Goniodomin A, one of the polyether macrolides, isolated from the dinoflagellate Goniodoma pseudogoniaulax stimulated skeletal actomyosin ATPase activity 2-3 times, with a peak at 3 x 10(-8) to 3 x 10(-7) M. The effect of goniodomin A was dependent on the concentration of actin, but not of myosin. The actomyosin ATPase activity was increased by pretreatment of actin (but not of myosin) with goniodomin A. Goniodomin A induced a sustained and concentration-dependent increase in the fluorescence intensity (excitation wavelength, 277 nm; emission wavelength, 329 nm) of actin. The maximum response was obtained with concentrations of goniodomin A in the 10(-5) to 10(-4) M range in the presence of 5 microM F-actin. However, the ATPase activity and fluorescence intensity of myosin were not changed by goniodomin A at concentrations from 10(-8) to 10(-5) M. Interestingly, goniodomin A induced a remarkable but transient increase in the fluorescence intensity of actomyosin in a concentration-dependent manner, with a peak at 3 x 10(-7) M. This profile was quite similar to that found in the stimulation of the actomyosin ATPase activity induced by goniodomin A. To investigate further the effect of goniodomin A, actin was labeled with N-(1-pyrenyl)iodoacetamide. Goniodomin A at 10(-6) M had no effect on the fluorescence intensity of pyrenyl-actin (excitation wavelength, 365 nm; emission wavelength, 407 nm), but increasing concentrations of goniodomin A to 3 x 10(-6) M remarkably decreased its intensity. This effect was potentiated by heavy meromyosin. Actin molecules treated with goniodomin A were completely sedimented by mild centrifugation (for 15 min at 12,000 x g). Electron microscopic observations suggest that actin filaments associate with each other to form a gel in the presence of 3 x 10(-6) M goniodomin A. The conformational change of actin molecules, resulting from stoichiometric binding of goniodomin A to actin monomers in filaments, may modify the interaction between actin and myosin.