, including cell-cell interactions, signal transduction, and molecular transportation. With the characteristics of low abundance, high hydrophobicity, and high heterogeneity, it is difficult to get a comprehensive view of cell surface proteome by direct analysis. Thus, it is important to selectively enrich the cell surface proteins before liquid chromatography with mass spectrometry analysis. In recent years, a variety of enrichment methods have been developed. Based on the separation mechanism, these methods could be mainly classified into three types. The first type is based on their difference in the physicochemical property, such as size, density, charge, and hydrophobicity. The second one is based on the bimolecular affinity interaction with lectin or antibody. And the third type is based on the chemical covalent coupling to free side groups of surface-exposed proteins or carbohydrate chains, such as primary amines, carboxyl groups, glycan side chains. In addition, metabolic labeling and enzymatic reaction-based methods have also been employed to selectively isolate cell surface proteins. In this review, we will provide a comprehensive overview of the enrichment methods for cell surface proteome profiling.

Modulation of αvβ3 Integrin via Transactivation of β3 Integrin Gene on Murine Bone Marrow Macrophages by 1,25(OH)2D3, Retinoic Acid and Interleukin-4

The interleukin (IL)-4, 1,25(OH)₂D₃ and retinoic acid, increase surface expression of functional integrin α_vβ₃ on murine osteoclast precursors. All three agonists stimulate transcription of the β₃ gene, leading to increased steady-state levels of mRNA this protein. By contrast, mRNA levels of α_v remain unchanged. In each instance, the increase in the surface expression of the integrin results in increased migration of the cells onto an α_vβ₃ substrate. Because β₃ subunit, except platelet where β₃ subunit conform a dimer with α_IIb, associates solely with α_v subunit monogamously, while promiscuous α_v subunit combines with various subunit, our present data support the idea that the β₃ subunit governs the surface-expressed functional integrin complex.

Biotinylation of platelets for transfusion purposes a novel method to label platelets in a closed system

BACKGROUND

Labeling of platelets (PLTs) is required to measure the recovery and survival of transfused PLTs in vivo. Currently a radioactive method is used to label PLTs. However, application of those radiolabeling methods is limited by both safety issues and the inability to isolate transfused PLTs from the circulation. Biotin‐labeled PLTs are an attractive nonradioactive option. However, no validated protocol to biotinylate PLTs is currently available for human studies.

STUDY DESIGN AND METHODS

Six PLT concentrates (PCs) were subaliquoted and biotinylated on Days 1 and 7 of storage. To distinguish the effect of the processing steps from the effects of biotin incubation, two control groups were used: 1) “sham” samples were processed without the biotinylation reagent and 2) control samples were assessed without any processing other than the PC isolation. For the biotinylation procedure, 50 mL of PCs was washed twice and incubated with 5 mg/L biotin for 30 minutes in a closed system. As measures of PLT activation, phosphatidylserine exposure and CD62p expression were assessed.

RESULTS

After biotinylation, 98.4% ± 0.9% of PLTs were labeled. PLT counts, pH, and “swirling” were within the range accepted by the Dutch blood bank for standard PLT products. Biotinylated PLTs were more activated compared than controles but not more than sham samples, but were more activated than the controls.

CONCLUSION

We developed a standardized and reproducible protocol according to Good Practice Guidelines standards, for biotin labeling of PLTs for clinical purposes. This method can be applied as nonradioactive alternative assess survival and recovery of transfused PLTs in vivo.

Biotinylation of oocyte cell surface proteins of the starfish Patiria miniata

Understanding the signal transduction processes that occur during oocyte maturation and fertilization requires knowledge of the constituent proteins from the cell surface to relevant intracellular compartments. To identify starfish oocyte and egg cell surface proteins, a biotinylation method was adapted from prior protocols using B cells, leukocytes, mouse oocytes, and sea urchin eggs (Cole et al. Mol Immunol 24:699-705, 1987; Flaherty and Swann NJ. Mol Reprod Dev 35:285-292, 1993; Haley and Wessel. Dev Biol 272:191-202, 2004; Hurley et al. J Immunol Methods 85:195-202, 1985). This method utilizes the water-soluble Sulfo-NHS-Biotin, which does not cross the egg plasma membrane. The process of biotinylation does not appear to have any effect on the process of oocyte maturation or fertilization. Furthermore, it can be used with either vitelline-intact or vitelline-free oocytes and allows the proteins to be visualized successfully through immunoblotting, immunoprecipitation, or by scanning confocal microscopy.

Cells and cell lysates: a direct approach for engineering antibodies against membrane proteins using yeast surface display

Membrane proteins (MPs) are often desirable targets for antibody engineering. However, the majority of antibody engineering platforms depend implicitly on aqueous solubility of the target antigen which is often problematic for MPs. Recombinant, soluble forms of MPs have been successfully employed as antigen sources for antibody engineering, but heterologous expression and purification of soluble MP fragments remains a challenging and time-consuming process. Here we present a more direct approach to aid in the engineering of antibodies to MPs. By combining yeast surface display technology directly with whole cells or detergent-solubilized whole-cell lysates, antibody libraries can be screened against MP antigens in their near-native conformations. We also describe how the platform can be adapted for antibody characterization and antigen identification. This collection of compatible methods serves as a basis for antibody engineering against MPs and it is predicted that these methods will mature in parallel with developments in membrane protein biochemistry and solubilization technology.

Determination of Biotinylated Proteins as an Index for Purification of Plasma Membrane using Surface Plasmon Resonance-based Optical Biosensor

Proteins of plasma membrane could be an index of purification of the plasma membrane of animal cells. A convenient method is proposed for determining the plasma membrane proteins by a surface plasmon resonance (SPR) biosensor. Biotinylated proteins were observed only in the peripheral areas of MOLT-4 cells which were treated by 5-[5-(N-succinimidyloxycarbonyl) pentylamido] hexyl-D: -biotinamide. The proteins on HeLa cells were also biotinylated. And then the membrane samples of the HeLa cells were injected onto the avidin-immobilized SPR-surface, and components bound non-specifically on the surface were removed by a washout solution. The amount of biotinylated protein (BP) was determined directly from the absolute resonance unit (RU) after injection of the washout solution. In the method a reference surface was not needed. The amount of BP bound to the surface was gradually attenuated with the repeated injection, and a method for calibrating the RU value was introduced by considering the ratio of attenuation by every injection. The correlation between the BP titer calculated by the calibration and the theoretically-estimated one was greatly improved. Three cycles of the BP determination on a sensor surface was performed successfully. During the purification process of membrane fractions, the degree of purification as judged by the BP titer was in good agreement with the degree of increase in aminopeptidase N activity in the membrane fraction. Thus, the BP titer could be used as an index for purification of plasma membrane.

Detecting the surface localization and cytoplasmic cleavage of membrane-bound proteins

Polycystin-1 (PC1) is a large, membrane-bound protein that localizes to the cilia and is implicated in the common ciliopathy autosomal-dominant polycystic kidney disease. The physiological function of PC1 is dependent upon its subcellular localization as well as specific cleavages that release soluble fragments of its C-terminal tail. The techniques described here allow visualization and quantification of these aspects of the biology of the PC1 protein. To visualize PC1 at the plasma membrane, a live-cell surface labeling immunofluorescence protocol paired with the labeling of an internal antigen motif allows a robust detection of the surface population of this protein. This technique is modified to generate a surface enzyme-linked immunosorbent assay (ELISA), which quantitatively measures the amount of surface protein as a fraction of the total amount of the protein expressed in that cell population. These assays are powerful tools in the assessment of the small but biologically important pool of PC1 that reaches the cell surface. The C-terminal tail cleavage of PC1 constitutes an interesting modification that allows PC1 to extend its functional role into the nucleus. A reporter assay based on Gal4/VP16 luciferase can be used to quantitate the amount of PC1 C-terminal tail that reaches the nucleus. This assay can be paired with quantitative measurement of the protein expression in the cell, allowing a more complete understanding of the pattern of PC1 cleavage and the nuclear localization of the resultant.

HIV-1 transforms the monocyte plasma membrane proteome

How HIV-1 affects the monocyte proteome is incompletely understood. We posit that one functional consequence of virus-exposure to the monocyte is the facilitation of protein transformation from the cytosol to the plasma membrane (PM). To test this, cell surface labeling with CyDye fluorophores followed by 2 dimensional differential in-gel electrophoresis (2D DIGE) and liquid chromatography tandem mass spectrometry (LC-MS/MS) was performed. Fifty three percent of HIV-1 induced proteins were PM associated. These were linked, in large measure, to cellular activation and oxidative stress. They included, but not limited to, biliverdin reductase, leukotriene hydrolase A(4), heat shock protein 70, and cystatin B. HIV-1 induced PM protein translocation was associated with cathepsin B- and caspase 9, 3-dependent apoptosis. In contrast, PMA-treated monocytes bypassed caspase 3, 9 pathways and lead to cathepsin B-dependent necrosis. These results demonstrate that HIV-1 uniquely affects monocyte activation and oxidative stress. These do not affect viral infection dynamics but are linked to stress-induced cell death.

Cell membrane proteomic analysis identifies proteins differentially expressed in osteotropic human breast cancer cells

Metastatic breast cancer cells are characterized by their high propensity to colonize the skeleton and form bone metastases, causing major morbidity and mortality. Identifying key proteins involved in the osteotropic phenotype would represent a major step toward the development of both new prognostic markers and new effective therapies. Cell surface proteins differentially expressed in cancer cells are preferred potential targets for antibody-based targeted therapies. In this study, using cell surface biotinylation and a mass spectrometric approach, we have compared the profile of accessible cell surface proteins between the human breast cancer cell line MDA-MB-231 and its highly osteotropic B02 subclone. This strategy allowed the identification of several proteins either up- or downregulated in the osteotropic cell line, and differential protein expressions were validated using antibody-based techniques. Class I HLAs were down-regulated in the bone metastatic variant, whereas alpha(v)beta(3) integrins, among others, were consistently up-regulated in this latter cell line. These results show that comprehensive profiling of the cell surface proteome of mother cancerous cell lines and derived organ-specific metastatic cell lines provides an effective approach for the identification of potential accessible marker proteins for both prognosis and antibody-based targeted therapies.

Proteomic identification of endothelial proteins isolated in situ from atherosclerotic aorta via systemic perfusion

The functional and structural alterations of vascular endothelium contribute to the initiation, progression, and complications of atherosclerotic plaque formation, but limited information is known about the molecular composition and pathways underlying pathological changes during atherosclerosis. We have developed an affinity proteomic strategy for in situ isolation and differential mapping of vascular endothelial proteins in normal and atherosclerotic aorta tissues. The selective labeling was carried out by perfusion of the blood vessels with an active biotin reagent for covalent modification of accessible vascular endothelial proteins. The biotinylated proteins were then enriched by streptavidin affinity chromatography, separated by SDS-PAGE, and subsequently characterized by LC-MS/MS. The described procedure led to the identification of 454 distinct proteins in normal and atherosclerotic aorta tissues. A majority of the proteins are plasma membrane associated and extracellular matrix proteins, and 81 showed altered expressions in atherosclerotic aorta tissue. The differentially expressed proteins are involved in immune and inflammatory responses, cell adhesion, and lipid metabolism. The method provides a new avenue for investigating the endothelial dysfunction and development of atherosclerosis.

Fluorescent immunoprecipitation analysis of cell surface proteins: A methodology compatible with mass-spectrometry

Radiolabelling and biotinylation of cell proteins followed by immunoprecipitation is a common procedure for biochemical characterization of cell-surface antigens recognized by monoclonal antibodies. Here we present a new method of cell labelling with fluorescent dyes followed by immunoprecipitation and SDS-PAGE with subsequent detection of specific bands by fluorescence imaging devices. Fluorescent immunoprecipitation analysis (FIPA) of cell surface proteins is a fast and sensitive alternative to conventional immunoprecipitation methods, eliminating the need to employ radioactive or biotin labels. The proposed method is compatible with mass spectrometry analysis and permits the identification of immunoprecipitated proteins.

Biochemical characterization and functional studies of Acanthamoeba mannose-binding protein

Acanthamoebae produce a painful, sight-threatening corneal infection. The adhesion of parasites to the host cells is a critical first step in the pathogenesis of infection. Subsequent to adhesion, the parasites produce a potent cytopathic effect (CPE) leading to target cell death. Recent studies showing that acanthamoebae express a mannose-binding protein (MBP) and that free alpha-mannose (alpha-Man) specifically inhibits the adhesion of parasites to host cells suggest that the MBP plays a key role in the pathogenesis of Acanthamoeba infection by mediating host-parasite interactions. However, direct evidence showing that Acanthamoeba MBP is a virulence protein has been lacking. In this study, we demonstrate that the polyclonal immunoglobulin Y (IgY) antibodies prepared against affinity-purified Acanthamoeba MBP markedly inhibit the adhesion of parasites to host cells. The antibody also inhibited the Acanthamoeba-induced CPE on host cells. In contrast, preimmune IgY did not influence either the adhesion of the parasites to host cells or the amoeba-induced CPE. Using a variety of approaches, including affinity chromatography on an alpha-Man gel, electrophoresis under native and denaturing conditions, biotinylation of cell surface proteins, and immunostaining, it was conclusively established that Acanthamoeba MBP is located on the surface membranes of the parasites. Neutral-sugar analysis and lectin binding experiments using succinylated concanavalin A, a plant lectin with high affinity for mannose, revealed that Acanthamoeba MBP is itself a mannose-containing glycoprotein. N-Glycanase treatment to remove N-linked oligosaccharides shifted the subunit molecular mass of MBP from 130 kDa to 110 kDa. Hexosamine analysis revealed that Acanthamoeba MBP lacks detectable levels of GalNAc, suggesting the absence of O-linked oligosaccharides. In summary, we have characterized Acanthamoeba MBP and have shown that it is a major virulence protein responsible for host-parasite interactions and the parasite-induced target cell destruction.

Targeting quantum dots to surface proteins in living cells with biotin ligase

Escherichia coli biotin ligase site-specifically biotinylates a lysine side chain within a 15-amino acid acceptor peptide (AP) sequence. We show that mammalian cell surface proteins tagged with AP can be biotinylated by biotin ligase added to the medium, while endogenous proteins remain unmodified. The biotin group then serves as a handle for targeting streptavidin-conjugated quantum dots (QDs). This labeling method helps to address the two major deficiencies of antibody-based labeling, which is currently the most common method for targeting QDs to cells: the size of the QD conjugate after antibody attachment and the instability of many antibody-antigen interactions. To demonstrate the versatility of our method, we targeted QDs to cell surface cyan fluorescent protein and epidermal growth factor receptor in HeLa cells and to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors in neurons. Labeling requires only 2 min, is extremely specific for the AP-tagged protein, and is highly sensitive. We performed time-lapse imaging of single QDs bound to AMPA receptors in neurons, and we compared the trafficking of different AMPA receptor subunits by using two-color pulse-chase labeling.

A novel method to isolate and map endothelial membrane proteins from pulmonary vasculature

Vascular endothelium has attracted extensive attention due to its important role in many physiological and pathological processes. Many methods have been developed to study the components and their functions in vascular endothelium. Here we report a novel approach to investigate vascular endothelium using normal rat lungs as the model. We perfused lung vascular beds with sulfosuccinimidyl-6-(biotinamido) hexanoate, a biotin analog, to label endothelial membrane proteins. The biotinylated proteins were isolated from lung homogenate with immobilized monomeric avidin and confirmed to be highly pure endothelial membrane proteins with little contamination of intracellular proteins. These biotinylated proteins were used as immunogens for development of monoclonal antibodies. Indeed, newly generated monoclonal antibodies have revealed different expression patterns of proteins across tissues. Some proteins were found highly specifically expressed to capillary vessels of pulmonary vasculature. This method has also been proven useful for investigating vasculature of other organs, as this study explored.

Purification of N-terminally truncated histone H2A-monoubiquitin conjugates from leukemic cell nuclei: probable proteolytic products of ubiquitinated H2A

To gain insight into the significance of nuclear ubiquitinated proteins, two serial extracts prepared from various leukemic cells were analysed by western blotting with anti-ubiquitin antibody. Two previously unidentified ubiquitinated proteins with molecular masses of 10 and 17 kDa were found in 8 M urea-soluble extracts, obtained from Tris-buffer-insoluble materials, of acute myeloid leukemia OCI/AML 1a cells and the cells from the leukemia patients. Both proteins were successfully purified from the OCI/AML 1a cells and identified as monoubiquitin-truncated H2A conjugates, the 10 kDa ubiquitinated H2A(115-129) and the 17 kDa ubiquitinated H2A(54-129), suggesting that both proteins were produced by limited proteolysis of an intact form (23 kDa) of ubiquitinated H2A(1-129). The 17 kDa protein as well as the 23 kDa ubiquitinated histone H2A were localised in chromatin fractions of the OCI/AML cells and released by high concentrations of salt in a micrococcal nuclease-sensitive manner, suggesting their association with chromatin. In contrast, the 10 kDa protein remained insoluble even when the nuclei were treated with nuclease under high salt concentrations, presumably due to binding to the nuclear matrix. An antibody recognising H2A(70-81) also detected the 17 kDa protein in anti-ubiquitin immunoprecipitates obtained from the OCI/AML cell nuclei. In addition, the 17 kDa protein levels in THP-1 cells were transiently increased, concomitant with a decrease in the 23 kDa ubiquitinated H2A, by treatment with phorbol 12-myristate 13-acetate or all-trans-retinoic acid, both of which induce differentiation. This is the first report of probable proteolytic products of ubiquitinated H2A, which might have a role in nuclear functions.

Expression profiling of lymphocyte plasma membrane proteins

The physicochemical properties of plasma membrane proteins of mammalian cells render them refractory to systematic analysis by two-dimensional electrophoresis. We have therefore used in vivo cell surface labeling with a water-soluble biotinylation reagent, followed by cell lysis and membrane purification, prior to affinity capture of biotinylated proteins. Purified membrane proteins were then separated by solution-phase isoelectric focusing and SDS-PAGE and identified by high-pressure liquid chromatography electrospray/tandem mass spectrometry. Using this approach, we identified 42 plasma membrane proteins from a murine T cell hybridoma and 46 from unfractionated primary murine splenocytes. These included three unexpected proteins; nicastrin, osteoclast inhibitory lectin, and a transmembrane domain-containing hypothetical protein of 11.4 kDa. Following stimulation of murine splenocytes with phorbol ester and calcium ionophore, we observed differences in expression of CD69, major histocompatibility complex class II molecules, the glucocorticoid-induced TNF receptor family-related gene product, and surface immunoglobulin M and D that were subsequently confirmed by Western blot or flow cytometric analysis. This approach offers a generic and powerful strategy for investigating differential expression of surface proteins in many cell types under varying environmental and pathophysiological conditions.

Asparagine 81, an invariant glycosylation site near apyrase conserved region 1, is essential for full enzymatic activity of ecto-nucleoside triphosphate diphosphohydrolase 3

N-linked glycosylation is important for the function, cellular localization, and oligomerization of membrane-bound ecto-nucleoside triphosphate diphosphohydrolases (eNTPDases). NTPDase3 is a prototypical cell membrane-associated eNTPDase, which is equally related and enzymatically intermediate to the other two cell surface membrane NTPDases (NTPDase1 and 2). The protein sequence of NTPDase3 contains seven putative N-glycosylation sites located in the ecto-domain. Only one of these putative glycosylation sites, asparagine 81 in NTPDase3, which is located near apyrase conserved region 1 (ACR1), is invariant in all the cell surface membrane eNTPDases. Using site-directed mutagenesis, mutants were constructed to eliminate this highly conserved N-glycosylation site in NTPDase3. The results indicate that glycosylation at this position is essential for full enzymatic activity, with mutant ATPase activity decreased more than ADPase activity. Enzymatic deglycosylation of this site is shown to be responsible for the inactivation of the wild-type enzyme by treatment with peptide N-glycosidase-F. In addition, glycosylation of this conserved site is necessary for the stabilization/stimulation of nucleotidase activity upon treatment with the lectin concanavalin A. However, lack of glycosylation at this site did not result in large changes in tertiary or quaternary structure, as measured by Cibacron blue binding, chemical cross-linking, and native gel electrophoretic analysis. Since this N-glycosylation site is invariant in cell membrane eNTPDases, it is postulated that glycosylation of this residue near ACR1 is crucial for full enzymatic activity of the cell membrane NTPDases.

A biochemical and immunological comparative study on Trypanosoma equiperdum and Trypanosoma evansi

Trypanosoma equiperdum and Trypanosoma evansi were purified by three or four cycles of low-speed centrifugation and final filtration through DEAE cellulose. The purified trypanosomes were used in comparative biochemical and immunological studies. Comparative polypeptide pattern analysis revealed that T. equiperdum showed 21 polypeptide bands, whose Mr ranged from >200 to 14.8 kDa. T. evansi showed 25 polypeptide bands in the Mr range 97-14.8 kDa. The main differences were associated with the presence of secondary bands, relative intensity and the number of bands. Both species gave seven glycoprotein bands; those of 97 and 68 kDa were present in T. equiperdum but absent in T. evansi. Bands of 61 and 28 kDa were present in T. evansi but not in T. equiperdum. Anti-T. equiperdum sera recognized four homologous antigens and cross-reacted with three antigens of T. evansi. Anti-T. evansi sera recognized three homologous antigens and cross-reacted with four T. equiperdum antigens. Four identical proteolytic protease bands were present for both species, while only one surface protein was detected for each species: 66 kDa for T. equiperdum and 62 kDa for T. evansi.

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.

Detection of biotinylated proteins in polyacrylamide gels using an avidin-fluorescein conjugate

Biotinylated proteins are widely used as a molecular tool in biotechnological applications. In this paper, we demonstrated that biotinylated proteins after electrophoresis were detected directly in gels using an avidin-fluorescein conjugate with a fluorescence image analyzer. Upon analysis of the purified and chemically biotinylated protein, the sensitivity of this method was almost equal to that of silver staining. Chemically biotinylated proteins of Escherichia coli cell surfaces could also be specifically detected with our method. Furthermore, recombinant proteins fused with the biotin acceptor domain and biotinylated enzymatically in vivo were also detected in a lysate of E. coli specifically. The sensitivity and specificity of our method are high, and the procedure is simple. Therefore, our method would benefit detection of biotinylated proteins via gel electrophoresis and also various fields of study using avidin-biotin technology.

Radioiodination of glycoprotein-conjugated liposomes by using the Bolton-Hunter reagent and biodistribution in tumor-bearing mice

We have developed a suitable radiolabeling method for our new type of glycoprotein-liposome conjugate (GCL), in order to investigate its potential utility as a drug carrier that can target the cellular functions of carbohydrate-binding proteins. In order to obtain radiolabeled GCL with high labeling efficiency, we introduced p-hydroxyphenylpropyl groups into the liposome membrane through the amine moiety of a constitutive phospholipid, dipalmitoylphosphatidylethanolamine (DPPE) by using Bolton-Hunter reagent (BHR). Radioiodination of the introduced tyrosyl groups was performed by the Chloramine-T method. The labeling efficiency of the BHR-treated liposome conjugate was high in comparison with that of the BHR-untreated liposome conjugate. An in vitro inhibition study showed that the binding affinity of 125I-labeled BHR-treated GCL (125I-F3S-BH) with lectin was twice as high as that of untreated conjugate (125I-F3S). The biodistribution of 125I-F3S-BH in mice was considerably different from that of 125I-F3S. 125I-F3S-BH was more rapidly taken up by the liver and was more rapidly excreted from the liver than 125I-F3S. Moreover, 125I-F3S-BH accumulated more rapidly into the kidneys, which resulted a lower radioactivity in the blood circulation at an earlier time point than in the case of 125I-F3S. The characteristics of tumor accumulation of 125I-F3S-BH and 125I-F3S were similar to those in blood. If F3S is to be employed as an in vivo targeting ligand in biodistribution studies, BHR would be a suitable tool for radiolabeling because it allows GCL to retain the biological activity and characteristics of the unmodified conjugate.

Complications in cell-surface labelling by biotinylation of Candida albicans due to avidin conjugate binding to cell-wall proteins

Initial contact between the opportunistic fungal pathogen Candida albicans and host tissue occurs at the cell surface. Biotin derivatives have been used to label the cell-surface proteins of yeasts, with labelled proteins subsequently detected by avidin-reporter conjugates. Previous work has indicated that avidin can bind to C. albicans proteins in the absence of biotin, suggesting a possible host-cell-recognition mechanism by fungal cell-surface proteins. To investigate this mechanism, Western blots of proteins extracted from biotinylated and mock-treated cells were probed with avidin or modified-avidin reagents. Each avidin reagent bound to cell-wall proteins extracted from non-biotinylated cells. Binding did not appear to be due to the lectin-like activity of the cell-wall proteins of C. albicans or to the presence of biotin in the sample itself. Binding was inhibited by added biotin, by the chaotrope KSCN and by NaCl in a concentration-dependent manner, although inhibition varied among the avidin conjugates tested. Thus, the non-specific binding of avidin to the cell-wall proteins of C. albicans appears to involve hydrophobic and electrostatic interactions, depending on the particular avidin species. These observations demonstrate potential pitfalls in the use of avidin-biotin complexes to identify cell-surface molecules and could provide insights into protein-protein interactions at the C. albicans cell wall.

Development of a time-resolved fluorescence resonance energy transfer assay (cell TR-FRET) for protein detection on intact cells

An assay named Cell TR-FRET based on time-resolved fluorescence resonance energy transfer, here utilized for detection of receptor proteins on intact cells, is described. In this assay, intact membrane-biotinylated Sf9 cells expressing human interleukin-2Ralpha due to infection with a recombinant baculovirus were prelabeled with a streptavidin-europium (Eu(3+)) chelate, the donor. These prelabeled cells were used in a homogeneous assay by addition of a fluorochrome-labeled anti-hIL-2Ralpha-specific antibody, 7G7B6-Cy5, the acceptor. Binding of 7G7B6-Cy5 to hIL-2Ralpha expressed on the cell surface and europium-labeled streptavidin to surface biotin esters brings the donor and the acceptor in close proximity, allowing transfer of energy from the excited state donor to the acceptor. This energy transfer was specifically inhibited by unlabeled antibody and by free biotin. The described assay constitutes a general method since no specific component of the cell membrane is labeled, thereby allowing a number of binding studies on the cell membrane, including receptor density determinations, to be performed. In addition, due to the rapid fashion in which the Cell TR-FRET assay is accomplished, it can be a valuable method not only for identifying novel membrane-associated proteins, but also for drug screening of large samples in high-throughput format.

Detection of carbohydrate recognition molecules on the plasma membrane of boar sperm by dextran-based multivalent oligosaccharide probes

Two kinds of molecules, one recognizing the sialo-/asialo-N-acetyllactosamine structures and the other recognizing the Lewis X structure in a divalent cation-independent manner, were detected on the head of boar sperm prepared from cauda epididymis by fluorescence-labeled or biotinylated dextran-based multivalent oligosaccharide probes. The N-acetyllactosamine recognition molecule(s) is weakly detected on uncapacitated sperm and becomes strongly detectable on capacitated sperm. On the other hand, the Lewis X recognition molecule is detected at a moderate level before capacitation and at a high level after capacitation. Both molecules disappear from the sperm head after induction of acrosome reaction and also by mild detergent treatment. Thus, the two kinds of carbohydrate molecules are expressed on the plasma membrane of boar sperm depending on their physiological state. Inhibition study of the oligosaccharide-dextran probe binding to isolated sperm plasma membrane by various glycoproteins, oligosaccharides, and sulfated polysaccharides also supported the occurrence of the two distinct kinds of molecules.

Hemifusion activity of a chimeric influenza virus hemagglutinin with a putative fusion peptide from hepatitis B virus

Entry of enveloped viruses is often mediated by an aminoterminal hydrophobic fusion peptide of a viral surface protein. The S domain of the hepatitis B virus surface protein contains a putative fusion peptide at position 7-18, but no systems are available to study its function directly. We tested the functionality of this peptide and a related peptide from another hepadnavirus in the context of the well-characterized influenza virus hemagglutinin H7 using gene mutation. The chimeric hemagglutinins could be expressed stably in CV 1 cells and were transported to the cell surface. The chimeras were incompletely cleaved by cellular proteases but cleavage could be completed by trypsin treatment of the cells. The chimeras did not differ in receptor binding, i.e. erythrocyte binding. Hemifusion and fusion pore formation were detected with membrane or cytosolic fluorescent dye-labeled erythrocytes as target structures of the hemagglutinin. Five of six different chimeras mediated hemifusion in 20-54% of the hemagglutinin-expressing cells, complete fusion and syncytium formation was not observed. The data suggest that the sequence 7-18 of the hepatitis B S domain may indeed initiate the first step of viral entry, i.e. hemifusion.

Evaluation of biotinylated cells as a source of antigens for characterization of their molecular profile

Biotinylated lymphoid cells have been suggested as a useful source of antigen for the immunochemical characterization of their molecular profile. Labelling with biotin eliminates the problems associated with the use of radioactivity. However, this method has not been widely used. This reflects: (1) difficulties in optimizing the signal/background ratio because of the lack of a simple method to quantify biotinylated proteins in a cell lysate, (2) the loss of reactivity with monoclonal antibody of antigen following biotinylation, because of steric hindrance, and (3) the lack of information about the utility of other biotinylated cells as an antigen source. To overcome these limitations, we developed an ELISA to quantify biotinylated proteins in cell lysates and optimized the signal/background ratio. The validity of this approach was confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of a number of cell surface antigens immunoprecipitated from lymphoid cells by an optimal amount of monoclonal antibody. Furthermore, we showed that biotinylated melanoma cells are a useful source of antigen for immunoprecipitation experiments and that ligation of biotin to antigen does not affect reactivity with monoclonal antibody. Lastly, biotinylated antigens in cell lysates stored at -80 degrees C for 6 months maintained their reactivity with monoclonal antibodies. Biotinylated cells thus represent a useful source of antigen for characterizing the immunochemical profile and analyzing the specificity of antibodies with immunochemical methods.

Macrophages recognize and adhere to an OmpD-like protein of Salmonella typhimurium

Murine peritoneal macrophages bind to Salmonella typhimurium in vitro in the absence of exogenous opsonins. We have identified an outer membrane protein of S. typhimurium that mediates this adhesion. Biotin-labeled macrophages were used to probe electroblotted envelope proteins of S. typhimurium that had been previously resolved by polyacrylamide electrophoresis under denaturing and reducing conditions. Macrophages bound to an outer membrane protein with an apparent molecular mass of 44 kDa. The protein was purified to homogeneity and free of detectable lipopolysaccharide. Limited microsequencing of this protein resulted in a 15-amino acid query sequence of A-E-V-Y-N-K-D-G-N-K-L-D-L-Y-G, which shares complete identity with a 15-mer of both the OmpD of S. typhimurium SH 7454 and the OmpC polypeptide of Escherichia coli K-12. Picomolar concentrations of this purified protein significantly inhibited the subsequent adherence of 35S-labeled S. typhimurium to macrophages in monolayers. We propose that this 44-kDa protein is involved in the recognition of S. typhimurium by macrophage during the initial stages of infection.

Immobilization of active facilitated glucose transporters (GLUT-1) in supported biological membranes

Membrane fragments or membrane proteins within a lipid mixture were immobilized over metal electrodes. This procedure has been developed to study biological membranes without interference from cell machinery. To obtain a smooth hydrophilic biomembrane support and a mode of binding of the membrane, either a crosslinked gel or an aromatic polyamine-polymer doped with avidin was deposited at the metal electrode by electropolymerization. This layer (less than 10 nm thick) also served as a submembrane compartment. The facilitated glucose transporter (GLUT-1) purified from human erythrocytes was integrated into a lipid membrane containing artificial biotinylated lipids and reacted with the activated surface of the glucose sensitive electrode. It was demonstrated that the lipid layer was attached to the polymer-containing avidin and could only be removed by detergent extraction. The presence of an active membrane transporter was demonstrated by electrochemical detection of glucose in the submembrane compartment, and by inhibition of glucose transport with the specific inhibitor Cytochalasin-B.

Ultrastructural and biochemical detection of biotin and biotinylated polypeptides in Schistosoma mansoni

Biotinylation is proposed for the identification of surface proteins in Schistosoma mansoni using the streptavidin-HRP conjugate for the detection of labeled polypeptides. However, control samples also showed several endogenous biotinylated polypeptides. In an attempt to determine the possibility of nonspecific binding between the streptavidin-HRP conjugate and polypeptides from S. mansoni, the conjugate was blocked with biotinamidecaproate-N-hydroxysuccinimide ester (BcapNHS) before biotin-streptavidin blotting. No bands were detected on the nitrocellulose sheet, demonstrating the specific recognition of biotin by the streptavidin present in the conjugate. Whole cercariae and cercarial bodies and tails showed several endogenous biotinylated polypeptides. The biotin concentration was 13 micrograms/190,000 cercariae. Adult worms presented less endogenous biotinylated polypeptides than cercariae. These results may be due to changes in the environment from aerobic to anaerobic conditions when cercarial bodies (schistosomula) are transformed into adult worms and a decrease in CO2 production may occur. Cercariae, cercarial bodies and adult male worms were examined by transmission electron microscopy employing an avidin-colloidal gold conjugate for the detection of endogenous biotin. Gold particles were distributed mainly on the muscle fibers, but dispersed granules were observed in the tegument, mitochondria and cytosol. The discovery of endogenous biotin in S. mansoni should be investigated in order to clarify the function of this vitamin in the parasite.

Estrogen effects on protein expressed by marrow stromal osteoblasts

The aim of this study was to follow the changes of estrogen treatment on osteoblastic MBA-15 cells derived from marrow stromal origin. Following exposure to estrogen, the cells' patterns of protein synthesis and expression were monitored. The proteins synthesized by MBA-15 cells were identified in cell lysate fractionated to soluble proteins (SOL), cytoskeleton (CK), membrane and nuclei, and intermediate filaments (PL) fractions. These cellular fractions of the osteoblastic MBA-15 cell cultures were assayed on SDS-PAGE of total proteins or following radiolabeling of cells by [35S]-Methionine. Changes in cytoskeletal and membrane proteins of the control and treated cells were monitored by these assays. Reduction in expression of tubulin (TUB) and thropomyosin (TM) were observed by western blot analysis and of actin by fluorescein staining. A reduction in expression of an antigen highly expressed by osteogenic cells and detected by MoAb 85/12 was also observed in these cells. These experiments showed reduction in cytoskeletal and other cellular proteins in the stromal osteoblastic MBA-15 cells treated with 17beta-Estradiol in comparison to untreated cells.

Pathogenesis of Acanthamoeba keratitis: carbohydrate-mediated host-parasite interactions

Acanthamoeba keratitis is a sight-threatening corneal infection. In a recent study, the saccharide mannose has been shown to inhibit the binding of Acanthamoeba organisms to the epithelium of the cornea (L. D. Morton, G. L. McLaughlin, and H. E. Whiteley, Infect. Immun. 59:3819-3822, 1991). In an attempt to determine the molecular mechanism by which acanthamoebae adhere to the surface of the cornea, the present study was designed to determine whether Acanthamoeba castellanii derived from an infected human cornea (i) binds to mannose-containing glycoproteins (mannose-GPs) of corneal epithelium and (ii) expresses one or more mannose-binding proteins. Mannose-GPs of primary cell cultures of rabbit corneal epithelium were isolated by using three different agarose-conjugated, mannose-specific lectins. By electrophoresis blot-overlay assays, 35S-labeled acanthamoebae were shown to bind to mannose-GPs of corneal epithelium and to a neoglycoprotein, mannose-bovine serum albumin (mannose-BSA). 35S-labeled acanthamoebae also bound to microtiter wells coated with mannose-BSA in a concentration-dependent manner. The binding of amoebae to mannose-GPs was blocked by free methyl-alpha-D-mannopyranoside. The parasites did not bind to galactose-BSA or to many other proteins lacking mannose residues. A membrane-associated mannose-binding protein (136 kDa) of A. castellanii was isolated by affinity chromatography of detergent extracts of unlabeled parasites and of cell surface biotin-labeled parasites on a p-aminophenyl alpha-D-mannopyranoside-agarose column. The affinity-purified protein of the amoeba was shown to bind specifically to mannose-BSA. In summary, a mannose-binding protein is present on the surface membranes of Acanthamoeba, and corneal epithelial cells express Acanthamoeba-reactive GPs. One of the mechanisms of Acanthamoeba adhesion to the corneal surface may involve interactions between the mannose-binding protein of Acanthamoeba and mannose-GPs on the surface of corneal epithelium.

Nuclear Translocation of fibroblast growth factor (FGF) receptors in response to FGF-2

Members of the FGF family of growth factors localize to the nuclei in a variety of different cell types. To determine whether FGF receptors are also present within nuclei and if this localization is regulated by FGFs, nuclei were prepared from quiescent and FGF-2-treated Swiss 3T3 fibroblasts and examined for the presence of FGF receptors by immunoblotting with an antibody produced against the extracellular domain of FGF receptor-1 (FGFR-1). Little or no FGFR-1 is detected in nuclei prepared from quiescent cells. When cells are treated with FGF-2, however, there is a time- and dose-dependent increase in the association of FGFR-1 immunoreactivity with the nucleus. In contrast, treatment with either EGF or 10% serum does not increase the association of FGFR-1 with the nucleus. When cell surface proteins are labeled with biotin, a biotinylated FGFR-1 is detected in the nuclear fraction prepared from FGF-2-treated, but not untreated, cells indicating that the nuclear-associated FGFR-1 immunoreactivity derives from the cell surface. The presence of FGFR-1 in the nuclei of FGF-2-treated cells was confirmed by immunostaining with a panel of different FGFR-1 antibodies, including one directed against the COOH-terminal domain of the protein. Fractionation of nuclei from FGF-2-treated cells indicates that nuclear FGFR-1 is localized to the nuclear matrix, suggesting that the receptor may play a role in regulating gene activity.

Biotinylation of cell surface MHC molecules: a complementary tool for the study of MHC class II polymorphism in cattle

Biotinylation of cell surface proteins is often used as an alternative to radioactive labelling procedures, but very little is known about the labelling of the different allelic products of polymorphic antigenic systems. In this report, we demonstrate that NHS-LC-biotin labels bovine MHC class II molecules with different efficiencies for several allelic forms of this polymorphic system compared to conventional metabolic labelling with [35S]methionine. This was shown after immunoprecipitation and one-dimensional isoelectric focusing (1D-IEF). The avidity of the monoclonal antibody Bo139 (alpha-bovine MHC class II) was not affected after in situ biotinylated of bovine PBMC, as revealed by flow cytometric analyses, immunoblotting after SDS-PAGE and immunoprecipitation. The biotinylation did not affect the apparent isoelectric points of polymorphic bovine MHC class II beta chains. This was demonstrated by double labelling of cells with [35S]methionine and subsequent biotinylation and comparison of the banding pattern after immunoprecipitation and 1D-IEF. 1D-IEF of 11 unrelated animals resulted in the demonstration of 29 polymorphic bands of which eight were detected by both labelling procedures, six only after biotinylation and 15 only after metabolic labelling with [35S]methionine. Hence, biotinylation alone cannot serve as an alternative for radioactive labelling of bovine MHC class II molecules but can reveal expressed allelic forms not detectable by metabolic labelling with [35S]methionine.