STRUCTURAL STUDIES ON THE RIBBON-TO-HELIX TRANSITION IN PROFILIN-ACTIN CRYSTALS

Knowledge of the structure of actin in its various conformational stat es is important for understanding the diverse motile activities carrie d out by eukaryotic cells. Profilin:actin crystals provide a unique sy stem for studying conformational states of actin, because they exhibit a high degree of polymorphism in response to environmental conditions while maintaining crystalline order. A preliminary comparison of two states of profilin:beta-actin crystals shows that crystal polymorphism involves movements of actin subdomains at hinge points homologous to those found in hexokinase, a protein whose polypeptide fold is related to actin. The homology of the hinge points in actin to those in hexok inase suggests that actin subdomain movements in profilin:beta-actin c rystals have functional significance. We discuss how these movements c ould be related to structural transitions between states of filamentou s actin in muscle contraction.