The most significant difference between PKA and PKG family members is that in PKA, the regulatory and catalytic activities are performed by separate gene products known as PKA-R and PKA-C, respectively, whereas in PKG the cNMP-binding (cyclic nucleotide-binding domain) and catalytic domains are usually present in the same polypeptide [ 35].
These include a central hydrophobic core region consisting of six membrane-spanning α-helices (S1 S6), and partially overlapping cyclic nucleotide-binding and calmodulin-binding domains situated near their C-termini (Fig. 1).
Furthermore, cytoplasmic domains like the cyclic nucleotide binding domain and the PAS domain have been positioned in the overall structure.
Many cellular functions in eukaryotic pathogens are mediated by the cyclic nucleotide binding (CNB) domain, which senses second messengers such as cyclic AMP and cyclic GMP.
These data, together with the evidence that partially liganded channels open significantly, suggested strong coupling between cyclic nucleotide binding and MloK1 channel opening.
Cyclic AMP binding to PKA-R leads to dissociation of the holoenzyme into a PKA-R subunit dimer (with four cAMP molecules bound) and two active C monomers.
In addition, a cyclic nucleotide binding domain (cNBD) immediately C terminal to the pore domain is thought to contribute to the stabilization of the tetrameric structure [6].
Despite the high degree of similarity between PKA and PKG, our structures reveal that the molecular interactions that mediate cyclic nucleotide binding are distinct from PKA.
Indeed, unlike what is seen in other cGMP- and cAMP-pockets, our structures show that the nucleotides are partially exposed to solvent, whereas in PKA RIα "capping" residues increase cAMP affinity by covering the cyclic nucleotide binding pocket.
This point mutation is located downstream of the cyclic nucleotide binding domain (residues 750 870) [25] and the R-X-R ER-retention signal sequence (1005 1007) [26], but overlaps with the tetramerizing coiled-coil domain (residues 1018 1122) [1018 1122
