Purification and Characterization of an Actin-, Calmodulin- and Tropomyosin-Binding Protein from Chicken Gizzard Smooth Muscle.

An actin-binding protein (p33) has been purified from chicken gizzard smooth muscle. The homogenous protein has a molecular weight near 33000 as determined by both sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and size exclusion chromatography. Its binding ability to F-actin remained after heating at 95 degrees C for 4 min. Immunoblot analyses indicated that p33 was not a degradation product from higher molecular components. The binding of p33 to F-actin was saturable in a molar ratio of about one p33 to 2-3 actin molecules with an apparent binding constant of 6.6 x 10(7) M-1. p33 also bound to calmodulin and tropomyosin. The bindings of p33 to F-actin and tropomyosin were regulated by calmodulin in a Ca(2+)-dependent fashion. In addition to actin, caldesmon and tropomyosin, p33 was contained in the native thin filaments prepared from smooth muscle. Other actin-binding proteins, including alpha-actinin, caldesmon and filamin, had little effect on p33 binding to actin filaments. These results demonstrate that p33 may function in actin-based cellular processes which are mediated by Ca2+ and calmodulin.