AbstractThis study report the first development of a fluorescently labeled filamin. Smooth muscle was labeled with fluorscent dyes in order to study its interaction with stress fibers and myofibrils, both in living cells and in permeabilized cells. The labeled filamin bounds to the Z bands of isolated cross‐striated myofibrils and to the Z bands and intercalated discs in both permeabilized embryonic cardiac myocytes and in frozen sections of adult rat venticle. In permeabilized embryonic chick myotubes, filamin bound to early myotubes but was absent at later stages. In living embryonic chick myotubes, the fluorescently labeled filamin was incorporated into the Z bands of myofibirls during early and late stages of develoment but was absent during an intermediate stages. In living cardiac myocytes, filamin‐IAR was incorporated into nascent as well as fully formed sarcomeres throughout develoment. In permeabilized nonmuslce cells, labeled filamin bound to attachment plaques and foci of polygonal networks and to the dense bodies in stress fibers. The periodic bands of filamin in stress fibers had a longer spacing in fibroblasts than in epithelial cells. When injected into living cells, filamin was readily incorporated into stress fibers in a striated pattern. The fluorescent filamin bands were broader in injected cells, however, than they were in permeabilized cells. We have interpreted these results from living and permeabilized cells to mean that native filamin is distributed along the full lengh of the actin filaments in the stress fibers, with a higher concentration present in the dense bodies. A sarcomeric model is presented indicating the position of filamin with respect to other proteins in the stress fib
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