Direct visualization of bipolar myosin filaments in stress fibers of cultured fibroblasts.

The authors examined the molecular organization of myosin in stress fibers (microfilament bundles) of cultured mouse embryo fibroblasts. To visualize the organization of myosin filaments in these cells, fibroblast cytoskeletons were treated with gelsolin-like protein from bovine brain (hereafter called brain gelsolin), which selectively disrupts actin filaments. As shown earlier [Verkhovsky et al., 1987], this treatment did not remove myosin from the stress fibers. The actin-free cytoskeletons then were lightly sonicated to loosen the packing of the remaining stress fiber components and fixed with glutaraldehyde. Electron microscopy of platinum replicas of these preparations revealed dumbbell-shaped structures of approximately 0.28 μm in length, which were identified as bipolar myosin filaments by using antibodies to fragments of myosin molecule (subfragment I and light meromyosin) and colloidal gold label. Bipolar filaments of myosin in actin-free cytoskeletons were often organized in chains and lattices formed by end-to-end contacts of individual filaments at their head-containing regions. Therefore, after extraction of actin, it was possible for the first time to display bipolar myosin filaments in the stress fibers of cultured cells.