A Ca2+-sensitive actin regulatory protein from smooth muscle

Using a procedure involving DNase I affinity chromatography and Sephadex G-200 gel filtration, we partially purified a Ca2+-sensitive actin regulatory 90-kDa protein from bovine aorta. The 90-kDa protein existed in the form of a complex with actin on a DNase I column even in the presence of 5 mM EGTA, indicating that the 90-kDa protein binds tightly to actin in a Ca2+-insensitive manner. The isolation procedure described above indicates that the 90-kDa protein is present in smooth muscles including aorta, uterus and bladder, but not in skeletal and heart muscles. When added to G-actin before polymerization, the 90-kDa protein increases the initial rate of actin polymerization and lowers the final viscosity at Ca2+ concentrations greater than 10(-7) M. This decrease in viscosity is due to the generation of filaments which cannot be readily sedimented.