A membrane cytoskeleton from Dictyostelium discoideum. III. Plasma membrane fragments bind predominantly to the sides of actin filaments

Ponticulin is an atypical membrane protein

We have cloned and sequenced ponticulin, a 17,000-dalton integral membrane glycoprotein that binds F-actin and nucleates actin assembly. A single copy gene encodes a developmentally regulated message that is high during growth and early development, but drops precipitously during cell streaming at approximately 8 h of development. The deduced amino acid sequence predicts a protein with a cleaved NH2-terminal signal sequence and a COOH-terminal glycosyl anchor. These predictions are supported by amino acid sequencing of mature ponticulin and metabolic labeling with glycosyl anchor components. Although no alpha-helical membrane-spanning domains are apparent, several hydrophobic and/or sided beta-strands, each long enough to traverse the membrane, are predicted. Although its location on the primary sequence is unclear, an intracellular domain is indicated by the existence of a discontinuous epitope that is accessible to antibody in plasma membranes and permeabilized cells, but not in intact cells. Such a cytoplasmically oriented domain also is required for the demonstrated role of ponticulin in binding actin to the plasma membrane in vivo and in vitro (Hitt, A. L., J. H. Hartwig, and E. J. Luna. 1994. Ponticulin is the major high affinity link between the plasma membrane and the cortical actin network in Dictyostelium. J. Cell Biol. 126:1433-1444). Thus, ponticulin apparently represents a new category of integral membrane proteins that consists of proteins with both a glycosyl anchor and membrane-spanning peptide domain(s).

Ponticulin is the major high affinity link between the plasma membrane and the cortical actin network in Dictyostelium

Interactions between the plasma membrane and underlying actin-based cortex have been implicated in membrane organization and stability, the control of cell shape, and various motile processes. To ascertain the function of high affinity actin-membrane associations, we have disrupted by homologous recombination the gene encoding ponticulin, the major high affinity actin-membrane link in Dictyostelium discoideum amoebae. Cells lacking detectable amounts of ponticulin message and protein also are deficient in high affinity actin-membrane binding by several criteria. First, only 10-13% as much endogenous actin cosediments through sucrose and crude plasma membranes from ponticulin-minus cells, as compared with membranes from the parental strain. Second, purified plasma membranes exhibit little or no binding or nucleation of exogenous actin in vitro. Finally, only 10-30% as much endogenous actin partitions with plasma membranes from ponticulin-minus cells after these cells are mechanically unroofed with polylysine-coated coverslips. The loss of the cell's major actin-binding membrane protein appears to be surprisingly benign under laboratory conditions. Ponticulin-minus cells grow normally in axenic culture and pinocytose FITC-dextran at the same rate as do parental cells. The rate of phagocytosis of particles by ponticulin-minus cells in growth media also is unaffected. By contrast, after initiation of development, cells lacking ponticulin aggregate faster than the parental cells. Subsequent morphogenesis proceeds asynchronously, but viable spores can form. These results indicate that ponticulin is not required for cellular translocation, but apparently plays a role in cell patterning during development.

Association of the Dictyostelium 30 kDa actin bundling protein with contact regions

‘Contact regions’ are plasma membrane domains derived from areas of intercellular contact between aggregating Dictyostelium amebae (H.M. Ingalls et al. (1986). Proc. Nat. Acad. Sci. USA 83, 4779). Purified contact regions contain a prominent actin-binding protein with an M(r) of 34,000. Immunoblotting with monoclonal antibodies identifies this polypeptide as a 34,000 M(r) actin-bundling protein (known as 30 kDa protein), previously shown to be enriched in filopodia (M. Fechheimer (1987). J. Cell Biol. 104, 1539). About four times more 30 kDa protein by mass is associated with contact regions than is found in total plasma membranes isolated from aggregating cells. In agreement with these observations, immunostaining of the 30 kDa protein in aggregating cells reveals a prominent localization along the plasma membrane at sites of intercellular contact. By contrast, alpha-actinin does not appear to be significantly enriched at sites of cell to cell contact. Binding experiments using purified plasma membranes, actin and 30 kDa protein indicate that the 30 kDa protein is associated with the plasma membrane primarily through interactions with actin filaments. Calcium ions are known to decrease the interaction of actin with 30 kDa protein in solution. Surprisingly, membrane-associated complexes of actin and the 30 kDa protein are much less sensitive to dissociation by micromolar levels of free calcium ions than are complexes in solutions lacking membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of actin to liver cell membranes: the state of membrane-bound actin

Previous work has shown that actin binds specifically and saturably to liver membranes stripped of endogenous actin (Tranter, M. P., S. P. Sugrue, and M. A. Schwartz. 1989. J. Cell Biol. 109:2833-2840). Scatchard plots of equilibrium binding data were linear, indicating that binding is not cooperative, as would be expected for F- or G-actin. To determine the state of membrane-bound actin, we have analyzed the binding of F- and G-actin to liver cell membranes. G-actin in low salt depolymerization buffer and EF-actin, a derivative that polymerizes very poorly in solution, bind to liver cell membranes as well as untreated actin in polymerization buffer. Phalloidin-stabilized F-actin binds, but to a lesser extent. The binding of F- and G-actins are mutually competitive and are inhibited by ATP, suggesting that both forms of actin bind to the same sites. For untreated actin in polymerization buffer, the time course of binding is biphasic, with an initial rapid component which is followed by a plateau phase, then a second, slower component. The binding kinetics of pure F-actin and pure G-actin are both monophasic and match the fast and slower components, respectively, of untreated actin. In the reconstituted system, membrane-bound actin does not stain with rhodamine-phalloidin, nor are actin filaments detected by EM. Distinct regions of amorphous material, however, are visible, which stain with an anti-actin antibody. The exact nature of this material has yet to be determined. A model of actin binding is presented.

Direct binding of F-actin to ponticulin, an integral plasma membrane glycoprotein

We have developed an 125I-labeled F-actin blot overlay assay for the identification of F-actin-binding proteins after transfer to nitrocellulose from SDS-polyacrylamide gels. Two major F-actin-binding proteins from Dictyostelium discoideum, a cytoplasmic 30 kDa protein and a 17 kDa integral membrane protein, and two minor membrane polypeptides of 19 kDa and 15 kDa were detected by this method. Using F-actin affinity and immunoaffinity chromatography, the 17 kDa polypeptide was identified as ponticulin, a previously described actin-binding glycoprotein from D. discoideum plasma membranes (Wuestehube, L.J., and Luna, E.J., [1987]: J. Cell Biol. 105:1741-1751). The binding of F-actin to ponticulin on blots is specific because unlabeled F-actin competes with 125I-labeled F-actin and because G-actin does not bind. Nitrocellulose-bound ponticulin displays binding characteristics similar to those of purified plasma membranes in solution, e.g., F-actin binding is sensitive to high salt and to elevated temperatures. Under optimal conditions, 125-I-labeled F-actin blot overlays are at least as sensitive as are immunoblots with an antibody specific for ponticulin. When blotted onto nitrocellulose after 2-D gel electrophoresis, all isoforms of ponticulin and of the 19 kDa and 15 kDa polypeptides appear to bind F-actin in proportion to their abundance. Thus the actin-binding activies of these proteins do not appear to be regulated by modifications that affect isoelectric point. However, the actin-binding activity of nitrocellulose-bound ponticulin is diminished when the protein is exposed to reducing agents, suggesting an involvement of disulfide bond(s) in ponticulin function. The 125I-labeled F-actin blot overlay assay also may enable us to identify F-actin-binding proteins in other cell types and should provide a convenient method for monitoring the purification of these proteins.

Dictyostelium discoideum plasma membranes contain an actin-nucleating activity that requires ponticulin, an integral membrane glycoprotein

In previous equilibrium binding studies, Dictyostelium discoideum plasma membranes have been shown to bind actin and to recruit actin into filaments at the membrane surface. However, little is known about the kinetic pathway(s) through which actin assembles at these, or other, membranes. We have used actin fluorescently labeled with N-(1-pyrenyl)iodoacetamide to examine the kinetics of actin assembly in the presence of D. discoideum plasma membranes. We find that these membranes increase the rate of actin polymerization. The rate of membrane-mediated actin polymerization is linearly dependent on membrane protein concentrations up to 20 micrograms/ml. Nucleation (the association of activated actin monomers into oligomers) appears to be the primary step of polymerization that is accelerated. A sole effect on the initial salt-induced actin conformational change (activation) is ruled out because membranes accelerate the polymerization of pre-activated actin as well as actin activated in the presence of membranes. Elongation of preexisting filaments also is not the major step of polymerization facilitated by membranes since membranes stripped of all peripheral components, including actin, increase the rate of actin assembly to about the same extent as do membranes containing small amounts of endogenous actin. Acceleration of the nucleation step by membranes also is supported by an analysis of the dependence of polymerization lag time on actin concentration. The barbed ends of membrane-induced actin nuclei are not obstructed by the membranes because the barbed end blocking agent, cytochalasin D, reduces the rate of membrane-mediated actin nucleation. Similarly, the pointed ends of the nuclei are not blocked by membranes since the depolymerization rate of gelsolin-capped actin is unchanged in the presence of membranes. These results are consistent with previous observations of lateral interactions between membranes and actin filaments. These results also are consistent with two predictions from a model based on equilibrium binding studies; i.e., that plasma membranes should nucleate actin assembly and that membrane-bound actin nuclei should have both ends free (Schwartz, M. A., and E. J. Luna. 1988. J. Cell Biol. 107:201-209). Integral membrane proteins mediate the actin nucleation activity because activity is eliminated by heat denaturation, treatment with reducing agents, or proteolysis of membranes. Activity also is abolished by solubilization with octylglucoside but is reconstituted upon removal or dilution of the detergent. Ponticulin, the major actin-binding protein in plasma membranes, appears to be necessary for nucleation activity since activity is not reconstituted from detergent extracts depleted of ponticulin.

Ponticulin, a developmentally-regulated plasma membrane glycoprotein, mediates actin binding and nucleation

Ponticulin is a 17,000-dalton transmembrane glycoprotein that is involved in the binding and nucleation of actin filaments by Dictyostelium discoideum plasma membranes. The major actin-binding protein isolated from these membranes by F-actin affinity chromatography, ponticulin also binds F-actin on blot overlays. The actin-binding activity of ponticulin in vitro is identical to that observed for purified plasma membranes: it resists extraction with 0.1 N NaOH, is sensitive to high salt concentrations, and is destroyed by heat, proteolysis, and thiol reduction and alkylation. A cytoplasmic domain of ponticulin mediates binding to actin because univalent antibody fragments directed against the cytoplasmic surface of this protein inhibit 96% of the actin-membrane binding in sedimentation assays. Antibody specific for ponticulin removes both ponticulin and the ability to reconstitute actin nucleation activity from detergent extracts of solubilized plasma membranes. Levels of plasma membrane ponticulin increase 2- to 3-fold during aggregation streaming, when cells adhere to each other and are highly motile. Although present throughout the plasma membrane, ponticulin is preferentially localized to some actin-rich membrane structures, including sites of cell-cell adhesion and arched regions of the plasma membrane reminiscent of the early stages of pseudopod formation. Ponticulin also is present but not obviously enriched at phagocytic cups of log-phase amebae. These results indicate that ponticulin may function in vivo to attach and nucleate actin filaments at the cytoplasmic surface of the plasma membrane. A 17,000-dalton analogue of ponticulin has been identified in human polymorphonuclear leukocyte plasma membranes by immunoblotting and immunofluorescence microscopy.(ABSTRACT TRUNCATED AT 250 WORDS)

Actin-associated proteins in Dictyostelium discoideum

The cellular slime mold Dictyostelium discoideum is becoming the premier system for the explication of the biochemical and cellular events that occur during motile processes. Proteins associated with the actin cytoskeleton, in particular, appear to play key roles in cellular responses to many external stimuli. This review summarizes our present understanding of the actin-associated proteins in Dictyostelium, including their in vitro activities and their structural and/or functional analogues in mammalian cells.

Indirect immunofluorescence localization of ponticulin in motile cells

Ponticulin is the major actin-binding integral glycoprotein in plasma membranes isolated from log-phase Dictyostelium discoideum amebae. As such, this protein appears to be an important link between the plasma membrane and actin filaments (Wuestehube and Luna: Journal of Cell Biology 105:1741-1751, 1987). In this study, indirect immunofluorescence microscopy was used to examine the distribution of ponticulin in randomly moving D. discoideum amebae and in amebae engaged in cell migration and phagocytosis. Ponticulin is distributed throughout the plasma membrane and also is present in intracellular vesicles associated with the microtubule-organizing center-Golgi complex adjacent to the nucleus. In aggregating amebae, ponticulin is concentrated in regions of lateral cell-cell contact and in arched regions of the plasma membrane. Ponticulin also is present, but not obviously enriched, in filopodia, in the actin-rich anterior end of polarized cells, and in detergent-insoluble cytoskeletons. In amebae engaged in phagocytosis of yeast, ponticulin is present but not enriched in phagocytic cups and is associated with intracellular vesicles around engulfed yeast. These results suggest that ponticulin is stably associated with actin filaments in certain regions of the plasma membrane and that the actin-binding activity of ponticulin may be tightly controlled. Indirect immunofluorescence microscopy and immunoblot analysis demonstrate that human polymorphonuclear leukocytes also contain a 17 kD protein that specifically cross-reacts with antibodies affinity-purified against D. discoideum ponticulin. As in D. discoideum, the mammalian 17 kD ponticulin-analog appears to be localized in plasma membrane and is evident in actin-rich cell extensions. These results indicate that ponticulin-mediated linkages between the plasma membrane and actin may be present in higher eukaryotic cells.

How actin binds and assembles onto plasma membranes from Dictyostelium discoideum

We have shown previously (Schwartz, M. A., and E. J. Luna. 1986. J. Cell Biol. 102: 2067-2075) that actin binds with positive cooperativity to plasma membranes from Dictyostelium discoideum. Actin is polymerized at the membrane surface even at concentrations well below the critical concentration for polymerization in solution. Low salt buffer that blocks actin polymerization in solution also prevents actin binding to membranes. To further explore the relationship between actin polymerization and binding to membranes, we prepared four chemically modified actins that appear to be incapable of polymerizing in solution. Three of these derivatives also lost their ability to bind to membranes. The fourth derivative (EF actin), in which histidine-40 is labeled with ethoxyformic anhydride, binds to membranes with reduced affinity. Binding curves exhibit positive cooperativity, and cross-linking experiments show that membrane-bound actin is multimeric. Thus, binding and polymerization are tightly coupled, and the ability of these membranes to polymerize actin is dramatically demonstrated. EF actin coassembles weakly with untreated actin in solution, but coassembles well on membranes. Binding by untreated actin and EF actin are mutually competitive, indicating that they bind to the same membrane sites. Hill plots indicate that an actin trimer is the minimum assembly state required for tight binding to membranes. The best explanation for our data is a model in which actin oligomers assemble by binding to clustered membrane sites with successive monomers on one side of the actin filament bound to the membrane. Individual binding affinities are expected to be low, but the overall actin-membrane avidity is high, due to multivalency. Our results imply that extracellular factors that cluster membrane proteins may create sites for the formation of actin nuclei and thus trigger actin polymerization in the cell.

Actin polymerization and pseudopod extension during amoeboid chemotaxis

Amoebae of the cellular slime mold Dictyostelium discoideum are an excellent model system for the study of amoeboid chemotaxis. These cells can be studied as a homogeneous population whose response to chemotactic stimulation is sufficiently synchronous to permit the correlation of the changes in cell shape and biochemical events during chemotaxis. Having demonstrated this synchrony of response, we show that actin polymerization occurs in two stages during stimulation with chemoattractants. The assembly of F-actin that peaks between 40 and 60 sec after the onset of stimulation is temporally correlated with the growth of new pseudopods. F-actin, which is assembled by 60 sec after stimulation begins, is localized in the new pseudopods that are extended at this time. Both stages of actin polymerization during chemotactic stimulation involve polymerization at the barbed ends of actin filaments based on the cytochalasin sensitivity of this response. We present a hypothesis in which actin polymerization is one of the major driving forces for pseudopod extension during chemotaxis. The predictions of this model, that localized regulation of actin nucleation activity and actin filament cross-linking must occur, are discussed in the context of current models for signal transduction and of recent information regarding the types of actin-binding proteins that are present in the cell cortex.

F-actin binds to the cytoplasmic surface of ponticulin, a 17-kD integral glycoprotein from Dictyostelium discoideum plasma membranes

F-actin affinity chromatography and immunological techniques are used to identify actin-binding proteins in purified Dictyostelium discoideum plasma membranes. A 17-kD integral glycoprotein (gp17) consistently elutes from F-actin columns as the major actin-binding protein under a variety of experimental conditions. The actin-binding activity of gp17 is identical to that of intact plasma membranes: it resists extraction with 0.1 N NaOH, 1 mM dithiothreitol (DTT); it is sensitive to ionic conditions; it is stable over a wide range of pH; and it is eliminated by proteolysis, denaturation with heat, or treatment with DTT and N-ethylmaleimide. gp17 may be responsible for much of the actin-binding activity of plasma membranes since monovalent antibody fragments (Fab) directed primarily against gp17 inhibit actin-membrane binding by 96% in sedimentation assays. In contrast, Fab directed against cell surface determinants inhibit binding by only 0-10%. The actin-binding site of gp17 appears to be located on the cytoplasmic surface of the membrane since Fab against this protein continue to inhibit 96% of actin-membrane binding even after extensive adsorption against cell surfaces. gp17 is abundant in the plasma membrane, constituting 0.4-1.0% of the total membrane protein. A transmembrane orientation of gp17 is suggested since, in addition to the cytoplasmic localization of the actin-binding site, extracellular determinants of gp17 are identified. gp17 is surface-labeled by sulfo-N-hydroxy-succinimido-biotin, a reagent that cannot penetrate the cell membrane. Also, gp17 is glycosylated since it is specifically bound by the lectin, concanavalin A. We propose that gp17 is a major actin-binding protein that is important for connecting the plasma membrane to the underlying microfilament network. Therefore, we have named this protein "ponticulin" from the Latin word, ponticulus, which means small bridge.

Calcium-sensitive, lipid-binding cytoskeletal proteins of the human placental microvillar region

In this study we describe a group of Ca2+-sensitive proteins located in the microvillar region of the human placental syncytiotrophoblast. By following the distribution of proteins between the particulate and supernatant phases of detergent-solubilized microvilli in the presence of defined concentrations of free Ca2+, we demonstrate a class of proteins of subunit molecular weights 72,000, 69,000, 38,000, 36,000, and 32,000 that associate with both the cytoskeleton and lipid at high concentrations of free Ca2+. These proteins can be released from microvilli using EGTA-containing buffers. Although they do not bind to phenyl-Sepharose, they will bind to phospholipids immobilized on phenyl-Sepharose columns in a Ca2+-dependent manner and show a marked preference for phospholipids with negatively charged headgroups. The results provide evidence for a sequence of events which may occur within the microvillus as the localized concentration of intracellular free Ca2+ rises.

Cell surface proteoglycan associates with the cytoskeleton at the basolateral cell surface of mouse mammary epithelial cells

The cell surface proteoglycan on normal murine mammary gland mouse mammary epithelial cells consists of an ectodomain bearing heparan and chondroitin sulfate chains and a lipophilic domain that is presumed to be intercalated into the plasma membrane. Because the ectodomain binds to matrix components produced by stromal cells with specificity and high affinity, we have proposed that the cell surface proteoglycan is a matrix receptor that binds epithelial cells to their underlying basement membrane. We now show that the proteoglycan surrounds cells grown in subconfluent or newly confluent monolayers, but becomes restricted to the basolateral surface of cells that have been confluent for a week or more; Triton X-100 extraction distinguishes three fractions of cell surface proteoglycan: a fraction released by detergent and presumed to be free in the membrane, a fraction bound via a salt-labile linkage, and a nonextractable fraction; the latter two fractions co-localize with actin filament bundles at the basal cell surface; and when proteoglycans at the apical cell surface are cross-linked by antibodies, they initially assimilate into detergent-resistant, immobile clusters that are subsequently aggregated by the cytoskeleton. These findings suggest that the proteoglycan, initially present on the entire surface and free in the plane of the membrane, becomes sequestered at the basolateral cell surface and bound to the actin-rich cytoskeleton as the cells become polarized in vitro. Binding of matrix components may cross-link proteoglycans at the basal cell surface and cause them to associate with the actin cytoskeleton, providing a mechanism by which the cell surface proteoglycan acts as a matrix receptor to stabilize the morphology of epithelial sheets.

Binding and assembly of actin filaments by plasma membranes from Dictyostelium discoideum

The binding of native, 125I-Bolton-Hunter-labeled actin to purified Dictyostelium discoideum plasma membranes was measured using a sedimentation assay. Binding was saturable only in the presence of the actin capping protein, gelsolin. In the presence of gelsolin, the amount of actin bound at saturation to three different membrane preparations was 80, 120, and 200 micrograms/mg of membrane protein. The respective concentrations of actin at half-saturation were 8, 12, and 18 micrograms/ml. The binding curves were sigmoidal, indicating positive cooperativity at low actin concentrations. This cooperativity appeared to be due to actin-actin associations during polymerization, since phalloidin converted the curve to a hyperbolic shape. In kinetic experiments, actin added as monomers bound to membranes at a rate of 0.6 microgram ml-1 min-1, while pre-polymerized actin bound at a rate of 3.0 micrograms ml-1 min-1. Even in the absence of phalloidin, actin bound to membranes at concentrations well below the normal critical concentration. This membrane-bound actin stained with rhodamine-phalloidin and was cross-linked by m-maleimidobenzoyl succinimide ester, a bifunctional cross-linker, into multimers with the same pattern observed for cross-linked F-actin. We conclude that D. discoideum plasma membranes bind actin specifically and saturably and that these membranes organize actin into filaments below the normal critical concentration for polymerization. This interaction probably occurs between multiple binding sites on the membrane and the side of the actin filament, and may be related to the clustering of membrane proteins.

Chemotaxis and cell motility in the cellular slime molds

Chemotaxis and cell motility have essential roles to play throughout the developmental cycle of the cellular slime molds. The particular emphasis of this review, however, will be on the amoeboid stages of the life cycle. The nature of the chemoattractants and their detection will be discussed as will the possible mechanisms that may account for the directed locomotion of amoebae. Intracellular chemoattractant-elicited molecular responses thought to play a role in transduction of extracellular signals into a motility response will also be examined. Furthermore, relationships of these transduction pathway components with changes in assembly states of the cytoskeletal proteins contributing to shape change and cell movement will be assessed. Theories of amoeboid movement involving these cytoskeletal proteins will be compared and discussed in terms of their relevance to cellular slime mold motility.

An integral glycoprotein associated with the membrane attachment sites of actin microfilaments

An integral membrane protein associated with sites of microfilament-membrane attachment has been identified by a newly developed IgG1 monoclonal antibody. This antibody, MAb 30B6, was derived from hybridoma fusion experiments using intact mitotic cells of chick embryo fibroblasts as the immunization vehicle as well as the screening probe for cell surface antigens. In immunofluorescent experiments with fixed cells, MAb 30B6 surface labeling is uniquely correlated with microfilament distributions in the cleavage furrow region of dividing chick embryo fibroblasts and cardiac myocytes in culture. The MAb 30B6 antigen in addition is associated with microfilament-membrane attachment sites in interphase fibroblasts at the dorsal surface, the adhesion plaque region at the ventral surface, and at junction-like regions of cell-cell contact. It is also found co-localized with the membrane-dense plaques of smooth muscle. The MAb 30B6 antigen is expressed in a wide number of chicken cell types (particularly smooth muscle cells, platelets, and endothelial cells), but not in erythrocytes. Some of the molecular characteristics of the MAb 30B6 antigen have been determined from immunoblotting, immunoaffinity chromatography, immunoprecipitation, cell extraction, and charge shift electrophoresis experiments. It is an integral sialoglycoprotein with an apparent molecular mass of 130 kD (reduced form)/107 kD (nonreduced form) in SDS PAGE. Another prominent glycoprotein species with an apparent molecular mass of 175 kD (reduced form)/165 kD (nonreduced form) in SDS PAGE is co-isolated on MAb 30B6 affinity columns, but appears to be antigenically distinct since it is not recognized by MAb 30B6 in immunoblotting or immunoprecipitation experiments. By virtue of its surface distributions relative to actin microfilaments and its integral protein character, we propose that the MAb 30B6 antigen is an excellent candidate for the function of directly or indirectly anchoring microfilaments to the membrane.

Ligand-induced changes in the location of actin, myosin, 95K (alpha-actinin), and 120K protein in amebae of Dictyostelium discoideum

In this study we investigated concanavalin A (Con A) induced changes in the locations of actin, myosin, 120K, and 95K (alpha-actinin) to determine the extent to which actin and myosin are reorganized during capping and the roles that 120K and 95K might play in this reorganization. We observed the location of each protein by indirect immunofluorescence using affinity purified antibodies. Four morphological states were distinguished in vegetative Dictyostelium amebae: ameboid cells before Con A binding, patched cells, capped cells, and ameboid cells with caps. The location of each protein was distinct in ameboid cells both before and after capping Actin and 120K were found in the cell cortex usually associated with surface projections, and myosin and 95K were diffusely distributed. Myosin was excluded from surface projections in ameboid cells. During patching, all four proteins were localized below Con A patches. During capping, actin, myosin, and 95K protein moved with the Con A patches into the cap whereas 120K protein was excluded from the cap. During the late stages of cap formation actin and myosin were progressively lost from the cap, and 120K became concentrated in new actin-filled projections that formed away from the cap. However, 95K remained tightly associated with the cap. Poisoning cells with sodium azide inhibited capping but not patching of ligand. In azide-poisoned cells, myosin and 95K did not co-patch with Con A, whereas copatching of 120K and actin with Con A occurred as usual. Our results support the hypothesis that capping is an actomyosin-mediated motile event that involves a sliding interaction between actin filaments, which are anchored through the membrane to ligand patches, and myosin in the cortex. They are also consistent with a role for 120K in the formation of surface projections by promoting growth and/or cross-linking of actin filaments within projections, and with a role for 95K in regulating actomyosin-mediated contractility, earlier proposals based on the in vitro properties of these two proteins (Condeelis, J., M. Vahey, J. M. Carboni, J. DeMey, S. Ogihara, 1984, J. Cell Biol., 99:119s-126s).

Decoration with myosin subfragment-1 disrupts contacts between microfilaments and the cell membrane in isolated Dictyostelium cortices

We used isolated cortices from ameboid cells of Dictyostelium discoideum to examine the structural nature of attachments between microfilaments and the cell membrane and to determine the effect of myosin subfragment-1 (S-1) on such contacts. By varying several parameters in our previously described isolation procedure (Condeelis, J., 1979. J. Cell Biol., 80:751-758), we have improved this procedure and have been able to isolate stable cortices. In this paper we identify two types of contact sites between microfilaments and the cell membrane similar to those seen in the brush border of intestinal epithelial cells: (a) an end-on attachment between the barbed end of actin filaments and the cell membrane; and (b) a lateral attachment mediated by rod-shaped bridges measuring approximately 6 X 15 nm. The spacing between bridges averages 36 nm, which suggests that the helical twist of the actin filament influences bridge location. Together these contacts account for an average of approximately 25,000 attachments per cell. Incubation of cortices with concentrations of S-1 sufficient to saturate binding sites on the microfilaments caused disruption of the contacts. This observation was confirmed by quantitative morphometry to show a threefold loss in the number of contact sites following S-1 decoration. These results indicate that S-1 decoration should be used with caution when information about the precise location of microfilaments and their attachment to the membrane is required.

A membrane cytoskeleton from Dictyostelium discoideum. II. Integral proteins mediate the binding of plasma membranes to F-actin affinity beads

In novel, low-speed sedimentation assays, highly purified, sonicated Dictyostelium discoideum plasma membrane fragments bind to F-actin beads (fluorescein-labeled F-actin on antifluorescein IgG-Sephacryl S-1000 beads). Binding was found to be (a) specific, since beads containing bound fluorescein-labeled ovalbumin or beads without bound fluorescein-labeled protein do not bind membranes, (b) saturable at approximately 0.6 microgram of membrane protein per microgram of bead-bound F-actin, (c) rapid with a t1/2 of 4-20 min, and (d) apparently of reasonable affinity since the off rate is too slow to be measured by present techniques. Using low-speed sedimentation assays, we found that sonicated plasma membrane fragments, after extraction with chaotropes, still bind F-actin beads. Heat-denatured membranes, proteolyzed membranes, and D. discoideum lipid vesicles did not bind F-actin beads. These results indicate that integral membrane proteins are responsible for the binding between sonicated membrane fragments and F-actin on beads. This finding agrees with the previous observation that integral proteins mediate interactions between D. discoideum plasma membranes and F-actin in solution (Luna, E.J., V. M. Fowler, J. Swanson, D. Branton, and D. L. Taylor, 1981, J. Cell Biol., 88:396-409). We conclude that low-speed sedimentation assays using F-actin beads are a reliable method for monitoring the associations between F-actin and membranes. Since these assays are relatively quantitative and require only micrograms of membranes and F-actin, they are a significant improvement over other existing techniques for exploring the biochemical details of F-actin-membrane interactions. Using F-actin beads as an affinity column for actin-binding proteins, we show that at least 12 integral polypeptides in D. discoideum plasma membranes bind to F-actin directly or indirectly. At least four of these polypeptides appear to span the membrane and are thus candidates for direct transmembrane links between the cytoskeleton and the cell surface.