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.

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.

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.

Isolation of a calcium-sensitive, 35,000-dalton microfilament- and liposome-binding protein from ascites tumor cell microvilli: identification as monomeric calpactin

Microvilli isolated from the MAT-C1 ascites subline of the 13762 rat mammary adenocarcinoma contain a major calcium-sensitive microfilament-binding protein, AMV-p35 (ascites microvillar p35). Association of AMV-p35 with microfilament cores during Triton X-100 extraction of the microvilli is half-maximal at 0.1-0.2 mM calcium. The protein, which comprises 6% of the total microvillar protein, can be isolated from microfilament cores prepared in the presence of calcium by extraction with EGTA and purification by ion-exchange chromatography. Alternatively, the protein can be isolated from Triton extracts of microvilli prepared in the absence of calcium by precipitation with calcium, solubilization of the precipitate with EGTA, and chromatography on an ion-exchange column. AMV-p35 binds to phosphatidylserine liposomes and F-actin with half-maximal calcium concentrations of about 10 microM and 0.2 mM, respectively. Treatment of AMV-p35 with chymotrypsin yields a 33,000-dalton fragment, behavior similar to the tyrosine kinase substrates calpactins I and II and lipocortins I and II. Immunoblot analyses using antibodies directed against calpactin I, lipocortin I, and lipocortin II showed strong reactivity of AMV-p35 with anti-calpactin I and anti-lipocortin II, but little reactivity toward anti-lipocortin I. The close relationship between AMV-p35 and calpactin I was verified by amino acid sequence analyses of peptides isolated from cyanogen bromide digests of AMV-p35. By gel filtration and velocity sedimentation analyses purified AMV-p35 is a 35,000-dalton monomer. Moreover, AMV-p35 extracted directly from microvilli in Triton/EGTA also behaves as a 35,000-dalton menomer. These findings indicate that AMV-p35 is closely related to the pp60src kinase substrate calpactin I (p36). However, AMV-p35 occurs in the microvilli as a monomer rather than as the heterotetrameric calpactin found in several other cell types.

Structural features of absorptive cell and microvillus membrane preparations from rat small intestine

Absorptive cells of the small intestine are highly polarized cells with distinct microvillus membrane (MVM) and basolateral plasma membrane domains. We compared membrane structure in the following preparations of rat small intestine commonly used for in vitro study of MVM function: epithelial sheets, isolated epithelial cells, and four different MVM vesicle preparations, using electron microscopy of thin sections and freeze fracture replicas. We also quantitated mean vesicle diameter of the four MVM preparations by quasielastic light scattering and determined their actin content. Epithelial sheets maintained their plasma membrane polarity as judged by intramembrane particle (IMP) distribution for at least 30 min after isolation. In contrast, the plasma membrane of isolated cells showed redistribution of IMPs, indicating considerable loss of polarity in the few minutes required for cell recovery. The P-face IMPs in MVM prepared by Ca++ precipitation were randomly distributed but became aggregated after exposure to potassium thiocyanate, which removed approximately 50% of core actin. The P-face IMPs in Mg++ precipitated MVM were aggregated whether or not core actin was depleted with potassium thiocyanate. The shape and size of MVM vesicles differed considerably with different preparative techniques. The extremely rapid loss of plasma membrane polarity of isolated intestinal epithelial cells and the striking structural heterogeneity of MVM vesicles prepared by commonly used techniques should be considered in the interpretation of functional studies with these preparations.

Demonstration of the association of the cell-surface enzyme, 5'-nucleotidase, with microvillar microfilaments by phalloidin shift on velocity sedimentation gradients

The cell-surface enzyme 5'-nucleotidase in microvilli from 13762 rat mammary adenocarcinoma cells remains largely associated with microfilament-containing high-speed pellets from Triton X-100 extracts of the microvilli. The fraction remaining with the insoluble portion is higher under ionic conditions which enhance microfilament stability. To minimize trapping and cosedimentation we have analyzed the distribution of microfilaments and 5'-nucleotidase activity on velocity sedimentation sucrose gradients of the microvillar extracts. A large fraction of the total enzyme activity is found in the filament fractions in the middle of the gradient. When phalloidin is included in the extraction buffer to stabilize the microfilaments, both the microfilaments and the bulk of the nucleotidase activity are shifted further into the gradients. Both the position of the filament fraction and the percentage of the total nucleotidase activity remaining with the filament fraction varies with extraction buffer composition and conditions. Nonetheless, under all conditions tested, a large percentage of the activity was shifted, along with the microfilaments, in the presence of phalloidin. These results are consistent with a specific association of 5'-nucleotidase with microfilaments in the ascites tumor cell microvilli.

Phalloidin shift on velocity sedimentation sucrose gradient centrifugation for identification of microfilament-associated proteins

Velocity sedimentation by sucrose density gradient centrifugation has been used to characterize ascites microvillar microfilament cores and to identify microfilament-associated proteins. Fluoride, calcium, phalloidin and chemical cross-linking treatments of microvilli during Triton X-100 extractions increase the sedimentation rate of the microfilament core, compared with untreated control samples. Electrophoretic analyses of the distributions of actin, alpha-actinin and other microfilament-associated proteins across the gradients indicate that the primary mechanism for stabilization of the microfilament core is the reduction of fragmentation of the microfilaments. Significantly, alpha-actinin could be completely removed from the microfilaments by calcium treatment without causing a decrease in the size of the microfilament core. Because of the specificity of phalloidin in the stabilization of microfilaments, the shift on the gradients of microfilaments and their associated proteins in the presence of phalloidin provides a diagnostic tool for the identification of microfilament-associated proteins. This phalloidin shift technique should have widespread utility in the analysis of actin forms and microfilament-associated proteins from complex cell fractions.

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.

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.