Abstract
A new type of actin rods formed in both of the nucleus and the cytoplasm are implicated in the maintenance of dormancy and viability of Dictyostelium discoideum spores. The rods are composed of hexagonally cross-linked actin tubules with 13 nm diameter, and include S-adenosyl-L-homocysteine hydrolase. In addition to actin rods, the complex of G-actin and fat droplets are formed in the spore cytoplasm. Half of the actin molecules in the spores are tyrosine-phosphorylated. The high level of the phosphorylation is required for stabilizing spores. D-glucose is a trigger molecule for the actin dephosphorylation. In the contact-sensitive plant Mimosa pudica L., actin of the main pulvinus is heavily tyrosine-phosphorylated and changes in the extent of phosphorylation correlate with the degree of bending of the plant’s petioles. Also in dormant forms of the true slime mold Physarum polycephalum actin is phosphorylated. In D. discoideum, SrfA, a homologous gene to the MADS-box family, is involved in the formation of the actin rods as well as actin phosphorylation.
