Growth control in cultured 3T3 fibroblasts. Assays of cell proliferation and demonstration of a growth inhibitory activity

Treatment of sparse, proliferating cultures of 3T3 cells (target cells) with medium conditioned by exposure to density-inhibited 3T3 cultures resulted in an inhibition of growth and division in the target cells when compared to similar treatment with unconditioned medium (UCM). This differential effect of conditioned medium (CM) and UCM on target cells was demonstrated using three assay systems: (a) assessment of total cell number; (b) measurement of [3H]thymidine incorporated into acid-precipitable DNA; and (c) determination of the percentage of radioactively labeled nuclei in individual cells after incorporation of [3H]thymidine. The difference in the total incorporation of [3H]thymidine in CM-treated and UCM-treated cells was reflected by a difference in the percent of labeled cells. There was no differences in the average number of grains per labeled cell in the two cultures. Moreover, the inhibitory effect of the CM on target cell proliferation was reversible. Finally, this growth inhibitory activity can be collected in serum-free medium, precipitated by ammonium sulfate, and fractionated by gel filtration. In these purification procedures, the inhibitory activity was consistently found to be associated with the protein-containing fractions of the CM. No activity was found upon similar treatment with UCM. These results suggest that a system has been developed for the purification and molecular analysis of growth inhibitory factors that may mediate growth control in culture fibroblasts.

IgA and IgG antibody activities of serum and bronchoalveolar fluid from symptomatic and asymptomatic pigeon breeders

Serum IgA and IgG antibody activities against pigeon serum were measured in 16 symptomatic pigeon breeders, 20 asymptomatic pigeon breeders, and 3 normal subjects by radioimmunoassay. The IgA and IgG antibody activities against pigeon antigen of the group of patients with disease was significantly greater than those of patients in the asymptomatic and the control group. The overlap of results for the symptomatic and asymptomatic breeders limits the diagnostic value of these individual IgA or IgG antibody determinations. Bronchoalveolar fluid and serum samples from a smaller group of pigeon breeders who underwent lung lavage were available for studies of antibody activity against pigeon serum. Ten asymptomatic and 6 symptomatic breeders were available for study. Both IgG and IgA antibody activities were detected by radioimmunoassay in serum samples and bronchoalveolar fluid. The IgA antibody activity determined by the radioimmunoassay was higher in the respiratory secretions.

The management of allergic bronchopulmonary aspergillosis

Twenty-five patients with allergic bronchopulmonary aspergillosis (ABPA) were observed for periods of 12 months to 10 years (average duration, 2.6 years) after initial therapy with prednisone, which was then tapered and discontinued unless maintained at minimal doses as required for control of asthma. Thirteen patients have had no recurrence, 4 patients did not comply with the initial regimen and could not be considered to be controlled, and 8 patients had 12 recurrent episodes of ABPA characterized by pulmonary infiltrates with no explanation other than ABPA. The exacerbations were closely correlated with sharp increases in total serum IgE, which subsequently decreased after resumption of prednisone therapy. The increase of IgE preceded the pulmonary infiltrates in 7 or 12 exacerbations. The exacerbations, characterized by increased serum IgE and pulmonary infiltrates, may be associated with minimal symptoms. Acute asthma without pulmonary infiltrates was not associated with increased IgE. Four exacerbations occurred during administration of beclomethasone diproprionate used for control of asthma, and therefore, this agent does not appear to prevent or reverse exacerbations of ABPA. Twelve exacerbations occurred in 8 persons, with 2 patients having 4 and 2 recurrences, respectively. This suggests that exacerbations are more likely to occur in certain patients. Serial measurements of total serum IgE appears to be a useful index of disease activity in ABPA. In the 4 patients who did not comply with the prednisone therapy regimen or regular physician visits, patterns of IgE changes, clinical evaluations, and chest roentgenograms were not of use in evaluation of the clinical state or progress of the patient. A treatment regimen is suggested for initial therapy and recurrences of ABPA on the basis of these observations.

Allergic bronchopulmonary aspergillosis in pediatric practice

Twelve cases of allergic bronchopulmonary aspergillosis in the pediatric age group are reported. The average age of diagnosis was 14.5 years with a range from 6 to 18 years. All patients had a history of pulmonary infiltrations or atelectasis or both documented by chest radiographs. Eight patients had bronchograms or tomograms, and seven of them showed proximal bronchiectasis. Total serum IgE concentrations were elevated in all patients. Preciptitating antibodies against Aspergillus fumigatus were positive in all patients at the time of diagnosis, and became negative in some after therapy. The specific IgE or IgG antibody activity agaist Af was elevated in all 12 patients. After prednisone was started the total serum IgE sharply declined to a plateau and remained at this level until a flare of allergic aspergillosis occurred. A flare of allergic aspergillosis is characterized by an increasing total serum IgE concentration followed by pulmonary infiltration. Clinical and roentgenologic improvements were observed after steroid therapy. The importance and methods of early diagnosis in the pediatric population are discussed.

Binding and functional properties of concanavalin A and its derivatives. II. A proteolytic product with saccharide-binding activity

Limited digestion of the intact subunit of concanavalin A (Mr = 26,000) with trypsin followed by affinity chromatography on Sephadex G-100 has yielded a highly purified product designated here as Tn-Con A. Chemical studies have shown that Tn-Con A is composed of several components: a large fragment (Tn I, Mr = 19,000) spanning residues 1 to 172, and lower molecular weight polypeptides that are noncovalently associated with Tn I to form the active molecule. The molecular weight of Tn-Con A at pH 7 was 90,000, suggesting that, like native concanavalin A, it was a tetramer at physiological pH. Equilibrium dialysis experiments showed that Tn-Con A bound 1 molecule of alpha-methyl-D-glucoside/22,000 g atoms of protein and therefore that four saccharides are bound by the tetrameric molecule. Tn-Con A and native concanavalin A competed for the same receptors on the lymphocyte surface. Moreover, Tn-Con A was mitogenic for both mouse and human lymphocytes with dose-response curves similar to those of the native lectin. All of these results indicate that tryptic hydrolysis of concanavalin A produces a fragmented molecule retaining the saccharide-binding, subunit association, and mitogenic capacity of the native protein.

Binding and functional properties of concanavalin A and its derivatives. III. Interactions with indoleacetic acid and other hydrophobic ligands

The binding of concanavalin A to various structures via hydrophobic interactions has been studied using a variety of physicochemical assays. It was found that concanavalin A binds to nonpolar compounds such as the plant auxin beta-indoleacetic acid and its structural analogue tryptophan and that this binding is independent of the saccharide-binding activity normally associated with the lectin. The results of equilibrium dialysis experiments on the binding of beta-indoleacetic acid were consistent with the presence of a single weak binding site per subunit of protein, having an association constant of about 7 X 10(2) M-1. Competition experiments using various nonpolar compounds such as o-iodobenzoic acid suggested that this hydrophobic binding site is located in the same cavity which binds the iodine-containing ligand as shown by x-ray crystallography. Concanavalin A also binds to lipid vesicles composed of dipalmitoylphosphatidylcholine or 12-O-tetradecanoyl phorbol-13-O-acetate. This binding to lipid membranes raises the possibility that the synergistic effects of concanavalin A and tetradecanoyl phorbol acetate on lymphocyte mitogenesis may be due in part to an interaction between lectin and the phorbol ester.

Serum IgE and IgG antibody activity against Aspergillus fumigatus as a diagnostic aid in allergic bronchopulmonary aspergillosis

Serum IgE and IgG antibody activity against Aspergillus fumigatus was measured in 3 groups of subjects by 2 different immunologic methods. Group A consisted of 23 patients with allergic bronchopulmonary aspergillosis (ABPA). Group B was composed of 19 patients with extrinsic asthma who had marked immediate type skin reactivity to A. fumigatus (prick skin test, 3 or 4+) but no other manifestation of ABPA. Group C, the control group, was composed of 12 healthy subjects. Two immunological methods, including a solid-phase polystyrene tube radioimmunoassay and an iodine-125-labeled, A. fumigatus antigen radioimmunoassay, were used to study each patient's serum sample, so as to demonstrate IgE antibody activity against A. fumigatus (IgE-Af) and IgG antibody activity against A. fumigatus (IgG-Af). Both IgE-Af and IgG-Af were significantly greater among patients in Group A than among those in Group B and Group C, as measured by both methods (P is less than 0.001). The results of this study suggest that either method can be used as a diagnostic aid for ABPA. These methods may provide a laboratory test permitting diagnosis of ABPA in its early stages before bronchial or pulmonary destruction occurs.

Formation of hybrid concanavalin A molecules by subunit exchange

Mixing of native concanavalin A (Con A) and its dimeric succinylated derivative (succinyl-Con A) in glycine-HCl buffer, pH 4.5, resulted in the formation of a new chemical species that could be separated as a unique fraction by DEAE-cellulose chromatography or by gel electrophoresis. The molecular weight of this new component, which contained the subunits (Mr= 26,000) of native Con A and its succinyl derivative in equimolar amounts, was 50,000 at both pH 5 and pH 7. These data suggest that subunit exchange between 2 chemically distinct Con A molecules yields a hybrid molecule consisting of one protomer of native Con A and one protomer of succinyl-Con A. Similar exchange reactions and hybrid molecules were also observed after mixing acetyl-Con A and succinyl-Con A. These procedures provide several new chemical variants of the Con A molecule that may be useful for the analysis of lectin-cell surface interactions.

Initiation of replication in chromosomal DNA induced by extracts from proliferating cells

Addition of an extract prepared from a proliferating cell line to nuclei isolated from resting tissues such as frog liver and spleen resulted in the stimulation of DNA synthesis as assayed by [3H]dTTP incorporation. This stimulated incorporation of [3H]dTTP required ATP and depended on Mg2+ and deoxynucleoside triphosphates. Pulse-chase experiments showed that the synthesis of DNA in this system was discontinuous, resulting in the appearance of approximately 4S fragments and their ligation to yield higher molecular weight DNA. In addition, electron microscopic analysis of the DNA molecules from the reaction mixture showed that the frequency of replication "eyes" in the extract-stimulated reaction was 10-fold higher than that observed in controls. All of these results strongly suggest that the extract stimulated initiation of DNA replication in the chromatin of normally resting cells. Preliminary characterization by dialysis, heating, and enzyme treatments indicated that the activity is associated with one or more proteins of high molecular weight (greater than 50,000). Comparison of the levels of stimulatory activity in extracts from various mammalian and avian sources showed that the activity was present in cells proliferating either in vivo or in tissue culture. In contrast, extracts from normally resting tissues and cells had no activity. The level of activity present did not appear to be directly related to the levels of DNA polymerase. These results suggest a use for this system in studying regulation of the initiation of DNA synthesis and control of the various phases of the cell cycle.

Lymphocyte activation by monovalent fragments of antibodies reactive with cell surface carbohydrates

Antibodies reactive with cell surface carbohydrates were isolated from normal chicken serum and were found to be mitogenic for mouse splenic lymphocytes as assayed by both blast transformation and [3H]thymidine incorporation. The Fab' fragments of these carbohydrate-binding immunoglobulins were just as mitogenic as the divalent native antibody. Moreover, succinylated Fab' fragments, which probably would not form self-associating aggregates, showed similar mitogenic properties. All of these results indicate that, at least for saccharide-specific ligands, multipoint attachment and receptor cross-linkage on the cell to which the ligand is attached may not be a stringent requirement for activation.

Monovalent derivatives of concanavalin A

Monovalent dimers of concanavalin A (Con A) have been prepared by a combination of succinylation and photoaffinity labeling. Partial derivatization of native Con A using the photoaffinity label, p-azidophenyl-alpha-D-mannopyranoside, followed by affinity chromatography yielded a fraction that consisted of dimers with a single saccharide-binding site at pH 5. These monovalent dimers formed divalent tetramers at pH 7. In order to achieve a monovalent dimer at this pH, the divalent tetramers were succinylated by previously developed methods. Ultracentrifugation, equilibrium dialysis, and chromatographic experiments indicated that the resultant preparations consisted mainly of monovalent dimers which showed subunit exchange to yield about 15% divalent dimers after 12 hr at physiological pH. Freshly prepared material failed to agglutinate sheep erythrocytes at concentrations 500-fold higher than native tetravalent Con A. In addition, they showed saturating dose-response curves of mitogenic stimulation of mouse splenic lymphocytes. These curves resembled those of divalent succinyl-Con A but not those of the native molecule. Further development of methods for preparing stable monovalent derivatives of Con A should allow a refined analysis of the effects of lectin valence at the cell surface.

Isolation of lectins of different specificities on a single affinity adsorbent

Affinity chromatography on Sepharose-fetuin columns was used in a single step procedure to isolate the lectins concanavalin A, Favin, phytohemagglutinin, wheat germ agglutinin, and Limulus hemagglutinin. New lectins with unknown binding specificities were also purified by the same procedure from extracts of small California white beans, Idaho red beans, and white pea beans. The purified lectins exhibited different cell surface mapping properties on erythrocytes, lymphocytes, and sperm cells. It was particularly striking that neither 131I-labeled concanavalin nor 125I-labeled wheat germ agglutinin had any effect on the binding of the other to mouse spleen cells. In accord with this observation, gel electrophoretic analysis of radiolabeled lymphocyte receptors for these two lecithins yielded different patterns. These results indicate that highly purified lectins prepared by affinity chromatography on the same adsorbent can possess strikingly different binding specificities for cell surface receptors.

Inhibition by colchicine of the mitogenic stimulation of lymphocytes prior to the S phase

Colchicine, vinblastine, and vincristine inhibit the mitogenic stimulation of lymphocytes by concanavalin A as measured by the incorporation of [3H]thymidine and the appearance of blast cells. The inhibitory effect of colchicine could not be accounted for by diminution in cell viability or by metaphase arrest of mitosis in the stimulated cells. Moreover, the inhibition of [3H]thymidine incorporation was not due to blockage of thymidine transport or inhibition of DNA synthesis inasmuch as addition of colchicine had no effect on cells in the S phase of the cell cycle. The time of inhibition was correlated with the kinetics of cellular commitment to lectin activation and the kinetic data indicated that colchicine blocks stimulation early in the sequence of events following addition of the mitogen. These findings support the hypothesis that cytoplasmic microtubular function plays a role in the commitment of resting cells to undergo mitotic division.

Modulation of lymphocyte mitogenesis

Comparisons of the stimulation of normal lymphocytes and lymphoma cells by tetrameric concanavalin a (Con ta) and dimeric succinyl-Con A suggest that both stimulatory and inhibitory signals operate to modulate mitogenesis. Synergistic effects can be obtained for the stimulatory event using lectins, phorbol esters, and calcium ionophores, all of which are independently mitogenic for lymphocytes. The inhibitory effects of high doses of Con A could be mimicked by the simultaneous addition of the phorbol ester and Con A under conditions in which both reagents are optimally mitogenic when used alone. No inhibition of stimulation was found, however, when succinyl-Con A was used with phorbol ester under the same conditions. Moreover, when lymphocytes were cultured with Con A in the presence of succinyl-Con A, the inhibitory effect of the native lectin was seen at lower doses than in the absence of the derivative. These observations suggest that the stimulatory and inhibitory portions of the dose-response curve can be manipulated independently and may be mediated by two distinct signals. It is likely the signal for the inhibition of cell proliferation is regulated by the same cell surface modulating assembly that controls the mobility of cell surface receptors.

Antibodies reactive with cell surface carbohydrates

Normal and immune sera from various animal species were fractionated on columns of Sepharose covalently coupled with the glycoprotein fetuin. Elution of the material bound to fetuin yielded low but reproducible amounts of protein, ranging from 0.02 to 0.2% of the protein mass of the input sera. This material has been identified by immunoelectrophoresis in agar and by zone electrophoresis on cellulose acetate as immunoglobulin. The Ig fractions bound and agglutinated erythrocytes of various species, and also bound to cells from various mouse tissues including heart, kidney, thymus, and spleen. In all cases, the binding was inhibited by glycoproteins such as fetuin and thyroglobulin, by a glycopeptide isolated from fetuin, and by some bacterial lipopolysaccharides. When the binding of these Ig fractions to mouse splenocytes was tested in the presence of 17 saccharides, no inhibition of binding was observed except by sialic acid, D-galactose, N-acetyl-D-glucosamine, and D-mannose, all of which showed partial inhibition. Inasmuch as these four saccharides are present on the carbohydrate moiety of fetuin, the results suggest that the isolated material is a carbohydrate-specific Ig (CS-Ig) fraction of serum capable of binding to the carbohydrate portion of cell surface receptors and glycoproteins. When bound to lymphocytes, these CS-Ig molecules induced redistribution (patching and capping) of cell surface receptors. Moreover, the CS-Ig fractions from chicken and rabbit sera were weakly mitogenic for mouse splenic lymphocytes. CS-Ig fractions are useful new reagents for studying glycoproteins and the interactions and activities of cell surface carbohydrates.

The covalent and three-dimensional structure of concanavalin A. III. Structure of the monomer and its interactions with metals and saccharides

The three-dimensional structure of the lectin concanavalin A (Con A) has been determined at 2.0-A resolution by x-ray diffraction analysis. The protomers are ellipsoidal domes of dimensions 42 times 40 times 39 A. Folding of the polypeptide backbone is dominated by the presence of two antiparallel pleated sheets, a twisted sheet of seven strands passing through the center of the molecule and a bowed sheet of six strands which forms the back surface of the monomer. Manganese and calcium ions bind to the protein at adjoining sites to form a binuclear complex of two octahedra sharing a common edge. The ligands for each metal ion are four groups from the NH2-terminal region of the protein and 2 water molecules. The binding site for the inhibitor beta-(o-iodophenyl)-D-glucopyranoside is in a deep cavity which contains distinct hydrophobic and hydrophilic binding subsites. Studies of the binding of beta-(o-iodophenyl)-D-glucopyranoside to Con A in the crystalline state and in solution have indicated that the binding behavior of the protein is somewhat different in the two states.

The covalent and three-dimensional structural of concanavalin A. I. Amino acid sequence of cyanogen bromide fragments F1 and F2

Concanavalin A is a lectin composed of identical subunits, each containg 237 amino acid residues. The complete amino acid sequence of the first 129 residues of the polypeptide chain has been determined by analysis of peptides obtained from digests of CNBr Fragments F1 (residues 1 to 42) and F2 (residues 40 to 129). Correlation of the chemical sequence with x-ray crystallographic results indicates that Fragment F1 contains all of the protein ligands for the binding of 2 metal ions, Mn2+ and Ca2+, and that Fragment F2 contains many of the residues involved in the interactions of the subunits to form dimers and tetramers. The site of cleavage of the polypeptide chain to yield the naturally occurring Fragments A1 and A2 has also been identified as the peptide bond between residues 118 and 119.

The covalent and three-dimensional structure of concanavalin A. II. Amino acid sequence of cyanogen bromide fragment F3

The amino acid sequence of the COOH-terminal CNBr fragment, F3 (residues 130 to 237), of concanavalin A has been established, completing the determination of the covalent structure of this lectin. Analysis of the chemical sequence showed that the distribution of charged residues is generally more dense in the NH2-terminal half of the polypeptide chain than in the COOH-terminal portion and that in the latter region there is a linear stretch composed of many hydrophobic residues. Correlation with x-ray crystallographic results indicates that the hydrophobic region is located in the interior of the molecule, and that it forms a part of a deep cavity which is the binding site for the inhibitor, beta-(o-iodophenyl)-D-glucopyranoside. In conjunction with the three-dimensional structure, the amino acid sequence reported here provides new data for analysis of variables involved in predicting the three-dimensional folding of proteins from the primary structure. The sequence of concanavalin A is the first determined for a lectin and it serves as a reference structure for comparisons with other lectins.

Structure and function of concanavalin A

Lectins have been extensively used to analyze a variety of fundamental processes in cell biology. In conjuntion with our studies on the cell surface and mitosis, we have determined the amino acid sequence and three-dimensional struction of concanavalin A (Con A), the mitogenic lectin from the jack bean. Knowledge of the structure has been helpful in interpreting experiments on lymphocyte mitogenesis and the effects of Con A on cell surface receptor mobility. Con A subunits for molecular weight 25,500 are folded into dome-like structures of maximum dimensions 42 times 40 times 39 A. The domes are related by 222 symmetry to form roughly tetrahedral tetramers. Each subunit contains two large antiparallel pleated sheets, and subunits are joined to form dimers and tetramers by interactions involving one of these pleated sheets. We have examined the binding of a variety of carbohydrates to Con A and have obtained preliminary data which suggest that there are differences in the saccharide-binding behavior of Con A in solution and in the crystalline state. Dimeric chemical derivatives of Con A have been prepared and shown to have biological activities different from those of the native tetrameric protein. Under different conditions, native Con A exhibits two antagonistic activities on the lymphoid cell surface: the induction of cap formation by its own receptors and the inhibition of the mobility of a variety of receptors, including its own receptors. The dimeric derivative, succinyl-Con A, is just as effective a mitogen as the native lectin, but it lacks the ability to modulate cell surface receptor mobility. The data suggest that neither extensive immobilization of cell surface receptors nor cap formation is required for cell stimulation. Further studies on modulation of receptor translocation suggest that hypothesis that there exists a connecting network of colchicine-sensitive proteins that links receptors of different kinds and mediates their rearrangement. The degree of connectivity of this postulated network appears to be altered by changes in the state of attachment of various surface receptors to the network. Thus the network might provide the cell with a means of transmitting signals such as the stimulus for mitosis by lectins or antigens.

The kinetics of cellular commitment during stimulation of lymphocytes by lectins

The kinetics of cellular commitment in the stimulation of lymphocytes by concanavalin A (Con A) has been analyzed by measurement of DNA synthesis, autoradiography, and histologic staining techniques. If the competitive inhibitor alpha-methyl-D-mannoside (alphaMM) is introduced into cultures of mouse spleen cells at various times after the addition of Con A, there is a gradual decrease in its capacity to inhibit the lectin-stimulated incorporation of [(3)H]thymidine. Addition of the saccharide 20 h after exposure of the cells to Con A had no effect on the level of the cellular response to the lectin. With increasing periods of contact with Con A, the percentage of blast cells and the percentage of [(3)H]thymidine-labeled blast cells increased in parallel with the total radioactive thymidine incorporated while the average number of autoradiographic grains per labeled blast cell remained relatively constant. These observations suggest that the rising level of [(3)H]thymidine incorporation results from an increase in the number of cells that respond to lectin stimulation and become refractory to inhibition with alphaMM. Once such cells become committed, they synthesize DNA at a rate independent of the length of exposure to the lectin. The combined results indicate that mouse splenic lymphocytes are heterogeneous in their capacities to respond to Con A and that different cells require different induction periods to be stimulated.

Receptor mobility and receptor-cytoplasmic interactions in lymphocytes

An analysis of the inhibition by concanavalin A of the mobility of lymphocyte surface receptors is used to construct an hypothesis on membrane receptor-cytoplasmic interactions. It is proposed that binding of multivalent lectins alters the interaction of an assembly of colchicine-binding proteins with lectin receptors and other receptors, and reciprocally that the state of the colchicine-binding assembly alters the mobility and distribution of surface receptors on the cell membrane. Observations of the effect of colchicine and related drugs on the inhibition of receptor mobility by concanavalin A lend support to this hypothesis. The proposed model has several implications for studies of the initial events of mitogenesis in lymphocytes as well as for cell-cell interactions in general.

Concanavalin A derivatives with altered biological activities

Chemical derivatization of tetrameric concanavalin A (Con A) with succinic anhydride or acetic anhydride converts the protein to a dimeric molecule without altering its carbohydrate-binding specificity. At low concentrations, the dose-response curves for the mitogenic stimulation of mouse spleen cells by native Con A and succinyl-Con A are similar. Above lectin concentrations of 10 mug/ml, however, the response to Con A is diminished, while that for succinyl-Con A does not decrease until much higher doses are reached. We have attributed this difference mainly to the higher rate of cell death induced by the native Con A molecule. Con A also shows a greater capacity than succinyl-Con A to agglutinate sheep erythrocytes and to inhibit cap formation by immunoglobulin receptors on spleen cells. Moreover, at low concentrations, Con A induced its glycoprotein receptors to form caps, but succinyl-Con A did not induce cap formation. Addition of antibodies directed against Con A to succinyl-Con A bound on cells restored the properties of agglutination, inhibition of immunoglobulin receptor cap formation, and induction of cap formation by Con A receptors. Similar results have been obtained for acetyl-Con A. These data suggest that the altered biological activities of succinyl-Con A and acetyl-Con A are attributable to their reduced valence.

The covalent and three-dimensional structure of concanavalin A

The tentative amino-acid sequence and three-dimensional structure of the lectin concanavalin A have been determined. The amino-acid sequence, which was determined chemically, contains 238 residues. The sequences of three short stretches were assigned on the basis of x-ray crystallographic data. Interpretation of an electron density map at 2-A resolution indicates that the predominant structural element is extended polypeptide chain arranged in two anti-parallel pleated sheets or beta-structures. Residues not included in the beta-structures are arranged in regions of random coil. One of the pleated sheets contributes extensively to the interactions among the monomers to form both dimers and tetramers. The positions at which Mn(2+), Ca(2+), and saccharide are bound to the protein, and the point of cleavage for the formation of the naturally occurring fragments A(1) and A(2), have been tentatively assigned. Both metal-binding sites are at least 20-A removed from the position at which saccharides are bound. The saccharide-binding site is a deep pocket of approximately 6A x 7.5A x 18A, the inner portion of which is occupied by hydrophobic residues.

Unusual fragments in the subunit structure of concanavalin A

Gel electrophoresis in sodium dodecyl sulfate and gel filtration in guanidine. HCl indicate that native concanavalin A contains several molecular species. An intact subunit of molecular weight 27,000 has been purified from this mixture. In addition, three fragments of the intact subunit have been isolated and characterized. A working model of the concanavalin A molecule has been constructed based on pairings of the intact subunit and of subunits consisting of fragments.