Actin-based motility in a normal and Aip-1 mutant neutrophil. (A) A normal neutrophil polarized after exposure to a chemoattractant (placed in the top right corner). The lamellipodia to the right is filled with dendritic (branched) networks of actin filaments. The actin filaments are magnified to the right. In step 1, cofilin binds to the actin filaments and breaks apart the dendritic networks, leaving long actin filaments. In step 2, activation by Aip1 results in efficient severing and complete disassembly of actin filaments. (B) A mutant Aip1 neutrophil. A dense network of actin filaments is found throughout the cell, preventing effective polarization or directional amoeboid movement. The nuclei are pushed to the periphery of the cell. Note in the magnified view that the actin filaments remain intact. The amount of mutant Aip1 protein is decreased and the reduced amounts of mutant protein fail to activate cofilin. Professional illustration by Somersault18:24.