Single Turnover Autophosphorylation Cycle of the PKA RIIβ Holoenzyme

To provide tight spatiotemporal signaling control, the cyclic adenosine monophosphate (CAMP)-dependent protein kinase (PKA) holoenzyme typically nucleates a macromolecular complex or a PKA signalosome. Using the RII beta holoenzyme as a prototype, we show how autophosphorylation/dephosphorylation of the RII beta subunit, as well as cAMP and metal ions, contribute to the dynamics of PKA signaling. While we showed previously that the RII beta holoenzyme could undergo a single turnover autophosphorylation with adenosine triphosphate and magnesium (MgATP) and trap both products in the crystal lattice, we asked here whether calcium could trap an ATP: RII beta holoenzyme since the RII beta holoenzyme is located close to ion channels. The 2.8 angstrom structure of an RII beta p(2):C-2:(Ca(2)ADP)(2) holoenzyme, supported by biochemical and biophysical data, reveals a trapped single phosphorylation event similar to MgATP. Thus, calcium can mediate a single turnover event with either ATP or adenosine-5'-(beta,gamma-imido) triphosphate (AMP-PNP), even though it cannot support steady-state catalysis efficiently. The holoenzyme serves as a product trap because of the slow off-rate of the pRII beta subunit, which is controlled by cAMP, not by phosphorylation of the inhibitor site. By quantitatively defining the RII beta signaling cycle, we show that release of pRII beta in the presence of cAMP is reduced by calcium, whereas autophosphorylation at the phosphorylation site (P-site) inhibits holoenzyme reassociation with the catalytic subunit. Adding a single phosphoryl group to the preformed RII beta holoenzyme thus creates a signaling cycle in which phosphatases become an essential partner. This previously unappreciated molecular mechanism is an integral part of PKA signaling for type II holoenzymes.