Actin polymerization and pseudopod extension during amoeboid chemotaxis

Amoebae of the cellular slime mold Dictyostelium discoideum are an excellent model system for the study of amoeboid chemotaxis. These cells can be studied as a homogeneous population whose response to chemotactic stimulation is sufficiently synchronous to permit the correlation of the changes in cell shape and biochemical events during chemotaxis. Having demonstrated this synchrony of response, we show that actin polymerization occurs in two stages during stimulation with chemoattractants. The assembly of F-actin that peaks between 40 and 60 sec after the onset of stimulation is temporally correlated with the growth of new pseudopods. F-actin, which is assembled by 60 sec after stimulation begins, is localized in the new pseudopods that are extended at this time. Both stages of actin polymerization during chemotactic stimulation involve polymerization at the barbed ends of actin filaments based on the cytochalasin sensitivity of this response. We present a hypothesis in which actin polymerization is one of the major driving forces for pseudopod extension during chemotaxis. The predictions of this model, that localized regulation of actin nucleation activity and actin filament cross-linking must occur, are discussed in the context of current models for signal transduction and of recent information regarding the types of actin-binding proteins that are present in the cell cortex.