Enhancement of actin-depolymerizing factor/cofilin-dependent actin disassembly by actin-interacting protein 1 is required for organized actin filament assembly in the Caenorhabditis elegans body wall muscle

Regulated disassembly of actin filaments is involved in several cellular processes that require dynamic rearrangement of the actin cytoskeleton. Actin-interacting protein (AIP) 1 specifically enhances disassembly of actin-depolymerizing factor (ADF)/cofilin-bound actin filaments. In vitro, AIP1 actively disassembles filaments, caps barbed ends, and binds to the side of filaments. However, how AIP1 functions in the cellular actin cytoskeletal dynamics is not understood. We compared biochemical and in vivo activities of mutant UNC-78 proteins and found that impaired activity of mutant UNC-78 proteins to enhance disassembly of ADF/cofilin-bound actin filaments is associated with inability to regulate striated organization of actin filaments in muscle cells. Six functionally important residues are present in the N-terminal β-propeller, whereas one residue is located in the C-terminal β-propeller, suggesting the presence of two separate sites for interaction with ADF/cofilin and actin. In vitro, these mutant UNC-78 proteins exhibited variable alterations in actin disassembly and/or barbed end-capping activities, suggesting that both activities are important for its in vivo function. These results indicate that the actin-regulating activity of AIP1 in cooperation with ADF/cofilin is essential for its in vivo function to regulate actin filament organization in muscle cells.