Molecular biology of gelsolin, a calcium-regulated actin filament severing protein

Gelsolin is a Ca²⁺-binding protein of mammalian leukocytes, platelets and other cells which has multiple and closely regulated powerful effects on actin. In the presence of micromolar Ca²⁺, gelsolin severs actin filaments, causing profound changes in the consistency of actin polymer networks. A variant of gelsolin containing a 25-amino acid extension at the NH₂-terminus is present in plasma where it may be involved in the clearance of actin filaments released during tissue damage. Gelsolin has two sites which bind actin cooperatively. These sites have been localized using proteolytic cleavage and monoclonal antibody mapping techniques. The NH₂-terminal half of the molecule contains a Ca²⁺-insensitive actin severing domain while the COOH-terminal half contains a Ca²⁺-sensitive actin binding domain which does not sever filaments. These data suggest that the NH₂-terminal severing domain in intact gelsolin is influenced by the Ca²⁺-regulated COOH-terminal half of the molecule. The primary structure of gelsolin, deduced from human plasma gelsolin cDNA clones, supports the existence of actin binding domains and suggests that these may have arisen from a gene duplication event, and diverged subsequently to adopt their respective unique functions. The plasma and cytoplasmic forms of gelsolin are encoded by a single gene, and preliminary results indicate that separate mRNAs code for the two forms. Further application of molecular biological techniques will allow exploration into the structural basis for the multifunctionality of gelsolin, as well as the molecular basis for the genesis of the cytoplasmic and secreted forms of gelsolin.