Cyclic Adenosine 3′,5′-Monophosphate-dependent Protein Kinase of Human Erythrocyte Membranes

Abstract A cyclic AMP (adenosine 3', 5'-monophosphate)-dependent protein kinase has been found in membranes derived from human erythrocytes which accounts for greater than 70% of the total cyclic AMP-dependent protein kinase activity of these cells. Components of the erythrocyte membranes were phosphorylated by membrane-bound protein kinase, but stimulation of the rate of phosphorylation by cyclic AMP was observed only when histones or protamine served as substrates. The membrane-associated cyclic AMP-dependent protein kinase resembles many soluble protein kinases; it requires 20 mm Mg2+ for maximal activity, has apparent Km values of 26 µg per ml and 8.3 µm for protamine and ATP, respectively, and a Ka for cyclic AMP of 28 nm. Erythrocyte membranes specifically bind cyclic AMP and exhibit a dissociation constant of 3.3 nm for the membrane-cyclic AMP complex. Cyclic AMP-independent protein kinase activity was dissociated from the membrane by treatment with 1 m NH4Cl, but the cyclic AMP-binding activity was retained by the particulate fraction. These findings suggest that the cyclic AMP-binding moiety is firmly integrated into the membrane structure, whereas the catalytic moiety may only be loosely associated with the membrane.