The 2.0-angstrom structure of the cyclic adenosine monophosphate (cAMP)–dependent protein kinase (PKA) catalytic subunit bound to a deletion mutant of a regulatory subunit (RIα) defines a previously unidentified extended interface. The complex provides a molecular mechanism for inhibition of PKA and suggests how cAMP binding leads to activation. The interface defines the large lobe of the catalytic subunit as a stable scaffold where Tyr²⁴⁷ in the G helix and Trp¹⁹⁶ in the phosphorylated activation loop serve as anchor points for binding RIα. These residues compete with cAMP for the phosphate binding cassette in RIα. In contrast to the catalytic subunit, RIα undergoes major conformational changes when the complex is compared with cAMP-bound RIα. The inhibitor sequence docks to the active site, whereas the linker, also disordered in free RIα, folds across the extended interface. The β barrel of cAMP binding domain A, which is the docking site for cAMP, remains largely intact in the complex, whereas the helical subdomain undergoes major reorganization.