Phosphorylation of the G protein alpha-subunit, G alpha 2, of Dictyostelium discoideum requires a functional and activated G alpha 2.

The G alpha 2-subunit of Dictyostelium discoideum is essential to the initial stage of the cell's developmental life cycle. In response to the extracellular chemoattractant, cAMP, G alpha 2 is activated and transiently phosphorylated on serine-113 [Chen et al. (1994): J Biol Chem 269:20925-20930]. The role of G alpha 2 phosphorylation remains elusive; cells expressing the S113A, nonphosphorylated mutation of G alpha 2 appear to proceed through the developmental phase normally. To gain insight into the function of G alpha 2 phosphorylation, the conditions for G alpha 2 phosphorylation were examined using a variety of alpha-subunit point mutations and chimeras. Mutations that block the G protein activation cycle prior to or at the hydrolysis of GTP (G alpha 2-S45A, G alpha 2-G207A, and G alpha 2-Q208L) preclude G alpha 2 phosphorylation in vivo. Phosphorylation of the G alpha 2-Q208L mutation does however occur in an in vitro phosphorylation assay. It appears that G alpha 2 phosphorylation, shown previously in vivo to require the cAMP receptor, also requires signaling through the G2 pathway. Results from the in vitro assay suggest that the substrate for phosphorylation is the alpha-subunit monomer.