EXCITATION ENERGY TRANSFER AND THE QUANTITATIVE STUDY OF THE ANTIBODY HAPTEN REACTION

Monoclonal antibody on-rate constant determined from time-course data of ligand binding by capture ELISA: Evaluation of eight data analysis methods

We describe an ELISA method with which to determine monoclonal antibody (mAb) on-rate constants (k₊₁) based on time-course data of ligand (L) binding to plate-bound mAb. The assay was performed in pseudo-first order kinetic conditions ([L] > > [mAb]) and at various starting ligand concentrations. Time-course initial velocity was analyzed by several methods to derive the pseudo-first order (k_obs) and second order (k₊₁) association rate constants of the antibody; the methods included i) an exponential first order rate equation, ii) reaction half-time from the Michaelis-Menten relationship, iii) the V_max/K_m tangent of the time-course curve, iv) Boeker's extrapolated-v_o method, v-vi) modified Hanes-Woolf and Lineweaver-Burk linear plots, vii) a LOS plot, and viii) initial velocity gradient. Due to k₊₁ value dispersion associated with the methods of analysis, the on-rate constant of mAb SIM 253-19 anti-cholera toxin was estimated as an average value of 1.79 ± 0.11 × 10⁶ M^-1 s^-1, 95% CL (1.68-1.89) and 5.8 (%CV [coefficient of variation]), which is similar to the k₊₁ obtained by surface plasmon resonance, 1.60 ± 0.17 × 10⁶ M^-1 s ^-1 (mean ± half range). This kinetic ELISA is a sensitive, quantitative method by which to determine antibody association rate constants.

Quantitative FRET (qFRET) Technology for the Determination of Protein-Protein Interaction Affinity in Solution

Protein-protein interactions play pivotal roles in life, and the protein interaction affinity confers specific protein interaction events in physiology or pathology. Förster resonance energy transfer (FRET) has been widely used in biological and biomedical research to detect molecular interactions in vitro and in vivo. The FRET assay provides very high sensitivity and efficiency. Several attempts have been made to develop the FRET assay into a quantitative measurement for protein-protein interaction affinity in the past. However, the progress has been slow due to complicated procedures or because of challenges in differentiating the FRET signal from other direct emission signals from donor and receptor. This review focuses on recent developments of the quantitative FRET analysis and its application in the determination of protein-protein interaction affinity (K_D), either through FRET acceptor emission or donor quenching methods. This paper mainly reviews novel theatrical developments and experimental procedures rather than specific experimental results. The FRET-based approach for protein interaction affinity determination provides several advantages, including high sensitivity, high accuracy, low cost, and high-throughput assay. The FRET-based methodology holds excellent potential for those difficult-to-be expressed proteins and for protein interactions in living cells.

Protein-Protein Affinity Determination by Quantitative FRET Quenching

The molecular dissociation constant, K_d, is a well-established parameter to quantitate the affinity of protein-protein or other molecular interactions. Recently, we reported the theoretical basis and experimental procedure for K_d determination using a quantitative FRET method. Here we report a new development of K_d determination by measuring the reduction in donor fluorescence due to acceptor quenching in FRET. A new method of K_d determination was developed from the quantitative measurement of donor fluorescence quenching. The estimated K_d values of SUMO1-Ubc9 interaction based on this method are in good agreement with those determined by other technologies, including FRET acceptor emission. Thus, the acceptor-quenched approach can be used as a complement to the previously developed acceptor excitation method. The new methodology has more general applications regardless whether the acceptor is an excitable fluorophore or a quencher. Thus, these developments provide a complete methodology for protein or other molecule interaction affinity determinations in solution.

Mechanics of antigen extraction in the B cell synapse

B cell encounter with antigen displayed on antigen-presenting cells leads to B cell immune synapse formation, internalisation of the antigen, and stimulation of antibody responses. The sensitivity with which B cells detect antigen, and the quality and quantity of antigen that B cells acquire, depend upon mechanical properties of the immune synapse including interfacial tension, the strength of intermolecular bonds, and the compliance of the molecules and membranes that participate in antigen presentation. In this review, we discuss our current understanding of how these various physical parameters influence B cell antigen extraction in the immune synapse and how a more comprehensive understanding of B cell mechanics may promote the development of new approaches to stimulate the production of desired antibodies.

Remembrance of immunology past: conversations with Herman Eisen

Herman Eisen and Sondra Schlesinger spent several days together in September 2007 in Woods Hole, Massachusetts, talking about immunology, focusing on his remembrances of the field over the more than 60 years of his involvement. This article is an abridged version of those discussions (the full version is available on the Annual Reviews website). It is both an oral history and a written memory of some important but selected areas of immunology.

Affinity enhancement of antibodies: how low-affinity antibodies produced early in immune responses are followed by high-affinity antibodies later and in memory B-cell responses

The antibodies produced initially in response to most antigens are high molecular weight (MW) immunoglobulins (IgM) with low affinity for the antigen, while the antibodies produced later are lower MW classes (e.g., IgG and IgA) with, on average, orders of magnitude higher affinity for that antigen. These changes, often termed affinity maturation, take place largely in small B-cell clusters (germinal center; GC) in lymphoid tissues in which proliferating antigen-stimulated B cells express the highly mutagenic cytidine deaminase that mediates immunoglobulin class-switching and sequence diversification of the immunoglobulin variable domains of antigen-binding receptors on B cells (BCR). Of the large library of BCR-mutated B cells thus rapidly generated, a small minority with affinity-enhancing mutations are selected to survive and differentiate into long-lived antibody-secreting plasma cells and memory B cells. BCRs are also endocytic receptors; they internalize and cleave BCR-bound antigen, yielding peptide-MHC complexes that are recognized by follicular helper T cells. Imperfect correlation between BCR affinity for antigen and cognate T-cell engagement may account for the increasing affinity heterogeneity that accompanies the increasing average affinity of antibodies. Conservation of mechanisms underlying mutation and selection of high-affinity antibodies over the ≈200 million years of evolution separating bird and mammal lineages points to the crucial role of antibody affinity enhancement in adaptive immunity.

Optical spectroscopy in studies of antibody-hapten interactions

This article describes the use of optical spectroscopy in studying antibody-hapten interactions and in determining the equilibrium binding constants. Along with equilibrium binding data, spectroscopic tools often deliver structural information on binding-induced conformational changes of antibodies (or haptens). Structural implications of results from example antibody-hapten systems are included. Fluorescence spectroscopy has been particularly useful in the area of ligand binding, and thus steady-state fluorescence quenching and fluorescence polarization are the primary techniques under discussion. A brief description of fluorescence correlation spectroscopy is also provided. Absorption techniques, including circular dichroism, are mentioned to a lesser extent. A basic description of the mathematical models involved in the analysis of binding equilibria is provided along with references to more complete works. Simulated and experimental data are used to illustrate the various experimental protocols and the appropriate analytical methods. Typical sources of errors and experimental precautions are indicated throughout the general discussion.

Studies on the dimensions of the rabbit antibenzylpenicilloyl antibody-combing sites

Rabbit antisera prepared against conjugates of the benzylpenicilloyl (BPO) bifunctional haptenic group were analyzed to determine whether the antibodies are adapted to only a portion of the large BPO molecule, or to the entire molecule, and whether specificity extends to the lysine side chain and adjoining structures of the immunizing carrier protein. No antibodies adapted to the phenylacetylamine portion of the BPO group could be detected in a pooled rabbit anti-BPO serum globulin fraction by PCA and quantitative precipitin analysis using several phenylacetylamine-protein conjugates as antigens. No antibodies adapted only to the thiazolidine carboxylic acid portion of the BPO molecule were detected in the anti-BPO globulin fraction using quantitative precipitin and hapten inhibition methods. At least the bulk of the anti-BPO antibodies was found to be adapted to the entire BPO haptenic group. By quantitative hapten inhibition of precipitation of the anti-BPO globulin fraction, the anti-BPO antibodies were found to show specificity for a 6 carbon amide side chain corresponding to the lysine side chain through which BPO groups are bound predominantly to protein. The contribution of this 6 carbon chain to antibody-hapten binding was small; (-DeltaF degrees ) was calculated to be 460 calories per mole (average). Rabbit anti-BPO antibodies prepared against BPO-rabbit serum albumin conjugates showed specificity also toward structures of the immunizing carrier protein, and possibly toward secondary or tertiary structural configurations. Penicilloyl conjugates of rabbit serum albumin precipitated from 3 individual rabbit antisera more anti-BPO antibodies than did penicilloyl conjugates of heterologous carriers (poly-L-lysine, human serum albumin, and human gamma-globulin). Anti-BPO antibodies demonstrated heterogeneity with regard to closeness of fit to the haptenic group, or with regard to the dimensions of the combining sites, or both. It was concluded that at least a large part of anti-BPO antibodies are specifically adapted to a large antigenic unit comprised of the entire BPO group, the lysine side chain, and structural configurations of the immunizing carrier protein.

Monoclonal antibody binding affinity determined by microchip-based capillary electrophoresis

The affinity constant of a monoclonal antibody to fluorescently labeled bovine serum albumin (BSA) was measured in diluted mouse ascites fluid using a microfluidic chip to perform affinity capillary electrophoresis. Borofloat glass-based devices could be used repeatedly with samples for many months. On-chip separations were performed in less than 60 s, and 30-60 s was required for manual sample exchange. The change in peak height for BSA with increasing BSA/anti-BSA concentration ratio was used to determine concentration changes in bound and free BSA. A Scatchard plot analysis gave an affinity constant (more exactly the intrinsic association constant) of 3.5+/-0.6 x 10(7) M(-1) for a 1:1 stoichiometric ratio. Two affinity complexes were separated. One complex was identified by the Scatchard method as having a 1:1 stoichiometric ratio. The other complex is proposed to have a stoichiometry with an excess of anti-BSA to BSA, most likely (anti-BSA)2-BSA, on the basis of a faster migration time than the 1:1 complex, a decrease in the amount of this complex with increasing [BSA], and predictions of theoretical models for multi-valent antigens. Potential applications of microchip-based devices in affinity measurements are discussed.

Polyfunctional dinitrophenyl haptens as reagents for elicitation of immediate type allergic skin responses

Dinitrophenyl derivatives of differing molecular weights and degrees of substitution have been contrasted with respect to their ability to elicit immediate type allergic responses and their capacity to induce antibody formation in the guinea pig. In contradistinction to dinitrophenyl-proteins, bis-DNP-lysine and DNP-polylysines (including a 100,000 molecular weight derivative) failed to induce antibody detectable by guinea pig passive cutaneous anaphylaxis. Dinitrophenyl-polylysines evoked urticarial responses non-specifically, but after succinylation were about as effective as dinitrophenyl-proteins in eliciting specific cutaneous reactions. An important factor influencing the effectiveness of bis-DNP-lysine in evoking specific wheal-and-erythema responses is antibody affinity for the dinitrophenyl-lysyl determinant.

Hypersensitivity to penicillenic acid derivatives in human beings with penicillin allergy

Multifunctional derivatives of penicillenic acid are effective elicitors of wheal-and-erythema skin responses in humans allergic to penicillin. Of the effective derivatives, penicilloyl-polylysines are particularly attractive as skin test reagents because they appear to be incapable of inducing antibody formation. The skin responses are specifically inhibitable in most instances by homologous unifunctional haptens. The penicillenic acid derivatives which appear to be determinants of human allergic reactions to penicillin are: penicilloyl, penicillenate, and groups of the penamaldate-penilloaldehyde type. Of these, the most significant appears to be the penicilloyl-lysyl determinant.

Determination of the dissociation constant of phosvitin-anti-phosphoserine interaction by affinity capillary electrophoresis

We used affinity capillary electrophoresis (ACE) to study the interaction of a monoclonal anti-phosphoserine antibody (mAb) to a homopolyvalent antigen (hpAg), phosvitin. A model system, which allows the measurement of the true dissociation constant (Kd) in Ag excess based on measurement of migration shifts of mAb-hpAg complexes at different Ag concentrations in solution, is presented for the study of the interactions between a mAb and an Ag that has identical determinants. The experimental value of Kd (22.4 x 10(-6) M) obtained by ACE is shown to be in close agreement with the value (17.8 x 10(-6) M) obtained by the conventional immunoassay based on indirect competition enzyme-linked immunosorbent assay (ELISA). Moreover, the Kds of mAb-hpAg complexes were measured and shown to be independent of the applied electrical field strength. Thus, under conditions where the total Ag concentration is in large excess over the total Ab concentration and when certain requirements are fulfilled, this method offers the advantage of dealing with the determination of Kd for unlabeled mAb and homopolymeric Ag molecules in free solution rather than at the liquid-solid interface.

DIFFERENCES IN THE ELECTROPHORETIC MOBILITIES OF GUINEA PIG 7S ANTIBODIES OF DIFFERENT SPECIFICITIES

The electrophoretic mobilities of guinea pig gamma(1) and gamma(2) antibodies bearing different specificities were compared in agar gel at pH 8.2. Specifically purified antibodies bearing the same immunological specificity showed the same electrophoretic mobility, but significant differences in mobility were observed when antibodies with certain selected different specificities were compared. The specificity of the carrier protein appeared not to affect the mobilities of antihapten antibodies. Differences in mobility have also been shown between gamma(1) antihapten antibodies produced by individual guinea pigs immunized concomitantly with 2,4-dinitrophenyl bovine gamma globulin and arsanilic acid azo guinea pig albumin. The differences in the net electrophoretic charge between antibodies with different specificities roughly paralleled that of their S fragments produced by papain digestion.

The autoantigenic response to rabbit cytochrome c

In this report we describe the production and characterization of autoantibody responses to rabbit cytochrome c (cyt c) in rabbits immunized with either the native monomeric or polymerized form of rabbit cyt c. Fine-specificity analyses of the response indicated that the majority of the response was to the evolutionarily conserved amino-terminal region of the molecule. The relative affinity of the autoantibody interaction with rabbit cyt c was assessed by a solid-phase assay and was found to be lower than that observed for rabbit anti-horse cyt c antibody populations. These findings are consistent with the prediction that low-affinity self-reactive B cells may escape tolerance induction.

Evaluation of performance parameters of a membrane-based dot immunoassay for meningococcal polysaccharide

Increasingly, membrane-based enzyme immunoassays are being developed as the preferred solid-phase enzyme immunoassay format. We describe the rate kinetics of a polyvinylidene difluoride membrane-based dot immunoassay for meningococcal group A polysaccharide. Antigen detection sensitivity decreased logarithmically with linear decreases in incubation time. The sensitivity of a 30-min assay (5-min incubation steps) was increased to nearly the level of the standard assay (1-h incubation steps) by increasing the concentration of assay reagents fourfold. These results support the idea that existing microtiter plate assays can be transferred to rapid dot immunoassay formats with little or no loss of sensitivity.

The effects of pH, calcium and chloride ions on the binding of tolmetin to human serum albumin: circular dichroic, dialysis and fluorometric measurements

The binding of the non-steroidal anti-inflammatory drug, tolmetin (1-methyl-5-p-toluoylpyrrole-2-acetic acid) to human serum albumin (HSA) has been shown by circular dichroism, fluorescence and equilibrium dialysis to be dependent on the N-B conformational change of the albumin. The influence of calcium and chloride ions on the interaction was also investigated using the same techniques. Experiments suggested that calcium ions increased the binding constant of tolmetin to HSA whereas chloride ions decreased it. The displacement study showed that tolmetin caused a significant increase in the affinity of diazepam to HSA whereas it decreased the binding of warfarin to HSA. Tolmetin seems to cause an allosteric change in the diazepam binding site in spite of it sharing a primary site with warfarin.

Somatic evolution of variable region structures during an immune response

Immunization of strain A mice with p-azophenylarsonate-conjugated protein stimulates B cells that synthesize anti-p-azophenylarsonate antibodies. A large fraction of these cells produce antibodies with variable (V) regions encoded by a single heavy chain V gene segment together with multiple combinations of diversity, heavy chain joining, light chain variable, and light chain joining gene segments. Early in the immune response, these V regions are not somatically mutated. One of these V regions is initially expressed by only a minority of the responding B cells but binds p-azophenylarsonate with the highest affinity. After a secondary immunization, B cells synthesizing mutated derivatives of this single V region dominate the response and bind p-azophenylarsonate with even higher affinity than does the unmutated V region. These results suggest that antigen directs both the expression of the immune repertoire and the amplification of V region diversity by a sequential process of clonal selection of B cells expressing receptor antibodies encoded by unmutated V genes, induction of mutation in the V genes expressed by the selected cells, and reselection of B cells expressing antibodies with mutated V regions of higher affinity.

Interaction of purified precipitating and non-precipitating (coprecipitating) antibodies with hapten and with haptenated protein. Evidence of an asymmetric antibody molecule

The interaction of monovalent hapten dinitrophenyl epsilon-amino caproic acid (DNP-EACA) with purified IgG1 sheep anti-DNP precipitating and non-precipitating antibodies, and their F(ab')2, F(ab') and Fab fragments, was studied by fluorescence quenching and by a radioimmunoassay. The Scatchard plots of whole non-precipitating antibody and its F(ab')2 fragment showed a bi-modal curve that could be interpreted as due to the existence of two populations of sites with very different affinity for the ligand, each population representing 50% of the total number of sites. The F(ab) fragments of the non-precipitating antibody could be fractionated by immunoadsorption into two populations of high and low affinity whose association constants differed by more than 2 logs. The study of the interaction of whole antibodies with DNP-bovine serum albumin (BSA) demonstrated that each molecule of precipitating antibody can combine with two molecules of antigen but non-precipitating antibody cannot combine with more than one molecule of antigen. It is concluded that the molecule of non-precipitating antibody is asymmetric and has a site of high affinity and another of low affinity. As a consequence of this structure the non-precipitating antibody behaves functionally as univalent and is unable to form precipitates with the multivalent antigen and to activate effector mechanisms.

Activation specificity of arsonate-reactive T cell clones. Structural requirements for hapten recognition and comparison with monoclonal antibodies

We describe clones of hapten-specific inducer T cells from (BALB/c X A/J)F1 mice that respond to the p-azobenzenearsonate hapten conjugated to carrier proteins or directly conjugated to antigen-presenting cells. Some of the clones are also activated by haptens structurally related to arsonate. All activating analogues are recognized by each clone in association with the same major histocompatibility complex (MHC) protein as is arsonate. Weakly activating and nonactivating analogues are immunogenic in D2.GD amd (BALB/c X A/J)F1 mice, since they can effectively activate primed lymph node cells or long-term hapten-reactive cell lines. Hence the specificities of these clones may reflect their intrinsic recognition of arsonate and its analogues, rather than more efficient presentation of certain analogues than of others by antigen-presenting cells, or differential recognition of associated MHC epitopes by the clones. We compare the activation specificities of the clones with the binding specificities of monoclonal antibodies to arsonate, and discuss structural features of the analogues that may be important for activation and binding. Our results suggest that a site (or subsite) on arsonate-reactive T cell clones may interact directly with hapten, and may be experimentally separable from the site (or subsite) for MHC determinants.

Dinitrophenyl-pepstatins as active-site-directed localization reagents for cathepsin D

1. N-Pepstatinyl-N'-dinitrophenyl-1,6-diaminohexane, a potential active-site-directed localization reagent for cathepsin D, was found to bind non-specifically to immuno-precipitates containing cathepsin D. 2. Three new water-soluble localization reagents were synthesized, by using NN'-bis-(3-aminopropyl)piperazine, 3-oxa-1,5-diamino-pentane or 3,6-dioxa-1,8-diamino-octane, as spacer arms between the pepstatin and dinitrophenyl moieties. 3. The hydrophilic dinitrophenyl-pepstatins were all tight-binding inhibitors of cathepsin D at pH 3.5, but showed little or no binding to immuno-precipitates containing the inactive enzyme at pH 7.4. 4. Gel-chromatographic experiments showed that, at pH 5.0, all the dinitrophenyl-pepstatins were bifunctional reagents able to bind cathepsin D and anti-dinitrophenyl antibody at the same time. Enzyme-inhibitor-antibody complexes were not formed at pH 7.4, thus confirming that the reagents were active-site-directed. 5. Cultured human synovial cells were fixed and incubated with the dinitrophenyl-pepstatins at pH 5.0 or pH 7.4. After washing briefly, the cells were incubated at the appropriate pH value with anti-dinitrophenyl antibody labelled with fluorescein. When examined by fluorescence microscopy the cells stained at pH 5.0 showed fluorescent perinuclear granules, which were not seen in the cells treated at pH 7.4. The distribution of cathepsin D, determined by indirect immuno-fluorescence at pH 7.4, closely resembled that revealed by the dinitrophenyl-pepstatins at pH 5.0. 7. NN'-(3-Pepstatinylaminopropyl-3'-dinitrophenylaminopropyl)piperazine gave the most intense lysosomal staining and showed no non-specific binding. We conclude that this reagent is suitable for the subcellular localization of the active conformation of cathepsin D.

Dynamics of antibody- and lectin-mediated endocytosis of hapten-containing liposomes by murine macrophages

The uptake by murine macrophages of liposomes, exhibiting one of a variety of haptenic groups on their surfaces, was greatly enhanced by the addition of an intact antibody or a lectin specific for the incorporated hapten. The uptake of untreated liposomes was slow and linear over long periods, whereas upon addition of the antibody or lectin, over 30-fold increase in the maximal rate of uptake was observed. The process reached a plateau after 90-120 min. The interaction of the antibody- or lectin-treated liposome with the macrophages apparently resulted in an active endocytosis of soluble fluorescent, intraliposomal marker had a granular intracellular pattern in treated cells. The uptake was sensitive to azide and the liposome constituents could not be detected at the cell surface. The size of the liposomes as well as the state of stimulation of the macrophages (thioglycollate stimulated vs. normal) did not seem to have a major effect on the phagocytic process. The time required to reach the plateau in uptake was independent of liposome composition or antibody concentration and is, apparently, an intrinsic property of the cells. The implication of this phenomenon on the dynamics of the relevant macrophage receptors is discussed.

Investigation of enzyme immunoassay time courses: development of rapid assay systems

An enzyme immunoassay (EIA) consists of a series of antigen-antibody reactions which result in the binding of an enzyme-labeled antibody to a solid phase. The performance time of an EIA determination is thus largely dependent upon the time required for the antigen-antibody reactions. In an attempt to develop a rapid EIA system, we investigated the time course of an EIA system for the measurement of Haemophilus influenzae type b polysaccharide. We found that, although the use of short incubations led to a decrease in sensitivity, an assay system utilizing 10-min incubation periods was still capable of detecting antigen at a concentration of 1 ng/ml. Important factors in the sensitivity of EIAs with short incubation times were the performance of the reaction at 37 degrees C and the incubation of the solid phase with constant agitation. Utilizing these techniques, we developed an EIA system for the measurement of H. influenzae type b polysaccharide which could be completed in less than 30 min. This system was sufficiently sensitive to detect H. influenzae polysaccharide in the cerebrospinal fluids of nine patients with proven H. influenzae meningitis. Thus, EIA systems utilizing short incubation times might be useful for the rapid detection of infectious antigens in body fluids.

Interaction of dinitrophenyl-pepstatins with human cathepsin D and with anti-dinitrophenyl antibody. Development of potential reagents for the localization in vivo of active proteinases at sites of tissue injury

Extracellular cathepsin D has been observed by various cytochemical methods at sites of tissue injury. However, the role of this enzyme in connective tissue matrix degradation is uncertain because there are no histochemical methods for determining whether or not the cathepsin D is active at such sites in living tissues. We considered that the combined use of a labelled tight-binding inhibitor with immunoprecipitation of the enzymes might overcome this problem. We have explored the application of derivatives of the inhibitor pepstatin, as only active cathepsin D binds pepstatin tightly. A series of N-pepstatinyl-N'-dinitrophenyl-alpha, omega-diaminoalkanes were synthesized with alkyl-chain lengths of two, four and six carbon atoms. These compounds were tight-binding inhibitors of human cathepsin D. In fluorescence-quenching titrations the dinitrophenyl groups were also fully available to bind high-affinity anti-dinitrophenyl antibody. It was shown by immunodiffusion in gels and by gel permeation chromatography that N-pepstatinyl-N'-dinitrophenyl-1,6-diaminohexane was a bifunction inhibitor able to bind cathepsin D and anti-dinitrophenyl antibody at the same time.

Diffusion, patching, and capping of stearoylated dextrans on 3T3 cell plasma membranes

Fluorescence-labeled trinitrophenylated stearoylated dextrans have been used as controllable analogues of cell membrane proteins on model membranes and on a variety of natural cell membranes. This paper reports their behavior on 3T3 mouse fibroblast plasma membranes. Spatial distribution on the membrane was studied by fluorescence microscopy, and molecular mobility was measured by fluorescence photobleaching recovery. At concentrations from 10(2) to 3 X 10(3) molecules/micron2 essentially homogeneous fluorescence was observed after treatment with these stearoyldextrans in culture. Diffusion coefficients and fractional recovery of fluorescence after photobleaching were cvoncentration independent. For 3 X 10(3) molecules/micron2 we found at 23 degrees C D = (3.0 +/- 1.8) X 10(-10) cm2/s with 65 +/- 17% recovery and at 37 degrees C D = (7.0 +/- 5.0) X 10(-10) cm2/s without a change of the fractional recovery. Cross-linking with antibodies stopped diffusion on a macroscopic scale and sometimes induced patching, mottling (defined as the development of gaps in the fluorescence layer), and capping (defined as the confinement of the fluorescence to less than 50% of the cell). Capping required approximately 3 h at 37 degrees C and was inhibited by metabolic poisons and cytochalasin B. These drugs did not affect stearoyldextran diffusion or fractional recovery. Colchicine, which did not dramatically affect capping, slowed diffusion two- to threefold but did not affect fractional recovery. The antibody inhibition of the diffusion of stearoyldextrans precedent to capping did not affect the diffusion of a lipid probe or fluorescein isothiocyanate labeled membrane proteins. When the trinitrophenylated stearoyldextran was cleared from most of the surface by capping and the surface subsequently relabeled with stearoyldextran, the diffusion coefficient and fractional recovery of the second label were identical with those of the first label prior to capping. Thus, capping does not clear an immobilizing factor from the membrane.

Cysteinyl-tRNA synthetase from Bacillus stearothermophilus. A structural and functional monomer

A procedure is described for the purification of cysteinyl-tRNA synthetase as a side product of a multi-enzyme isolation from Bacillus stearothermophilus. The native and denatured enzyme are both shown to have a molecular weight of 54000 by gel filtration and sodium dodecyl sulphate/polyacrylamide gel electrophoresis respectively. Fingerprinting and peptide counting indicate that the polypeptide chain has a nonrepeating primary structure. The enzyme has only one binding site for each of its substrates (cysteine, ATP and tRNACys) as judged by equilibrium dialysis, active-site titration and fluorescence quenching. No evidence for the dimerisation of the enzyme in the presence of these substrates could be found. We conclude that cysteinyl-tRNA synthetase, which is the smallest aminoacyl-tRNA synthetase yet described, is both structurally and functionally monomeric.