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S J Singer

Publications and source records attributed to S J Singer.

At least 91 records · Page 5Linked to original sources
PubMed1983

Immunoelectron microscopic studies of the intracellular transport of the membrane glycoprotein (G) of vesicular stomatitis virus in infected Chinese hamster ovary cells.

An immunoelectron microscopic study was undertaken to survey the intracellular pathway taken by the integral membrane protein (G-protein) of vesicular stomatitis virus from its site of synthesis in the rough endoplasmic reticulum to the plasma membrane of virus-infected Chinese hamster ovary cells. Intracellular transport of the G-protein was synchronized by using a temperature-sensitive mutant of the virus (0-45). At the nonpermissive temperature (39.8 degrees C), the G-protein is synthesized in the cell infected with 0-45, but does not leave the rough endoplasmic reticulum. Upon shifting the temperature to 32 degrees C, the G-protein moves by stages to the plasma membrane. Ultrathin frozen sections of 0-45-infected cells were prepared and indirectly immunolabeled for the G-protein at different times after the temperature shift. By 3 min, the G-protein was seen at high density in saccules at one face of the Golgi apparatus. No large accumulation of G-protein-containing vesicles were observed near this entry face, but a few 50-70-mm electron-dense vesicular structures labeled for G-protein were observed that might be transfer vesicles between the rough endoplasmic reticulum and the Golgi complex. At blebbed sites on the nuclear envelope at these early times there was a suggestion that the G-protein was concentrated, these sites perhaps serving as some of the transitional elements for subsequent transfer of the G-protein from the rough endoplasmic reticulum to the Golgi complex. By 3 min after its initial asymmetric entry into the Golgi complex, the G-protein was uniformly distributed throughout all the saccules of the complex. At later times, after the G-protein left the Golgi complex and was on its way to the plasma membrane, a new class of G-protein-containing vesicles of approximately 200-nm diameter was observed that are probably involved in this stage of the transport process. These data are discussed, and the further prospects of this experimental approach are assessed.

Animals↗
PubMed1982

Mitochondria are associated with microtubules and not with intermediate filaments in cultured fibroblasts.

Triple-immunofluorescence experiments with antibodies to cytochrome c oxidase, tubulin, and vimentin have been used to immunolabel the mitochondria, microtubules, and intermediate filaments inside the same cultured fibroblasts. In particular, fibroblasts were immunolabeled after they had either been transformed by infection with Rous sarcoma virus or given long-term treatment with cycloheximide. These treatments induced redistribution of the intermediate filaments into a perinuclear arrangement, segregated away from the microtubules, which remained extended to the cell periphery. In such cells, many labeled mitochondria were observed to be codistributed with the peripherally located microtubules. From these results, we infer that an association, probably involving some type of chemical linkage(s), between mitochondria and microtubules exists in these cells that is independent of the intermediate filaments.

Animals↗
PubMed1982

Immunofluorescent localization of the transforming protein of Rous sarcoma virus with antibodies against a synthetic src peptide.

Antisera were raised against a synthetic peptide (src-c) containing the six COOH-terminal amino acids of p60src, the transforming protein of Rous sarcoma virus (RSV). Antibodies specific for the src-c peptide were purified by affinity chromatography and then used to study the location of p60src in transformed cells. The distribution of p60src was compared to that of vinculin, a candidate cytoskeletal substrate of p60src, by indirect double immunofluorescence microscopy. In RSV-transformed rat, mouse, and chicken cells, an extensive codistribution of p60src with vinculin was observed. Both proteins were concentrated in the few remaining focal adhesion plaques, in transformation-induced rosette clusters at the ventral cell surface, and in cell-cell contact areas. In addition, antibodies to both proteins stained the cytoplasm diffusely. In all cells examined, the immunofluorescent staining patterns produced by antibodies to the src-c peptide were indistinguishable from those obtained by immunolabeling of p60src with sera from RSV-infected tumor-bearing rabbits. The excellent agreement of the results obtained with two completely independent antibody preparations indicates strongly that the observed immunolabeling patterns correctly define the intracellular distribution of p60src. The significance of the intracellular location of p60src to the transforming activities of the protein is discussed.

Animals↗
PubMed1982

On the nature of crossreactions observed with antibodies directed to defined epitopes.

Antibodies directed against a synthetic peptide (src-c) containing the six carboxyl-terminal amino acids of p60src, the transforming protein of Rous sarcoma virus, recognize p60src. However, when used at sufficiently high concentrations they also react with a number of constituents of untransformed cells. These reactions can be completely inhibited by src-c peptide. Crossreactivities are to different components in cells from different species and cannot be attributed to p60c-src, the ubiquitous cellular homologue of p60src. By indirect immunofluorescence microscopy and immunochemical techniques we have identified three cytoskeletal proteins, myosin, tubulin, and vimentin, as well as an unknown intranuclear antigen, as major targets of anti-src-c antibodies in different untransformed cells. These crossreactivities probably reflect identities or similarities in the amino acid sequence of the immunogenic peptide and segments of the otherwise unrelated crossreactive proteins. These findings are discussed with respect to the interpretation of crossreactivities that are occasionally observed with anti-peptide sera and with monoclonal antibodies.

Amino Acid Sequence↗
PubMed1982

Polarization of the Golgi apparatus and the microtubule-organizing center in cultured fibroblasts at the edge of an experimental wound.

We have used the technique of experimental wounding of confluent monolayers of normal fibroblasts to induce essentially unidirectional and synchronous cell movement at the edge of the wound. The intracellular location of the Golgi apparatus and the microtubule-organizing center was determined by double indirect immunofluorescence microscopy, using antibodies specific for the membranes of the Golgi apparatus and antibodies specific for tubulin, respectively. In cells at the wound edge, the immunolabeled Golgi apparatus and microtubule-organizing center were in close proximity to one another and located predominantly forward of the cell nucleus facing the wound. In the same cultures in cells removed from the wound, the two organelles were also coordinately located; however, they were randomly oriented with respect to the wound edge. This reorientation of the two organelles in cells at the wound edge was evident within minutes after wounding and persisted as cell extension subsequently occurred into the wound. These results suggest that both the Golgi apparatus and the microtubule-organizing center may participate in directing cell movement. The possible mechanisms involved are discussed in the light of previous hypotheses and experimental evidence concerning cell motility.

Animals↗
PubMed1982

Immunoelectron microscopic studies of the sites of cell-substratum and cell-cell contacts in cultured fibroblasts.

Our object was to obtain information about the molecular structures present at cell-substratum and cell-cell contact sites formed by cultured fibroblasts. We have carried out double immunoelectron-microscopic labeling experiments on ultrathin frozen sections cut through such contact sites to determine the absolute and relative dispositions of the three proteins fibronectin, vinculin, and alpha-actinin with respect to these sites. (a) Three types of cell-substratum and cell-cell contact sites familiar from plastic sections could also be discriminated in the frozen sections by morphological criteria alone, i.e., the gap distances between the two surfaces, and the presence of submembranous densities. These types were: (i) focal adhesions (FA); (ii) close contacts (CC); and (iii) extracellular matrix contacts (ECM). This morphological typing of the contact sites allowed us to recognize and assign distinctive immunolabeling patterns for the three proteins to each type of site on the frozen sections. (b) FA sites were immunolabeled intracellularly for vinculin and alpha-actinin, with vinculin labeling situated closer to the membrane than alpha-actinin. Fibronectin was not labeled in the narrow gap between the cell surface and the substratum, or between two cells, at FA sites. Control experiments showed that this could not be ascribed to inaccessibility of the FA narrow gap to the immunolabeling reagents but indicated an absence or severe depletion of fibronectin from these sites. (c) CC sites were labeled intracellularly for alpha-actinin but not vinculin and were labeled extracellularly for fibronectin. (d) ECM sites were characterized by large separations (often greater than 100 nm) between the cell and substratum or between two cells, which were connected by long cables of extracellular matrix components, including fibronectin. In late (24-36 h) cultures, ECM contacts predominated over the other types. ECM sites appeared to be of two kinds, one labeled intracellularly for both alpha-actinin and vinculin, the other for alpha-actinin alone. (e) From these and other results, a coherent but tentative scheme is proposed for the molecular ultrastructure of these contacts sites, and specific functional roles are suggested for fibronectin, vinculin, and alpha-actinin in cell adhesion and in the linkage of intracellular microfilaments to membranes at the different types of contact sites.

Actinin↗
PubMed1982

Molecular interactions at the cell surface.

In this article, I have dealt briefly with the structures and properties of the proteins associated with membranes. The categorization of these proteins into integral and peripheral, and the recognition of several distinct types of transmembrane integral proteins, should help to clarify the problems of the functions and biosynthesis (particularly of the membrane attachment) of these proteins. It is likely that genetic defects in the structures of many of these proteins, or in their association with their respective membranes, as well as in the regulation of their membrane concentrations, can lead to various kinds of cellular dysfunction and diseases.

Animals↗
PubMed1982

Structure of the cell surface.

The cell surface is the locus for many important biochemical functions of cells and for the interactions of cells with one another and with their environment. The structure of the cell surface may be thought of as three-layered, with a central plasma membrane to which certain macromolecular components are attached on the outer face (the exoskeleton) and other components on the inner face (the membrane cytoskeleton). In the last decade, the basic molecular structure of the plasma membrane has been elucidated and can be represented by the fluid mosaic model as a first approximation. The binding of specific integral proteins of the membrane to individual peripheral proteins outside or inside the cell is most likely the basis for the three-layered structure of the cell surface. Studies of the last several years on the molecular structures of these three-layered cell surfaces of cultured normal fibroblasts and of fibroblasts transformed by oncogenic viruses are beginning to shed light on the molecular mechanisms responsible for changes in cell shape, adhesiveness, and in contact inhibition of motility associated with neoplastic transformation.

Animals↗
PubMed1981

Member-associated changes during erythropoiesis. On the mechanism of maturation of reticulocytes to erythrocytes.

The mature mammalian erythrocyte has a unique membranoskeleton, the spectrin-actin complex, which is responsible for many of the unusual membrane properties of the erythrocyte. Previous studies have shown that in successive stages of differentiation of the erythropoietic series leading to the mature erythrocyte there is a progressive increase in the density of spectrin associated with the membranes of these cells. An important stage of this progression occurs during the enucleation of the late erythroblast to produce the incipient reticulocyte, when all of the spectrin of the former cell is sequestered to the membrane of the reticulocyte. The reticulocyte itself, however, does not exhibit a fully formed membranoskeleton. In particular, the in vitro binding of multivalent ligands to specific membrane receptors on the reticulocyte was shown to cause a clustering of some fractions of these ligand-receptor complexes into special mobile domains on the cell surface. These domains of clustered ligand-receptor complexes became invaginated and endocytosed as small vesicles. By immunoelectron microscopic experiments, these invaginations and endocytosed vesicles were found to be specifically free of spectrin on their cytoplasmic surfaces. These earlier findings then raised the possibility that the maturation of reticulocytes to mature erythrocytes in vivo might involve a progressive loss of reticulocyte membrane free of spectrin, thereby producing a still more concentrated spectrin-actin membranoskeleton in the erythrocyte than in the reticulocyte. This proposal is tested experimentally in this paper. In vivo reticulocytes were observed in ultrathin frozen sections of spleens from rabbits rendered anemic by phenylhydrazine treatment. These sections were indirectly immunolabeled with ferritin-antibody reagents directed to rabbit spectrin. Most reticulocytes in a section had one or more surface invaginations and one or more intracellular vesicles that were devoid of spectrin labeling. The erythrocytes in the same sections did not exhibit these features, and their membranes were everywhere uniformly labeled for spectrin. Spectrin-free surface invaginations and intracellular vesicle were also observed with reticulocytes within normal rabbit spleens. Based on these results, a scheme for membrane remodeling during reticulocyte maturation in vivo is proposed.

Actins↗
PubMed1981

Vinculin: a cytoskeletal target of the transforming protein of Rous sarcoma virus.

Vinculin, a protein associated with the cytoplasmic face of the focal adhesion plaques which anchor actin-containing microfilaments to the plasma membrane and attach a cell to the substratum, contains 8-fold more phosphotyrosine in cells transformed by Rous sarcoma virus than in uninfected cells. Because the transforming protein of RSV, p60src, is a protein kinase that modifies cellular proteins through the phosphorylation of tyrosine and because phosphotyrosine is a very rare modified amino acid, this result is a very rare modified amino acid, this result suggests that vinculin is a primary substrate of p60src. Only trace amounts of phosphotyrosine were detected in myosin heavy chains, alpha-actinin, filamin, and the intermediate filament protein vimentin. The modification of vinculin by p60src may be responsible in part for the disruption of the microfilament organization and for the changes in cell shape and adhesiveness which accompany transformation by Rous sarcoma virus.

Amino Acids↗
PubMed1981

Association of microtubules and intermediate filaments in normal fibroblasts and its disruption upon transformation by a temperature-sensitive mutant of Rous sarcoma virus.

By double indirect immunofluorescence, using primary rabbit antibodies to tubulin and guinea pig antibodies to vimentin, we have simultaneously labeled microtubules and intermediate filaments in several types of cultured normal fibroblasts. With well-spread interphase cells there was an extensive but not complete correspondence of the labeling patterns for the two filamentous structures out to the cell periphery. This correspondence existed both at a gross level, where parallel but not coincident arrays of thickly labeled strands of the two types of filaments were observed, and at a fine level, where thinly labeled strands of the two were superimposed. The results suggest that there may be some type(s) of molecular linkages between microtubules and vimentin intermediate filaments that is under metabolic control. With NRK fibroblasts infected with a temperature-sensitive mutant (LA23) of Rous sarcoma virus, cells grown at the nonpermissive temperature (39 degrees C) showed the correspondence of the distributions of the microtubules and intermediate filaments characteristic of the normal phenotype but within 1 hr after a shift to the permissive temperature (33 degrees C) there was an extensive retraction of the intermediate filaments around the cell nucleus whereas the microtubules remained dispersed into the cell periphery. These results suggest that one of the functions carried out by p60src, the protein kinase responsible for transformation by Rous sarcoma virus, may be to modify the component(s) involved in the putative linkages between microtubules and intermediate filaments in the normal cells.

Animals↗
PubMed1981

Ultrastructure of chicken cardiac muscle as studied by double immunolabeling in electron microscopy.

The ultrastructural localization of alpha-actinin and vinculin in chicken cardiac muscle was studied by double indirect immunoelectron microscopy, using ferritin and iron-dextran (Imposil) as the electron-dense markers conjugated to the secondary antibodies, on ultrathin frozen sections of fixed tissue. Fixation and immunolabeling procedures were developed that permitted maximal retention of the two proteins at their natural sites as well as their adequate labeling. alpha-Actinin was found both on the Z-bands, as expected, and near the fascia adherens of the intercalated discs, whereas vinculin was confined to the latter sites. At the fascia adherens, the double labeling results clearly showed that vinculin was situated closer to the membrane than was alpha-actinin. These results, coupled with earlier observations, suggest that vinculin may participate in the linkage of actin-containing microfilament bundles to membranes in a variety of cell types.

Actinin↗
PubMed1981

Passage of an integral membrane protein, the vesicular stomatitis virus glycoprotein, through the Golgi apparatus en route to the plasma membrane.

The intracellular pathway of biogenesis of the vesicular stomatitis virus transmembrane glycoprotein was investigated in situ by using indirect immunofluorescence of whole infected Chinese hamster ovary cells and immunoelectron microscopy of ultrathin frozen sections of infected cells. Transport of the glycoprotein was synchronized by using the temperature-sensitive virus mutant Orsay-45 and a temperature shift-down protocol. Sequential appearance of the glycoprotein in the rough endoplasmic reticulum, Golgi apparatus, and plasmalemma was demonstrated. The potential of this system for further studies is discussed.

Animals↗
PubMed1981

Immunoelectron microscope studies of membrane-microfilament interactions: distributions of alpha-actinin, tropomyosin, and vinculin in intestinal epithelial brush border and chicken gizzard smooth muscle cells.

The ultrastructural localization of three cytoskeletal proteins, alpha-actinin, tropomyosin, and vinculin, in the brush border of epithelial cells of chicken small intestine and the smooth muscle cells of chicken gizzard was studied by immunofluorescence and immunonelectron microscope labeling of frozen sections of lightly fixed, intact tissues. In the immunoelectron microscope studies, a recently described new type of electron-dense antibody conjugate, imposil-antibody, has been successfully used, along with ferritin-antibody conjugates, in single and double immunolabeling experiments. In the intestinal brush border shows that vinvulin is sharply confined to the junctional complex close to the membrane region of the zonula adherens, in distinct contrast to the more diffuse distributions of the other two proteins. In the smooth muscle cells, the labeling patterns show that vinculin is sharply confined to the membrane-associated dense plaques, closer to the membrane than the alpha-Actinin is also present in the cytoplastic dense bodies, from which vinculin is absent. Tropomyosin is present diffusely distributed in the cytoplasm, but absent from both dense plaques and dense bodies. These findings with the muscle cells demonstrate, therefore, that the dense plaques and dense bodies are chemically and structurally distinct entities. The results with both tissues, along with those in previous papers (Geiger, 1979, Cell. 18:193-205.; Geiger et al., 1980, Proc. Natl. Acad. Sci. U. S. A. 77:4127-4131), suggest that vinculin may play an important and widespread role in the linkage of actin-containing microfilament bundles to membranes.

Actinin↗