Microfilament association of ASGP-2, the concanavalin A-binding glycoprotein of the cell-surface sialomucin complex of 13,762 rat mammary ascites tumor cells

Microfilament-associated proteins and membrane-microfilament interactions are being investigated in microvilli isolated from 13,762 rat mammary ascites tumor cells. "Phalloidin shift" analyses on velocity sedimentation gradients of Triton X-100 extracts of [3H]-glucosamine-labeled microvilli identified a 120-kDa cell-surface glycoprotein associated with the microvillar microfilament core. The identification was verified by concanavalin A (Con A) blots of one- and two-dimensional (2D) electrophoresis gels of sedimented microfilament cores. By 2D-electrophoresis and lectin analyses the 120-kDa protein appeared to be a fraction of ASGP-2, the major Con A-binding glycoprotein of the sialomucin complex of the 13,762 cells. This identity was confirmed by immunoblot analyses using immunoblot-purified anti-ASGP-2 from anti-membrane serum prepared against microvillar membranes. Proteolysis of the microvilli with subtilisin or trypsin resulted in an increase in the amount of ASGP-2 associated with the microfilament cores. An increase was also observed with sialidase treatment of the microvilli, suggesting that negative charges, probably present on the highly sialated sialomucin ASGP-1 of the ASGP-1/ASGP-2 sialomucin complex, reduce ASGP-2 association with the microfilament core. Proteolysis of isolated microvillar membranes, which contain actin but not microfilaments, also increased the association of ASGP-2 with a Triton-insoluble, actin-containing membrane fraction. Purified ASGP-2 does not bind to microfilaments in sedimentation assays. Since the Triton-insoluble membrane residue is enriched in an actin-containing transmembrane complex, which contains a different glycoprotein, we suggest that the ASGP-2 is binding indirectly via this complex to the microfilament core in the intact microvilli.     

Cell surface properties of ascites sublines of the 13762 rat mammary adenocarcinoma : Relationship of the major sialoglycoprotein to xenotransplantability

The relationship between cell surface sialoglycoprotein and xenotransplantation has been investigated in ascites sublines of the 13762 rat mammary adenocarcinoma. Two of the five sublines (MAT-C and MAT-C1) can be transplanted into mice. These two sublines also have the greatest amounts of total, trypsin-releasable and neuraminidase-releasable sialic acid. Chemical labeling using periodate treatment followed by [³H]borohydride reduction indicates that most of the protein-bound sialic acid is associated with a single major sialoglycoprotein (or family of glycoproteins) with a low mobility on polyacrylamide gels in dodecyl sulfate (SDS). This glycoprotein, denoted ASGP-1, is also labeled by lactoperoxidase and ¹²⁵I, indicating its presence at the cell surface. Metabolic labeling with [³H]glucosamine shows that ASGP-1 is the major glycosylated protein in both xenotransplantable (MAT-C1) and non-xenotransplantable (MAT-B1) sublines, representing >70% of the protein-bound label in each. The labeling studies indicate that the non-xenotransplantable subline does not have a substantially greater amount of ASGP-1 on its cell surface. Likewise cationized ferritin labeling and transmission electron microscopy (TEM) do not show substantially greater amounts of negatively charged groups distributed along the cell surfaces of MAT-C1 than of MAT-B1 cells. The results indicate that the transplantation differences between these sublines cannot be explained solely by the presence of a major sialoglycoprotein at the cell surface.     

Sialoglycoprotein differences between xenotransplantable and nonxenotransplantable ascites sublines of the 13762 rat mammary adenocarcinoma

MAT-B1 and MAT-CI rat ascites mammary adenocarcinoma cells differ in morphology, lectin receptor mobility, and xenotransplantability. Since these properties may be related to cell surface organization, the predominant sialoglycoproteins of these sublines have been investigated by chemical labeling, proteolysis, and alkaline borohydride elimination. Treatment of both sublines with periodate and tritiated borohydride labels one major sialoglycoprotein (ASGP-1) with a low electrophoretic mobility on polyacrylamide gels in dodecyl sulfate. Treatment of labeled or unlabeled cells with trypsin releases about 30% of the total cell sialic acid without significant decrease in cell viability. Gel filtration in pyridine-acetate buffer or in dodecyl sulfate indicates that the released materials are very heterogeneous, and that most of the MAT-C1 sialoglycopeptides are larger than sialoglycopeptides of MAT-B1. Amino acid compositions are quite similar for the released material from the two sublines, but they differ substantially in sialic acid. Further degradation of trypsin-released material with Pronase gives products which are included in a column of mixed Bio-Gel P-10 and P-30 and which also indicate a larger average size for MAT-C1 sialoglyco-peptides. Oligosaccharides from the sialoglycopeptides were obtained by alkaline borohydride treatment of trypsin-released, labeled material and fractionated by chromatography on Bio-Gel P-2. The oligosaccharide(s) comprising the major peak from MAT-C1 cells was larger in size than most of the material from MAT-B1 cells and contained galactosaminitol, galactose, glucosamine, sialic acid, and fucose. These results suggest that MAT-C1 ASGP-1 has more complex oligosaccharides than MAT-B1 ASGP-1, a difference which may play an important role in the differences in cell behavior between the sublines, including transplantability. Regardless of whether the ASGP-1 plays a role in transplantation, investigations of the sialoglycoproteins of these sublines provide a potentially valuable tool for understanding some of the mechanisms by which tumor cells control their cell surface properties.     

Structures of the O-linked oligosaccharides of the major cell surface sialoglycoprotein of MAT-B1 and MAT-C1 ascites sublines of the 13762 rat mammary adenocarcinoma

Structures of the principal O-glycosides from the major cell surface sialoglycoprotein (ASGP-1) of the MAT-B1 and MAT-C1 ascites sublines of the 13762 rat mammary adenocarcinoma have been determined. Oligosaccharitols were released by alkaline borohydride treatments of ASGP-1 and purified by gel filtration, DEAE-Sephadex ion exchange chromatography, and high performance liquid chromatography. On the basis of carbohydrate composition, methylation analysis, periodate oxidation, and exoglycosidase digestion, the five major oligosaccharides released by mild alkaline borohydride were assigned the following structures: Component II-3: (NeuAc alpha 2----3Gal beta 1----4GlcNAc beta 1----6)Ga 1 NAcOH(3----1 betaGa 1 3----2 alpha NeuAc) III-2a: (Ga 1 beta 1----4G1cNAc beta 1----6)Ga 1 NAcOH(3----1 beta Ga 1 3----2 alpha NeuAc) III-2c: (Ga 1 alpha 1----3Ga 1 beta 1----4G1cNAc beta 1----6) Ga 1 NAcOH(3----1 beta Ga 1 3----2 alpha NeuAc) IV-1a: (Ga 1 beta 1----4G 1 cNAc beta 1----6)Ga 1 NAcOH(3----1 beta Ga 1) IV-1c: (Ga 1 alpha 1----3Ga 1 beta 1----4G 1 cNAc beta 1----6) Ga 1 NAcOH(3----1 beta Ga 1) Fucosylated derivatives of III-2a, IV-1a, and IV-1c were found in smaller amounts with the fucose tentatively assigned to the 2-position of the lactosamine galactose. Components II-3, III-2a, and the fucosylated derivative of III-2A were found in both MAT-B1 and MAT-C1 sublines. The alpha-galactosides were found in detectable quantities only in subline MAT-B1. Oligosaccharides from MAT-C1 cells were enriched in sialic acid when compared to those from MAT-B1 cells. These results suggest that the 13762 ascites sublines, which bear different oligosaccharides, will provide models useful for the investigation of mechanisms regulating the expression of structures of the larger O-linked oligosaccharides.     

N-Acetylneuraminic acid and N-glycolylneuraminic acid in the O-linked oligosaccharides of a tumor cell glycoprotein. Incorporation and distribution

The MAT-B1 and MAT-C1 ascites sublines of the 13762 rat mammary adenocarcinoma, which differ in several cell surface properties, contain a major mucin-type glycoprotein, termed ASGP-1. The sialic acid content of MAT-C1 ASGP-1 is 2-3-fold greater than MAT-B1 ASGP-1 (Sherblom, A. P., Buck, R. L., and Carraway, K. L. (1980) J. Biol. Chem. 255, 783-790). Sialic acid analysis demonstrated that, whereas MAT-C1 ASGP-1 contained approximately equal amounts of N-acetylneuraminic acid (NeuAc) and N-glycolylneuraminic acid (NeuGl), MAT-B1 ASGP-1 was devoid of NeuGl. MAT-B1 microsomes also did not contain NeuGl. MAT-B1 cells incubated with [3H]N-acetylmannosamine did not synthesize either labeled CMP-NeuGl or free NeuGl, even though the CMP-sialic acid synthetase was active with the substrate NeuGl. Thus, MAT-B1 cells may be deficient in the enzyme N-acetylneuraminate monooxygenase. The O-linked oligosaccharides from both MAT-B1 and MAT-C1 ASGP-1 have been shown to contain a core tetrasaccharide Gal(beta 1-4)GlcNAc(beta 1-6)(Gal(beta 1-3]GalNAc in which both galactose residues may be linked to additional sugars (Hull, S. R., Laine, R. A., Kaizu, T., Rodriquez, I., and Carraway, K. L. (1984) J. Biol. Chem. 259, 4866-4877). The distribution of NeuAc and NeuGl between the two galactose termini of the core tetrasaccharide was examined for MAT-C1 ASGP-1. Oligosaccharides were released by alkaline-borohydride treatment of MAT-C1 ASGP-1 which had been labeled with [14C]glucosamine and galactose oxidase/B3H4. Following fractionation by Bio-Gel P-4, DEAE-Sephadex, and high-performance liquid chromatography, oligosaccharides were analyzed for NeuAc and NeuGl and for susceptibility to digestion with beta-galactosidase. Three disialylated oligosaccharides were identified containing 2 mol of NeuAc (5.5% recovery), 2 mol of NeuGl (4.5%), or 1 mol each of NeuAc and NeuGl (11.1%). For monosialylated oligosaccharides, NeuGl appeared preferentially associated with the Gal(beta 1-4)GlcNAc terminus (9.0%), whereas significant amounts of oligosaccharide containing NeuAc at both the Gal(beta 1-3)GalNAc (2.6%) and Gal(beta 1-4)GlcNAc (4.5%) termini were detected. Each of the major qualitative differences between MAT-B1 and MAT-C1 oligosaccharides, including the presence of NeuGl (MAT-C1), sulfate (MAT-B1), and alpha-linked galactose (MAT-B1), occurs at the Gal(beta 1-4)GlcNAc terminus.     

Shedding of the major plasma membrane sialoglycoprotein from the surface of 13762 rat ascites mammary adenocarcinoma cells

ASGP-1 (ascites Sialoglycoprotein 1) the major sialoglycoprotein of 13762 rat ascites mammary adenocarcinoma cells, is shed from MAT-B1 (nonxenotransplantable) and MAT-C1 (xenotransplantable) sublines when incubated in vitro after labeling in vivo with [³H]glucosamine. The rates of shedding of label in both particulate and soluble form are similar for the two sublines, but the turnover of label in the cells is 80% greater for MAT-C1 cells ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ 2.4 days) than for MAT-B1 cells ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ 4.1 days). Shed soluble ASGP-1 was smaller than ASGP-1 in the particulate fraction by gel filtration in dodecyl sulfate. By CsCl density gradient centrifugation, gel filtration, and sucrose density gradient centrifugation, all in 4 m guanidine hydrochloride, the shed soluble ASGP-1 was found to be slightly more dense and smaller than ASGP-1 purified from membranes. No differences in sialic acid or oligosaccharides released by alkaline borohydride treatment were found between the shed soluble ASGP-1 and purified ASGP-1. These results suggest that the shed soluble ASGP-1 is released from the membrane by a proteolytic cleavage. This mechanism is supported by the inhibition of the release of soluble shed ASGP-1 by aprotinin, a protease inhibitor. Soluble ASGP-1 in ascites fluid is also smaller by gel filtration, but is more heterogeneous, suggesting a similar release mechanism in vivo followed by more extensive degradation in the ascites fluid.     

Topography and microfilament core association of a cell surface glycoprotein of ascites tumor cell microvilli

Membrane-microfilament interactions are being investigated in microvilli isolated from 13762 rat mammary ascites tumor cells. These microvilli are covered by a sialomucin complex, composed of the sialomucin ascites sialoglycoprotein-1 (ASGP-1) and the associated concanavalin A (Con A)-binding glycoprotein ASGP-2. Limited proteolysis of the microvilli releases large, highly glycosylated fragments of ASGP-1 from the microvilli and increases the association of ASGP-2 with the Triton-insoluble microvillar microfilament core (Vanderpuye OA, Carraway CAC, Carraway, KL: Exp Cell Res 178:211, 1988). To analyze the topography of ASGP-2 in the membrane and its association with the microfilament core, microvilli were treated with proteinase K for timed intervals and centrifuged. The pelleted microvilli were extracted with Triton X-100 for the preparation of microfilament cores and Triton-soluble proteins or with 0.1 M carbonate, pH 11, for the preparation of microvillar membranes depleted of peripheral membrane proteins. These microvilli fractions were analyzed by dodecyl sulfate gel electrophoresis, lectin blotting with Con A and L-phytohemagglutinin, and immunoblotting with anti-ASGP-2. The earliest major proteolysis product from this procedure was a 70 kDa membrane-bound fragment. At longer times a 60 kDa released fragment, 30-40 kDa Triton-soluble fragments, and 25-30 kDa membrane- and microfilament-associated fragments were observed. Phalloidin shift analysis of microfilament-associated proteins on velocity sedimentation gradients indicated that the 25-30 kDa fragments were strongly associated with the microfilament core. From these studies we propose that ASGP-2 has a site for indirect association with the microfilament core near the membrane on a 15-20 kDa segment.     

Actin-associated cell-surface glycoprotein from ascites cell microvilli: a disulfide-linked multimer

Isolated microvilli of the MAT-C1 subline of the 13762 rat mammary adenocarcinoma contain a transmembrane complex composed of a cell surface, cytoskeleton-associated glycoprotein (CAG), actin, and a 58,000-dalton polypeptide (58K). The behavior of CAG has been studied by differential centrifugation and velocity sedimentation gradient centrifugation of detergent extracts of microvilli. CAG can be pelleted along with a fraction of the microvillar actin even in the presence of ionic detergents and under microfilament-depolymerizing conditions. By velocity sedimentation analysis CAG in Triton/PBS extracts sediments as a large, heterogeneous species (sedimentation coefficient greater than 25S). In Sarkosyl and sodium dodecyl sulfate (SDS) the size and heterogeneity are somewhat reduced. In SDS CAG sediments as a 20S species in the absence of mercaptoethanol and as a 5S species in the presence of mercaptoethanol. These results indicate that CAG is a disulfide-linked multimer in the microvillus membrane. We suggest that the stable multimeric structure of CAG permits it to act as the membrane association site for several microfilaments and plays an important role in the formation and stabilization of the microvillus structure.     

Effects of cytoskeletal perturbant drugs on ecto 5''-nucleotidase, a concanavalin A receptor

Differences in cell morphology, concanavalin A-induced receptor redistributions, and the cooperativity of the inhibition of 5'-nucleotidase (AMPase) by concanavalin A (Con A) have been investigated in ascites sublines of the 13762 rat mammary adenocarcinoma cells treated with microfilament- and microtubule-perturbing drugs. By scanning electron microscopy MAT-C1 cells exhibit a highly irregular surface, covered with microvilli extending as branched structures from the cell body. MAT-A, MAT-B, and MAT-B1 cells have a more normal appearance, with unbranched microvilli, ruffles, ridges, and blebs associated closely with the cell body. MAT-C cells have an intermediate morphology. Treatment of MAT-A, MAT-B, or MAT-B1 cells with Con A causes rapid redistribution of Con A receptors. Both cytochalasins and colchicine cause alternations in the receptor redistributions. Receptors on MAT-C1 cells are highly resistant to redistribution, even in the presence of cytoskeletal perturbant drugs. The cooperativity of the inhibition of AMPase by Con A was investigated in MAT-A and MAT-C1 cells. Untreated cells exhibit no cooperativity. If either subline is treated with colchicine, cytochalasin B or D, or dibucaine, cooperativity is observed. Lumicolchicine has no effect. Theophylline or dibutyryl cyclic AMP prevents the effects of either colchicine or cytochalasin. The concentration required for half-maximal induction of cooperativity is 0.3--0.4 microM for both colchicine and cytochalasin D, which is in the appropriate range for specific microtubule and microfilament disruptions. The effectiveness of the cytochalasins (E greater than D greater than B) is consistent with their known effects on microfilaments. No direct correlation was observed between the induction of cooperativity and drug-induced changes in Con A receptor redistribution or cell morphology. The morphology of MAT-A cells is grossly altered by cytochalasins or dibucaine and somewhat less by colchicine. MAT-C1 cells exhibit more minor alterations in morphology as a result of these drug treatments. The results of this study indicate that the inhibition of AMPase, which is a Con A receptor, is a different process from the redistribution of the bulk of the Con A receptors, possibly short range membrane interactions rather than global effects on the cell.     

Membrane-microfilament interactions in ascites tumor cell microvilli. Identification and isolation of a large microfilament-associated membrane glycoprotein complex

[14C]Glucosamine metabolic labeling and concanavalin A blots were used to identify four major glycoprotein species associated with ascites tumor cell microvillar microfilament cores and with a transmembrane complex containing actin. Phalloidin shift analysis of glucosamine-labeled microvilli showed that glycoproteins of 110-120, 80, 65, and 55 kDa are stably associated with the microfilament cores. Analysis of large (greater than 10(6) kDa) transmembrane complexes from microvillar membranes made under microfilament-depolymerizing conditions (Carraway, C. A. C., Jung, G., and Carraway, K. L. (1983) Proc. Natl. Acad. Sci. U. S. A. 80, 430-434) revealed glycoproteins of the same Mr values, showing the same relative staining or labeling patterns as those observed with the microfilament cores. Gel filtration of high salt, high pH extracts of intact microvilli, microfilament cores, or transmembrane complexes showed that in all of these fractions the glycoproteins are associated in a very large, stable complex. The glycoprotein multimer was isolated essentially free of actin and other components by Sephacryl S-1000 chromatography of microvilli, microvillar membranes prepared at pH 11, microfilament cores, or transmembrane complex fractions in Triton X-100, 1 M KCl, glycine, pH 9.5. Purified glycoprotein complex bound actin when incubated under polymerizing conditions. The presence of the glycoprotein heteromultimer in both microfilament cores and transmembrane complex from isolated membranes and the association of the purified glycoprotein complex with actin are consistent with our hypothesis that the glycoprotein-containing transmembrane complex is an association site for microfilaments at the plasma membrane.     

Identification of a cytoskeleton-associated glycoprotein from isolated microvilli of a mammary ascites tumor

Microvilli isolated from MAT-C1 13762 ascites tumor cells were extracted with Triton X-100 in phosphate-buffered saline (PBS) to yield cytoskeletal residues. Analysis of the residues by two-dimensional isoelectric focusing-dodecyl sulfate electrophoresis and silver staining suggested that one of the major components is a glycoprotein (CAG). Neuraminidase treatments and glucosamine labeling demonstrated that CAG is a glycoprotein, and lactoperoxidase iodination showed its presence at the microvillar surface. DNase treatments and myosin affinity analysis suggested an association between CAG and the microvillar microfilaments. Thus, CAG has the properties expected of a transmembrane-linking molecule connecting the cell surface to the cytoskeleton.     

Isolation of microvillar microfilaments and associated transmembrane complex from ascites tumor cell microvilli

The association of microvillar microfilaments with the microvillar membrane actin-containing transmembrane complex of MAT-C1 13762 ascites tumor cell microvilli has been investigated by differential centrifugation, gel electrophoresis and electron microscopy of detergent extracts of the isolated microvilli. Several methods have been used to reduce breakdown and solubilization of the microfilament core actin during the detergent extractions for preparation of microvillar core microfilaments. Gel electrophoresis of differential centrifugation fractions demonstrated that over 70% of the total microvillus actin could be pelleted with microfilament cores at 10 000 g under extraction conditions which reduce filament breakdown. Transmission electron microscopy (TEM) of all of the core preparations showed arrays of microfilaments and small microfilament bundles. The major protein components of the microfilament cores, observed by sodium dodecyl sulfate (SDS) electrophoresis, were actin and alpha-actinin. Among the less prominent polypeptide components was a 58 000 Dalton polypeptide (58 K), previously identified as a member of the MAT-Cl transmembrane complex. This three-component complex contains, in addition to 58 K, actin associated directly and stably with a cell surface glycoprotein (Carraway, CAC, Jung, G & Carraway, K L, Proc. natl acad. sci. US 80 (1983) 430). Evidence that the apparent association of complex with the microfilament core was not due simply to co-sedimentation was provided by myosin affinity precipitation. These results provide further evidence that the transmembrane complex is a site for the interaction of microfilaments with the microvillar plasma membrane.     

Separation of microvilli from Tubifex eggs upon activation: Its inhibition by concanavalin A hinders ooplasmic segregation and cleavage

The surface of mature eggs of the freshwater oligochaete Tubifex exhibits numerous microvilli. Upon activation, microvilli become narrower at their base and separated from the ooplasmic surface. Here it is shown that concanavalin A (Con A) reversibly inhibits the separation of microvilli from activated Tubifex eggs. The Con A-treated eggs undergo meioses and mitoses at a normal rate. Microvilli on these eggs change their length in a meiotic cycle-dependent manner; their core bundles of microfilaments elongate significantly during the second meiosis. The Con A-treated eggs fail to complete polar body formation, ooplasmic segregation and cleavages. Treatment with Con A of eggs that have accomplished microvillar separation does not exert any inhibitory effect on their development. Succinyl-Con A, a dimeric derivative of Con A, does not prevent microvillar separation, suggesting that the tetravalent form of Con A is essential for Con A to exert its inhibitory effect on microvillar separation.     

Strong association of bovine IgM with microvilli and their microfilament core from 13762 ascites tumor cells

Microfilament cores, obtained by extracting 13762 mammary ascites tumor cell microvilli with Triton X-100, contain a major glycoprotein migrating at an apparent molecular weight of 80 kDa by dodecyl sulfate-polyacrylamide gel electrophoresis. The 80-kDa component is a disulfide-linked multimer, as demonstrated by velocity sedimentation and agarose-acrylamide gel electrophoresis analyses under nonreducing conditions. This 80-kDa species is not metabolically labeled, as is a minor 80-kDa glycoprotein found in the cores, membranes, and an isolated transmembrane complex with actin. Antibodies prepared against the 80-kDa glycoprotein react strongly with bovine IgM and more weakly with rat IgM. These antibodies were used to demonstrate that the 80-kDa component is present in microvilli, microvillar microfilament cores, and microvillar membranes only if the microvilli are prepared in the presence of calf serum. The 80-kDa component, purified by velocity sedimentation in dodecyl sulfate, reacts with anti-rat IgM by immunoblot analyses. Moreover, immunoprecipitation of detergent extracts of microvilli with anti-rat IgM specifically sediments the 80-kDa component. The 80-kDa glycoprotein fractionates with the actin-containing transmembrane complex prepared by gel filtration of Triton-solubilized microvillar membranes. These results indicate that the disulfide-linked, multi-meric 80-kDa component is bovine IgM, which binds strongly to a cell-surface component of the microvilli, and is indirectly associated with the microfilament cores. Thus, the IgM provides a marker by which the transmembrane complexes to the microfilaments can be identified.     

Transmembrane modulation of the concanavalin A inhibition of 5'-nucleotidase is not due to a direct association of the enzyme with the cytoskeleton

The inhibition of the cell surface enzyme 5'-nucleotidase by concanavalin A is being studied as a model for understanding transmembrane modulation of cell surface functions. Nucleotidase of 13762 MAT-C1 ascites rat mammary adenocarcinoma cells is inhibited by concanavalin A in a noncooperative process. When cells are treated with the cytoplasmic effectors cytochalasins, colchicine, energy poisons, calcium plus ionophore or hypotonic buffers, the concanavalin A inhibition of the enzyme becomes cooperative. 5'-Nucleotidase of isolated MAT-C1 microvilli is also inhibited by concanavalin A in a noncooperative process; however, treatment of the microvilli with the same cytoplasmic effectors does not induce cooperativity. Since previous studies in several systems have suggested an association of nucleotidase with actin-containing microfilaments or the cell cytoskeleton, one explanation for the cooperativity changes is that they result from a change in the association of the enzyme with the cytoskeleton. However, Triton X-100 extractability of nucleotidase is the same for MAT-C1 cells exhibiting cooperative or noncooperative concanavalin A inhibition. Moreover, enzyme from cells exhibiting cooperative inhibition can be extracted into the zwitterionic detergent Zwittergent in a cooperative form, while enzyme exhibiting noncooperative behavior can be extracted into Zwittergent in a noncooperative form. Gel filtration and rate-zonal sucrose density gradient centrifugation showed little discernible size or sedimentation difference between enzyme samples exhibiting noncooperative and cooperative inhibition. These results indicate that changes in the cooperativity of the concanavalin A inhibition of nucleotidase are not a result of changes in the association of the enzyme with the cytoskeleton. These studies emphasize the caution which must be exercised in interpreting the effects of cytoskeletal perturbants on cell surface functions.     

Transmembrane modulation of the concanavalin A inhibition of 5′-nucleotidase is not due to a direct association of the enzyme with the cytoskeleton

The inhibition of the cell surface enzyme 5′-nucleotidase by concanavalin A is being studied as a model for understanding transmembrane modulation of cell surface functions. Nucleotidase of 13762 MAT-C1 ascites rat mammary adenocarcinoma cells is inhibited by concanavalin A in a noncooperative process. When cells are treated with the cytoplasmic effectors cytochalasins, colchicine, energy poisons, calcium plus ionophore or hypotonic buffers, the concanavalin A inhibition of the enzyme becomes cooperative. 5′-Nucleotidase of isolated MAT-C1 microvilli is also inhibited by concanavalin A in a noncooperative process; however, treatment of the microvilli with the same cytoplasmic effectors does not induce cooperativity. Since previous studies in several systems have suggested an association of nucleotidase with actin-containing microfilaments or the cell cytoskeleton, one explanation for the cooperativity changes is that they result from a change in the association of the enzyme with the cytoskeleton. However, Triton X-100 extractability of nucleotidase is the same for MAT-C1 cells exhibiting cooperative or noncooperative concanavalin A inhibition. Moreover, enzyme from cells exhibiting cooperative inhibition can be extracted into the zwitterionic detergent Zwittergent in a cooperative form, while enzyme exhibiting noncooperative behavior can be extracted into Zwittergent in a noncooperative form. Gel filtration and rate-zonal sucrose density gradient centrifugation showed little discernible size or sedimentation difference between enzyme samples exhibiting noncooperative and cooperative inhibition. These results indicate that changes in the cooperativity of the concanavalin A inhibition of nucleotidase are not a result of changes in the association of the enzyme with the cytoskeleton. These studies emphasize the caution which must be exercised in interpreting the effects of cytoskeletal perturbants on cell surface functions.     

Proteolytic enhancement of gelation of ascites tumor cell extracts. Relationship to actin binding protein

Proteolysis of cytoplasmic extracts of sarcoma 180 and MAT-C1 adenocarcinoma ascites cells enhances the rate of gelation. Only high molecular weight polypeptides, including actin binding protein and myosin, are cleaved during the process; actin is not cleaved. In MAT-B1 adenocarcinoma extracts the gelation rate was not enhanced by proteolysis and actin binding protein was not readily cleaved. Electrophoretic comparisons of trypsin-treated and untreated extracts of MAT-B1 and MAT-C1 cells show that actin binding protein is the only readily discernible polypeptide which is cleaved in the C1 cells but not in the B1 cells. These results suggest that actin binding protein may act as an inhibitor of gelation.     

Light-induced structural changes of cytoskeleton in squid photoreceptor microvilli detected by rapid-freeze method

The cytoskeleton in squid photoreceptor microvilli was studied by freeze-substitution electron microscopy combined with rapid freezing using liquid helium, under dark-adapted and light-illuminated conditions. In the dark-adapted microvilli, actin filaments were regularly associated with granular structures on their surface; these granular structures were cross-linked to the rhodopsin-bearing plasma membranes through slender strands. Upon exposure to light, the granular components detached from the actin filaments, which then appeared to be fragmented and/or depolymerized. These observations have led us to conclude that light stimulation triggers the breakdown of the microvillar actin filament complex in squid photoreceptor cells. The results are discussed with special reference to the physiological role of actin filaments in photoreception.     

ERM (Ezrin/Radixin/Moesin)-based Molecular Mechanism of Microvillar Breakdown at an Early Stage of Apoptosis

Breakdown of microvilli is a common early event in various types of apoptosis, but its molecular mechanism and implications remain unclear. ERM (ezrin/radixin/moesin) proteins are ubiquitously expressed microvillar proteins that are activated in the cytoplasm, translocate to the plasma membrane, and function as general actin filament/plasma membrane cross-linkers to form microvilli. Immunofluorescence microscopic and biochemical analyses revealed that, at the early phase of Fas ligand (FasL)–induced apoptosis in L cells expressing Fas (LHF), ERM proteins translocate from the plasma membranes of microvilli to the cytoplasm concomitant with dephosphorylation. When the FasL-induced dephosphorylation of ERM proteins was suppressed by calyculin A, a serine/threonine protein phosphatase inhibitor, the cytoplasmic translocation of ERM proteins was blocked. The interleukin-1β–converting enzyme (ICE) protease inhibitors suppressed the dephosphorylation as well as the cytoplasmic translocation of ERM proteins. These findings indicate that during FasL-induced apoptosis, the ICE protease cascade was first activated, and then ERM proteins were dephosphorylated followed by their cytoplasmic translocation, i.e., microvillar breakdown. Next, to examine the subsequent events in microvillar breakdown, we prepared DiO-labeled single-layered plasma membranes with the cytoplasmic surface freely exposed from FasL-treated or nontreated LHF cells. On single-layered plasma membranes from nontreated cells, ERM proteins and actin filaments were densely detected, whereas those from FasL-treated cells were free from ERM proteins or actin filaments. We thus concluded that the cytoplasmic translocation of ERM proteins is responsible for the microvillar breakdown at an early phase of apoptosis and that the depletion of ERM proteins from plasma membranes results in the gross dissociation of actin-based cytoskeleton from plasma membranes. The physiological relevance of this ERM protein–based microvillar breakdown in apoptosis will be discussed.     

Membrane-associated actin from the microvillar membranes of ascites tumor cells

A membrane fraction (MF2) has been purified from isolated microvilli of the MAT-C1 subline of the 13762 rat mammary ascites adenocarcinoma under conditions which cause F-actin depolymerization. This membrane preparation contains actin as a major component, although no filamentous structures are observed by transmission electron microscopy. Membranes were extracted with a Triton X-100-containing actin-stabilizing buffer (S buffer) or actin-destabilizing buffer (D buffer). In D buffer greater than 90% of metabolically labeled protein and glycoprotein was extracted, and 80-90% of these labeled species was extracted in S buffer. When S buffer extracts of MF2 were fractionated by either gel filtration on Sepharose 6 B or rate-zonal sucrose density gradient centrifugation, most of the actin was found to be intermediate in size between G- and F-actin. In D buffer most of the MF2 actin behaved as G-actin. Extraction and gel filtration of intact microvilli in S buffer also showed the presence of the intermediate form of actin, indicating that it did not arise during membrane preparation. When [35S]methionine-labeled G-actin from ascites cells was added to S buffer extracts of MF2 and chromatographed, all of the radioactivity chromatographed as G-actin, indicating that the intermediate form of actin did not result from an association of G-actin molecules during extraction or chromatography. The results of this study suggest that the microvillar membrane fraction is enriched in an intermediate form of actin smaller than F-actin and larger than G-actin.