PKAC1

Toxoplasma gondii PKAc1 is the catalytic subunit of a cAMP-dependent protein kinase that acts as a master switch enforcing the intracellular replicative state and repressing premature host cell egress (PMID:29030485). The kinase is held inactive at the parasite pellicle through interaction with the dually acylated regulatory subunit PKAr1, and conditional depletion of PKAc1 or expression of a cAMP-insensitive dominant-negative PKAr triggers premature, acidification-dependent egress (PMID:29030485, PMID:30208022). Shortly after invasion, adenylate-cyclase-derived cAMP activates PKAc1, which suppresses cytosolic Ca2+ signalling and the PKG/CDPK motility pathway to lock the parasite into replication, with loss of PKAc1 sensitising parasites to cGMP-induced Ca2+ release (PMID:30208022, PMID:39903669). Epistasis places PKAc1 action between guanylate cyclase and calcium release and upstream of cGMP metabolism, consistent with PKAc1 modulating a cGMP-phosphodiesterase to coordinate cAMP/cGMP crosstalk governing the growth-versus-egress switch (PMID:29030485, PMID:39903669). PKAc1 additionally activates the cAMP-specific phosphodiesterase PDE2 to consume cAMP, generating a negative feedback loop that renders PKAc1 activation transient and inhibits motility (PMID:35107337, PMID:39903669).

1. 2017 High

   Established that PKAc1 is a pellicle-tethered kinase whose activity level dictates the choice between intracellular division and premature egress, defining it as a checkpoint controller of the lytic cycle.

   Evidence Conditional knockdown, overexpression, and dominant-negative PKAr stabilisation with phosphoproteomics in T. gondii

   PMID:29030485

   Open questions at the time

   • Direct kinase substrates driving the egress block not identified
   • Molecular basis of pellicle tethering only partly resolved
2. 2017 High

   Positioned PKAc1 upstream of cGMP metabolism and PKG in the egress pathway, revealing cAMP/cGMP crosstalk through a putative cGMP-phosphodiesterase.

   Evidence Phosphoproteomics plus pharmacological PKG inhibition, phosphodiesterase inhibitor treatment, and genetic epistasis in T. gondii

   PMID:29030485

   Open questions at the time

   • Identity of the cGMP-phosphodiesterase not confirmed
   • Direct phosphorylation of the phosphodiesterase by PKAc1 not demonstrated
3. 2018 High

   Showed PKAc1 actively suppresses cytosolic Ca2+ immediately after invasion, mechanistically linking loss of the kinase to PLP-1-dependent premature exit.

   Evidence Conditional depletion, live cytosolic Ca2+ imaging, localization, and PLP-1 epistasis in T. gondii

   PMID:30208022

   Open questions at the time

   • Effectors linking PKAc1 to Ca2+ suppression unknown
   • How PKAr1 acylation is regulated not addressed
4. 2018 Medium

   Demonstrated that PKAc1 loss sensitises parasites to cGMP-induced Ca2+ signalling, functionally coupling the cAMP and cGMP/Ca2+ arms.

   Evidence Genetic depletion of PKAc1 with pharmacological cGMP elevation and Ca2+ imaging in T. gondii

   PMID:30208022

   Open questions at the time

   • Single lab, no independent replication
   • Molecular node of convergence between cGMP and Ca2+ undefined
5. 2022 Medium

   Connected PKAc1-mediated motility suppression to the cAMP-specific phosphodiesterase TgPDE2, defining a component that shapes the cAMP pool PKAc1 responds to.

   Evidence Knockdown screen, in vitro substrate specificity of purified TgPDE2, interactome and motility/growth assays in T. gondii

   PMID:35107337

   Open questions at the time

   • Direct regulatory relationship between PKAc1 and PDE2 not established in this study
   • Single lab, no independent replication
6. 2025 Medium

   Resolved the temporal logic: cAMP transiently activates PKAc1, which suppresses PKG/CDPK and activates PDE2 in a negative feedback loop, placing PKAc1 between guanylate cyclase and calcium release.

   Evidence Genetic and pharmacological perturbation of cyclic nucleotide pathways with epistasis analysis in T. gondii

   PMID:39903669

   Open questions at the time

   • Direct PKAc1 substrates in the PKG/CDPK and PDE2 branches not identified
   • Single lab, no independent replication

• The direct phosphorylation substrates through which PKAc1 suppresses Ca2+ and the PKG/CDPK pathway, and the molecular mechanism coupling it to cGMP/cAMP phosphodiesterases, remain uncharacterized.
• No direct kinase substrate identified
• No structural model of the PKAc1/PKAr1 complex in the corpus
• Mechanism of adenylate cyclase-PKAc1 coupling undefined