Studies on thyroid cell surface glycoproteins: isolation of plasma membranes and characterization of carbohydrate units

Calmodulin-binding proteins in granule and plasma membranes from bovine chromaffin cells

Calmodulin-binding proteins in chromaffin granule membrane and chromaffin cell plasma membranes have been investigated and compared. Chromaffin granules were purified by centrifugation over a 1.7 M sucrose layer. Plasma membranes were obtained in a highly purified form by differential and isopycnic centrifugation. Enzymatic determinations of 5'-nucleotidase, a generally accepted plasma membrane marker, showed a 40-50-fold enrichment as compared to the cell homogenate. Marker enzyme studies demonstrated only minimal contamination by other subcellular organelles. After solubilization with Triton X-100, calmodulin-binding proteins were isolated from chromaffin granule membranes and plasma membranes by affinity chromatography on a calmodulin/Sepharose 4B column. On two-dimensional polyacrylamide gelelectrophoresis a prominent protein (Mr = 65,000, pI ranging from 5.1 to 6) consisting of multiple spots, was present in the calmodulin-binding fraction from chromaffin granule membranes as well as from plasma membranes. Besides this 65 kDa protein both fractions had at least four groups of proteins in common. Also, proteins typical for either preparation were observed. In the calmodulin-binding protein preparations from chromaffin granule membranes a prominent spot with Mr = 80,000 and a pH ranging from 5.0 to 5.7 was present. This protein was enzymatically and immunologically identified as dopamine-beta-monooxygenase.

Studies of normal and prematurely ruptured human amniotic membranes: low calcium and magnesium in prematurely ruptured membranes

Prematurely ruptured amniotic membranes at term, and membranes from normal term pregnancies were studied. The membranes were analyzed for neutral sugars, hexosamines, sialic acid, uronic acid, DNA, RNA, cholesterol, phospholipids, calcium, magnesium, sodium and potassium. The only significant difference found between the normal and the prematurely ruptured membranes was a significantly lower concentration of calcium and magnesium in the latter group. (Calcium 0.32 +/- 0.07, vs. 0.24 +/- 0.07, p less than 0.05; Magnesium 0.30 +/- 0.19 vs. 0.15 +/- 0.04, p less than 0.05; in mg/100 mg protein). The possible metabolic effects of low calcium and magnesium may be of relevance to the pathogenesis of premature rupture of the amniotic membranes.

Topography, purification and characterization of thyroidal 5'-nucleotidase

1. Subcellular studies of bovine thyroid indicate that 5'-nucleotidase is predominantly associated with plasma membranes, although a considerable part of this ectoenzyme is also found internalized. 2. The enzyme displaying the features of a glycoprotein has been purified 1400 times by detergent solubilization and two subsequent affinity chromatographic steps. 3. Thyroidal 5'-nucleotidase can be classified as an unspecific metallo-dependent 5'-ribonucleotide phosphohydrolase. The native enzyme exists as a dimer (MW 150 kDalton), composed of two similar or identical subunits.

Differentiation-associated decrease in the proportion of fucosylated polylactosaminoglycans of CaCo-2 human colonic adenocarcinoma cells

CaCo-2 cells are human colonic adenocarcinoma cells which can differentiate spontaneously into enterocytes when maintained confluent for extended periods of time. Cells kept in culture for 4 days (rapidly growing), 7-9 days (early confluence) and 19-22 days (late confluence) were incubated for 24 h with L-[5,6-3H]fucose or D-[6-3H]glucosamine in order to examine the changes in glycoprotein carbohydrate structure that occur during this differentiation. Labelled glycopeptides obtained by exhaustive Pronase digestion of the cell-surface and cell-pellet fractions were fractionated on Bio-Gel P-6. A high-Mr glycopeptide fraction which was excluded from Bio-Gel P-6 was present in all cases. These glycopeptides were then fractionated by affinity chromatography on Datura stramonium agglutinin-agarose. The glycopeptides which were specifically bound to the lectin column were largely degraded by endo-beta-galactosidase, thereby indicating that they consisted of fucosylated polylactosaminoglycans. The proportion of labelled polylactosaminoglycans decreased with increasing time in culture, whereas sucrase activity, which is characteristic of differentiated enterocytes, increased. These results demonstrate that a relatively large decrease in the proportion of fucosylated polylactosaminoglycans occurs with differentiation of CaCo-2 cells.

Isolated erythroglycans have a high-affinity interaction with wheat germ agglutinin but are poorly accessible in situ

The sugar and cell specificities of wheat germ agglutinin have been studied extensively. In particular, it is well established that wheat germ agglutinin will interact with highly sialylated glycoconjugates of the type carried by the erythrocyte glycoprotein, glycophorin (Adair, W.L. and Kornfeld, S. (1974) J. Biol. Chem. 249, 4696-4704). We have found that polylactosamines isolated from adult and fetal erythrocytes can have a high-affinity interaction with immobilized wheat germ agglutinin. In fact, this interaction is much stronger than the sialic acid-dependent interaction. Using flow microfluorimetry in conjunction with various serological and enzymatic pretreatments, we have measured the extent to which polylactosamines contribute to wheat germ agglutinin binding. We have found that most of the neuraminidase-resistant receptors on erythrocytes are polylactosamine in nature. However, this residual binding of wheat germ agglutinin to neuraminidase-treated erythrocytes is of much lower apparent affinity than the sialic acid-dependent interaction. The lower reactivity of polylactosamines at the erythrocyte surface suggests that these large glycans are actually poorly accessible.

Fluidity characteristics of bovine thyroid plasma membranes

Highly purified plasma membranes of bovine thyroid were obtained by differential pelleting followed by discontinuous gradient centrifugation in a swing-out rotor. Subfractions of plasma membranes were prepared by affinity chromatography on Con A-Sepharose. The final membrane fractions were enriched 25-30-fold over homogenate in 5'-nucleotidase and alkaline phosphatase and displayed a protein to phospholipid ratio of 1.67 and a cholesterol to phospholipid molar ratio of 0.55. The phospholipid composition did not deviate appreciably from that of whole tissue except for the higher sphingomyelin level (22.5 vs. 14.0%). The predominant fatty acids were palmitic (16:0), oleic (18:1), stearic (18:0) and linoleic (18:2) acid. The physical state of the membrane was studied by (i) calculation of the lipid structural order parameter SDPH from steady-state fluorescence anisotropy determinations of the hydrophobic probe 1,6-diphenyl-1,3,5-hexatriene (DPH); (ii) estimation of the lateral diffusion coefficient of pyrene following excimer formation. These parameters were determined in native thyroid plasma membranes and in reconstituted vesicles, obtained by detergent dialysis from octylglucoside solubilized membrane components. The presence of membrane protein or neutral lipids induced more restraint on the movements of the fluorophores. The lipid order parameter, SDPH was mainly determined by the neutral lipids. Subfractions of plasma membrane enriched in luminal membranes have a slightly lower fluidity (higher SDPH and lower Ddiff values) than subfractions enriched in basolateral membranes. This difference appears to be due to both differences in lipid as well as protein composition. Under physiological conditions, no significant alterations in probe dynamics could be observed upon addition of thyrotropin or cholera toxin, even at micromolar concentrations.

Purification and characterization of a receptor for the 987P pilus of Escherichia coli

A receptor-containing fraction that caused visible aggregation of piliated Escherichia coli strain 987 bacteria was released into solution when brush borders from adult rabbit small intestines were stored. The receptor-containing fraction was separated into more than 50 bands by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, 4 of which contained carbohydrate as demonstrated by staining with periodic acid-Schiff. Receptor activity in the receptor-containing fraction was associated with a low-molecular-weight (less than 14,000) glycoprotein as measured by Western blots. Extraction of brush borders with chloroform-methanol-water (4:8:3) resulted in increased pilus 987P receptor activity in the aqueous extract. Gel filtration chromatography of sodium dodecyl sulfate-solubilized aqueous extract yielded a fraction with 987P receptor activity that contained only the low-molecular-weight glycoprotein. The purified 987P receptor contained 81% carbohydrate and 19% amino acids by weight. Isoelectric focusing separated the 987P receptor receptor into 3 peaks corresponding to isoelectric points of 2.2, 3.8, and 4.1. Each of the peaks contained 987P receptor activity. The 987P receptor activity was sensitive to periodate oxidation and to digestion with pronase. The strain 987-agglutinating activity of the receptor was inhibited by amino sugars and their N-acetylated derivatives, by compounds having a free amino group, by high concentrations of salt, and by lectins that recognize D-galactose or L-fucose.

NADPH-dependent H2O2 generation and peroxidase activity in thyroid particular fraction

A NADPH-dependent H2O2 generating system associated with a thyroid particular fraction is described. H2O2 is measured by two different methods: iodination of NADPH itself when the system is supplemented with lactoperoxidase and [125I]iodide, and by the scopoletin method. It is shown that: H2O2 generation is inhibited by catalase and is dependent on NADPH or particulate protein concentration; radical scavengers of OH and of singlet oxygen have no effect while superoxide dismutase has only a marginal effect; disruption of the particular fraction by phospholipase A2 or digitonin treatment completely abolished H2O2 generation activity while thyroid peroxidase activity appears, suggesting different sites for the two activities in the membrane vesicles.

Improved yield of plasma membrane from mammalian cells through modifications of the two-phase polymer isolation procedure

Modifications to the two-phase polymer gradient procedure for isolating plasma membrane from mammalian cells have resulted in greatly increased yields of purified plasma membrane. First, the cells were not treated with a membrane stabilizer (ZnCl2) prior to homogenization. This reduced the severity of homogenization required for disruption and allowed a greater proportion of the surface membrane to form large, flattened sheets that are more easily purified than the smaller fragments formed during more severe homogenization. Second, three crude fractions obtained from the homogenate (600g, 2000g, and 12,000g pellets), rather than a single, low-speed pellet (600g) containing only large sheets of membrane, were subjected to gradient centrifugation to obtain plasma membrane. This modification allowed purification of small as well as large fragments of plasmalemma and greatly increased the yield of purified membrane. Mg+2-dependent, Na+-K+-stimulated ATPase, a marker enzyme for plasma membrane, was enriched in the purified fraction by congruent to 17-fold relative to homogenate on a specific activity basis, and the yield of isolated plasma membrane averaged 70%, and was occasionally as high as 90%.

Isolation from thyroid cells or purified plasma membranes with associated actin microfilaments. Proteins bound to actin

Plasma membranes of thyroid cells were purified from hog thyroid glands following two procedures. Their homogeneity was tested by electron microscopy and by measurements of the activity of membrane-bound enzyme markers. According to the procedure used the membrane fractions obtained present some differences in their morphological features as well as in the repartition of the activities of the membrane-bound enzyme markers. However, whatever the composition of the membrane fraction examined (membrane vesicles, single membrane sheets with junctional complexes), decoration with heavy meromyosin clearly shows the presence of actin filaments attached to these fragments. Analysis of proteins by polyacrylamide gel electrophoresis indicates the presence of about twelve major components with actin. Treatment of membranes with Triton X-100 results in an insoluble core which contains all the actin and most of the major proteins. The selective extraction of these components by buffers differing in their ionic strength, pH, or the presence or absence of ATP X Mg has been used to characterize some of the proteins associated to actin; among them are filamin, myosin, alpha-actinin, tropomyosin.