Redistribution of contractile and cytoskeletal components induced by cytochalasin. I. In Hmf cells, a nontransformed fibroblastoid line.

Hmf cells are normal rat fibroblastoid cells of large size having an extensive stress fiber (cable) system. On exposure to cytochalasin D (CD), shortening and segmentation of the actin-based cables and diffusion of the normal periodic distribution of tropomyosin and myosin occur, concomitant with generalized cell retraction. During retraction, areas of extended cytoplasm may be pulled apart and torn. The actin, tropomyosin, and myosin of the stress fibers become localized in dispersed masses represented in the electron microscope as compact filamentous felt-like networks. Many of these are derived from shortening of stress fibers at their insertions into the persisting attachment plaques. In a few cells, rod-like elements of variable length remain; these are CD-resistant segments of uncontracted stress fibers. Inhibitors of energy metabolism prevent these changes. Microtubules remain unaltered but are passively displaced in the CD-deformed cells. Bundles of 10 nm filaments maintain close relations with the actomyosin masses resulting from CD-treatment. Evidence is considered for the hypothesis that cellular retraction, the apparent disorganization of stress fibers, and the redistribution of contractile proteins result from unremittent energy-dependent contraction induced by CD, and that the compact forms may be analogous to rigor complexes. A mechanism for these actions of cytochalasin is proposed.