An N-terminally truncated form of cyclic GMP–dependent protein kinase Iα (PKG Iα) is monomeric and autoinhibited and provides a model for activation

Abstract

The type I cGMP-dependent protein kinases (PKG I) serve essential physiological functions, including smooth muscle relaxation, cardiac remodeling, and platelet aggregation. These enzymes form homodimers through their N-terminal dimerization domains, a feature implicated in regulating their cooperative activation. Previous investigations into the activation mechanisms of PKG I isoforms have been largely influenced by structures of the cAMP-dependent protein kinase (PKA). Here, we examined PKG I activation by cGMP and cAMP by engineering a monomeric form that lacks N-terminal residues 1-53 (53). We found that the construct exists as a monomer as assessed by whole-protein MS, size-exclusion chromatography, and small-angle X-ray scattering (SAXS). Reconstruction of the SAXS 3D envelope indicates that 53 has a similar shape to the heterodimeric RI-C complex of PKA. Moreover, we found that the 53 construct is autoinhibited in its cGMP-free state and can bind to and be activated by cGMP in a manner similar to full-length PKG I as assessed by surface plasmon resonance (SPR) spectroscopy. However, we found that the 53 variant does not exhibit cooperative activation, and its cyclic nucleotide selectivity is diminished. These findings support a model in which, despite structural similarities, PKG I activation is distinct from that of PKA, and its cooperativity is driven by in trans interactions between protomers.