CK1

PF3D7_1136500 (CK1) is an essential, catalytically active serine/threonine protein kinase of the intraerythrocytic asexual cycle of Plasmodium falciparum (PMID:9334178, PMID:26629826). The recombinant and native enzyme display canonical CK1 biochemistry: phosphorylation of a CK1-specific peptide substrate, strong preference for ATP over GTP, and inhibition by CK1-7, with active-site residues defined by mutagenesis (PMID:9334178). Enzymatic activity is stage-dependent, peaking in trophozoites (trophozoite > ring >> schizont) (PMID:9334178). CK1 shows dynamic subcellular distribution across the asexual cycle — a pool at the host erythrocyte membrane in early infection, predominantly intra-parasite localisation in trophozoites and schizonts, and association with micronemes in merozoites — and a catalytically active pool is secreted into the culture supernatant as an ectokinase (PMID:26629826). Trafficking of CK1 to the infected-erythrocyte plasma membrane is mediated by PfRab5B (PMID:24498355). The gene could not be disrupted, indicating essentiality for asexual proliferation, and interactome plus in vitro phosphorylation data implicate CK1 in mRNA splicing, protein trafficking, ribosomal processes, and host cell invasion (PMID:26629826). CK1 is also an intracellular binding target of trisubstituted purine compounds such as purvalanol B (PMID:10873834). Beyond these findings, the direct substrates that account for its cellular roles have not been defined in the available corpus.

1. 1997 High

   Establishing whether the P. falciparum CK1 gene encodes a functional kinase resolved that it is a genuine, catalytically active CK1-family enzyme with defined substrate and cofactor preferences rather than a predicted ORF.

   Evidence Bacterial recombinant expression with in vitro peptide kinase assay, CK1-7 inhibition, ATP-vs-GTP preference, active-site mutagenesis, and corroboration by native enzyme purified from parasite extracts; Northern blot for stage expression

   PMID:9334178

   Open questions at the time

   • physiological substrates in the parasite not identified
   • no structural determination of the active site
   • stage regulation shown at activity level but mechanism of regulation unknown
2. 2000 Medium

   Identifying CK1 as a purvalanol B-binding protein established it as an intracellular target of trisubstituted purine compounds in the parasite, relevant to small-molecule targeting.

   Evidence Affinity chromatography on immobilised purvalanol B from parasite extracts with protein identification

   PMID:10873834

   Open questions at the time

   • binding not shown to inhibit CK1 catalytic activity
   • single pulldown method without orthogonal confirmation of direct binding
3. 2014 Medium

   Determining how CK1 reaches the host cell surface showed that PfRab5B mediates its trafficking to the infected-erythrocyte plasma membrane, linking CK1 export to a specific vesicular pathway.

   Evidence Transgenic GFP-fusion localisation, immuno-electron microscopy, and reverse-genetics deletion/complementation of Rab5 paralogs in P. berghei

   PMID:24498355

   Open questions at the time

   • CK1-specific trafficking conclusion secondary to the main Rab5B study
   • direct interaction between CK1 and PfRab5B not demonstrated
   • functional consequence of surface trafficking not tested
4. 2015 High

   Endogenous tagging and reverse genetics resolved CK1's essentiality, dynamic localisation, secretion as an ectokinase, and candidate cellular processes, moving it from a biochemically characterised enzyme to a functionally contextualised one.

   Evidence Endogenous GFP/His knock-in with IFA, gene-disruption attempt, supernatant ectokinase assay, co-IP interactome, and in vitro kinase assays on putative interactors

   PMID:26629826

   Open questions at the time

   • interactome-implicated roles (splicing, trafficking, ribosomal, invasion) not validated in vivo
   • function of the secreted ectokinase pool unknown
   • essentiality shown by failure to disrupt, not by conditional knockdown
5. 2016 Low

   A computational structural model addressed how ATP is accommodated in the CK1 active site, predicting a glycine-rich binding motif and conformational differences between apo and ATP-bound states.

   Evidence Homology modelling, molecular docking, MD simulation, MM/PBSA binding free energy, and principal component analysis

   PMID:27923179

   Open questions at the time

   • computational prediction only, no experimental structural validation
   • predicted motif not tested by mutagenesis in this study
6. 2020 Low

   Co-purification data proposed a specific route by which CK1 reaches the RBC membrane, implicating host membrane-trafficking/recycling machinery.

   Evidence Co-purification and reciprocal immunoprecipitation from parasite-infected RBCs with mass spectrometry identification of GAPVD1 and Sorting nexin 22

   PMID:32356940

   Open questions at the time

   • trafficking interaction not functionally validated
   • co-purification without demonstration of direct binding
   • single lab, single method

• The physiological substrates and the mechanistic basis for CK1's proposed roles in splicing, trafficking, ribosomal function, and invasion remain undefined.
• no validated in-vivo substrate identified
• function of the secreted ectokinase pool at the host membrane unknown
• no conditional-depletion phenotype linking activity to a specific process