title: A network of 2-4 nm filaments found in sea urchin smooth muscle. Protein constituents and in situ localization.
abstract: In this report the coisolation of two proteins from sea urchin smooth muscle of apparent molecular weights (Mr) 54 and 56 kD respectively, as determined on SDS-PAGE, is described. Like the intermediate filament proteins, these two proteins are insoluble in high ionic strength buffer solution. On two-dimensional gel electrophoresis and by immunological methods it is shown that these proteins are not related (by these criteria) to rat smooth muscle desmin (54 kD) or vimentin (56 kD). Furthermore, in conditions where both desmin and vimentin assemble in vitro into 10 nm filaments, the sea urchin smooth muscle proteins do not assemble into filaments. Ultrastructural studies on the sea urchin smooth muscle cell show that the thin and thick filaments organization resembles that described in the vertebrate smooth muscle. However, instead of 10 nm filaments, a network of filaments, 2-4 nm in diameter, is revealed, upon removal of the thin and thick filaments by 0.6 M KCl treatment. By indirect immunofluorescence microscopy, and in particular by immunocytochemical electron microscopy studies on the sea urchin smooth muscle cell, it is shown that the antibodies raised against both 54 and 56 kD proteins appear to specifically label these 2-4 nm filaments. These findings indicate that both the 54 and 56 kD proteins might be constituents of this category of filaments. The possible significance of this new cytoskeletal element, that we have named echinonematin filaments, is discussed.
mesh_terms: D000818:Animals; D003598:Cytoskeletal Proteins / Q000032:analysis* / Q000276:immunology / Q000302:isolation & purification; D003599:Cytoskeleton / Q000032:analysis / Q000648:ultrastructure; D004591:Electrophoresis, Polyacrylamide Gel; D005260:Female; D005455:Fluorescent Antibody Technique; D007158:Immunologic Techniques; D051379:Mice; D008854:Microscopy, Electron; D008856:Microscopy, Fluorescence; D008970:Molecular Weight; D009124:Muscle Proteins / Q000032:analysis* / Q000276:immunology / Q000302:isolation & purification; D009130:Muscle, Smooth / Q000032:analysis* / Q000648:ultrastructure; D011108:Polymers; D051381:Rats; D011919:Rats, Inbred Strains; D012617:Sea Urchins / Q000032:analysis / Q000648:ultrastructure*; D012995:Solubility
chemical_list: D003598:Cytoskeletal Proteins; D009124:Muscle Proteins; D011108:Polymers