Relationship of the cGMP-binding activity to the cGMP-specific phosphodiesterase in Dictyostelium discoideum
Optimal conditions for assaying and stabilizing the soluble cGMP-binding activity in Dictyostelium discoideum were established. Using these procedures, we investigated the relationship between the cGMP-binding activity and the cGMP-specific phosphodiesterase in this organism. In wild-type strains, the binding and phosphodiesterase activities were found to be regulated differently during development. Also, stmF mutants, which possess very low levels of cGMP-specific phosphodiesterase activity, exhibited normal levels of cGMP-binding activity. Fractionation studies revealed that the binding and phosphodiesterase activities could be resolved by DEAE-cellulose chromatography. Finally, the effect of pH on cGMP binding was different from that reported for cGMP-mediated activation of the phosphodiesterase. Taken together, these results indicate that the cGMP-binding protein and the cGMP-specific phosphodiesterase are probably unrelated. In addition, the cGMP-binding activity is not associated with cGMP-stimulated kinase activity and it does not elute from DEAE-cellulose like the highly conserved cGMP-dependent protein kinases found in other systems.