PK5

PK5 (PfPK5) is a cyclin-dependent kinase homologue of Plasmodium falciparum that functions in cell-cycle regulation during the intra-erythrocytic asexual stage, where it is implicated in the control of S-phase DNA synthesis (PMID:8143724, PMID:8943149). Recombinant PfPK5 is an intrinsically active kinase that phosphorylates histone H1 and casein and is present at approximately constant cytosolic levels across asexual development (PMID:8143724). Its activity is stimulated in vitro by cyclins — including the parasite cyclin homologue Pfcyc-1 and additional cyclin-related proteins (Pfcyc-2/3/4), the non-cyclin activator RINGO, and heterologous human cyclin H and p25 — and PfPK5 autophosphorylates in the presence of a cyclin, yet it is neither activated nor phosphorylated by the parasite CDK-activating kinase Pfmrk (PMID:10722743, PMID:12869562). PfPK5 is negatively regulated by CDK-inhibitor-type proteins, being inhibited in vitro by human p21(CIP1) but not by the CDK4/6-specific p16(INK4) (PMID:11700040). A direct substrate is the origin recognition complex subunit PfORC1: PfPK5-mediated phosphorylation of the ORC1 N-terminal domain abolishes its DNA-binding activity, alters its subcellular localization, and triggers proteasome-mediated degradation of ORC1 in schizonts, coupling PfPK5 activity to control of DNA replication licensing (PMID:26094711). The crystal structure of PfPK5, the first for a P. falciparum kinase, reveals a conserved monomeric CDK fold with inhibitor-binding-site differences from human CDKs that permit selective ATP-competitive inhibitor design (PMID:14604523).

1. 1994 Medium

   Establishing whether PK5 is a genuine functional kinase was the first requirement; recombinant expression showed it is an active enzyme, defining it as a bona fide protein kinase rather than a pseudokinase.

   Evidence Recombinant expression in E. coli with in vitro kinase assays against casein and histone H1, plus antibody-based localization

   PMID:8143724

   Open questions at the time

   • Physiological substrates not identified
   • In vitro activity not linked to a cellular process
   • Localization from a single antibody study without replication
2. 1994 Low

   Whether PfPK5 is functionally equivalent to yeast cdc2 was tested by complementation, but the assay was uninformative because the gene was not expressed in yeast.

   Evidence Yeast cdc2/cdc28 mutant complementation assay (negative, confounded)

   PMID:8143724

   Open questions at the time

   • Negative result confounded by lack of expression in yeast
   • No conclusion about cdc2 equivalence possible
3. 1996 Medium

   To place PfPK5 in a cell-cycle context, its activity was correlated with S-phase and probed pharmacologically, indicating a role in DNA synthesis during nuclear division.

   Evidence Immunoprecipitation kinase assays from aphidicolin-blocked parasites, CDK-inhibitor (flavopiridol, olomoucine) treatment of cultures, immunolocalization

   PMID:8943149

   Open questions at the time

   • Pathway placement rests on non-selective pharmacological inhibitors
   • No direct S-phase substrate identified at this stage
   • Inhibitor effects on DNA synthesis not proven to act through PfPK5 specifically
4. 2000 High

   The regulatory logic of PfPK5 activation was defined by reconstituting cyclin activation, showing it behaves as a CDK activated by cyclins including its own Pfcyc-1 but independent of the parasite CAK Pfmrk.

   Evidence In vitro kinase activation assays with recombinant human cyclin H/p25 and Pfcyc-1, autophosphorylation assay, negative test with Pfmrk

   PMID:10722743

   Open questions at the time

   • Physiological activating cyclin in vivo not established
   • Whether autophosphorylation is activating or regulatory in the parasite unclear
   • In vivo relevance of Pfmrk independence untested
5. 2001 Medium

   Negative regulation of PfPK5 was probed, showing it is sensitive to CIP/KIP-type but not INK4-type inhibitors, implying partial conservation of CDK-inhibitor control.

   Evidence In vitro kinase inhibition assays with recombinant human p21(CIP1) and p16(INK4)

   PMID:11700040

   Open questions at the time

   • No endogenous parasite CDK inhibitor identified
   • No in vivo validation of inhibitory regulation
   • Heterologous human inhibitors used as surrogates
6. 2003 Medium

   The repertoire of PfPK5 activators was broadened, showing multiple parasite cyclins and the non-cyclin activator RINGO can each stimulate the kinase in vitro.

   Evidence In vitro histone H1 kinase activation assays with recombinant Pfcyc-2/3/4 and RINGO

   PMID:12869562

   Open questions at the time

   • Which activator operates in vivo is unresolved
   • No cellular validation of these activator pairings
   • Specificity among the cyclins not defined
7. 2003 High

   The structural basis of PfPK5 and its druggability was established by solving the first P. falciparum kinase structure, revealing a conserved CDK fold and inhibitor-site differences exploitable for selective inhibition.

   Evidence X-ray crystallography with co-crystallization of three ATP-competitive CDK inhibitors

   PMID:14604523

   Open questions at the time

   • No cyclin-bound activated-state structure
   • Selectivity predictions not confirmed by cellular potency here
   • Structure of substrate-bound complex unavailable
8. 2015 High

   A direct physiological substrate and mechanistic output was identified, linking PfPK5 activity to control of replication licensing via phosphorylation-triggered inactivation and degradation of PfORC1.

   Evidence In vitro kinase assay on recombinant PfORC1 N-terminal domain, transgenic parasites expressing WT and phospho-mutant ORC1, DNA-binding, localization, and proteasome-inhibitor experiments

   PMID:26094711

   Open questions at the time

   • Whether ORC1 is the principal in vivo substrate unknown
   • Timing/coupling to the activating cyclin not resolved
   • Genetic requirement of PfPK5 for parasite viability not established here
9. 1998 Low

   Comparison with orthologs distinguished PfPK5 behaviour, since P. knowlesi and P. berghei CRK2 homologues were catalytically inactive as monomers and did not complement yeast cdc28.

   Evidence Sequence analysis, recombinant expression, in vitro kinase and yeast cdc28ts complementation assays in P. knowlesi and P. berghei proteins

   PMID:9803415

   Open questions at the time

   • Negative ortholog results; not direct evidence about PF3D7_1356900
   • Monomeric inactivity may reflect missing activating partner
   • Functional divergence across species unresolved

• Whether PfPK5 is genetically essential in the asexual cycle and which cyclin-substrate pairing governs its physiological S-phase function in vivo remains undetermined.
• No conditional knockout/knockdown of PF3D7_1356900 in the corpus
• In vivo activating partner not defined
• Substrate set beyond ORC1 not mapped