The cGMP-dependent protein kinase (PKG) has two tandem cyclic nucleotide binding (CNB) domains which act as the primary intracellular receptor for cGMP [1,2]. PKG exhibits a homodimeric rod-like structure which undergoes significant molecular rearrangements upon the binding of cGMP [3-5]. However, a detailed structural analysis of the core regulatory elements inherent to PKG is still required.
We recently solved a crystal structure of the two cGMP binding sites from PKG Iα in order to highlight the atomic details of the regulatory domain. This PKG78-355 structure is free of cGMP and presents the protein in an elongated conformation. A surprising dimeric arrangement between PKG78-355 protomers is orchestrated via hydrophobic contacts between a novel helical element C-terminal to the second cGMP binding site (the switch helix) and the opposite CNB domain B (Figure 1). Small angle X-ray scattering (SAXS) of PKG78-355 suggests an overall molecular dimension of ~130 Å, consistent with the maximal linear dimension observed in our crystal structure. Upon incubation with cGMP, PKG78-355 contracted to ~95 Å. This molecular compaction was not observed in a construct lacking the switch helix (PKG78-326), suggesting the additional importance of the switch helix in mediating cGMP-specific conformational changes inherent to the regulatory domain.
Figure 1. Overall fold of PKG78-355. The crystal structure of the PKG regulatory domain identifies a novel allosteric interface between PKG78-355 protomers.
Overall, these studies provide the first atomic resolution model of tandem cGMP binding domains and expand our understanding of the allosteric mechanisms surrounding PKG activation.
Zhao J, Trewhella J, Corbin J, Francis S, Mitchell R, Brushia R, Walsh D: Progressive cyclic nucleotide-induced conformational changes in the cGMP- dependent protein kinase studied by small angle X-ray scattering in solution.
Richie-Jannetta R, Busch JL, Higgins KA, Corbin JD, Francis SH: Isolated regulatory domains of cGMP-dependent protein kinase Ialpha and Ibeta retain dimerization and native cGMP-binding properties and undergo isoform- specific conformational changes.