Dual role of GDP in the regulation of the levels of p36 phosphorylation in Dictyostelium discoideum.

We examined the dephosphorylation of p36, a protein of D. discoideum that has previously been shown to be phosphorylated in a GDP-dependent manner (Anschutz et al., 1989). Specific dephosphorylation of p36 was found to occur in cell preparations but the activity responsible was strongly dependent upon the concentration of proteins in those extracts. When preparations were diluted, this activity was no longer detectable and the radiolabeled phosphate incorporated into p36 was stable. In contrast, p36 phosphorylation was seemingly unaffected by this treatment. Under the conditions where endogenous dephosphorylating activity was not detectable, the addition of GDP to the reaction resulted in substantial dephosphorylation of p36. The stimulation of this dephosphorylation process occurred at concentrations of GDP that were distinct from those that led to an increased p36 phosphorylation due to the previously reported stimulation of p36 protein kinase activity. Characterization of the dephosphorylation of p36 indicates that the same enzyme is responsible for the endogenous and GDP-stimulated activities. Additionally, these activities are identical when assayed with p36 that had been phosphorylated with ATP or GTP. In contrast to p36 kinase activity, the dephosphorylation of p36 did not display any developmental changes with respect to its regulatory features.