N3A motifs in RIβ mediate allosteric crosstalk between cAMP and ATP in PKA activation

The RIβ subunit of cAMP-dependent protein kinase (PKA) is highly expressed in the brain, yet it remains the least studied of the PKA regulatory subunits (R). As pathologic variants of its gene are increasingly implicated in neurodevelopmental disorders, neurodegeneration, and cancer, gaining more information about the structure/function of RIβ, and how it differs from RIα, has become increasingly important. We previously reported the structure of the RIβ₂C₂ holoenzyme, which revealed a novel conformation where ATP binding was stabilized by a head-to-head anti-parallel packing of the C-tail wrapped around the N-lobe of the catalytic subunit (C). Although visible, the Dimerization/Docking Domain was poorly folded and reduced. Since RIβ is oxidized in brain tissues, we asked if oxidation or binding of an A Kinase Anchoring Protein (AKAP) would affect the holoenzyme structure. Oxidation or addition of an AKAP peptide to crystals led to the release of nucleotide. To capture this at higher resolution we crystallized RIβ₂C₂ in the presence of an AKAP peptide. This new structure represents an RIβ:C heterodimer. Density for the D/D domain was missing; ATP was absent, the kinase adopted an open conformation, and the C-terminus of the RIβ subunit was no longer resolved. Because the crosstalk between ATP and cAMP in the R:C complex appears to be mediated by the two N3A motifs (N3A^A and N3A^B) as well as by the linker, which in free RIβ is intrinsically disordered, we describe the conserved features of these two motifs as well as the linker and show how each contributes in a unique but coordinated way to allosteric activation of RIβ holoenzymes by cAMP. A key difference in our RIβ:C structure is the rotation of the side chain of W260 at the N-terminus of the αA Helix in N3A^B. W260, at the R:C interface in the holoenzyme, is also the capping residue for cAMP bound to CNB-A, so we may have actually captured the first step in cAMP activation.