A SOLUBLE FACTOR AND GTP-GAMMA-S ARE REQUIRED FOR DICTYOSTELIUM-DISCOIDEUM GUANYLATE-CYCLASE ACTIVITY

Amoeba of Dictyostelium discoideum show a rapid, transient cGMP synthesis in response to chemotactic stimulation. Using Mg2+-GTP as a substrate, guanylate cyclase (E.C. 4.6.1.2.) activity is found exclusively in the particulate fraction of Dictyostelium cells. Here we show that the activity is dependent on the presence of the non-hydrolysable GTP-analogue GTP-gamma-S, which itself is only a poor substrate for the enzyme under the prevailing conditions. Evidence is presented that a transient exposure of the enzyme to GTP-gamma-S is sufficient to constitutively activate the enzyme. GTP-gamma-S-dependent activity is found to require a factor that can be separated from the enzyme by washing the particulate fraction with low salt buffer. Addition of the soluble cell fraction to these washed membranes restores enzyme activity.