Abstract
Cold extracts of Acantharnoeba castellanii in polymerizing buffer contain 32 μM unpolymerized actin of which about 20% polymerizes (as measured by ultracentrifugation) when the extract is warmed to 22°C. As quantified by the increase in fluorescence of pyrene-labeled actin, 16% of muscle G-actin and 46% of Acanthamoeba G-actin polynterized when 0.8 μM of each was added to warm extracts of Acanthamoeba. Added muscle F-actin (1.2 μM) rapidly and totally depolymerized and then partially repolymerized whereas 1.2 μM 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.
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