Upon removal of nutrients, the amoebae of the cellular slime mold Dictyostelium discoideum differentiate into dormant spores which survive starvation stress. In this study, we demonstrate that half of the actin molecules in the spores are tyrosine-phosphorylated. The phosphorylated actin is distributed around immobile crenate mitochondria and vesicles, as well as in the cytoplasm of the spores. The actin isolated from spore lysates contains phosphorylated and unphosphorylated forms at the same molar ratio as that of the original whole spore lysate. Under actin polymerizing conditions they form actin filaments and then they are completely depolymerized under actin depolymerizing conditions, indicating that tyrosine phosphorylation of actin may not prohibit actin polymerization nor stimulate depolymerization. The phosphorylation levels increase at the end of the culmination stage when spores have matured morphologically and physiologically, and reach maximum levels after an additional 12 hours of development. The levels are stable for 20 days following spore maturation, and decline to undetectable levels within the next 10 days. Spores having high levels of phosphorylation show high viability, and vice versa. Following activation of spores with nutrient medium containing spore germination promoters, the phosphorylation levels quickly decrease with a half-life of about 5 minutes. After 20 minutes spores begin to swell. At this later time, most of the phosphorylated actin already has been dephosphorylated. Also, in heat-activated spores actin dephosphorylation occurs prior to spore swelling. However, addition of phosphatase inhibitors following heat-activation, prevented spore swelling and dephosphorylation of actin. Our data indicate that the high levels of actin tyrosine phosphorylation, specific to the spore stage, may be required for maintaining dormancy to withstand starvation stress. The rapid dephosphorylation of actin leads to a reactivated dynamic actin system which participates in spore swelling, vesicle movement, and mitochondrial shape changes during the spore germination process.

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