Actin forming linear actin filaments builds up the acto-myosin cortex in cells providing mechanical stability, enabling vesicle transport, cell migration and cell division. This versality is achieved by interaction with many regulatory and actin binding proteins and the interplay with the plasma membrane to locally control actin filament length, stability and crosslinking to other filaments. Here, we report about a newly identified protein domain, Curly, within IQGAP family proteins that binds actin and causes bending of individual actin filaments into tight rings when anchored to a lipid membrane in vitro. Membrane anchored Curly induces reproducibly and efficiently F-actin ring formation of characteristic curvature (0.5-1 µm−1) and recognizes F-actin orientation. Interestingly, decoration of F-actin with the tropomyosin increases actin ring formation, while Fimbrin abrogates it. Addition of myosin II filaments results in further formation of actin rings by induced f-actin flow and, more importantly, in robust ring contraction. This constitutes, hence, a minimal (four components) contractile ring system. The discovery of Curly provides a new opportunity to address and control F-actin curvature in cells and in vitro and will open new approaches to generate synthetic cells capable of division and controlled shape changes.