We report changes in actin associated with the Triton-insoluble cytoskeleton following chemotactic stimulation in five species of cellular slime mould, Dictyostelium discoideum, D. mucoroides, D. purpureum, D. lacteum and Polysphondylium violaceum. Stimulation of amoebae with the chemoattractant specific for each species elicits a rapid increase in cytoskeletal actin content with a peak at 5 s, probably corresponding to pseudopodium formation. Subsequent changes consist of a second discrete

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... of actin accumulation and, in at least two species, D. discoideum and P. violaceum, a third peak. We correlate these later changes in actin content with the changes in amoebal shape that have been reported to be a consequence of chemotactic stimulation. We have also investigated species variation in actin content and find D. purpureum to have a remarkably low cytoskeletal actin content in comparison to other species. Whole cell and cytoskeletal actin levels during the first 12 h of D. discoideum development have also been analysed. While little change is found in total protein content, whole cell actin shows an abrupt decline during aggregation and this is accompanied by an increase in cytoskeletal actin, amounting to 100% over 0-12 h of development. These results are discussed in the light of a possible changing role for actin during the course of development, from being involved in the dynamic events of chemotaxis to assuming a more structural role in the multicellular aggregate.