ABSTRACT
The first indications of the inhibitor protem arose in experiments of J. B. Posner et al. in 1964, in which it was observed that muscle extracts contained a trypsin-labiie factor which interfered with an assay of cyclic adenosine 3', 5'-monophosphate (cAMP) based upon the cAMP-dependent activation of phosphorylase kinase. In many of the subsequent studies the inhibitor protein has been used as a means to examine cAMP-dependent phosphorylation, both in vitro and in intact cells via microinjection, liposome fusion, by transfection with a pseudo partial complementary DNA. The inhibitor protein has been used extensively as an effective means to evaluate the role of cAMP in mediating cell regulation, both in vitro to classify protein kinases and cAMP-mediated protein phosphorylation and also by its introduction into intact cells. The inhibitor protein has been purified to homogeneity from rabbit skeletal muscle, and bovine brain and has been partially purified from bovine heart and thyroid, rat brain, and chicken liver.
