A NEW SPORE DIFFERENTIATION FACTOR (SDF) SECRETED BY DICTYOSTELIUM CELLS IS PHOSPHORYLATED BY THE CAMP-DEPENDENT PROTEIN-KINASE

Upon starvation, Dictyostelium discoideum unicellular amoebae form a m ulticellular organism leading to the development of a fruiting body co ntaining spores. Single cells of sporogenous mutants, unlike wild type cells, are able to differentiate into spores under specific condition s. We show in this report that overexpression of the catalytic subunit of the cAMP dependent protein kinase (PKA), not only renders the cell s sporogenous, but is also accompanied by the production/release of a diffusible spore differentiation factor (SDF). SDF is a small, thermos table phospho-polypeptide. In vitro dephosphorylation reduces SDF spor e differentiation capacity, which can be regained in vitro by PKA phos phorylation. These results indicate that SDF is a PKA substrate and mi ght be activated in vivo by this protein kinase. Since spore different iation requires PKA catalytic subunit activation, we conclude that the response of prespore cells to SDF involves an intracellular pathway d ependent on PKA.