Characterization of a detergent-resistant surface lamina in cultured human fibroblasts

Wheat-germ-agglutinin and Ricinus communis-agglutinin-binding sites of BHK cells compared with each other and with 140 kDa fibronectin receptors

We compared the wheat-germ agglutinin (WGA) and Ricinus communis agglutinin (RCA) binding sites of baby-hamster kidney (BHK) cells. There were 1.01 X 10(8) WGA-binding sites per cell (Kd = 0.027 nM) and 6 X 10(6) RCA-binding sites per cell (Kd = 0.014 nM). Binding of WGA or RCA to BHK cells resulted in more than 75% of the cell-surface binding sites becoming associated with the cytoskeleton (i.e. resistant to extraction with detergent), although no more than 10% of these sites were associated with the cytoskeleton before addition of the lectins. After binding of WGA to the cells, the cell surface was cross-linked so extensively that it remained intact even after detergent extraction of the treated cells, and could be observed by electron microscopy. A similar cross-linking effect did not occur after binding of RCA to cells, which may be because there were so many more binding sites for WGA than for RCA. The composition of WGA- and RCA-binding molecules was analysed by lectin affinity chromatography of metabolically radiolabelled BHK cells. We found that in the WGA-binding-molecule preparations there were eight major polypeptides, ranging in molecular mass from 93 to 340 kDa, and that the RCA-binding molecules were a subpopulation of the WGA-binding molecules. A polyclonal antibody against the 140 kDa fibronectin (FN) receptors of Chinese-hamster ovary (CHO) cells immunoblotted a 145 kDa polypeptide component in both WGA- and RCA-binding-molecule preparations. The results indicated that the 145 kDa component was present in at least two FN-receptor complexes that differed in glycosylation, only one of which was able to bind to RCA affinity columns. The oligomeric nature of the FN-receptor complex, which contained three polypeptides with molecular masses of 120-145 kDa, was demonstrated by using anti-(CHO-cell FN receptor) antibodies to immunoprecipitate extracts prepared from radioiodinated BHK cells.

Agorins: major structural proteins of the plasma membrane skeleton of P815 tumor cells

Plasma membranes of P815 mastocytoma cells contain a set of proteins that remain selectively insoluble upon extraction of the membranes with Triton X-100, and appear to form a membrane skeletal matrix independent of the filamentous cytoskeletal systems. EGTA treatment of the matrix was found to release approximately 25% of the protein as polypeptides of 70, 69, 38, and 36 kD, all of which appear to be peripheral components associated with the cytoplasmic face of the plasma membrane via divalent cation-dependent interactions. About 75% of the total matrix protein was recovered in the EGTA-insoluble fraction. Actin accounted for approximately 5% of the total protein in the EGTA-insoluble fraction. The rest was accounted for by two novel proteins of 20 and 40 kD which, despite their relatively low molecular weights, do not enter SDS PAGE gels. Together these proteins account for approximately 15% of the total plasma membrane protein, and are thus present in much higher amounts than any other characterized protein of nucleated cell plasma membranes. Based on the extensive associations of these proteins to form very large detergent-insoluble structures, we propose that they may be named agorin I, the 20-kD protein, and agorin II, the 40-kD protein, from the Greek agora meaning assembly. The amount and properties of these proteins and the appearance of the EGTA-insoluble material in thin-section electron micrographs indicate that the agorins are the major structural elements of the membrane matrix, and thus of the putative membrane skeleton.

Formation of stress fibres and focal adhesion sites in monensin-exposed cultured human fibroblasts in response to exogenously added cellular fibronectin

Human fibroblasts, plated and cultured in serum-free conditions in the presence of monensin, a carboxylic ionophore, attach and attain a flattened morphology, although fibronectin deposition does not take place. They fail, however, to develop organized stress fibers, vinculin plaques, and focal adhesion sites. In the present study, we demonstrate that the actomyosin-vinculin system of the monensin-exposed cells can be restored by plating the cells on the culture substratum coated with cellular fibronectin. Direct overlay of the electrophoretically separated polypeptides of the cultured fibroblasts with iodinated fibronectin revealed several polypeptides which could represent putative cell-surface receptors for fibronectin.

Triton X-100 extraction of P815 tumor cells: evidence for a plasma membrane skeleton structure

It has been shown that a Triton X-100-insoluble protein matrix can be isolated from the plasma membranes of P815 tumor cells and murine lymphoid cells (Mescher, M. F., M. J. L. Jose and S. P. Balk, 1981, Nature (Lond.), 289:139-144). The properties of the matrix suggested that this set of proteins might form a membrane skeletal structure, stable in the absence of the lipid bilayer. Since purification of plasma membrane results in yields of only 20 to 40%, it was not clear whether the matrix was associated with the entire plasma membrane. To determine if a detergent-insoluble structure was present over the entire cell periphery and stable in the absence of the membrane bilayer or cytoskeletal components, we have examined extraction of whole cells with Triton X-100. Using the same conditions as those used for isolation of the matrix from membranes, we found that extraction of intact cells resulted in structures consisting of a continuous layer of protein at the periphery, a largely empty cytoplasmic space, and a nuclear remnant. Little or no lipid bilayer structure was evident in association with the peripheral layer, and no filamentous cytoskeletal structures could be seen in the cytoplasmic space by thin-section electron microscopy. Analysis of these Triton shells showed them to retain approximately 15% of the total cell protein, most of which was accounted for by low molecular weight nuclear proteins. 5'-Nucleotidase, a cell surface enzyme that remains associated with the plasma membrane matrix, was quantitatively recovered with the shells. Included among the polypeptides present in the shells was a set with mobilities identical to those of the set that makes up the plasma membrane matrix. The polypeptide composition of the shells further confirmed that cytoskeletal proteins were present to a very low extent, if at all, after the extraction. The results demonstrate that a detergent-insoluble protein matrix associated with the periphery of these cells forms a continuous, intact macrostructure whose stability is independent of the membrane bilayer or filamentous cytoskeletal elements, and thus has the properties of a membrane skeletal structure. Although not yet directly demonstrated, the results also strongly suggest that this peripheral layer is composed of the previously described set of plasma membrane matrix proteins. This article discusses possible roles for this proposed membrane skeletal structure in stabilizing the membrane bilayer and affecting the dynamics of other membrane proteins.

Isolation of an actin-binding protein from membranes of Dictyostelium discoideum

We prepared a probe of radiolabeled, glutaraldehyde cross-linked filamentous actin (F-actin) to study binding of actin to membranes of Dictyostelium discoideum. The probe bound to membranes or detergent extracts of membranes with a high affinity and in a saturable manner. The binding could be reduced by boiling of either the actin probe or the membranes, or by addition of excess native F-actin, but not by addition of an equivalent amount of bovine serum albumin, to the assay. The probe labeled several proteins when used to overlay sodium dodecyl sulfate gels of Dictyostelium membranes. One of these labeled proteins was a 24,000-mol-wt protein (p24), which was soluble only in the presence of a high concentration of sodium deoxycholate (5%, wt/vol) at room temperature or above. The p24 was purified by selective detergent extraction and column chromatography. When tested in a novel two-phase binding assay, p24 bound both native monomeric actin (G-actin) and F-actin in a specific manner. In this assay, G-actin bound p24 with a submicromolar affinity.

Changes in cell-substratum adhesion and nuclear-cytoskeletal anchorage during cytodifferentiation of BeWo choriocarcinoma cells

Changes in the substratum anchorage of cells and nuclei were examined during methotrexate (MTX)-induced cytodifferentiation of BeWo human choriocarcinoma cells. During this process cytotrophoblast-like cells (CTLs) transform into giant mono- and multinucleated syncytiotrophoblast-like cells (STLs). Cells treated with MTX for 24 h exhibited significantly faster rates of substratum detachment by EDTA, trypsin-EDTA, EDTA-glycine, and DMSO than did uninduced controls. The decrease in cell-substratum adhesiveness occurred prior to the onset of morphological transformation. By 48 h, when morphological transformation was first observed, there had occurred a marked change in nuclear-cytoskeletal anchorage to the substratum, as evidenced by a difference in sensitivity of Triton-extracted STL and CTL monolayers to detachment by KI. STL monolayers were completely detached within 5 min of exposure to 0.3 M KI, while CTL monolayers remained firmly attached to the substratum for at least 3 h. KI-extracted residues were examined by electron microscopy and found to consist of nuclear shells attached to intermediate filaments. When cytoskeletal residues and KI-extracted proteins of STL and CTL cells were compared by two-dimensional polyacrylamide gel electrophoresis (PAGE), qualitative and quantitative differences were seen in a number of minor components. Thus the sensitivity of STL nuclear-cytoskeletal monolayers to removal by KI, an effective actin depolymerizing agent, may involve changes in the organization, stability, or interactions of actin with other components of the cytoskeletal framework.

Distinct cytoskeletal domains revealed in sperm cells

Antibodies against different cytoskeletal proteins were used to study the cytoskeletal organization of human spermatozoa. A positive staining with actin antibodies was seen in both the acrosomal cap region and the principal piece region of the tail. However, no staining was obtained with nitrobenzoxadiazol-phallacidin, suggesting that most of the actin was in the nonpolymerized form. Most of the myosin immunoreactivity was confirmed to a narrow band in the neck region of spermatozoa. Tubulin was located to the entire tail, whereas vimentin was only seen in a discrete band-like structure encircling the sperm head, apparently coinciding with the equatorial segment region. Surface staining of the spermatozoa with fluorochrome-coupled Helix pomatia agglutinin revealed a similar band-like structure that co-distributed with the vimentin-specific staining. Instead, other lectin conjugates used labeled either the acrosomal cap region (peanut and soybean agglutinins), both the acrosomal cap and the postacrosomal region of the head (concanavalin A), or the whole sperm cell surface membrane (wheat germ and lens culinaris agglutinins and ricinus communis agglutinin l). In lectin blotting experiments, the Helix pomatia agglutinin-binding was assigned to a 80,000-mol-wt polypeptide which, together with vimentin, also resisted treatment with Triton X-100. Only the acrosomal cap and the principal piece of the tail were decorated with rabbit and hydridoma antibodies against an immunoanalogue of erythrocyte alpha-spectrin (p230). p230 appeared to be the major calmodulin-binding polypeptide in spermatozoa, as shown by a direct overlay assay of electrophoretic blots of spermatozoa with 125I-calmodulin. The results indicate that spermatozoa have a highly specialized cytoskeletal organization and that the distribution of actin, spectrin, and vimentin can be correlated with distinct surface specializations of the sperm cells. This suggest that cytoskeleton may regulate the maintenance of these surface assemblies and, hence, affect the spermatozoan function.

140 000 Dalton surface glycoprotein. A plasma membrane component of the detergent-resistant cytoskeletal preparations of cultured human fibroblasts

A 140 000 D glycoprotein (140 kD gp), labelled radioactively with surface-specific techniques, remained as the major cell surface glycoprotein in the detergent-resistant cytoskeletal preparations of cultured human fibroblasts. The 140 kD gp was present also in trypsinized cells and was not affected by treatment of the cells either with collagenase, chymotrypsin or thrombin. In density gradient fractionation of whole cells the 140 kD gp was recovered in the plasma membrane fraction together with small amounts of cytoskeletal components. In fractionation of cytoskeletal preparations, on the other hand, the 140 kD gp could not be dissociated from cytoskeletal proteins and together with vimentin it formed the major component of the oligomeric polypeptide complex generated by treating the surface-labelled cytoskeletal preparations with bifunctional cross-linking reagent, dithiobis succinimidyl propionate (DTPS). Moreover, the 140 kD gp seemed to copurify with vimentin upon reconstitution of intermediate filaments from urea-solubilized cytoskeletal preparations. On the other hand, low ionic-induced degradation of vimentin led to a decrease in the amount of the detergent-resistant 140 kD gp on the cell surface. In electron microscopy, a close apposition between bilayer-like plasma membrane remnants of the adherent cytoskeletons and cytoskeletal elements could be seen. The results indicate that the 140 kD gp is a plasma membrane glycoprotein which closely interacts with the detergent-resistant cytoskeleton of cultured human fibroblast. Possible mechanisms of the association are discussed.