A Ca2+-dependent actin modulator from vertebrate smooth muscle.

A protein of Mr approximately 85 000 has been isolated and purified from pig stomach smooth muscle that modulates the polymer state of actin in a Ca2+-dependent manner. When added either to performed F-actin filaments or to G-actin, prior to polymerisation, the modulator induces the formation of shorter filaments. The average filament length in the presence of the modulator is directly dependent on its molar ratio to actin indicating a stoichiometric rather than a catalytic type of interaction. When mixed with G-actin the modulator forms a stable complex with two actin monomers; this complex is presumed to act as a potent nucleus for actin polymerisation. The dynamics of the interaction with F-actin suggests a direct severing of actin filaments by the modulator via a binding to intrafilamentous actins.