Activation of cyclic AMP-dependent protein kinase isoenzymes: Studies using specific antisera

A novel method for rapidly determining the amount and degree of association-dissociation of the Type I and Type II cAMP-dependent protein kinases has been developed and validated. Antibodies directed against the regulatory subunits of Type I and Type II cAMP-dependent protein kinases were used. The antibodies formed complexes with holoenzymes and regulatory subunits which were precipitated by goat anti-rabbit IgG (immunoglobulin G). These complexes bound [³H]cAMP with an apparent K_b of 20 nm for protein kinase I and 80 nm for protein kinase II. Immunoprecipitated protein kinases I and II were catalytically active when incubated with cAMP, [γ-³²P]ATP, and histone H2B. When mixtures of the two kinase isoenzymes or cytosol were incubated with various amounts of [³H]cAMP and the isoenzymes were separated by precipitation with antisera specific for each isoenzyme, the amount of [³H]cAMP associated with immunoprecipitates was proportional to the concentration of [³H]cAMP. In contrast, the catalytic activity that was immunoprecipitated varied inversely with the concentration of [³H]cAMP, showing that the activation of protein kinase could be assessed by the disappearance of catalytic activity from the immunoprecipitates. In the absence of MgATP protein kinase I was activated by a 10-fold lower concentration of cAMP than protein kinase II. However, when MgATP was added to the incubation, there was no significant difference in the binding of [³H]cAMP or dissociation of catalytic subunits of the two isoenzymes. The anti-R antibodies were also used to rapidly quantitate the concentration of regulatory subunits and the relative ratio of protein kinases I and II in tissue cytosols.