1.1. INTRODUCTION

3', 5'-dependent protein kinases became a focus of attention after the discovery that they are required in the activation of the genetic apparatus and of the membrane processes (Langan, 1971" Allfi'ey et al., 1974: Machlas et al., 1974; Weller and Rodnight, 1970). The beginning of studies of these processes was laid down by the discovery of cyclic adenosine 3', 5'-monophosphate and by the recognition of its role in the activation of phosphorylase b. It became known that the activating effect of 3', 5'-AMP was due to the phosphorylation of a protein kinase, catalyzing the transfer of the terminal phosphate group of ATP to phosphorylase b (Sutherland et al., 1960; Walsh et al., 1968). The stimulating effect of 3', 5'-AMP on phosphorylase b includes at least two consecutive reactions of phosphorylation" one of them involves the phosphorylation of a modulator protein (protein kinase), while another represents the transfer of the phosphate group from the protein kinase to phosphorylase b. In both these reactions 3', 5'-AMP plays the role of the second messenger mediating the action of epinephrine on the phosphorylase (Krebs, 1972). Epinephrine triggers the reaction by activating adenyl cyclase and thus catalyzes the formation of 3, _-AMP from ATP (Corbin and Krebs, 1969'Kuo and Greengard, 1969).