The extraction of isolated vertebrate smooth muscle cells at high and low ionic strength yields cell ghosts which are seen in the electron microscope to be composed of a complex network of 10-nm filaments, together with residual actin. After SDS-gel electrophoresis of the cell ghosts only 2 bands may be recognized, one corresponding to actin and the other migrating at about 55 000 mol. wt that arises from the 10-nm filaments. The 10-nm filaments are extremely sensitive to proteolysis and are absent from cells exposed to crude collagenase in the presence of Triton X-100. Such cells, lacking 10-nm filaments, still contract in response to ATP. The data indicate that the 10-nm filaments are not essential for contraction, but rather form a specialized intracellular cytoskeleton. While completely insoluble in concentrated salt solutions the 55 000 mol. wt protein is readily extracted with acetic acid from homogenized and salt-extracted smooth muscle residue. The extracted protein reassembles, on dialysis, into filaments of about 10-nm diameter and has an amino acid composition almost identical to that deduced for vertebrate neurofilaments. From the cytoskeletal role that the 10-nm filaments play in smooth muscle and, as appears likely, in other cell types the filament protein has been tentatively termed ‘skeletin’. Results relating to the proportion of skeletin in smooth muscle and the structure of the 10-nm filaments are described and discussed.

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