Science 335, 712–716 (2012)

PKA is a cAMP-dependent kinase composed of two regulatory subunits (R) and two catalytic subunits (C) in its inactive holoenzyme state. Upon cAMP binding to the R subunits, the active C subunits are released. There are two classes of R subunits (RI and RII), each having two functionally nonredundant isoforms (α and β). The mechanism of PKA activation by cAMP and the reason the R subunits are nonredundant are not clear. Zhang et al. now report the crystal structure of the full-length RIIβ-C holoenzyme, stabilized by a mutation that abrogates cAMP binding, revealing a tetramer composed of two R-C heterodimers in which two residues that are known to interact with cAMP are kept from the cAMP binding site by a holoenzyme-specific salt bridge. The activation constant for the RIIβ tetramer by cAMP was higher than that for other holoenzymes, possibly owing to extensive interaction between the RIIβ-C heterodimers. By soaking Mg-ATP into their holoenzyme crystals, the authors were able to solve the structure of the enzyme—with reaction products Mg-ADP and a phosphorylated RIIβ subunit—which was similar to the holoenzyme structure, indicating that the holoenzyme contains a well-formed active site in the C subunit and that allosteric activation may be required to release the reaction products. This first structure of a full-length RIIβ-C holoenzyme provides insight into allosteric activation by cAMP and suggests that differential contacts among heterodimers may underlie nonredundancy of R subunits.