PK2

PK2 (PF3D7_1238900) is a Ca2+/calmodulin-dependent serine/threonine protein kinase of P. falciparum that functions as an upstream orchestrator of merozoite invasion of host erythrocytes (PMID:18762219, PMID:37988347). It was first identified as a putative serine/threonine kinase whose catalytic domain resembles calcium-calmodulin-dependent kinases, and is expressed in a stage-specific manner (PMID:1378403). Recombinant PK2 autophosphorylates and phosphorylates an exogenous substrate in a Ca2+- and calmodulin-dependent manner, an activity abolished by mutation of its active-site catalytic lysine or by calmodulin antagonists, which also reduce parasite invasion (PMID:18762219). Conditional knockdown establishes that PK2 is required for invasion: it drives microneme protein secretion including AMA1 release necessary for tight-junction formation, and acts upstream of the cGMP-calcium second-messenger axis by regulating cGMP generation and calcium release, with phosphoproteomics identifying downstream invasion and signalling substrates (PMID:37988347). Beyond these findings, no further mechanistic detail—including direct in vivo substrates or the biochemical link between PK2 and cGMP synthesis—has been characterized in the available corpus.

1. 1992 Medium

   Establishing whether P. falciparum encodes a distinct calcium-regulated kinase, this work identified PK2 as a putative serine/threonine kinase and localized it to the parasitic membrane.

   Evidence Molecular cloning, sequence homology analysis, and immunostaining with antiserum against a recombinant fragment, in P. falciparum

   PMID:1378403

   Open questions at the time

   • No in vitro kinase activity demonstrated in this study
   • Physiological substrates and cellular function undefined
   • Homology-based family assignment only, not biochemically validated
2. 2008 High

   Testing whether PK2 is a catalytically active, calcium-responsive kinase, this work demonstrated Ca2+/calmodulin-dependent autophosphorylation and substrate phosphorylation dependent on the active-site lysine, and linked its activity to invasion.

   Evidence Recombinant kinase assay with [γ-32P]ATP, catalytic-lysine mutagenesis, calmodulin-antagonist inhibition, immunoelectron microscopy, and in vitro invasion assay in P. falciparum merozoites

   PMID:18762219

   Open questions at the time

   • Physiological substrates not identified in this study
   • Invasion link inferred from calmodulin antagonists, not direct PK2 loss-of-function
   • How Ca2+/calmodulin signals connect to invasion machinery unresolved
3. 2018 Low

   Addressing when PK2 acts in the parasite cell cycle, transcriptomics placed pk2 among Ca2+-sensitive kinases upregulated during re-entry into the cycle after G1 arrest.

   Evidence Transcriptomic profiling following DFMO-induced G1 arrest and polyamine rescue in P. falciparum

   PMID:30409996

   Open questions at the time

   • Expression correlation only; no direct functional or biochemical assay on PK2 protein
   • Causal role of PK2 in cell-cycle re-entry not tested
   • Relationship between cell-cycle expression and invasion function unclear
4. 2023 High

   Resolving the in vivo role of PK2, genetic knockdown established it as essential for invasion through control of microneme (AMA1) secretion, tight-junction formation, and upstream regulation of the cGMP-calcium signalling axis.

   Evidence Conditional knockdown, invasion and AMA1 secretion assays, comparative phosphoproteomics, and cGMP/calcium measurements in P. falciparum

   PMID:37988347

   Open questions at the time

   • Direct in vivo PK2 substrates among phosphoproteomic hits not individually validated
   • Biochemical mechanism by which PK2 regulates cGMP generation unknown
   • No structural model of PK2 or its complexes

• The direct molecular targets that connect PK2's kinase activity to cGMP synthesis and calcium release, and how these are spatially organized in the invading merozoite, remain undefined.
• No validated direct substrate linking PK2 to guanylyl cyclase or calcium channels
• No reconstitution of the PK2-cGMP-calcium axis
• No structural characterization of PK2