title: Transmembrane orientation of the fibronectin receptor complex (integrin) demonstrated directly by a combination of immunocytochemical approaches.
abstract: The avian 140-KD cell adhesion receptor or "integrin," a complex of three glycoproteins with molecular masses averaging 140 KD, interacts with extracellular fibronectin and forms a linkage complex that co-localizes with intracellular actin. To probe the molecular interactions involved in this linkage complex, we used monoclonal antibodies and a combination of immunolocalization approaches to determine whether any component was transmembrane. Immunoadsorption and immunoblotting experiments demonstrated that anti-120-KD Mabs recognized the band 3 component of integrin isolated from chicken embryo fibroblasts (CEF) by JG22E immunoaffinity chromatography, and they co-localize with anti-fibronectin and polyclonal anti-integrin at cell contact sites in double-labeling experiments. Immunofluorescence experiments involved comparisons of double-labeled intact cells or substrate-attached, ventral plasma membrane "rip-off" fragments, using anti-fibronectin and each of the anti-120-KD Mabs. The extracellular faces of living intact cells were strongly labeled by a majority (approximately 70%) of the anti-120-KD Mabs at fibronectin-membrane attachment sites. The remainder (approximately 30%) labeled intact cells weakly or not at all. However, although the anti-120-KD Mab ES186 did not stain living cells, it did demonstrate positive staining above substratum contact sites over entire isolated rip-off membranes. In contrast, Mabs directed against putative extracellular epitopes and anti-fibronectin antibodies did not label these sites at the center of rip-offs unless the membranes were detergent permeabilized. Proteolysis experiments suggested that the ES186 epitope was located at one end of the molecule, since removal of short fragments from integrin band 3 concomitantly removed or destroyed the ES186 epitope, whereas the extracellular epitopes still remained. These experiments directly demonstrate that integrin band 3 is a transmembrane polypeptide with at least one epitope recognized by anti-120-KD Mabs on the cytoplasmic side of the plasma membrane and at least one epitope on the extracellular cell surface.
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal; D000954:Antigens, Surface / Q000276:immunology / Q000378:metabolism*; D015815:Cell Adhesion Molecules; D002462:Cell Membrane / Q000648:ultrastructure; D002478:Cells, Cultured; D002642:Chick Embryo; D003593:Cytoplasm / Q000276:immunology; D000939:Epitopes; D005110:Extracellular Space / Q000276:immunology; D007150:Immunohistochemistry; D008562:Membrane Glycoproteins / Q000276:immunology / Q000378:metabolism*; D008970:Molecular Weight; D010446:Peptide Fragments / Q000032:analysis; D017469:Receptors, Fibronectin; D011971:Receptors, Immunologic / Q000276:immunology / Q000378:metabolism*
chemical_list: D000911:Antibodies, Monoclonal; D000954:Antigens, Surface; D015815:Cell Adhesion Molecules; D000939:Epitopes; D008562:Membrane Glycoproteins; D010446:Peptide Fragments; D017469:Receptors, Fibronectin; D011971:Receptors, Immunologic