Isolation of an abundant 50,000-dalton actin filament bundling protein from dictyostelium amoebae

A monomeric actin bundling protein with a native molecular weight of approximately 50,000 (ABP-50) has been isolated from amoebae of Dictyostelium discoideum. ABP-50 cross-links F-actin to form tightly packed bundles, some of which are highly ordered. It exhibits a K_d of 2.1 μM and a molar ratio to actin of 1:1 in bundles. Calcium and ATP at physiological concentrations have no effect on these activities. ABP-50 is immunologically unrelated to 30-kDa protein, a previously described bundling protein from Dictyostelium. Immunofluorescence with affinity-purified polyclonal antibodies indicates that ABP-50 is localized in regions of the amoeboid cell cortex containing actin bundles. The molar ratio of ABP-50 to actin is approximately 1:5 in vivo. Therefore, the abundance of ABP-50 suggests that it may be responsible for the majority of the bundling activity in these cells.