Presence of the sites for interacting with cyclic AMP and with catalytic subunit on small fragments of protein kinase regulatory subunit.

During the purification of cyclic AMP binding proteins from rat liver, some smaller active fragments were obtained, possibly as the result of proteolysis. The binding proteins detected had approximate molecular weights of 50,000, 36,000, and 10,000. Each of these components bound cyclic [3H]AMP with high affinity (apparent dissociation constants ranging from 2 to 10 nM) and had a similar ability to inhibit the purified catalytic subunit of rat liver protein kinase. Cyclic AMP prevented this inhibition in each instance. These results suggest that the binding site for cyclic AMP and the site for interacting with catalytic subunit occur relatively close to one another on the regulatory subunit and can remain functional when a substantial fraction of the subunit is lost.