Calmodulin-binding proteins: visualization by 125I-calmodulin overlay on blots quenched with Tween 20 or bovine serum albumin and poly(ethylene oxide)

To streamline detection of calmodulin-binding proteins, blotting techniques for the electrophoretic transfer of proteins onto nitrocellulose filters, followed by overlay with 125I-calmodulin, have been adapted. Autoradiography of the 125I-calmodulin-labeled blots allows the identification and quantitation of proteins that possess affinity for calmodulin. Five protocols for suppressing nonspecific binding and for enhancing specific interactions of 125I-calmodulin with electrophoretically separated proteins were investigated. Tween 20 and bovine serum albumin alone, as well as combinations of bovine serum albumin and poly(ethylene oxide) or hemoglobin and gelatin, were evaluated as quenching and enhancing agents. Tween 20 proved highly effective for quenching nonspecific binding and for enhancing specific 125I-calmodulin binding of a 61,000-Mr rat brain protein, which was only faintly observed on blots quenched with proteins alone. However, Tween 20 dissociated 50% of 68,000-Mr proteins and 80% of 21,000-Mr 125I-labeled protein standards from the nitrocellulose filter. An alternative, the combination of bovine serum albumin followed by incubation with 15,000- to 20,000-Mr poly(ethylene oxide), proved satisfactory for the recovery of 61,000-Mr calmodulin-binding activity and for the detection of calmodulin-binding peptides (50,000 to 14,000 Mr) produced by limited proteolysis of rat brain 51,000-Mr calmodulin-binding protein. These blotting procedures for detection of calmodulin-binding proteins are compatible with a variety of one-dimensional and two-dimensional electrophoresis systems, including a two-dimensional electrophoresis system utilizing urea and sodium dodecyl sulfate in the first dimension and nonurea sodium dodecyl sulfate electrophoresis in the second, a system which proved useful for resolving calmodulin-binding proteins displaying anomalous electrophoretic migration in the presence of urea.     

Calcineurin: a member of a family of calmodulin-stimulated protein phosphatases

Calcineurin, a major calmodulin-binding protein of brain, is a heterodimer composed of a 61,000 Mr calmodulin-binding subunit, calcineurin A, and a 19,000 Mr Ca2+-binding subunit, calcineurin B. The discovery of a calmodulin-regulated protein phosphatase in rabbit skeletal muscle with a similar subunit structure led to the identification of calcineurin as a protein phosphatase (AA Stewart, TS Ingebritsen, A Manalan, CB Klee, P Cohen (1982) FEBS Lett 137:80-84). Using rabbit polyclonal antibodies to bovine brain calcineurin, both subunits of calcineurin can be identified in crude homogenates of bovine brain by an immunoblotting technique. In crude homogenates of bovine skeletal and cardiac muscle, a 59,000-61,000 Mr doublet and a 15,000 Mr species (the electrophoretic mobility of calcineurin B) are also detected by this technique. The cross-reactivity of these species with antibodies to brain calcineurin indicates antigenic similarity between the muscle proteins and calcineurin, and suggests the existence of a family of structurally related calmodulin-stimulated protein phosphatases. Like calcineurin, the 61,000 Mr subunits in skeletal and cardiac muscle bind calmodulin and are detected in crude tissue extracts by 125I-calmodulin gel overlay. Thus, both the 125I-calmodulin gel overlay method and the immunoblotting technique are useful in screening crude preparations, in which detection of calmodulin-stimulated protein phosphatase activity may be complicated by the many phosphatases present.     

Identification and characterization of calmodulin-binding proteins in mammalian sperm flagella

A calcium and calmodulin-regulated cyclic nucleotide phosphodiesterase has been shown to be an integral component of both rat and bovine sperm flagella. The calcium-activated enzyme was inhibited by both trifluoperazine (ID50 = 10 microM) and [ethylene-bis(oxyethylenenitrilo)]tetraacetic acid (EGTA), and the basal activity measured in the presence of EGTA was stimulated by limited proteolysis to that observed in the presence of calcium/calmodulin. 125I-Calmodulin binding to purified rat sperm flagella has been characterized and the flagellar-associated calmodulin-binding proteins identified by a combination of gel and nitrocellulose overlay procedures and by chemical cross-linking experiments using dimethyl suberimidate. 125I-Calmodulin bound to demembranated rat sperm flagella in a time- and concentration-dependent manner. At equilibrium, 30-40% of the bound 125I-calmodulin remains associated with the flagella after treatment with EGTA or trifluoperazine. The majority of the bound 125I-calmodulin, both the Ca2+-dependent and -independent, was displaced by excess calmodulin. A 67-kDa calmodulin-binding protein was identified by both the gel and nitrocellulose overlay procedures. In both cases, binding was dependent on Ca2+ and was totally inhibited by trifluoperazine, EGTA, and excess calmodulin. On nitrocellulose overlays, the concentration of calmodulin required to decrease binding of 125I-calmodulin by 50% was between 10(-10) and 10(-11) M. Limited proteolysis resulted in the total loss of all Ca2+-dependent binding to the 67-kDa polypeptide. Chemical cross-linking experiments identified a major calcium-dependent 125I-calmodulin:polypeptide complex in the 84-90-kDa molecular mass range and a minor complex of approximately 200 kDa. Immunoblot analysis showed that the major 67-kDa calmodulin-binding protein did not cross-react with polyclonal antibodies raised against either the calcium/calmodulin-regulated cyclic nucleotide phosphodiesterase or phosphoprotein phosphatase (calcineurin) from bovine brain.     

Phosphorylation-regulated calmodulin binding to a prominent cellular substrate for protein kinase C

We have evaluated the possibility that a major, abundant cellular substrate for protein kinase C might be a calmodulin-binding protein. We have recently labeled this protein, which migrates on sodium dodecyl sulfate-gel electrophoresis with an apparent Mr of 60,000 from chicken and 80,000-87,000 from bovine cells and tissues, the myristoylated alanine-rich C kinase substrate (MARCKS). The MARCKS proteins from both species could be cross-linked to 125I-calmodulin in a Ca2+-dependent manner. Phosphorylation of either protein by protein kinase C prevented 125I-calmodulin binding and cross-linking, suggesting that the calmodulin-binding domain might be located at or near the sites of protein kinase C phosphorylation. Both bovine and chicken MARCKS proteins contain an identical 25-amino acid domain that contains all 4 of the serine residues phosphorylated by protein kinase C in vitro. In addition, this domain is similar in sequence and structure to previously described calmodulin-binding domains. A synthetic peptide corresponding to this domain inhibited calmodulin binding to the MARCKS protein and also could be cross-linked to 125I-calmodulin in a calcium-dependent manner. In addition, protein kinase C-dependent phosphorylation of the synthetic peptide inhibited its binding and cross-linking to 125I-calmodulin. The peptide bound to fluorescently labeled 5-dimethylaminonaphthalene-1-sulfonyl-calmodulin with a dissociation constant of 2.8 nM, and inhibited the calmodulin-dependent activation of cyclic nucleotide phosphodiesterase with an IC50 of 4.8 nM. Thus, the peptide mimics the calmodulin-binding properties of the MARCKS protein and probably represents its calmodulin-binding domain. Phosphorylation of these abundant, high affinity calmodulin-binding proteins by protein kinase C in intact cells could cause displacement of bound calmodulin, perhaps leading to activation of Ca2+-calmodulin-dependent processes.     

Identification of albumin-binding proteins of thymocyte plasmalemma

In the present work we examined whether the interaction between albumin molecules and thymocytes involves albumin-binding proteins (ABP). Two plasmalemma-rich fractions obtained by differential centrifugation from rat thymus lymphocytes were characterized biochemically and morphologically. These fractions were examined by ligand-blotting and ligand affinity chromatography techniques. Plasmalemma proteins separated by SDS-PAGE were electrotransferred onto nitrocellulose membranes and incubated with¹²⁵I-albumin, in the presence or absence of excess native albumin. The autoradiogram revealed specific binding to two sets of polypeptides of 16–18 and 29–31 kDa, which could be blocked by native albumin. To elucidate whether albumin-binding proteins are exposed on the cell surface, intact lymphocytes were surface radioiodinated and membrane fractions prepared from them were subjected to affinity chromatography on albumin-agarose beads. The proteins thus purified had, like ABP, M_r of 16 and 31. These data indicate that ABP (i) are components of thymocyte plasma membrane, (ii) have apparent molecular mass of 16–18 and 29–31 kDa, and (iii) are exposed on the outer membrane surface.Abbreviations ABP albumin-binding proteins - Alb bovine serum albumin - Au gold - DAPI 4,6-diamidino-2-phenylindol - EM electron microscopy - NC nitrocellulose - PAGE polyacrylamidegel electrophoresis - PBS phosphate buffered saline - PEG polyethylene glycol - PMSF phenylmethylsulfonyl fluoride - WGA Wheat germ agglutinin     

Wheat germ agglutinin binding sites on human urothelial cells of different grades of transformation

¹²⁵I-Wheat germ agglutinin (WGA) binding parameters of human urothelial cell lines of different grades of transformation (TGrll and TGrlll) were compared. The values of association constant (K_a) and the number of binding sites/cell for HCV29 (TGrll) cell line were about 3×10⁶M^–1 and over 4×10⁷, respectively. Two TGrlll cell lines, HCV29T and Hu549 revealed lower values for K_a, and considerably higher numbers of binding sites/cell (about 3×10⁸ and 2×10⁸, respectively). Binding of¹²⁵I-WGA to total cellular proteins resolved by SDS-PAGE and transferred to nitrocellulose showed multiple diffused bands in the range of 58–180 kDa. Some of these bands were characteristic for TGrll cells (124 kDa) or TGrlll cells (135 and 148 kDa).Abbreviations TGr transformation grade - WGA wheat germ agglutinin - sWGA succinylated wheat germ agglutinin - GlcNAc N-acetyl-d-glucosamine - BSA bovine serum albumin - SDS-PAGE sodium dodecyl sulphate polyacrylamide gel electrophoresis     

Identification and characterization of calmodulin-binding proteins in islet secretion granules

The binding of 125I-calmodulin to intact secretion granules and protein gel blots of secretion granules from pancreatic islet tissue was examined. Binding of 125I-calmodulin to intact secretion granules was Ca2+-dependent and inhibited by the calmodulin inhibitors trifluoperazine and calmidazolium. Binding was inhibited by excess (200 nM) unlabeled calmodulin, but not by parvalbumin, a Ca2+-binding protein which has little sequence homology to calmodulin. In order to study the binding of calmodulin to specific secretion granule proteins, secretion granules were solubilized, and the solubilized proteins were resolved on sodium dodecyl sulfate-polyacrylamide gels, electrophoretically transferred to nitrocellulose, and incubated with 125I-calmodulin. Autoradiograms of the protein gel blots revealed the presence of three major calmodulin-binding proteins with approximate molecular weights of 73,000, 64,000, and 58,000. These proteins reversibly bound calmodulin in a calcium-dependent manner. Unlabeled calmodulin in the range of 0.1-1.0 nM competed with 125I-calmodulin for binding to these proteins, whereas troponin and parvalbumin were 100 and 1000-fold less effective, respectively. Trifluoperazine blocked binding to the granule proteins in a range of 10(-4) to 10(-5) M, and calmidazolium was effective between 10(-5) and 10(-6) M. Trypsin, at a concentration which did not lyse granules, markedly inhibited calmodulin binding to intact secretion granules. Protein blots from trypsin-treated granules showed that the three major calmodulin-binding proteins were absent. These results indicate that Ca2+-dependent calmodulin-binding proteins are present on the cytoplasmic surface of islet secretion granules and are consistent with the hypothesis that these proteins may play a role in secretion granule exocytosis.     

Calmodulin-binding proteins and calmodulin-regulated enzymes in dog pancreas.

Calmodulin was isolated and purified to homogeneity from dog pancreas. Highly purified subcellular fractions were prepared from dog pancreas by zonal sucrose-density ultracentrifugation and assayed for their ability to bind 125I-calmodulin in vitro. Proteins contained in these fractions were also examined for binding of 125I-calmodulin after their separation by polyacrylamide-gel electrophoresis in SDS. Calmodulin-binding proteins were detected in all subcellular fractions except the zymogen granule and zymogen-granule membrane fractions. One calmodulin-binding protein (Mr 240,000), observed in a washed smooth-microsomal fraction, has properties similar to those of alpha-fodrin. The postribosomal-supernatant fraction contained three prominent calmodulin-binding proteins, with apparent Mr values of 62,000, 50,000 and 40,000. Calmodulin-binding proteins, prepared from a postmicrosomal-supernatant fraction by Ca2+-dependent affinity chromatography on immobilized calmodulin, exhibited calmodulin-dependent phosphodiesterase, protein phosphatase and protein kinase activities. In the presence of Ca2+ and calmodulin, phosphorylation of smooth-muscle myosin light chain and brain synapsin and autophosphorylation of a Mr-50,000 protein were observed. Analysis of the protein composition of the preparation by SDS/polyacrylamide-gel electrophoresis revealed a major protein of Mr 50,000 which bound 125I-calmodulin. This protein shares characteristics with the calmodulin-dependent multifunctional protein kinase (kinase II) recently observed to have a widespread distribution. The possible role of calmodulin-binding proteins and calmodulin-regulated enzymes in the regulation of exocrine pancreatic protein synthesis and secretion is discussed.     

A change in the phosphorylation pattern of the 30000–33000 M r synaptonemal complex proteins of the rat between early and mid-pachytene

The lateral elements (LEs) of synaptonemal complexes (SCs) of the rat contain major components with relative electrophoretic mobilities (M _r , s) of 30000–33000, which are the products of a single gene. After one-dimensional separation of SC proteins on polyacrylamide-SDS gels, these components show up as two major bands, whereas upon two-dimensional electrophoresis they are resolved in at least 24 spots, which focus at pH 6.5 to 9.5. In this paper we show that these spots represent phosphorylation variants. For the analysis of the phosphorylation of the 30000-to 33000-M _r SC components during progression through meiotic prophase, we developed a procedure for isolation of fractions of testicular cells of the rat that are enriched in separate stages of meiotic prophase. Analysis of the 30000-to 33000-M _r SC components in these fractions by two-dimensional electrophoresis and immunoblotting showed that phosphorylated variants of the 30000-to 33000-M _r SC proteins occur throughout meiotic prophase. However, the extent of phosphorylation changes between early and mid-pachytene, when one phosphate group is probably added to each of the variants.     

Preparation of 125I-calmodulin with retention of full biological activity: Its binding to human erythrocyte ghosts

Of several methods employed for preparing ¹²⁵I-calmodulin, only the glucose oxidase-lactoperoxidase system under controlled conditions produced an iodinated derivative which retained complete biological activity. Unlabeled calmodulin and ¹²⁵I-calmodulin stimulated cyclic nucleotide phosphodiesterase from bovine brain interchangeably and both proteins displaced ¹²⁵I-calmodulin from high-affinity binding sites on human erythrocyte ghosts with equal effectiveness. This procedure yielded a labeling stoichiometry of 1.34. Scatchard plots of binding of ¹²⁵I-calmodulin to ghosts were consistent with the presence of a single class of high-affinity binding sites with the properties expected of (Ca²⁺ + Mg²⁺)-ATPase molecules. The binding showed positive cooperativity and occurred only in the presence of Ca²⁺. The maximum amount of binding seen in Scatchard plots corresponded to 4.1 × 10³ sites per ghost.     

A Similar Calmodulin-Binding Protein Expressed in Chromaffin, Synaptic, and Neurohypophyseal Secretory Vesicles

The presence of calmodulin-binding proteins in three neurosecretory vesicles (bovine adrenal chromaffin granules, bovine posterior pituitary secretory granules, and rat brain synaptic vesicles) was investigated. When detergent-solubilized membrane proteins from each type of secretory organelle were applied to calmodulin-affinity columns in the presence of calcium, several calmodulin-binding proteins were retained and these were eluted by EGTA from the columns. In all three membranes, a 65-kilodalton (63 kilodaltons in rat brain synaptic vesicles) and a 53-kilodalton protein were found consistently in the EGTA eluate. 125I-Calmodulin overlay tests on nitrocellulose sheets containing transferred chromaffin and posterior pituitary secretory granule membrane proteins showed a similarity in the protein bands labeled with radioactive calmodulin. In the presence of 10(-4) M calcium, eight major protein bands (240, 180, 145, 125, 65, 60, 53, and 49 kilodaltons) were labeled with 125I-calmodulin. The presence of 10 microM trifluoperazine (a calmodulin antagonist) significantly reduced this labeling, while no labeling was seen in the presence of 1 mM EGTA. Two monoclonal antibodies (mAb 30, mAb 48), previously shown to react with a cholinergic synaptic vesicle membrane protein of approximate molecular mass of 65 kilodaltons, were tested on total membrane proteins from the three different secretory vesicles and on calmodulin-binding proteins isolated from these membranes using calmodulin-affinity chromatography. Both monoclonal antibodies reacted with a 65-kilodalton protein present in membranes from chromaffin and posterior pituitary secretory granules and with a 63-kilodalton protein present in rat brain synaptic vesicle membranes. When the immunoblotting was repeated on secretory vesicle membrane calmodulin-binding proteins isolated by calmodulin-affinity chromatography, an identical staining pattern was obtained. These results clearly indicate that an immunologically identical calmodulin-binding protein is expressed in at least three different neurosecretory vesicle types, thus suggesting a common role for this protein in secretory vesicle function.     

Structural differences between insulin and somatomedin-C/insulin-like growth factor-1 receptors revealed by autoantibodies to the insulin receptor

Polyadenylated RNA prepared from first trimester human placenta was translated in a membrane-free cell-free system derived from wheat germ. Analysis of the [³⁵S]methionine-labeled products by SDS-polyacrylamide electrophoresis demonstrated two proteins with apparent M_rs of 14,500 and 16,000 that were specifically immunoprecipitated by antiserum to reduced and carboxylated bovine LH_α, and two different proteins with apparent M_rs of 18,500 and 21,000 that were specifically immunoprecipitated by antiserum to hCG_β. None of these products was sensitive to cleavage by endoglycosidase H, whereas the M_r 21,000 product precipitated by antisera to bovine LH_α and to hCG_α from translations supplemented by canine pancreatic microsomes was processed to a product with M_r 13,000 by endoglycosidase H. We suggest that the two forms of the α and β subunit precursors could arise from the translation of two distinct mRNAs encoding each subunit.     

Characterization of the Hepatic Glucagon Receptor

Abstract

The hepatic glucagon receptor was covalently labeled with [¹²⁵I-Tyr¹⁰]-monoiodoglucagon by use of the heterobifunctional crosslinker hydroxysuccini-midyl-p-azidobenzoate and analyzed by SDS-gel electrophoresis. The autoradio-gram of the gel showed one band at M_r=63,000 that was sensitive to excess unlabeled glucagon and GTP. The labeled receptor was solubilized with Lubrol-PX and the hydrodynamic characteristics of the receptor were determined. The molecular parameters of the solubilized receptor are S_{20, w} = 4.3 ± 0.1, Stokes radius = 6.3 ± 0.1 nm, frictional coefficient f/f° = 1.8 and a calculated M_r = 33,000 fragment, that retains guanine nucleotide sensitivity. Elastase treatment of vacant receptors results in a M_r = 24,000 fragment that binds hormone in a GTP-sensitive manner. The M_r = 24,000 fragment is contained within the M_r = 33,000 fragment. The M_r = 63,000 receptor upon treatment with endo-β-N-acetylglucosamine F for 4 h yields four fragments of apparent M_r = 61,000, 56,000, 51,000, and 45,000; 24 h treatment results in the accumulation of the last two fragments. Neither M_r = 33,000 and 24,000 fragment appear to be substrates for endo-β-N-acetylglucosaminidase F.

These data allow us to conclude that the hepatic glucagon receptor in the membrane is a dimer of ～ 60,000 dalton hormone binding subunit which is a glycoprotein containing at least four N-linked glycans accounting for 18,000 daltons of its mass. Both the hormone binding function and the capacity for the interaction with the stimulatory regulator of adenylyl cyclase are contained within a fragment of only ～ 21,000 daltons that does not contain any N-linked glycans.     

Isolation of 3-methylcrotonyl-coenzyme A carboxylase from bovine kidney

3-Methylcrotonyl-CoA carboxylase (MCase), an enzyme of the leucine oxidation pathway, was highly purified from bovine kidney. The native enzyme has an approximate molecular weight of 835,000 as measured from exclusion limits by polyacrylamide gel electrophoresis at pH 7.3. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate demonstrated two subunits, identified as a biotin-free subunit (A subunit; M_r = 61,000) and a biotin-containing subunit (B subunit; M_r = 73,500). The biotin content of the enzyme was 1 mol/ 157,000 g protein, consistent with an AB protomeric structure for the enzyme. The isoelectric point of the enzyme was found to be 5.4. Maximal MCase activity was found at pH 8 and 38 °C in the presence of Mg²⁺ and an activating monovalent cation such as K⁺. Kinetic constants (K_m values) for the enzyme substrates were: 3-methylcrotonyl-CoA, 75 μm; ATP, 82 μm; HCO₃^?, 1.8 mm. Certain acyl-CoA derivatives, including crotonyl-CoA, (2Z)-3-ethylcrotonyl-CoA, and acetoacetyl-CoA, were also substrates for the enzyme. Some data on inhibition of the enzyme by acyl-CoA derivatives, and sulfhydryl- and arginyl-reagents, are presented.     

Subcellular distribution of carbonic anhydrase in Solanum tuberosum L. leaves

The intracellular compartmentation of carbonic anhydrase (CA; EC 4.2.1.1), an enzyme that catalyses the reversible hydration of CO₂ to bicarbonate, has been investigated in potato (Solanum tuberosum L.) leaves. Although enzyme activity was mainly located in chloroplasts (87% of total cellular activity), significant activity (13%) was also found in the cytosol. The corresponding CA isoforms were purified either from chloroplasts or crude leaf extracts, respectively. The cytosolic isoenzyme has a molecular mass of 255 000 and is composed of eight identical subunits with an estimated M _r of 30000. The chloroplastic isoenzyme (M _r 220000) is also an octamer composed of two different subunits with M _r estimated at 27 000 and 27 500, respectively. The N-terminal amino acid sequences of both chloroplastic CA subunits demonstrated that they were identical except that the M _r-27 000 subunit was three amino acids shorter than that of the M _r-27 500 subunit. Cytosolic and chloroplastic CA isoenzymes were found to be similarly inhibited by monovalent anions (Cl^–, I^–, N ₃ ^- and NO ₃ ^- ) and by sulfonamides (ethoxyzolamide and acetozolamide). Both CA isoforms were found to be dependent on a reducing agent such as cysteine or dithiothreitol in order to retain the catalytic activity, but 2-mercaptoethanol was found to be a potent inhibitor. A polyclonal antibody directed against a synthetic peptide corresponding to the N-terminal amino acid sequence of the chloroplastic CA monomers also recognized the cytosolic CA isoform. This antibody was used for immunocytolocalization experiments which confirmed the intracellular compartmentation of CA: within chloroplasts, CA is restricted to the stroma and appears randomly distributed in the cytosol.Abbreviations BSA bovine serum albumin - CA carbonic anhydrase - PMSF phenylmethylsulphonyl fluoride - BAM benzamidine - DTT dithiothreitol - 2-ME 2-mercaptoethanol - PVDF polyvinylidene difluoride The authors thanks P. Carrier and Dr. B. Dimon for technical assistance with the mass-spectrometry measurements.     

Studies on 125I-labeled proteins in rat plasma using an air-driven ultracentrifuge: protein-protein interactions and nonideality

The molecular weights of a number of ¹²⁵I-labeled plasma proteins have been determined from an analysis of their sedimentation equilibrium behavior in an air-driven ultracentrifuge. The values obtained agree well with results obtained by other methods. Molecular weights obtained for ¹²⁵I-labeled bovine serum albumin and the rat serum proteins albumin, α₁-acid glycoprotein, and major acute-phase α₁-protein were unaffected by the addition of 7% rat plasma. Direct evidence for protein-protein interactions was obtained for mixtures of ¹²⁵I-labeled rat α₁-acid glycoprotein and the plant lectin concanavalin A and for mixtures of ¹²⁵I-labeled protein A from Staphylococcus aureus and 7% rat plasma. Interactions of a different type were observed when the sedimentation equilibrium profiles of ¹²⁵I-labeled proteins were determined in concentrated solutions of other proteins. Under these conditions the effects of molecular exclusion or nonideality became significant and low estimates were obtained for the molecular weights of the labeled proteins. Analysis of the data obtained for ¹²⁵I-labeled bovine serum albumin in concentrated solutions of bovine serum albumin (20–80 mg/ ml) yielded nonideality coefficients in good agreement with literature values. Analysis of the behavior of ¹²⁵I-labeled rat serum albumin, transferrin, and α₁-acid glycoprotein yielded nonideality coefficients and hence activities of these proteins in undiluted rat plasma.     

The high-molecular-weight proteins of bovine brain plain synaptic vesicles

High molecular mass polypeptides (M _r >100,000) of plain synaptic vesicles from bovine cerebral cortex were separated using porous polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Four major bands, ofM _r 262,000, 249,000, 216,000, and 173,000, were resolved. Investigations into the membrane association of theM _r 216,000 and 173,000 proteins by means of solubilization experiments and Sepharose 4B chromatography indicate that the former is a peripheral protein and the latter is more firmly attached, possibly an integral protein. Finally, theM _r 216,000 protein was shown to be highly enriched in synaptic vesicles compared to other brain subfractions. It thus appears to be specifically associated with synaptic vesicles and therefore may have an important role specific to synaptic vesicle function or structure.     

Affinity labeling of high-affinity α-thrombin binding sites on the surface of hamster fibroblasts

The serine proteinase α-thrombin potently stimulates reinitiation of DNA synthesis in quiescent Chinese hamster fibroblasts (CCL39 line). ¹²⁵I-labeled α-thrombin binds rapidly and specifically to CCL39 cells with high affinity (K_d ≈ 4 nM). Binding at 37°C was found to remain stable for 6 h or more during which time no receptor down-regulation, ligand internalization and/or degradation could be detected. The structure of α-thrombin receptors on CCL39 cells was identified by covalently coupling ¹²⁵I-α-thrombin to intact cells using a homobifunctional cross-linking agent, ethylene glycol bis(succinimidyl succinate). By resolution in sodium dodecyl sulfate polyacrylamide gel electrophoresis we observed the specific labeling of a major α-thrombin-binding site of M_r ≈ 150 000 revealed as a ¹²⁵I-α-thrombin cross-linked complex of M_r ≈ 180 000. Independent of chemical cross-linking, ¹²⁵I-α-thrombin also formed a covalent complex with a minor, 35 000 M_r, membrane component identified as protease nexin. Two derivatives of α-thrombin modified at the active site are 1000-fold less than α-thrombin for mitogenicity. These two non-mitogenic derivatives bound to cells with similar affinity and maximal binding capacity as native α-thrombin, and affinity-labeled the receptor subunit of M_r 150 000. When present in large excess, during incubation of cells with α-thrombin, these binding antagonists were ineffective in blocking α-thrombin-induced DNA synthesis. These data suggest that the specific 150 000 M_r binding sites that display high affinity for α-thrombin do not mediate induction of the cellular mitogenic response.     

Biotin-mediated delivery of exogenous macromolecules into soybean cells

We have demonstrated that attachment of biotin to a variety of macromolecules allows the uptake of those macromolecules into cultured soybean cells (Glycine max Merr cv Kent). Macromolecules that were nondestructively delivered into intact cells in large numbers (>10⁶/cell) by this technique include bovine insulin (M_r about 5,700), bovine ribonuclease (M_r about 14,000), human hemoglobin (M_r about 64,000), and bovine serum albumin (M_r about 68,000). It is hypothesized that this methodology may be useful for delivering antibodies, toxins, enzymes, and genetic material into living plant cells without requiring prior removal of the cell wall or infection with Agrobacterium.     

Two major components of synaptonemal complexes are specific for meiotic prophase nuclei

Monoclonal antibody II52F10 was elicited against purified synaptonemal complexes (SCs); it recognizes two major components of the lateral elements of SCs, namely an M_r=30 000 and an M_r=33000 protein. We studied the distribution of the antigens of II52F10 within tissues and cells of the male rat by immunoblot analysis and immuno-cytochemical techniques. Nuclear proteins from various cell types, including spermatogonia and spermatids, did not react with antibody II52F10 on immunoblots; the same holds for proteins from isolated mitotic chromosomes. As expected, an M_r=30 000 and an M_r=33 000 protein from spermatocyte nuclei did react with the antibody. In cryostat sections of liver, brain, muscle and gut we could not detect any reaction with II52F10. In the testis the reaction was confined to SCs or SC fragments. Partly on the basis of indirect evidence we identified the antigen-containing cells as zygotene up to and including post-diffuse diplotene spermatocytes. The persistence of some antigen-containing fragments in the earliest stages of spermatids could not be excluded. We conclude that the lateral elements (LEs) of SCs are not assembled by rearrangement of pre-existing components of the nucleus: at least two of their major components are newly synthesized, presumably during zygotene. Furthermore we conclude partly from indirect evidence that the major components of the LEs of SCs are not involved in the chromosome condensation processes that take place during the earliest stages of meiotic prophase.Abbreviations BSA bovine serum albumin - CE central element - FITC fluorescein isothiocyanate - LE lateral element - PBS phosphate-buffered saline (140 mM NaCl, 10 mM sodium phosphate, pH 7.3) - SC synaptonemal complex - TBST Tris-buffered saline with Tween (50 mM Tris-HCl, pH 7.4, 500 mM NaCl, 0.05% Tween-20)