Purification and characterization of actobindin, a new actin monomer-binding protein from Acanthamoeba castellanii

Actobindin is a new actin-binding protein isolated from Acanthamoeba castellanii. It is composed of two possibly identical polypeptide chains of approximately 13,000 daltons, as determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis, and with isoelectric points of 5.9. In the native state, actobindin appears to be a dimer of about 25,000 daltons by sedimentation equilibrium analysis. It contains no tryptophan and probably no tyrosine. Actobindin reduces the concentration of F-actin at steady state and inhibits the rate of filament elongation to extents consistent with the formation of a 1:1 actobindin-G-actin complex in a reaction with a KD of about 5 microM. The available data do not eliminate the possibility of other stoichiometries for the complex, but they are not consistent with any significant interaction between actobindin and F-actin. Despite the similarities between the effects of actobindin and Acanthamoeba profilin on the polymerization of Acanthamoeba actin, the two proteins are quite distinct with different native and subunit molecular weights, different isoelectric points, and different amino acid compositions. Also, unlike profilin, actobindin binds as well to rabbit skeletal muscle G-actin and to pyrenyl-labeled G-actin as it does to unmodified Acanthamoeba G-actin.