Actin associated with membranes from 3T3 mouse fibroblast and HeLa cells

Human renal fibroblast contraction of collagen I lattices is an integrin-mediated process

BACKGROUND

Expression of the beta1 family of integrins allows dermal fibroblasts in wounds to contribute to the healing process through migration, adhesion, synthesis, and rearrangement of extracellular matrix. To date the ability of human renal fibroblasts to reorganize collagens and the role of cell surface receptors in this process remain unknown.

METHODS

Renal fibroblasts were grown from the cortical tissue of surgically removed human kidneys. The ability of human renal fibroblasts to reorganize interstitial collagen I was examined in vitro using solidified collagen I lattices. Integrin function was blocked by incubating fibroblasts with isotype-specific antibodies prior to addition to collagen I lattices.

RESULTS

Human renal fibroblasts embedded in collagen I lattices progressively decreased lattice diameter to 60.6+/-11.4% of initial diameter at 48 h post-release (P:<0.01). Fibroblasts incubated in the presence of antibody to beta1 integrin failed to contract collagen I lattices, whilst fibroblasts incubated with non-specific antibody reduced lattice diameter to 60.1+/-12.4% of initial diameter at 48 h post-release (P:<0.01). Further characterization of integrin alpha subunits showed that blocking alpha2beta1 integrin prevented lattice contraction (P:<0.05, alpha2beta1 integrin antibody vs non-specific antibody), whilst blocking of alpha5beta1, alpha3beta1 and alpha1beta1 integrins did not influence this process.

CONCLUSIONS

We postulate that collagen I fibril rearrangement by human renal fibroblasts in vitro appears to be an integrin-mediated process involving the alpha2beta1 integrin.

Mechanisms responsible for F-actin stabilization after lysis of polymorphonuclear leukocytes

While actin polymerization and depolymerization are both essential for cell movement, few studies have focused on actin depolymerization. In vivo, depolymerization can occur exceedingly rapidly and in a spatially defined manner: the F-actin in the lamellipodia depolymerizes in 30 s after chemoattractant removal (Cassimeris, L., H. McNeill, and S. H. Zigmond. 1990. J. Cell Biol. 110:1067-1075). To begin to understand the regulation of F-actin depolymerization, we have examined F-actin depolymerization in lysates of polymorphonuclear leukocytes (PMNs). Surprisingly, much of the cell F-actin, measured with a TRITC-phalloidin-binding assay, was stable after lysis in a physiological salt buffer (0.15 M KCl): approximately 50% of the F-actin did not depolymerize even after 18 h. This stable F-actin included lamellar F-actin which could still be visualized one hour after lysis by staining with TRITC-phalloidin and by EM. We investigated the basis for this stability. In lysates with cell concentrations greater than 10(7) cells/ml, sufficient globular actin (G-actin) was present to result in a net increase in F-actin. However, the F-actin stability was not solely because of the presence of free G-actin since addition of DNase I to the lysate did not increase the F-actin loss. Nor did it appear to be because of barbed end capping factors since cell lysates provided sites for barbed end polymerization of exogenous added actin. The stable F-actin existed in a macromolecular complex that pelleted at low gravitational forces. Increasing the salt concentration of the lysis buffer decreased the amount of F-actin that pelleted at low gravitational forces and increased the amount of F-actin that depolymerized. Various actin-binding and cross-linking proteins such as tropomyosin, alpha-actinin, and actin-binding protein pelleted with the stable F-actin. In addition, we found that alpha-actinin, a filament cross-linking protein, inhibited the rate of pyrenyl F-actin depolymerization. These results suggested that actin cross-linking proteins may contribute to the stability of cellular actin after lysis. The activity of crosslinkers may be regulated in vivo to allow rapid turnover of lamellipodia F-actin.

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.

Cell surface expression of the 70-kD component of Ku, a DNA-binding nuclear autoantigen

The Ku complex, a heterodimer of 86- and 70-kD proteins, is a nuclear DNA-binding autoantigen. However, hydrophobicity analysis of the deduced amino acid sequence of the 70-kD protein had strongly suggested that this might also be a membrane protein. In the present study, using antibodies to synthetic peptides and a polyclonal antiserum to the purified protein, we show that the 70-kD protein of the Ku complex is present in isolated plasma membranes of human cells. By indirect immunofluorescence microscopy and fluorescein-activated cell sorting, we demonstrate that this autoantigen is exposed on the cell surface. In addition, we have identified several domains of the protein that are exposed. Our study provides one of the first demonstrations of a eukaryotic, nuclear DNA-binding protein that is also on the cell membrane. Moreover, our results might help explain how autoantibodies to the Ku autoantigen could target cells for an autoimmune attack.

Clonal selection by fluorescence redistribution after photobleaching (FRAP)--a "fast" lateral mobility fibroblast mutant (E7G1)

Fluorescence redistribution after photobleaching (FRAP) was utilized to select a "fast" lateral mobility clone from Kirsten murine sarcoma virus-transformed 3T3 (KMSV-3T3) fibroblasts. The clone, E7G1, demonstrated a lateral mobility for membrane wheat germ agglutinin (WGA) and succinylated concanavalin A (sCon A) receptors of (2.1 +/- 1.6) X 10(-9) cm2/s and (2.7 +/- 2.3) X 10(-9) cm2/s, respectively. These mobilities were approximately equivalent to phospholipid mobility (2.8 +/- 1.9 X 10(-9) cm2/s). The fast mobility phenotype is observed when the cells are unattached and spherical. Upon attachment, the mobility decreases to (0.19 +/- 0.19) X 10(-9) cm2/s. In addition, the ability of Con A to initiate global modulation was completely lost in spread as well as spherical cells in the E7G1 fast mobility clone. A comparison of F-actin patterns between untransformed Balb/c fibroblasts and the E7G1-transformed line suggests a correlation between well-developed stress fiber assemblies and the ability to induce global modulation. The fast mobility clone was stable for at least 23 passages.

A novel isoform of cytoplasmic actin that binds poly-L-proline

An actin-like protein was purified to apparent homogeneity from chick-embryo homogenates and chick-embryo fibroblasts by the use of poly-L-proline-agarose affinity chromatography; we therefore refer to this protein as PBP (poly-L-proline-binding protein). PBP binds to deoxyribonuclease-agarose, co-migrates with known actin standards on SDS/polyacrylamide-gel electrophoresis, and has an amino acid composition similar to that of actin. Linear peptide maps after digestion with Staphylococcus aureus proteinase reveal its apparent homology with gamma-actin; however, isoelectric-focusing experiments show that PBP is clearly more acidic than any of the three major isoforms of actin. PBP polymerizes in the presence of ATP to form fibrillar structures resembling actin paracrystalline aggregates. In chick-embryo fibroblasts, immunofluorescence with antibodies to PBP shows that its distribution is cytoplasmic: perinuclear staining of the cytoplasm, generalized cytoplasmic staining and peripheral fibrillar structures are evident. In contrast, antibodies specific for the (alpha, gamma)-actins reveal the typical stress fibre structures characteristic of fibroblastic cells. PBP appears to constitute a novel isoform of cellular actin, distinct from the known actin isoforms in terms of its lower isoelectric point, its ability to bind poly-L-proline and its distinct subcellular localization.

Compartmentation of the rough endoplasmic reticulum

It has become evident during recent years that a wide variety of proteins are synthesized on membrane-bound polysomes, very many of which are not ultimately secreted from the cell. The majority of proteins appear to go through some form of post-translational modification before the final appearance of an 'active' product, and in some cases the polypeptide chain may be modified before the completed protein molecule is released from the ribosome. This then raises the question concerning the possibility of the organization of the rough endoplasmic reticulum into individual domains, or compartments, each of which may have the responsibility of performing definite and well defined functions. During recent years the behaviour of two subfractions of the rough endoplasmic reticulum in a variety of cell types and under a variety of conditions has been studied in order to gain insight into a possible compartmentation of this organelle. Throughout the studies disruption of cells has been performed by nitrogen cavitation. This technique was chosen in order to provide conditions of homogenization which were extremely reproducible since shearing forces, mechanical damage and the effects of local heating were eliminated. Endoplasmic reticulum (ER) membranes isolated from the post-mitochondrial supernatant have been separated into subfractions by centrifugation on discontinuous sucrose gradients. By virtue of their high density imparted by the association of ribosomes, rough ER (RER) membranes penetrate 1.4 M sucrose accumulating above either 2.0 M sucrose (light rough -LR membranes) or a cushion of 2.3 M sucrose (heavy rough -HR membranes). Smooth (S) membranes, which are virtually devoid of ribosomes, collect above 1.4 M sucrose. The HR, LR and S subfractions in MPC-11 cells differ in a number of respects: RNA/protein and RNA/phospholipid ratios, polysome profiles and marker enzymes. When cells were homogenized in buffer containing 25 mM KCl then all three ER subfractions were observed, however, when the buffer contained 100 mM KCl then only the LR and S subfractions were observed in gradients, radioactivity equivalent to that in the HR fraction was not recovered in the other two subfractions. Four times as many light chain immunoglobulin polypeptides were found associated with polysomes of HR membranes compared to LR membranes. The nuclear associated ER (NER), though very active in protein synthesis, was only 20% as active in the synthesis of light chain as the combined LR/HR fraction. Studies with MPC-11 cells showed that the relative amounts of the three ER subfractions were related to the phase of the cell cycle.(ABSTRACT TRUNCATED AT 400 WORDS)

The proteins of human lung surfactant

Human pulmonary surfactant was purified from bronchoalveolar lavage of patients. The proteins present in surfactant were analyzed by SDS-polyacrylamide gel electrophoresis into serum and non-serum components. One non-serum surfactant protein (Mr = 43 000) was then identified in the 100 000 X g supernatant of a lung homogenate on the basis of phospholipid binding. This lung protein was purified and partially characterized. The presence of 3-methyl histidine and reaction in Western blot analysis with antibody against chicken muscle actin both strongly suggested that the 43 000 Da protein of human surfactant is indeed cytoplasmic actin. It is proposed that this surfactant protein is involved in the secretion and not necessarily in the function of surfactant.

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.

Redistribution of membrane-bound and cytosolic action in rabbit polymorphonuclear leucocytes during phagocytosis

In the analysis of highly purified surface membrane from both resting and phagocytosing neutrophils an increase in the surface membrane associated actin has been demonstrated. This change at the cell periphery is associated with a coincident increase in the F-actin content of the cells following stimulation of the cells by exposure to opsonized Oil Red O droplets. The actin which is newly associated with the surface membrane of the phagocytosing cells was more susceptible to removal by detergent than the membrane-associated actin in resting cells and it was also noted that the F-actin associated with phagosomes was readily disrupted by detergent. A redistribution of the surface membrane glycoprotein 5'-nucleotidase was observed during phagocytosis, but no change in distribution of a 125I-labelled Lens culinaris lectin was observed during the entire phagocytic process.

Ultracytochemistry of the cell surface and microfilaments in dimethylhydrazine-induced colonic tumor cells

Studies were made of the ultracytochemical changes in the cell membrane and microfilaments of colonic epithelial cells during tumorigenesis induced by 1,2-dimethylhydrazine (DMH) in mice fed a high fat diet. The tumor cells showed reduced membrane ATPase activity and loss of contact with neighboring cells. Microfilaments in tumor cells showed an irregular intensity of fluorescent staining. Their actin filaments bound with heavy meromyosin (HMM) had an arrowhead pattern as in normal cells, but these complexes were shortened and detached from the cell membrane. The arrowheads were directed toward the interior in the terminal web of tumor cells. Microfilaments with long rootlets extended to the apical surface of some tumor cells. These results indicate that during development of colonic tumors, the structures of the cell membrane and microfilaments of the cells changes.

Studies on rat parotid-cell actomyosin

Actomyosin was partially purified from rat parotid cells dispersed by collagenase digestion and found to possess different solubility characteristics from that from (undispersed) rat parotid tissue. This is attributed to the decrease in vascular contamination effected by the isolation of parotid cells, yielding a non-muscle actomyosin [Adelstein, Conti, Johnson, Pastan & Pollard (1972) Proc. Natl. Acad. Sci. U.S.A. 69, 3693-3697]. Myosin light-chain kinase was partially purified from dispersed rat parotid cells by calmodulin affinity chromatography and shown to be activated by Ca2+-calmodulin. The calmodulin content of dispersed rat parotid cells was shown to be 6.50 +/- 0.59 ng of calmodulin/micrograms of rat parotid-cell protein (mean +/- S.E.M.), as determined by the activation of purified bovine brain phosphodiesterase by heat-treated extracts of dispersed rat parotid cells.

Membrane-associated actin in the rhabdomeral microvilli of crayfish photoreceptors

Infiltration of compound eyes of crayfish, Cherax destructor, with the thiol protease inhibitor Ep-475 or with trifluoperazine prior to fixation for electron microscopy was found to stabilize an axial filament of 6-12 nm diam within each rhabdomeral microvillus of the photoreceptors. Rhabdoms isolated from retinal homogenates by sucrose gradient centrifugation under conditions that stabilize cytoskeletal material contained large amounts of a 42-kd polypeptide that co-migrated with insect flight muscle actin in one- and two-dimensional PAGE, inhibited pancreatic DNase l, and bound to vertebrate myosin. Vertebrate skeletal muscle actin added to retinal homogenates did not co-purify with rhabdoms, implying that actin was not a contaminant from nonmembranous structures. DNase l inhibition assays of detergent-lysed rhabdoms indicated the presence of large amounts of filamentous actin provided ATP was present. Monomeric actin in such preparations was completely polymerizable only after 90 min incubation with equimolar phalloidin. More than half of the actin present could be liberated from the membrane by sonication, indicating a loose association with the membrane. However, a large proportion of the actin was tightly bound to the rhabdomeral membrane, and washing sonicated membrane fractions with solutions of a range of ionic strengths and nonionic detergents failed to remove it. Antibodies to scallop actin only bound to frozen sections of rhabdoms after gentle permeabilization and very long incubation periods, probably because of steric hindrance and the hydrophobicity of the structure. The F-actin probe nitrobenzoxadiazol phallacidin bound to rhabdoms and labeled F-actin aggregates in other retinal components, but rhabdom fluorescence was not abolished by preincubation with phalloidin. The biochemical data indicate the existence of two distinct actin-based cytoskeletal systems, one being closely membrane associated. The other may possibly constitute the axial filament, although the evidence for this is equivocal.

Altered distribution profiles of endoplasmic reticulum subfractions after incubation of Krebs II ascites cells with different concentrations of cytochalasin B

Information on the interaction between endoplasmic reticulum (ER) membranes and components of the skeletal network of the cell was gained by treating cells with the antimicrofilament agent cytochalasin B prior to cell disruption by nitrogen cavitation. Treatment of Krebs II ascites cells with cytochalasin B (5-10 micrograms ml-1) resulted in an increased yield of three ER membrane subfractions--heavy rough (HR), light rough (LR) and smooth (S) membranes, as judged by 3H-choline incorporation in gradient fractions following discontinuous sucrose gradient centrifugation. The major increase was observed in the HR fraction. These results indicate that the actual yield of the respective ER membrane subfractions after cell disruption is dependent on the degree of direct and/or indirect interaction between individual ER membranes and actin containing filaments of the cytoskeleton in the intact cell.

The quantitation of G- and F-actin in cultured cells

An improved method to quantitate the amounts of filamentous (F-actin) and monomeric (globular) actin (G-actin) in cultured cells was developed. Cells are lysed into a myosin-containing buffer and F-actin is removed by centrifugation. The pelleted F-actin is then depolymerized to G-actin in a 1 mM ATP-containing buffer for 1 h before measuring the levels of G-actin using the DNase I inhibition assay. Partitioning of G-actin in the supernatant (greater than 95%) and recovery of actin in both fractions (greater than 85%) were measured by adding [3H]actin to cultured cells. Actin in the separated fractions is stable for at least 72 h at 0 degree C. Asynchronous monolayer cultures of Chinese hamster ovary (CHO) cells contain 2.5 +/- 0.2% of the total protein as actin with 72.4 +/- 5.7% as F-actin. About 10% of this F-actin is not associated with the readily sedimented Triton-cytoskeleton. CHO cells grown in suspension contain 55.8% of the actin as F-actin; following plating about 90 min is required for these cells to flatten and for the F-actin level to reach the monolayer value of about 70%.

Purification of a HeLa cell high molecular weight action binding protein and its identification in HeLa cell plasma membrane ghosts and intact HeLa cells

The high molecular weight protein (HMWP) which was previously observed to be a major component of the actin based gels formed by incubating cytoplasmic extracts of HeLa cells at 25 degrees C [Weihing, R. R. (1977) J. Cell Biol. 75, 95-103] has now been purified by gel filtration of 0.6 M KCl extracts of precipitated gels. A few hundred micrograms of HMWP, which is about 90% pure, can be isolated from 4 X 10(9) cells. HMWP can gel muscle actin and cross-link it into filament bundles. Its subunit molecular weight is 250 0000, its Stokes radius is 125 A, and its sedimentation coefficient is 9 S. A native molecular weight of 480 000 was calculated by using the latter two parameters, and therefore the native molecule is a dimer. Its amino acid analysis is nearly indistinguishable from that of macrophage actin binding protein and of mammalian and avian filamins. All of these findings indicate that HMWP is homologous to the latter proteins. However, HeLa cell HMWP and avian filamin must differ in their primary sequences because their partial peptide maps are distinct and because an antiserum against HMWP reacts only weakly with filamin. For studies on the intracellular location of HMWP, a goat antiserum against purified HMWP was prepared and characterized and then used to localize HMWP in suspension grown cells. The technique of immunoblotting revealed that the antiserum reacted virtually exclusively with the high molecular weight polypeptide that comigrates with HMWP in cell lysates and in ZnCl2-stabilized plasma membrane ghosts prepared from HeLa cells [Gruenstein, E., Rich, A., & Weihing, R. R. (1975) J. Cell Biol. 64, 223-234] and that it did not react with rabbit myosin heavy chain, microtubule proteins (MAPS and tubulin) from HeLa cells and calf brain, or the proteins of human erythrocyte ghosts including spectrin. Suspension-grown cells which were stained with the antiserum by the technique of indirect immunofluorescence showed bright fluorescence at the rim of the cells and less intense generalized fluorescence. If preimmune serum or immune serum treated with HMWP was substituted for the immune serum, then staining at the rim was not observed, but the generalized fluorescence was only slightly reduced; unpermeabilized cells were not stained. These results indicate that HMWP is a component of the cortical cytoplasm of HeLa cells. Possible functions of cortical HMWP are discussed briefly.

Ultrastructural localization of actin in muscle, epithelial and secretory cells by applying the protein A-gold immunocytochemical technique

Actin-immunoreactive sites have been localized at the electron microscope level by the protein A-gold technique in striated and smooth muscle cells as well as in epithelial and secretory cells. The combination of the highly sensitive protein A-gold technique with the good ultrastructural preservation and retention of antigenicity obtained using low-temperature embedding conditions has allowed a very precise identification of the labelled structures with high resolution. In striated muscle cells the labelling was obtained over the myofilaments and the Z-band, mainly at its periphery. Labelling was also observed at the edge of the intercalated discs of the cardiac muscle cells. In smooth muscle cells the labelling was present over the myofilaments; the dense plaques associated with the plasma membrane were labelled at their periphery where actin filaments have been reported to anchor. In epithelial cells of the duodenum and the renal convoluted proximal tubule, the labelling occurred over the filamentous core of the microvilli and over the cell web. Gold particles were often present over, or closely associated with, the cell membrane at the tip of the microvilli or of invaginations and vesicular structures. At the level of the junctional complexes the gold particles were aligned at the edge of the dense zones. In pancreatic endocrine and exocrine secretory cells, actin-immunoreactive sites were revealed over the Golgi apparatus, mainly at the level of the inner cisternae in the maturing face over or closely associated with the membranes of the condensing vacuoles and secretory granules, and also over the plasma membrane. Microvilli and cell web were also labelled. Finally, in fibroblasts, gold particles were associated with the membrane of vesicular structures. The consistent finding of actin-immunoreactive sites closely associated with membranes of secretory granules and vesicular structures brings support to the proposal that contractile proteins might play an important role in transcellular transport and protein secretion.

Organizational forms of actin in 13762 ascites mammary tumor cell microvilli

The organization of microvillus actin and its associated proteins have been investigated in sublines of mammary ascites tumors (MAT) with mobile (MAT-B1) and immobile (MAT-C1) cell surface receptors. Microvilli isolated from these sublines differ in morphology (branched for MAT-C1 versus unbranched for MAT-B1) and the presence of a 58,000-dalton polypeptide (58K). 58K is found associated with MAT-C1 microvilli, microvillar cytoskeletons obtained by nonionic detergent extractions, and microvillar membranes prepared under conditions which depolymerize actin microfilaments. By extraction with actin-stabilizing buffers (isotonic Triton-Mg-ATP) microvillar actin can be fractionated into four forms. About 40% of the actin is sedimented at low speed (7,500g, 15 min). The pellets contain microfilaments; actin and alpha-actinin are the predominant proteins. High-speed pellets from these low-speed supernates contain about 10% of the actin as a transmembrane complex with a cell surface glycoprotein (cytoskeleton-associated glycoprotein, [CAG] 75-80,000 daltons) in MAT-B1 cells or with CAG and 58K in MAT-C1 cells. Transmembrane complexes can be purified from MAT-B1 and MAT-C1 microvillar membranes in Triton-containing buffer by gel filtration or sucrose density gradient centrifugation. The presence of only CAG and actin in the MAT-B1 transmembrane complex strongly suggests the direct interaction of actin and a cell surface component. The high-speed supernates contain soluble actin. By gel filtration or rate-zonal sucrose density gradient centrifugation about 30% of the microvillar actin is found as small oligomers and about 10% as G-actin in this extraction buffer. We suggest that the actin-containing transmembrane complexes may serve as membrane-association sites for oligomeric actin segments and microfilaments and as initiation sites for actin polymerization.

Microtubule-associated proteins of HeLa cells: heat stability of the 200,000 mol wt HeLa MAPs and detection of the presence of MAP-2 in HeLa cell extracts and cycled microtubules

One of the major groups of microtubule-associated proteins (MAPs) found associated with the microtubules isolated from HeLa cells has a molecular weight of just over 200,000. Previous work has demonstrated that these heLa MAPs are similar in several properties to MAP-2, one of the major MAPs of mammalian neural microtubules, although the two types of proteins are immunologically distinct. The 200,000 mol wt HeLa MAPs have now been found to remain soluble after incubation in a boiling water bath and to retain the ability to promote tubulin polymerization after this treatment, two unusual properties also shown by neural MAP-2. This property of heat stability has allowed the development of a simplified procedure for purification of the 200,000 HeLa MAPs and has provided a means for detection of these proteins, even in crude cell extracts. These studies have also led to the detection of a protein in crude extracts of HeLa cells and in cycled HeLa microtubules which has been identified as MAP-2 on the basis of (a) comigration with calf brain MAP-2 on SDS PAGE, (b) presence in purified microtubules, (c) heat stability, and (d) reaction with two types of antibodies prepared against neural high molecular weight-MAPs, one of these a monoclonal antibody against hog brain MAP-2, although present in HeLa cells, is at all stages of microtubule purification a relatively minor component in comparison to the 200,000 HeLa MAP's.

Thioredoxin, glutaredoxin, and thioredoxin reductase from cultured HeLa cells

Thioredoxin and glutaredoxin may be important in regulating cell metabolism by mediating interchanges between sulfhydryl and disulfide groups. Components of the thioredoxin/glutaredoxin system from cultured HeLa cells have been partially purified and characterized by using Escherichia coli adenosine 3'-phosphate 5'-phosphosulfate reductase, a thioredoxin/glutaredoxin-dependent enzyme on the pathway of sulfate reduction, as an assay system. In HeLa cells, a NADPH-thioredoxin reductase and three heat-labile proteins (designated PI, PII, and PIII) that have thioredoxin- or glutaredoxin-like properties are found. Both PI and PIII have molecular masses of approximately 12,000 daltons and are readily reduced by their homologous HeLa thioredoxin reductase. However, only PI can be reduced efficiently by the glutathione system and neither PI nor PIII has inherent glutathione-disulfide oxidoreductase activity. PII has a molecular mass of greater than 30,000 daltons and appears to be associated with a reductase activity. The HeLa NADPH-thioredoxin reductase has been purified to near homogeneity and found to be a 116,000-dalton flavoprotein composed of two 58,000-dalton subunits. The HeLa enzyme has low species and substrate specificity and can reduce HeLa PI and PIII, E. coli thioredoxin and glutaredoxin, and the disulfide bond in 5,5'-dithiobis(2-nitrobenzoic acid). The exact in vivo roles of the HeLa thioredoxin/glutaredoxin system remain to be determined.

A characterization of abrin A from the seeds of the Abrus precatorius plant

Abrin A was purified from the seeds of the Abrus precatorius plant and its physical and biological properties were studied. The biological properties of abrin A were found to be similar to the better studied Abrus protein, abrin C, in that it is toxic to cell-free protein synthesis and binds D-galactose. Abrin A contains carbohydrate moieties including both neutral and amine sugars but no metals, similar to the other two Abrus proteins (abrin C and the Abrus agglutinin). Amino acid compositions of the subunits of abrin A indicated that it consists of two different subunits of comparable size. Furthermore, one of the subunits showed microheterogeneity suggesting that abrin A is a mixture of isolectins. A comparative study of abrin A and abrin C based on compositions and tryptic maps reveals them to be closely related. The evidence suggests that the two abrins may have the same mechanisms of toxic action. Far-ultraviolet circular dichroic studies of abrin A show it to contain 47% beta-pleated sheet and 10% alpha-helix, again similar to the other two Abrus proteins.

Actin-containing matrix associated with the plasma membrane of murine tumour and lymphoid cells

A detergent-insoluble matrix has been isolated from murine tumour cell and lymphocyte plasma membranes. The major components of this matrix include actin and four additional proteins not previously identified as cytoskeletal proteins. Labelling studies indicate that the matrix is located on the inner face of the plasma membrane. A cell-surface glycoprotein, 5'-nucleotidase, remains associated with the isolated matrix.

Putative actin genes in the macronucleus of Oxytricha fallax

Previous work has shown that the macronuclear DNA of the hypotrichous ciliate Oxytricha fallax is arranged as short achromosomal pieces, 22 to 0.5 kilobase pairs (kb) in length. Micronuclear DNA has a typical chromosomal organization. Macronuclear DNA is derived from micronuclear DNA through a process of polytene chromosome fragmentation with a resultant decrease in DNA sequence complexity. Three putative actin genes have been identified in macronuclear DNA by using a cloned yeast actin gene as a hybridization probe. A restriction fragment of the yeast gene containing both actin coding and noncoding DNA hybridizes strongly to two macronuclear DNA pieces, 1.6 and 1.4 kb in length, and weakly to a 1.2-kb piece. The entire 1.6-kb piece has been cloned in plasmid pBR322 and the resulting recombinant plasmid has been designated pOfACT(1.6). The 1.6-kb pOfACT(1.6) insert hybridizes only to those restriction fragments of the yeast actin gene containing actin coding sequences. When hybridized to macronuclear DNA under conditions that allow the yeast probe to hybridize to all three macronuclear pieces, the pOfACT(1.6) insert hybridizes only to the 1.6-kb piece. Under less stringent conditions the insert also hybridizes to the 1.4-kb piece, but it shows no hybridization to the 1.2-kb DNA. The three macronuclear pieces homologous to the yeast actin gene thus differ in sequence and are interpreted as a related family of actin genes. Each of these pieces could accommodate an actin coding sequence, which in yeast, Dictyostelium discoideum, and Drosophila melanogaster is 1.1 kb, and an additional 0.1-0.5 kb of noncoding DNA.

Purification and reconstitution of HeLa cell microtubules

Microtubules from suspension cultures of HeLa cells have been purified by carrying them through four complete cycles of polymerization at 37 degrees C and depolymerization at 4 degrees C. These microtubules show, in addition to the major alpha- and beta-tubulin components, major proteins with molecular weights of 201 000-206 000 (comprising 4.5% of the total protein), proteins with molecular weights of 97 000, 100 000, 104 000, and 114 000 (together comprising approximately 2% of the total protein), and minor components with molecular weights of 68 000 and 151 000. HeLa microtubules have also been reconstituted from purified HeLa tubulin and proteins from HeLa microtubules separated from tubulin by DEAE-cellulose column chromatography. Experiments on the fractionation and reconstitution of both two- and four-cycle microtubules suggest that the 201 000-206 000-dalton proteins are incorporated into microtubules and promote tubulin polymerization. Microtubules formed by fractionationand reconstitution of two-cycle microtubules also contain several other proteins with molecular weights of 132 000, 146 000, 151 000, 160 000, and 284 000, although these are not present in microtubules carried through four assembly-disassembly cycles. Evidence is also presented which shows that a 68 000-dalton protein which is a prominent component of HeLa microtubules after two polymerization-depolymerization cycles does not stoichiometrically copurify with tubulin through repeated assembly--disassembly cycles and does not stimulate tubulin polymerization. On the other hand, the sedimentation of this 68 000-dalton protein is apparently influenced by the presence of polymerized microtubules, suggesting that this protein may be a component of a system whjich interacts weakly with microtubules. Finally, evidence is presented suggesting that two-cycle microtubules contain a proteolytic activity that can digest the 201 000-206 000-dalton proteins.

Properties of a macrophage cell line transformed by simian virus 40. Morphological changes related to cell functions

A mouse macrophage clone (line nH-1) transformed by simian virus 40 (SV40) was examined by electron microscopy. In the growing phase of the cultures, NH-1 cells were non-phagocytic and SV40 T antigen-positive, and contained a large number of filament sheaths within their pseudopodia. In the late stationary phase, they became phagocytic, SV40 T antigen-negative and contained a filamentous network within their psudopodia. In addition, NH-1 cells in the late stationary phase were very similar to normal macrophages in other morphological properties.

Protein and glycoprotein components of phagosome membranes derived from mouse L cells

1. Sodium dodecylsulfate--polyacrylamide gel electrophoresis has been employed to analyze the protein and glycorprotein components of phagosome membranes prepared from mouse L cells by the polystyrene latex bead method. 2. These experiments showed that phagosome membranes contain at least 20 major membrane protein species having apparent molecular weights between 27,000 and 250,000; the most abundant proteins have molecular weights of 112,000, 103,000, 76,000, 68,000, 62,000, 42,000 and 36,000. 3. Phagosome membrane glycoproteins, which were detected on the gels by staining with periodic acid-Schiff's reagent, were found to migrate in two broad zones centered at positions on the gel corresponding to proteins of mol. wt 140,000 and 85,000. 4. Comparison of the phagosome membrane results with the results of similar experiments carried out with cell surface membranes revealed a high degree of similarity between the two. All major protein and glycoprotein components present in phagosome membranes could be identified in both of two types of cell surface membrane preparations analyzed. 5. These results strongly support the view that phagosome membranes contain a representative, not a highly selected, sample of surface membrane proteins and glycoproteins.

The association of alpha-actinin with the plasma membrane

The role of alpha-actinin in the attachment of actin to plasma membranes has been investigated. Specific antibody staining of SDS gels has indicated that alpha-actinin is a major component in isolated plasma membranes prepared from three different cell types by two different procedures. Using specific extraction conditions, most of the alpha-actinin can be selectively extracted from the membranes with relatively little parallel release of actin. This selective dissociation of alpha-actinin from the plasma membrane leads us to conclude that alpha-actinin is present in these membrane preparations, because it is bound to actin, and that alpha-actinin does not form a direct link between actin and the membrane.

Multiple actin-related sequences in the Drosophila melanogaster genome

In this paper we describe the isolation and characterization of a 7.2 kb D. melanogaster chromosomal DNA fragment (K1) which contains nucleotide sequences complementary to D. melanogaster actin mRNA. Plasmid K1 was identified using a Dictyostelium actin cDNA plasmid, B1, as a probe. D. melanogaster mRNA selected by hybridization with immobilized K1 DNA was translated in vitro to yield products which co-migrate with the D. melanogaster actins I, II and III in two-dimensional gel electrophoresis and bind to DNAase I agarose. A physical map localizing restriction endonuclease cleavage sites in the K1 DNA fragment and the direction of transcription is presented. The position of the coding region has been localized by hybridization with labeled B1 DNA and with labeled poly(A)-containing D. melanogaster RNA. On the basis of hybridization of labeled subfragments of plasmid K1 to restriction endonuclease-cleaved D. melanogaster embryo DNA, we conclude that the nucleotide sequence of the presumptive coding region is responsible for labeling of a pattern of multiple restriction fragments from embryo DNA. The chromosomal locus from which DNA fragment K1 is derived has been localized by in situ hybridization to two closely linked bands in the region 88F. Related DNA sequences corresponding to putative actin genes have also been mapped cytologically. These results support the hypothesis that the genes for actin in D. melanogaster are members of a closely related family of coding sequences.

Initial studies of the molecular organization of the cell-substrate adhesion site

Using selective extraction reagents and non-penetrating probes, studies have been initiated on the molecular organization of substrate-attached material, adhesion sites which pinch off from the cell surface of normal Balb/c 3T3 or SV40-transformed Balb/c 3T3 (SVT2) cells and which remain bound to the serum-coated substrate during EGTA-mediated detachment of cells. Extraction of SVT2 adhesion sites with non-ionic detergents resulted in (a) only small amounts of leucine-radiolabeled protein and glucosamine-radiolabeled polysaccharide being solubilized; (b) selective solubilization of 80% of the adhesion site actin, and (c) solubilization of 95% of the phospholipid from these membranous pools. ATP in combination with potassium chloride extracted 60% of the actin. The 3T3 and SVT2 adhesion site proteins which are accessible to lactoperoxidase-catalyzed iodination were also determined. Many of the serum-derived proteins, bound to the substrate, were iodinated during iodination treatment of serum-coated or substrate-attached material-coated substrates, whereas the cellular proteins in the adhesion sites were not iodinated even though they were present in larger quantity as revealed by Coomassie blue staining. Iodination of cells, followed by their EGTA-mediated detachment and reattachment to fresh serum-coated substrates, indicated that the principal iodinated cell surface component deposited in new adhesion sites is the large external transformation-sensitive glycoprotein (even though large external transformation-sensitive glycoprotein is not the only principal iodinated cell surface component of these cells). These studies further establish the selective enrichment in this adhesive material of specific cell surface components and indicate that they are tenaciously bound at the interface between the serum coating and the undersurface of the adhesion site membranous pools.

Effects of the actin-binding protein DNAase I on cytoplasmic streaming and ultrastructure of Amoeba proteus. An attempt to explain amoeboid movement

Microinjection of DNAase I, which is known to form a specific complex with G-actin, induces characteristic changes in cytoplasmic streaming, locomotion and morphology of the contractile apparatus of A. proteus. Light microscopical studies show pronounced streaming originating from the uroid and/or the retracting pseudopods, which ceases 10--15 min after injection of DNAase I, at a time when ultrasctructural studies show that the actin filament system is very much reduced. These results suggest that a controlled reversible equilibrium between soluble and polymerized forms of actin is a necessary requirement for amoeboid movement. The topographic distribution of contractile filaments beneath the plasma membrane visualized by correlated light- and electron microscopy of DNAase I-injected cells establishes the importance of the membrane-bound filamentous layer for three major aspects of streaming: (1) Streaming originates by local contractions of a cell membrane-associated filament layer at the uroid and/or retracting pseudopods, creating a pressure flow. (2) This flow continues beneath the membrane, which is stabilized by filaments in the lateral regions between the posterior end, with a high hydrostatic pressure, and the anterior end, with a low hydrostatic pressure. (3) Pseudopods or extending areas are created by a local destabilization of the cell periphery caused by the separation of the filamentous layer from the plasma membrane.

Pinocytosis and locomotion of amoebae: XII. Dynamics and motive force generation during induced pinocytosis in A. proteus

The mechanism of induced pinocytosis was investigated in Amoeba proteus by light and electron microscopy. The application of nine different inducing substances revealed that pinocytotic channel formation, elongation, vesiculation, shortening and disappearance are the result of the successive or simultaneous action of both traction and pressure forces, which are produced by the contractile activity of a plasma membrane-associated layer of filaments ranging from a few hundred nm to several micrometer in thickness. The initial phase of channel formation is caused by traction forces according to the membrane flow concept, whereas channel elongation and vesiculation mainly result from pressure forces in conjunction with the extrusion of small hyaline pseudopodia. Shortening and disappearance of the pinocytotic channels are brought about by local contractions of the cortical filament layer in the basal region of the hyaline pseudopodia. Experiments using latex beads as marker particles together with inducing substances show that a rapid membrane turnover duirng pinocytosis can be excluded, and that the plasma membrane slides as an entire structure over the underlying cytoplasm.

The cytoskeletal framework and poliovirus metabolism

The cytoskeletal framework prepared by detergent lysis of suspension-grown HeLa cells is compared to the structure obtained from poliovirus-infected cells. This framework, which retains major features of cell morphology and carries the cellular polyribosomes as well as the major structural filaments, is profoundly reorganized following virus infection. This reorganization underlies, at least in part, the morphological changes termed the "cytoplasmic effect." These cytoskeletal changes appear related to the involvement of the framework with viral-specific metabolism. Extensive cytoskeleton alterations occur even when guanidine inhibits viral replication, and thus result from small amounts of early viral products. The normally spheroidal nucleus deforms, allowing a modified region of the cytoplasm to occupy a central position in the cell, and many membrane-enclosed vesicles peculiar to the infected cell are elaborated here. The skeleton preparation reveals that this region contains intermediate filaments arranged in a pattern unique to infected cells. Further changes occur when viral replication is permitted. The central region filaments become coated with darkly staining material which may be viral RNA. Numerous small particles appear on the filaments which resemble partially assembled virions. Mature virions, however, have no affinity for the cytoskeleton and appear to be free in the cytoplasm. Host cell messenger RNA, normally attached to the skeletal framework, is released in infected cells and is replaced by the viral-specific polyribosomes. The trabecular network which carries polyribosomes appears to be rearranged; the viral polyribosomes are located principally at the cell periphery and are excluded from the central region. The viral replication complex with its double-stranded RNA is also attached to the skeletal framework and may comprise the dark staining material coating the filaments of the central cell region.