HIGH-LEVELS OF ACTIN TYROSINE PHOSPHORYLATION - CORRELATION WITH THE DORMANT STATE OF DICTYOSTELIUM SPORES

Upon removal of nutrients, the amoebae of the cellular slime mold Dict yostelium discoideum differentiate into dormant spores which survive s tarvation stress. In this study, we demonstrate that half of the actin molecules in the spores are tyrosine-phosphorylated. The phosphorylat ed actin is distributed around immobile crenate mitochondria and vesic les, as well as in the cytoplasm of the spores. The actin isolated fro m 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, i ndicating that tyrosine phosphorylation of actin may not prohibit acti n polymerization nor stimulate depolymerization. The phosphorylation l evels increase at the end of the culmination stage when spores have ma tured morphologically and physiologically, and reach maximum levels af ter an additional 12 hours of development. The levels are stable for 2 0 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 wi th nutrient medium containing spore germination promoters, the phospho rylation levels quickly decrease with a half-life of about 5 minutes. After 20 minutes spores begin to swell. At this later time, most of th e phosphorylated actin already has been dephosphorylated. Also, in hea t-activated spores actin dephosphorylation occurs prior to spore swell ing. However, addition of phosphatase inhibitors following heat-activa tion, prevented spore swelling and dephosphorylation of actin. Our dat a indicate that the high levels of actin tyrosine phosphorylation, spe cific to the spore stage, may be required for maintaining dormancy to withstand starvation stress. The rapid dephosphorylation of actin lead s to a reactivated dynamic actin system which participates in spore sw elling, vesicle movement, and mitochondrial shape changes during the s pore germination process.