The smg GDS-induced activation of smg p21 is initiated by cyclic AMP-dependent protein kinase-catalyzed phosphorylation of smg p21

We have shown that smg p21B is phosphorylated by cyclic AMP-dependent protein kinase (protein kinase A) and that membrane acidic phospholipids such as phosphatidic acid and phosphatidylinositol markedly inhibit the smg GDS-induced activation of smg p21B. However, we show here that phosphatidic acid and phosphatidylinositol exhibit a less inhibitory effect on the smg GDS-induced activation of the phosphorylated form of smg p21B. Thus, in the presence of membrane acidic phospholipids, the smg GDS-induced activation of smg p21B is totally dependent on the protein kinase A-catalyzed phosphorylation of smg p21B. Since smg p21B is located on the membranes in resting cells, it is likely that the smg GDS-induced activation of smg p21B is initiated by the protein kinase A activation.