Extremely potent competitive inhibitor of cAMP-dependent protein kinase activity, this protein interacts with the catalytic subunit of the enzyme after the cAMP-induced dissociation of its regulatory chains. The inhibitory site contains regions very similar to the hinge regions (sites that directly interact with the enzyme active site) and 'pseudosubstrate site' of the regulatory chains; but, unlike these chains, PKI does not contain cAMP-binding sites. The arginine residues within the inhibitory site are essential for inhibition and recognition of the enzyme active site (By similarity). Belongs to the PKI family.