Molecular-weight estimates of milk-fat-globule-membrane protein–sodium dodecyl sulphate complexes by electrophoresis in gradient acrylamide gels

The molecular weights of milk-fat-globule-membrane proteins solubilized in sodium dodecyl sulphate were estimated by gradient gel electrophoresis. Standard curves were calibrated from both protein and glycoprotein markers of known molecular weight. Six major proteins were observed with Coomassie Blue staining and six with periodic acid-Schiff staining. The behaviour of the membrane proteins and the marker proteins was compared on several different single strength sodium dodecyl sulphate-polyacrylamide gels between 3 and 12% (w/v). The results were used to calculate the free electrophoretic mobility and retardation coefficient of each protein. Glycoprotein markers had a significantly lower mean free electrophoretic-mobility value than the protein markers. Three of the milk-fat-globule-membrane glycoproteins were shown to be independent of any of the Coomassie Blue-stained bands. On the basis of a comparison of the free electrophoretic-mobility and retardation- coefficient values of markers and unknown proteins the most appropriate standard curve for molecular-weight estimation was chosen.     

Studies on the structure of milk fat globule membrane.

Milk fat globule membrane was solubilized with sodium dodecyl sulfate and mercaptoethanol and the membrane proteins were separated by SDS-polyacrylamide gel electrophoresis. The membrane preparations contained three major size classes of polypeptide of 155,000, 62,500 and 43,500 daltons. At least five glycopeptides were separated of which two stained intensely with periodic acid-Schiff reagent, but poorly with coomassie blue. Trypsin hydrolysis of whole cream and isolated milk fat globule membrane revealed major differences in the rates of protein hydrolysis. Many of the membrane proteins of whole cream resisted proteolysis compared with the same proteins in the isolated membrane. Two glycopeptides were resistant to trypsin digestion in either preparation. Treatment of whole cream with neuraminidase led to the release of at least 70% of the protein-bound sialic acid. Whole cream and isolated membrane samples were iodinated with 125I in the presence of lactoperoxidase and hydrogen peroxide. The membrane proteins were significantly more accessible to lactoperoxidase-125I i in isolated membrane compared with the proteins of whole cream. Polypeptides of molecular weight 43,500 and approximately 48,000 daltons were predominantly labelled in whole cream and could be eluted from the fat globules with magnesium chloride (1.5m). The results strongly suggest that the proteins of milk fat globule membrane are asymmetrically arranged in the membrane and that most of the protein-bound sialic acid is present on the external surface of milk fat globules.     

Isolation and characterization of two individual glycoprotein components from human milk-fat-globule membranes.

Two individual glycoprotein components from human milk-fat-globule membranes (MFGM) has been purified by selectively extracting the membrane glycoproteins followed by lectin affinity chromatography and gel filtration on Sephadex G-200 in the presence of protein-disaggregating agents. The purified glycoprotein components, termed 'epithelial-membrane glycoprotein' (EMGP-155 and EMGP-39) have estimated molecular weights of 155 000 and 39 000 respectively, and yield a single band under reducing conditions on sodium dodecyl sulphate/polyacrylamide gel. EMGP-155 and EMGP-39 contain 21.0% and 7.0% carbohydrate by weight, with fucose (13.5%, 12.4%), mannose (3.7%, 6.2%), galactose (28.5%, 22.6%), N-acetylglucosamine (17.8%, 7.4%) and sialic acid (36.4%, 51.4%) of the carbohydrate moiety respectively. For both the glycoprotein components, aspartic and glutamic acid and serine are the major amino acid residues.     

Virus-receptor interaction in the adenovirus system I. Identification of virion attachment proteins of the HeLa cell plasma membrane.

Plasma membranes from HeLa cells were isolated in a two-phase polymer system. To compare the efficiency of attachment protein extraction, a normalized assay for the assessment of adenovirus type 2 (Ad2) receptor-active components interfering with the attachment of Ad2 to HeLa cells was developed. An optimized detergent extraction procedure, 0.5% Triton X-100, was used, and solubilized membrane proteins were radioisotope labeled in vitro. Proteins with affinity for Ad2 virions were quantified and identified in a sucrose gradient sedimentation assay and by affinity chromatography with cross-linked Ad2 virions immobilized to AH-Sepharose 4B. From virions recovered in the sucrose gradient system, one major membrane component of high affinity was identified with a polypeptide molecular weight of around 40,000. Glycosylated proteins isolated by wheat germ lectin chromatography with high affinity for immobilized virus particles were isolated, and two major components with apparent molecular weights of 40,000 and 42,000 were identified. We suggest that a glycosylated protein with high affinity for Ad2 virions and a polypeptide molecular weight of 40,000 to 42,000 is one component of the Ad2 attachment site on HeLa cells.     

Cooperativity of the concanavalin A inhibition of bovine milk fat globule membrane 5'-nucleotidase. Response to extraction of nucleotidase and of putative cytoplasmic surface coat components

5'-Nucleotidase (5'-ribonucleotide phosphohydrolase, EC 3.1.3.5) of bovine milk fat globules can be solubilized by deoxycholate from either isolated globule membranes or washed cream. The solubilized and membrane-bound enzymes exhibit similar Km values and are inhibited by concanavalin A by an apparent noncompetitive process. The soluble enzyme shows positive cooperativity for the inhibition (Hill coefficient of 2) at 37 degrees C, but the membrane enzyme exhibits essentially no cooperation effect. At lower temperatures (5 or 20 degrees C) the cooperative effect in the inhibition of the soluble enzyme is lost. Colchicine and cytochalasin D failed to induce cooperativity of the concanavalin A inhibition of the membrane enzyme, but induction cooperativity occurred when membranes were extracted with glycine/EDTA/mercaptoethanol, releasing a major protein component with a polypeptide molecular weight of 155 000. We suggest that the interaction of this component with the membrane imposes restraints on the behavior of the nucleotidase which are reflected in the cooperativity of the inhibition of the enzyme by concanavalin A.     

Composition of major outer membrane proteins of Vibrio vulnificus isolates: effect of different growth media and iron deficiency

The outer membrane proteins of Vibrio vulnificus including isolates from humans, seawater and an asari clam were examined by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. A major outer membrane protein with an apparent molecular weight of 48,000 (48K protein) was common to all the strains grown in 3% NaCl-nutrient broth; however this 48K protein was not produced in any of the strains grown in chemically defined medium. Other major outer membrane proteins with molecular weights ranging from 33,000 to 40,000 varied in number, relative amount and molecular weight depending on the strain. One to three new outer membrane proteins with molecular weights ranging from 74,000 to 85,000 were produced in the cells grown in iron-deficient medium. The 48K protein and one or two major proteins with molecular weights ranging from 35,000 to 37,000 in the cells grown in 3% NaCl-nutrient broth were not solubilized by 2% SDS at 60 C for 30 min and were resistant to trypsin, indicating that they are porins. On the other hand, in cells grown in chemically defined medium, one or two major outer membrane proteins with molecular weights ranging from 33,000 to 40,000 might be porins.     

Identification of Xenopus laevis sperm and egg envelope binding components on nitrocellulose membranes

Interacting egg envelope and sperm surface components were identified for Xenopus laevis using blotting methods. Sperm were extracted with sodium dodecyl sulfate (SDS), the extracted proteins separated by gel electrophoresis and blotted, and the blots treated with 125I-labeled heat solubilized envelopes. The converse experiment was also performed where envelope components were separated by gel electrophoresis, blotted, and the blots treated with 125I-labeled sperm components. Blotted sperm components with apparent molecular weights of 14K, 19K, 25K, and 35K selectively bound the solubilized envelopes. All of the envelope binding components were found to be localized on the sperm surface by radioiodinating intact sperm using Iodo-Gen. The blotted egg envelope component with an apparent molecular weight of 37K selectively bound to solubilized sperm components, and this binding was due to the protein moiety of the glycoprotein. 125I-labeled heat solubilized envelopes from unfertilized and fertilized eggs showed the same pattern of binding to blotted sperm components. Selected sulfated carbohydrates (fucoidan, dextran sulfate, and heparin, but not chondroitin sulfate) inhibited fertilization and binding of 125I-labeled heat solubilized envelopes to blotted sperm extract. Thus, the binding of heat solubilized envelopes to electrophoretically separated and blotted sperm proteins may reflect cellular interactions.     

Separation of the proteins of bovine milk-fat-globule membrane by electrofocusing with retention of enzymatic and immunological activity

1. Proteins of fat-globule membrane from bovine milk were solubilized with the non-ionic detergent Triton X-100 in the presence of protease inhibitors. Approximately 25% of the total membrane protein was solubilized and the extracts were shown to contain a sample of most of the major membrane proteins and glycoproteins. 2. The solubilized proteins were separated in flat-beds of Ultrodex by electrofocusing and the pI values for the major proteins, glycoproteins and certain enzymes determined. Several of the proteins displayed marked heterogeneity indicating the existence of protein variants and isoenzymes. Principal pI values for the enzymes assayed were as follows: xanthine oxidase, 7.35--7.55; NADH2: iodonitrotetrazolium reductase, less than 4.5; 5'-nucleotidase, 7.15--7.4; alkaline phosphatase, 5.4--5.7; phosphodiesterase, 4.6--4.8; gamma-glutamyl transpeptidase, 4.4--4.55. 3. Fractions after electrofocusing were analyzed by 'fused rocket' immunoelectrophoresis and crossed immunoelectrophoresis after separation in polyacrylamide gels containing sodium dodecyl sulphate. Major antigens of the membrane include xanthine oxidase and glycoproteins of apparent molecular weights 67 000, 49 500 and 46 000. The latter two components share common antigenic determinants and could not be separated by gel filtration, ion-exchange chromatography, lectin-affinity chromatography or preparative electrofocusing.     

Purification and properties of the major sialoglycoprotein of the milk fat globule membrane

The major periodate-Schiff positive component (glycoprotein-2) of bovine milk fat globule membranes (MFGM) has been purified by extraction of washed cream with chloroform/methanol followed by chromatography on Sephadex G-200 in sodium dodecyl sulfate. The glycoprotein is > 95% pure by polyacrylamide electrophoresis in dodecyl sulfate and shows the same prominent component at gel percentages of from 5 to 12.5. The molecular weight obtained by extrapolation of the apparent molecular weights on these gels to higher gel percentages was 70,000. An apparent molecular weight of 105,000 was obtained by gel filtration in 1% dodecyl sulfate on Sepharose 4B. The glycoprotein contains 50% carbohydrate by weight, with sialic acid (30.5%), N-acetylglucosamine (22.3%), galactose (15.9%), N-acetylgalactosamine (14.0%), mannose (11.1%), and fucose (5.8%) being the major monosaccharides. Leucine, glutamic acid, and glycine are the major amino acids. Affinity chromatography of deoxycholate-solubilized MFGM indicates that glycoprotein-2 is not the major concanavalin A receptor of these membranes.     

Structural disulfide bonds in the Bacillus thuringiensis subsp. israelensis protein crystal.

We examined disulfide bonds in mosquito larvicidal crystals produced by Bacillus thuringiensis subsp. israelensis. Intact crystals contained 2.01 X 10(-8) mol of free sulfhydryls and 3.24 X 10(-8) mol of disulfides per mg of protein. Reduced samples of alkali-solubilized crystals resolved into several proteins, the most prominent having apparent molecular sizes of 28, 70, 135, and 140 kilodaltons (kDa). Nonreduced samples contained two new proteins of 52 and 26 kDa. When reduced, both the 52- and 26-kDa proteins were converted to 28-kDa proteins. Furthermore, both bands reacted with antiserum prepared against reduced 28-kDa protein. Approximately 50% of the crystal proteins could be solubilized without disulfide cleavage. These proteins were 70 kDa or smaller. Solubilization of the 135- and 140-kDa proteins required disulfide cleavage. Incubation of crystals at pH 12.0 for 2 h cleaved 40% of the disulfide bonds and solubilized 83% of the crystal protein. Alkali-stable disulfides were present in both the soluble and insoluble portions. The insoluble pellet contained 12 to 14 disulfides per 100 kDa of protein and was devoid of sulfhydryl groups. Alkali-solubilized proteins contained both intrachain and interchain disulfide bonds. Despite their structural significance, it is unlikely that disulfide bonds are involved in the formation or release of the larvicidal toxin.     

Production of novel proteins by Bacteroides ureolyticus grown under conditions of reduced iron availability

Protein profiles of whole cells of Bacteriodes ureolyticus grown in the presence or absence of the iron chelator desferrioxamine mesylate (Desferal) were compared. Each of four strains produced novel proteins of molecular weights 19, 25 and 41 kilodaltons (kDa) under conditions of reduced iron availability. Novel proteins of molecular weights 32, 52 and 58 kDa were also detected although there was interstrain variation in their expression. Outer membranes from three of the strains grown on iron-depleted medium also contained novel proteins with molecular weights of approximately 25, 41 and 52 kDa. When organisms were grown on medium containing Desferal saturated with excess iron, the novel proteins were not detected indicating that their expression was regulated by the level of available iron in the medium.     

Isolation of the outer membrane and characterization of the major outer membrane protein from Spirochaeta aurantia.

The outer membrane of Spirochaeta aurantia was isolated after cells were extracted with sodium lauryl sarcosinate and was subsequently purified by differential centrifugation and KBr isopycnic gradient centrifugation. The purified outer membrane was obtained in the form of carotenoid-containing vesicles. Four protein species with apparent molecular weights of 26,000 (26K), 36.5K, 41K, and 48.5K were readily observed as components of the vesicles. The 36.5K protein was the major polypeptide and constituted approximately 90% of the outer membrane protein observed on sodium dodecyl sulfate-polyacrylamide gels. Under mild denaturing conditions the 36.5K major protein exhibited an apparent molecular weight of approximately 90,000. This, together with the results of protein cross-linking studies, indicates that the 36.5K polypeptide has an oligomeric conformation in the native state. Reconstitution of solubilized S. aurantia outer membrane into lipid bilayer membranes revealed the presence of a porin, presumably the 36.5K protein, with an estimated channel diameter of 2.3 nm based on the measured single channel conductance of 7.7 nS in 1 M KCl.     

DNA binding of the mouse class II major histocompatibility CCAAT factor depends on two components.

A CCAAT-binding factor that recognizes a CCAAT sequence (Y box) located upstream of the major histocompatibility class II gene I-A beta has been partially purified. This CCAAT-binding factor was found to consist of two components, designated factors A and B, both of which were required for efficient binding to the DNA. Factor A had an apparent molecular size of 34 kilodaltons, and factor B had an apparent molecular size of 42 to 46 kilodaltons.     

Solubilization of plant membrane proteins for analysis by two-dimensional gel electrophoresis

A plasma membrane-enriched fraction prepared from barley roots was analyzed by two-dimensional gel electrophoresis. Four methods of sample solubilization were assessed on silver stained gels. When membranes were solubilized with 2% sodium dodecyl sulfate followed by addition of Nonidet P-40, gels had high background staining and few proteins because of incomplete solubilization. Gels of membranes solubilized in urea and Nonidet P-40 had a greater number of proteins but proteins with molecular weights greater than 85,000 were absent and proteins with low molecular weights were diffuse. High molecular weight proteins were present in gels of membranes solubilized in 4% sodium dodecyl sulfate followed by acetone precipitation but background staining and streaking remained a problem. Gels of the best quality were obtained when membrane proteins were extracted with phenol and precipitated with ammonium acetate in methanol; background staining and streaking were diminished and proteins were clearly resolved. This method makes possible the resolution required for meaningful qualitative and quantitative comparisons of protein patterns on two-dimensional gels of plant membrane proteins.     

Pupal and larval cuticle proteins of Drosophila melanogaster

Proteins, soluble in 7 M urea, were extracted from third-instar larval and pupal cuticles of Drosophila melanogaster. Both extracts contain a limited number of polypeptides resolved by one- or two-dimensional electrophoresis. The five major larval proteins have low molecular weights (less than 20000) and are not glycosylated. The major pupal cuticle proteins fall into two size classes: two with apparent molecular weights of 56K and 82K and four with molecular weights between 15K and 25K. The proteins with high apparent molecular weights are glycosylated. In nondenaturing gels, no components of the larval and pupal cuticle extracts comigrate. One-dimensional "fingerprints" indicate that cuticle proteins from these two stages have unique primary structures. Immunological results indicate that the major low molecular weight larval and pupal cuticle proteins are comprised of two families of proteins that share antigenic determinants. The high molecular weight pupal cuticle proteins are immunologically unrelated to the low molecular weight components. We conclude that the pupal and larval proteins are encoded in part by multigene families that have arisen by gene duplication and evolutionary divergence.     

Characterization of the Blood-Brain Barrier: Protein Composition of the Capillary Endothelial Cell Membrane

Microvessels were isolated from canine cerebral cortex, and the composition of the endothelial cell membrane was investigated. Endothelial cell membranes were separated from the surrounding basement membrane, solubilized, and subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis in 12% gels. Staining with Coomassie Blue revealed a characteristic banding pattern of at least 12 major proteins with apparent molecular weights between 14,000 and 250,000. When proteins from red blood cell ghosts were run simultaneously, no similarities were observed, except for proteins at apparent molecular weights of 43,000 (band 3) and 35,000 (band 4). These two proteins migrated exactly to the positions of the erythrocyte proteins actin and glyceraldehyde 3-phosphate dehydrogenase, respectively. Membrane glycoproteins in gels were also examined by the use of fluorescent lectins. Of the fluorescein isothiocyanate-conjugated (FITC) lectins tested, only FITC-concanavalin A had an affinity for any membrane components. Diazotized [125I]iodosulfanilic acid, a membrane-impermeable reagent, was used to label the internal (lumen) cell surface and the external (antilumen) cell surface. Autoradiography and determination of radioactivity levels in gel slices showed that several proteins were specifically labeled, and that major differences in radioactivity of proteins existed in internal and external labeling experiments. It is concluded that the protein composition of the luminal membrane is different from that of the antiluminal membrane.     

Characterization of the cell wall and cell wall proteins of Chromatium vinosum.

Highly purified cell walls of Chromatium vinosum were isolated by differential centrifugation, with or without Triton X-100 extraction. The isolated material had a protein composition similar to that of cell walls obtained by sucrose density gradient centrifugation. Twenty-two proteins were reproducibly detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A 42-kilodalton protein was shown to account for 65% of the total cell wall protein. The majority of cell wall proteins were solubilized in sodium dodecyl sulfate at room temperature; however, they existed as high-molecular-weight complexes unless heated to 45 degrees C or above. The cell wall contained one heat-modifiable protein which migrated with an apparent molecular weight of 37,400 when solubilized at 70 degrees C or below, but which migrated with an apparent molecular weight of 52,500 if solubilized at 100 degrees C. The electrophoretic mobility of three proteins was modified by 2-mercaptoethanol. The majority of C. vinosum cell wall proteins had isoelectric points between pH 4.5 and 5.5, and the 42-kilodalton protein focused at pH 4.9. No proteins were detected which were analogous to the lipoprotein or peptidoglycan-associated proteins of the Enterobacteriaceae. Nearest-neighbor analysis with a reducible, cross-linking reagent indicated that three proteins, including the 42-kilodalton protein, associated with themselves. Most of the cell wall proteins were partially accessible to proteases in both intact cells and isolated cell walls. Protease treatment of the whole cell or isolated cell wall digested approximately an 11,000-molecular-weight portion of the 42-kilodalton protein.     

Purification of transforming growth factor-beta 1 binding proteins from porcine uterus membranes.

We have identified several transforming growth factor-beta 1 (TGF-beta 1) binding proteins in solubilized and glycoprotein-enriched porcine uterus membrane fractions by affinity cross-linking and in-gel ligand binding using 125I-labeled TGF-beta 1. By a ligand affinity chromatography using a column of immobilized recombinant TGF-beta 1, four components of apparent molecular weights 160,000, 80,000, 50,000, and 40,000 under reducing conditions were eluted at a pH of 3.5; the 160-,80-, and 40-kDa components were demonstrated to bind TGF-beta 1 specifically by the 125I-TGF-beta 1 binding assays. Further purification was performed by gel chromatography using a Superose 12 column eluted in 70% formic acid. The 40-kDa component was purified to an apparently homogenous form, whereas the 160-kDa component eluted in a broad peak overlapping the peak of the 80-kDa component. It remains to be elucidated whether these TGF-beta 1 binding proteins are related to cell surface receptors for TGF-beta s.     

Fish plasminogen activators: their identification and characterization

Immunoblots of proteins extracted from the skin of a small viviparous fish (Xiphophorus) showed that a monoclonal antibody against human urokinase recognizes multiple molecular weight species of antigens. The immunoaffinity-purified antigens had serine-protease activity for the hydrolysis of a chromogenic substrate and could convert human plasminogen to plasmin in a manner similar to that for human urokinase in vitro. Two antigens with apparent molecular weights of 55 and 50 kilodaltons that had been purified on a fibrin-Celite column were separable on SDS-polyacrylamide gels and were characterized as major plasminogen activators on fibrin-agar indicator plates. The 125I-tryptic peptide maps of both antigens were similar to that of human urokinase; therefore, the fish activators and human urokinase are structurally and functionally related.     

Analysis of lipid and protein components of ejaculated bull sperm surface and seminal plasma

Surface components of ejaculated bull sperm were radiolabeled by enzymatic iodination with lactoperoxidase and Na¹²⁵I. The sperm were lysed and the labeled components analyzed on SDS-7.5% polyacrylamide gels. Electrophoresis of solubilized radioactivity resolved six components with approximate molecular weights of 77, 61, 44, 36, 24, and 15 kilodaltons. To identify components that might be adsorbed to the bull sperm surface from seminal secretions, seminal plasma was labeled. Electrophoresis of labeled seminal plasma resolved four components with approximate molecular weights of 74, 33, 24, and 15 kilodaltons, each of which comigrated with a labeled sperm surface component. To identify the chemical composition of the radiolabeled components, labeled sperm surface and labeled seminal plasma were submitted to isopycnic density gradient centrifugation in cesium chloride. The ¹²⁵I incorporated into bull sperm surface separated into two discrete areas of radioactivity, one having a density characteristic of protein and the other, of lipid. Iodinated seminal plasma banded in one discrete area that had a density characteristic of protein. Electrophoretic analysis of each area of radioactivity recovered from the gradients demonstrated that five of the six sperm surface and all of the seminal plasma components were in the protein fractions. The 15-kilodalton sperm surface component banded as a lipid, whereas the 15-kilodalton seminal plasma componènt banded as a protein.