PKAR1

PKAr1 is the regulatory subunit of the Toxoplasma gondii PKA holoenzyme that couples cAMP/PKA signalling to the control of intracellular Ca2+ and the timing of host-cell egress (PMID:30208022). Through dual acylation, PKAr1 anchors the PKA catalytic subunit PKAc1 to the parasite periphery, functioning as a membrane-targeting scaffold for the holoenzyme (PMID:30208022). Peripheral PKAr1-anchored PKAc1 activity suppresses cytosolic Ca2+ signalling immediately after host-cell invasion; depletion of PKAc1 blocks the rapid post-invasion down-regulation of cytosolic Ca2+, triggering premature, PLP-1-dependent exit of newly invaded parasites (PMID:30208022). This activity also opposes cGMP-induced Ca2+ signalling, placing the cAMP/PKA axis in functional opposition to the cGMP-Ca2+ signalling modality (PMID:30208022). Mechanistic detail beyond this signalling role, including direct PKAc1 substrates and the biochemistry of PKAr1 acylation, has not been characterized in the available corpus.

1. 2018 Medium

   Establishing how the PKA holoenzyme is spatially organized in the parasite, dual acylation of PKAr1 was shown to anchor PKAc1 to the parasite periphery, defining PKAr1 as the membrane-targeting subunit of PKA.

   Evidence Genetic/biochemical demonstration of PKAr1 dual acylation and peripheral localization of PKAc1 in T. gondii

   PMID:30208022

   Open questions at the time

   • Acylation sites and modifying enzymes not defined
   • Structural basis of PKAc1 sequestration by PKAr1 not resolved
2. 2018 High

   To determine the functional output of peripheral PKA anchoring, conditional depletion of PKAc1 revealed that PKAr1-anchored PKA suppresses cytosolic Ca2+ after invasion to prevent premature egress, linking PKA activity to control of the parasite lytic cycle.

   Evidence Conditional knockdown of PKAc1 with cytosolic Ca2+ measurement and PLP-1-dependent egress phenotype readout

   PMID:30208022

   Open questions at the time

   • Direct PKAc1 substrates mediating Ca2+ suppression not identified
   • Mechanism connecting PKA activity to PLP-1-dependent egress unresolved
3. 2018 Medium

   To test crosstalk between second-messenger pathways, combining cGMP stimulation with PKAc1 depletion showed that loss of PKA sensitizes parasites to cGMP-induced Ca2+, establishing functional opposition between the cAMP/PKA and cGMP-Ca2+ axes.

   Evidence Pharmacological cGMP stimulation combined with genetic PKAc1 depletion and Ca2+ signalling readout in T. gondii

   PMID:30208022

   Open questions at the time

   • Molecular node of convergence between cAMP/PKA and cGMP pathways not identified
   • Single study with pharmacological tools

• The direct substrates of PKAc1 and the biochemical mechanism by which PKA suppresses cytosolic Ca2+ and integrates cGMP signalling remain unknown.
• No PKAc1 substrate identified
• No structural model of the PKAr1-PKAc1 holoenzyme
• cAMP-dependent regulation of PKAr1-PKAc1 dissociation not characterized in the corpus