Cofilin is a low‐molecular weight actin‐modulating protein, which binds to, severs, and depolymerizes actin filaments in vitro. Aip1, an actin‐interacting protein, was recently identified as a product of a gene on a multicopy plasmid which suppresses the temperature‐sensitive phenotype of a cofilin mutant in Saccharomyces cerevisiae. Actin cytoskeleton plays an essential role in resistance to hyperosmotic stress in Dictyostelium discoideum. The roles of cofilin and Aip1 in this resistance are not known.

In response to hyperosmotic stress, D. discoideum cells round up. This stress‐induced morphological change involves the redistribution of cofilin, together with actin filaments, into cortical contractile portions of the cells, followed by their contraction. Over‐expression of cofilin increases and thickens cortical actin bundles in cells. The bundles become tight and are reorganized into a ring‐shaped structure in response to hyperosmotic stress. The ring structure of actin bundles had two characteristic bands across them; bright and dark bands, heavily stained and not stained with phalloidin. In the bundles, straight filaments with a diameter of 5.3‐nm were aligned parallel by cross‐bridge structures. In cells lacking the myosin‐II heavy chain, the bundles, which were induced by an over‐expression of cofilin, shortened and became straight following hyperosmotic stress, forming a polygonal structure. D. discoideum Aip1/Wrp2 enhanced the severing of actin filaments by cofilin in vitro and colocalized with cofilin in cells, including those that were over‐expressing cofilin before and after exposure to hyperosmotic stress.

Cofilin plays a pivotal role in concert with Aip1/Wrp2 in the reorganization of actin architectures into bundles that contract in a myosin‐II‐independent manner, in response to hyperosmotic stress.