Protein kinase A (PKA) holoenzyme is one of the major receptors for cyclic adenosine monophosphate (cAMP), where an extracellular stimulus is translated into a signaling response. We report here the structure of a complex between the PKA catalytic subunit and a mutant RI regulatory subunit, RIalpha(91-379:R333K), containing both cAMP-binding domains. Upon binding to the catalytic subunit, RI undergoes a dramatic conformational change in which the two cAMP-binding domains uncouple and wrap around the large lobe of the catalytic subunit. This large conformational reorganization reveals the concerted mechanism required to bind and inhibit the catalytic subunit. The structure also reveals a holoenzyme-specific salt bridge between two conserved residues, Glu261 and Arg366, that tethers the two adenine capping residues far from their cAMP-binding sites. Mutagenesis of these residues demonstrates their importance for PKA activation. Our structural insights, combined with the mutagenesis results, provide a molecular mechanism for the ordered and cooperative activation of PKA by cAMP.