Rabbit skeletal muscle actin behaves differently than Acanthamoeba actin when added to soluble extracts of Acanthamoeba castellanii.

Cold extracts of Acanthamoeba castellanii in polymerizing buffer contain 32 microM unpolymerized actin of which about 20% polymerizes (as measured by ultracentrifugation) when the extract is warmed to 22 degrees C. As quantified by the increase in fluorescence of pyrene-labeled actin, 16% of muscle G-actin and 46% of Acanthamoeba G-actin polymerized when 0.8 microM of each was added to warm extracts of Acanthamoeba. Added muscle F-actin (1.2 microM) rapidly and totally depolymerized and then partially repolymerized whereas 1.2 microM added Acanthamoeba F-actin was stable indefinitely. Furthermore, muscle actin subunits were completely removed from copolymers of muscle and Acanthamoeba F-actin while all the amoeba actin remained polymerized when the copolymers contained at least 50% amoeba actin. These results suggest that exogenous tracer actin may not be an accurate indicator of the dynamics of endogenous actin in extracts and cells.