Three-dimensional immunogold localization of alpha-actinin within the cytoskeletal networks of cultured cardiac muscle and nonmuscle cells.

The ultrastructural distribution of alpha-actinin was studied in cultured hamster heart cells by immunogold replica electron microscopy. This technique enabled us to localize alpha-actinin within the cytoskeletal networks at high resolution and in three dimensions. Colloidal gold, indicating the presence of alpha-actinin, was localized on the Z bands of nascent myofibrils in myocytes and on stress fiber bundles in nonmuscle cells. alpha-Actinin staining was also seen on stellate foci, where cytoskeletal filaments converged along the inner myocyte cell membranes. Intermediate filaments were associated with Z bands of myofibrils, stress fibers, and subplasmalemmal actin networks at the specific points where alpha-actinin was localized on these structures. Heavy meromyosin treatment prior to immunostaining confirmed that the thin filaments contained actin. These results suggest that alpha-actinin serves to interlink these various cytoskeletal elements. In addition, this protein may be involved in the initial phases of filament organization during myofibrillogenesis along the inner surface of the myocyte plasma membrane.