Functions as a negative regulator of actin polymerization. Modulates actin/myosin II at cortex actinomyosins to prevent excessive F-actin polymerization around the cell periphery, thereby maintaining proper cell shape during phagocytosis and chemotaxis. Associates with mhcA. Null mutants show the remarkable ability to engulf multiple particles simultaneously, also positively respond to a cAMP gradient but form spurious peudopods which are normally suppressed to promote efficient movement up a chemical gradient.