The activation of cell aggregation by phosphorylation in Dictyostelium discoideum

Single amoebae of D. discoideum are phosphorylated in the presence of external ATP. Phosphorylation is catalyzed by a cAMP independent cell membrane bound protein kinase. As a result of phosphorylation cell aggregation is induced and the chemotactic sensitivity of the amoebae to a cAMP gradient decreased. Cell membrane phosphorylation may be involved in triggering cell aggregation in vivo. The fact that the number of free phosphorylable sites per cell decreases at the onset of aggregation gives support to this hypothesis. The existence of a plasma membrane bound phosphoprotein phosphatase suggests a possible regulator role for this enzyme on the phosphorylation of the amoebae. Finally, ATP inhibits intercellular contact sites outside the aggregation center. Despite this inhibiting effect on cell adhesiveness, amoebal movement toward an aggregation center maintains its normal periodicity.