Characterization of a GDP-sensitive phosphorylation in plasma membranes of D. discoideum

In a previous study, we reported the GDP-dependent phosphorylation of a 36 kD membrane protein, p36, in D. discoideum membranes prepared from starved (aggregation competent) cells (Anschutz et al., 1989). Here we show that p36 can be phosphorylated when membranes are supplied either ATP or GTP as the phosphate donor, but that a greater level of p36 phosphorylation is achieved with GTP. The rate of phosphorylation of p36, using either nucleotide triphosphate, is enhanced by GDP. This reflects a decrease in the apparent Km of the enzyme for the particular nucleotide triphosphate. p36 can also be phosphorylated in membranes prepared from vegetative cells. However, the ability of GDP to stimulate p36 phosphorylation is not observed in vegetative cell membranes. Competition experiments indicate that there are also developmental differences in the nucleotide triphosphate site(s) available to phosphorylate p36.