The effect of chemoattractants such as cyclic AMP and folate on amoebae of the cellular slime mould Dictyostelium discoideum is to cause a series of rapid intracellular responses. One of the most rapid of these responses is the polymerization of actin associated with the cytoskeleton, an event correlated with pseudopodium formation, which occurs within 3–5 s of chemotactic receptor stimulation. We report that this response can be mimicked by addition of 5 microM-inositol 1,4,5-triphosphate (IP3) or by addition of 100 microM-Ca2+ to saponin-permeabilized amoebae. The data suggest that cytoskeletal actin polymerization occurs in normal cells as a result of IP3 formation in response to cell surface receptor stimulation and the consequent release of Ca2+ from internal stores.

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