Regulation of the gene encoding the p24 actin-binding protein in Dictyostelium discoideum

We have isolated cDNA clones on the basis of sequence similarity to the gene encoding the cyclic cAMP-binding protein CABP1 of Dictyostelium discoideum. The predicted amino acid sequence of the cloned cDNAs shows that the homology to CABP1 is restricted to a region rich in proline, glycine, glutamine, and tyrosine. Sequence comparison indicates that the cloned cDNAs encode the actin-binding protein p24. We have examined by RNA blot hybridization the expression of the gene encoding p24. For cells developed in suspension, the levels of p24 mRNA increase rapidly during early development, reaching a peak at 3-4 h. Addition of high concentrations of exogenous cAMP during the first 4 h of development produced little or no effect on the accumulation of p24 mRNA. Treatment with cAMP during subsequent stages of development reduced the levels of p24 mRNA. We attempted to determine if the synthesis of new proteins during early development is a requirement for the reduction in p24 mRNA levels by treating the cells with protein synthesis inhibitor. Unexpectedly, the addition of the inhibitor cycloheximide resulted in an increase in the level of p24 mRNA. The roles of cycloheximide and cAMP on the expression of the p24 gene are discussed.