Structure and allostery of the PKA RIIß tetrameric holoenzyme.

ABSTRACT

In its physiological state, cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) is a tetramer that contains a regulatory (R) subunit dimer and two catalytic (C) subunits. We describe here the 2.3 angstrom structure of full-length tetrameric RIIß(2)C(2) holoenzyme. This structure showing a dimer of dimers provides a mechanistic understanding of allosteric activation by cAMP. The heterodimers are anchored together by an interface created by the ß4-ß5 loop in the RIIß subunit, which docks onto the carboxyl-terminal tail of the adjacent C subunit, thereby forcing the C subunit into a fully closed conformation in the absence of nucleotide. Diffusion of magnesium adenosine triphosphate (ATP) into these crystals trapped not ATP, but the reaction products, adenosine diphosphate and the phosphorylated RIIß subunit. This complex has implications for the dissociation-reassociation cycling of PKA. The quaternary structure of the RIIß tetramer differs appreciably from our model of the RIα tetramer, confirming the small-angle x-ray scattering prediction that the structures of each PKA tetramer are different.