Modulates actin dynamics and cell motility. Expression peaks at early stages of development at the time cells become highly chemotactic and aggregation competent. Cells form smooth plaques on bacterial lawns, produce abnormally small fruiting bodies and display reduced migration towards the cAMP source in chemotactic assays. Analysis of cell motion in cAMP gradients reveals decreased speed suggesting a defect in the cellular machinery for motility. Cells exhibit changes in the actin cytoskeleton, showing aberrant distribution of F-actin in fluorescence cell staining and an increased amount of cytoskeleton associated actin. There is also excessive pseudopod formation. Expressing cosA or it's human counterpart restores wild-type like cAMP chemotaxis response. Belongs to the costars family.