Intermediate (skeletin) filaments in heart Purkinje fibers. A correlative morphological and biochemical identification with evidence of a cytoskeletal function.

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Abstract
Bovine Purkinje fibers contain free cytoplasmic filaments which consistently differ in ultrastructural appearance from actin and myosin filaments, irrespective of preparation technique. Fixation and staining techniques influenced the filament diameter, which was 7.4-9.5 nm for filaments in plastic-embedded material and 7.0 nm in cryo-sectioned material, thus intermediate compared to actin and myosin filaments. Cross-sectional profiles suggested that intermediate-sized filaments are composed of 4 subfilaments. Extraction procedures were carried out to remove other cell organelles to study the filaments. EM showed that undulating bundles of intermediate filaments converging towards desmosomes remained after the extractions with Z-disk material. The shape of individual cells and assemblies of cell bundles remained intact confirming that the intermediate filaments of cow Purkinje fibers together with desmosomes have a cytoskeletal function. On account of the cytoskeletal function of the filaments similarities to smooth muscle 100-.ANG. filament protein subunit skeletin and the inadequate and confusing existing terminology, the filaments should be named skeletin filaments.