Kaptin Functions as a Barbed-End Binding Protein to Control Actin Filament Dynamics

Precise regulation of the actin cytoskeleton is fundamental to cellular morphology, motility, and intracellular transport. While key classes of actin-binding proteins, including nucleators, capping proteins, and bundlers, have been well characterized, additional modulators likely contribute to the spatial and temporal control of actin dynamics. Here, we identify Kaptin (KPTN), a protein localized to actin-rich structures at the cell periphery, as a novel regulator of actin filament dynamics. Using biochemical reconstitution and single-molecule TIRF microscopy, we demonstrate that KPTN binds to actin filament barbed ends and suppresses filament elongation. This activity leads to filament stabilization and bundling, suggesting a dual role in filament architecture maintenance. Structural prediction via AlphaFold classifies KPTN within the WD-repeat-containing protein family and highlights a conserved, positively charged residue within its predicted N-terminal β-propeller domain as essential for actin interaction. These findings uncover a novel mechanism by which KPTN regulates actin dynamics and establish it as both a barbed-end and side-binding protein within the actin cytoskeletal network.