Effects of phosphorylation of inhibitory GTP‐binding protein by cyclic AMP‐dependent protein kinase on its ADP‐ribosylation by pertussis toxin, islet‐activating protein

Abstract

Pretreatment of rat cardiac myocytes with the β‐adrenergic agonist, db‐cAMP or forskolin decreased ADP‐ribosylation of 40–41 kDa protein by islet‐activating protein (IAP) in cell membranes. Addition of activated cyclic AMP‐dependent protein kinase (protein kinase A) catalytic subunit and MgCl₂ also decreased ADP‐ribosylation of 40–41 kDa protein by IAP in cell membranes. The α‐ and β‐subunits of partially purified inhibitory GTP‐binding protein (G_i) were both phosphorylated by protein kinase A. The amounts of phosphate incorporated into the subunits of G_i were 0.34 and 0.18 mol/mol protein. These show that phosphorylation of G_i by protein kinase A results in a decrease in its ADP‐ribosylation by IAP.