Protein phosphorylation in rat cardiac microsomes: Effects of inhibitors of protein kinase A and of phosphatases

The phosphorylation of rat cardiac microsomal proteins was investigated with special attention to the effects of okadaic acid (an inhibitor of protein phosphatases), inhibitor 2 of protein phosphatase 1 and inhibitor of cyclic AMP-dependent protein kinase (protein kinase A). The results showed that okadaic acid (5 µM) modestly but reproducibly augmented the protein kinase A-catalyzed phospholamban (PLN) phosphorylation, although exerted little effect on the calcium/calmodulin kinase-catalyzed PLN phosphorylation. Microsomes contained three other substrates (M_r 23, 19 and 17 kDa) that were phosphorylated by protein kinase A but not by calcium/calmodulin kinase. The protein kinase A-catalyzed phosphorylation of these three substrates was markedly (2-3 fold) increased by 5 µM okadaic acid. Calmodulin was found to antagonize the action of okadaic acid on such phosphorylation. Protein kinase A inhibitor was found to decrease the protein kinase A-catalyzed phosphorylation of microsomal polyp eptides. Unexpectedly, inhibitor 2 was also found to markedly decrease protein kinase A-catalyzed phosphorylation of phospholamban as well these other microsomal substrates. These results are consistent with the views that protein phosphatase 1 is capable of dephosphorylating membrane-associated phospholamban when it is phosphorylated by protein kinase A, but not by calcium/calmodulin kinase, and that under certain conditions, calcium/calmodulin-stimulated protein phosphatase (protein phosphatase 2B) is also able to dephosphorylate PLN phosphorylated by protein kinase A. Additionally, the observations show that protein phosphatase 1 is extremely active against the three protein kinase A substrates (M_r 23, 19 and 17 kDa) that were present in the isolated microsomes and whose state of phosphorylation was particularly affected in the presence of dimethylsulfoxide. Protein phosphatase 2B is also capable of dephosphorylating these three substrates. (Mol Cell Biochem 175: 109–115, 1997