PKAr

PKAr (PF3D7_1223100) is the cAMP-receptor regulatory subunit of Plasmodium falciparum protein kinase A, functioning as the allosteric gatekeeper that couples cAMP signaling to erythrocyte invasion and intra-erythrocytic development (PMID:27738107, PMID:30794532). Crystal structures of PKAr bound to cAMP and to the antimalarial analogue (Sp)-2-Cl-cAMPS establish that PKAr holds the catalytic subunit (PKA-C) inactive through allosteric inhibition, and that cooperative binding of two cAMP molecules drives a conformational change releasing PKA-C to phosphorylate downstream substrates such as AMA1 (PMID:27738107). This sequestration role is confirmed functionally: PKAr overexpression is toxic to parasites and its growth defect is rescued by raising intracellular cAMP, consistent with PKAr titrating free catalytic subunit; the same overexpression down-regulates the New Permeation Pathway anion channel of infected erythrocytes in a cAMP-dependent manner (PMID:18248092). Beyond its inhibitory function, PKAr is itself a signaling node: upon the high-K+ to low-K+ transition experienced by merozoites, it undergoes Ca2+-dependent phosphorylation, and the phosphorylated form is bound by Pf14-3-3I to nucleate a multiprotein complex required for microneme secretion and RBC invasion, since a phosphopeptide mimic of the PKAr phosphosite disrupts the complex and blocks invasion (PMID:32817103). Correct temporal restraint of PKA activity by PKAr is required for ordered invasion and early development, as genetic elevation of cAMP produces broad substrate hyperphosphorylation (PMID:30794532).

1. 2008 High

   It was unknown whether the PKA regulatory subunit had any function beyond inhibiting its catalytic partner; this work showed PKAr acts through the cAMP pathway to control an ion-transport phenotype of the host erythrocyte.

   Evidence Patch-clamp electrophysiology with recombinant PKAr addition and transgenic overexpression, with cAMP rescue of the growth defect, in P. falciparum

   PMID:18248092

   Open questions at the time

   • Whether PKAr directly contacts the anion channel or acts solely by sequestering catalytic subunit is not resolved
   • The molecular identity of the New Permeation Pathway channel is not established here
2. 2012 Low

   Before direct structural or interaction data, in silico analysis proposed that N-terminal PKAr phosphosites might serve as docking sites for regulatory proteins, framing PKAr as a potential signaling hub rather than a passive inhibitor.

   Evidence Bioinformatic reanalysis of published phosphoproteomic datasets and comparison of parasite versus mammalian PKA subunits

   PMID:22626931

   Open questions at the time

   • Docking of 14-3-3 to these phosphosites was not experimentally tested in this work
   • No new experiment on PKAr was performed; predictions only
3. 2016 High

   It was unclear how cAMP mechanistically triggers PKA activation in the parasite; the crystal structure defined the allosteric basis, showing two cAMP molecules stabilize a conformation that releases the catalytic subunit to phosphorylate AMA1.

   Evidence X-ray crystallography of PKAr–cAMP and PKAr–(Sp)-2-Cl-cAMPS complexes with agonist-induced growth-disruption assays in P. falciparum

   PMID:27738107

   Open questions at the time

   • Structure of the full PKAr–PKA-C holoenzyme was not solved
   • Direct in vitro phosphorylation of AMA1 by released catalytic subunit was not shown in this entry
4. 2019 Medium

   The physiological consequence of failing to restrain PKA was unknown; loss of cAMP degradation revealed that PKAr-mediated control of kinase activity is required for correct temporal ordering of invasion and early development.

   Evidence Conditional disruption of PDEβ producing elevated cAMP and hyperactivated PKA, with phosphoproteomic profiling and invasion/growth assays in P. falciparum

   PMID:30794532

   Open questions at the time

   • PKAr was not directly manipulated; its specific contribution is inferred from pathway-wide hyperactivation
   • Individual substrates whose dysregulation drives the phenotype are not resolved for PKAr specifically
5. 2020 High

   It was unknown whether PKAr had a signaling role beyond kinase inhibition; this work showed Ca2+-dependent phosphorylation of PKAr creates a 14-3-3 docking event that assembles an invasion-regulatory complex.

   Evidence Quantitative phosphoproteomics, co-immunoprecipitation, gel filtration, phosphopeptide competition, and microneme secretion/invasion assays in P. falciparum merozoites

   PMID:32817103

   Open questions at the time

   • The kinase phosphorylating PKAr in a Ca2+-dependent manner is not identified
   • The full composition and downstream effectors of the PKAr–Pf14-3-3I complex are not defined
   • How this phospho-dependent complex relates to cAMP-driven catalytic release is not integrated

• How the two regulatory modes of PKAr — cAMP-gated inhibition of the catalytic subunit and Ca2+-phosphorylation-driven 14-3-3 complex assembly — are coordinated during invasion, and which downstream substrates each controls, remains unresolved.
• No holoenzyme structure linking the inhibitory and docking functions
• The upstream Ca2+-activated kinase is unidentified
• The complete substrate set downstream of released PKA-C in the parasite is undefined