Phalloidin associates with microfilaments after microinjection into tissue culture cells

Phalloidin is a drug, which specifically binds to F-actin. Tissue culture cells were microinjected with phalloidin and the intracellular display of microfilament-associated proteins in such cells and in cytoskeletons prepared from them was followed by immunofluorescence microscopy using antibodies against actin, myosin, tropomyosin and alpha-actinin. When phalloidin concentrations of 0.2 mM were used, cells and cytoskeletons revealed in addition to the stress fibers aberrant microfilament arrangements ("islands") underneath the upper membrane. These islands contain in addition to actin all the microfilament-associated proteins. Cytockeletons were stabilized by phalloidin against the actin-depolymerization effect of 0.6 M Kl. This resistance of phalloidin-bound actin was used to localize the drug not only in the induced islands but also in the stress fibers. Phalloidin-injected cells showed the same response to cytochalasin B as normal cells and phalloidin did not interfere with ATP-induced contraction of glycerinated models. These results indicate that phalloid-in-stabilized filamentous actin can still give rise to contraction within the stress fiber system, and that F-actin bound to phalloidin may be translocated within the cell.