PK7

PK7 (PF3D7_0213400) is an atypical orphan protein kinase of P. falciparum that governs parasite proliferation and melatonin signal transduction across the life cycle (PMID:18083830, PMID:22348509). It is a constitutively active enzyme: its crystal structure shows an arginine residue in the activation loop substituting for the conserved phosphorylated residue that normally structures this region in canonical kinases, and it accommodates large insertions as excursions from the eukaryotic protein kinase fold (PMID:18275814). Recombinant PK7 phosphorylates diverse substrates in vitro but does not phosphorylate the P. falciparum MAPK homologues PfMAP1 and PfMAP2 and is insensitive to PKA and MEK inhibitors, distinguishing it from a canonical MAPKK despite C-terminal similarity to the MAPKK3/6 subgroup and N-terminal similarity to fungal PKA (PMID:15612927). Quantitative phosphoproteomics of pfpk7-null parasites identified 146 proteins with reduced phosphorylation and confirmed three of these as direct in vitro substrates, defining both direct targets and downstream signaling effects (PMID:29678115). Genetically, PK7 is required for schizogony, producing fewer daughter merozoites per schizont when disrupted, and for mosquito-stage oocyst formation, with both phenotypes rescued by complementation (PMID:18083830). Functionally, PK7 acts upstream in the melatonin transduction pathway, being necessary for melatonin-induced cytoplasmic calcium rise, up-regulation of ubiquitin/proteasome system genes, and PfMORC expression, and it also influences expression of mitochondrial fission genes and mitochondrial respiration during intra-erythrocytic development (PMID:22348509, PMID:29480948, PMID:33479315). No direct biochemical link between PK7 and the mitochondrial fission machinery or PfMORC has been established in the available corpus.

1. 2005 High

   Whether PK7, annotated by sequence as MAPKK-like, actually functions as a MAP kinase kinase was unknown; the biochemistry showed it is an active kinase but not a functional MAPKK.

   Evidence Recombinant kinase assay with defined substrates, inhibitor profiling, and blastp analysis in P. falciparum

   PMID:15612927

   Open questions at the time

   • Physiological substrates not identified
   • Lacks canonical MAPKK activation site — activation mechanism unexplained at this stage
2. 2007 High

   The cellular requirement for PK7 was unknown; gene disruption established that it is needed for asexual schizogony and for mosquito-stage sporogony.

   Evidence Reverse-genetic knockout with phenotyping and genetic complementation in P. falciparum

   PMID:18083830

   Open questions at the time

   • Molecular pathway linking PK7 to merozoite number and oocyst formation not defined
   • Substrates driving these phenotypes unidentified
3. 2008 High

   The activation state of an orphan kinase lacking a canonical T-loop phosphosite was unclear; the structure revealed a constitutively active conformation stabilized by an activation-loop arginine.

   Evidence X-ray crystallography and structural analysis of activation-loop conformation in P. falciparum PK7

   PMID:18275814

   Open questions at the time

   • No substrate-bound or inhibitor-bound structures
   • Regulation of an already-active kinase in vivo unresolved
4. 2012 High

   Where PK7 sits in a signaling pathway was unknown; knockout placed it upstream of melatonin-induced calcium signaling and ubiquitin/proteasome gene regulation.

   Evidence pfpk7- knockout and complementation with calcium measurement, UPS gene qPCR, and cell-cycle staging in P. falciparum

   PMID:22348509

   Open questions at the time

   • Direct substrates transmitting the melatonin signal not identified
   • Mechanism coupling PK7 to calcium mobilization unknown
5. 2018 Medium

   Whether PK7 influences organellar biogenesis was unknown; knockout showed altered mitochondrial fission gene expression and respiration during the intra-erythrocytic cycle.

   Evidence pfpk7- knockout with fission-gene qRT-PCR, MitoTracker/GFP reporter imaging, and oxygen consumption in P. falciparum

   PMID:29480948

   Open questions at the time

   • No direct biochemical link between PK7 and fission machinery (FIS1, DYN1, DYN2)
   • Effects may be indirect downstream consequences of the growth defect
6. 2018 High

   The direct substrates of PK7 were undefined; comparative phosphoproteomics of null parasites mapped PK7-dependent phosphosites and validated three direct substrates in vitro.

   Evidence TMT quantitative phosphoproteomics of wild-type vs pfpk7- schizonts/segmenters plus in vitro kinase assays in P. falciparum

   PMID:29678115

   Open questions at the time

   • Most of the 146 affected proteins are indirect and their upstream kinases unresolved
   • Identity/function of the three validated substrates not developmentally contextualized
7. 2021 Medium

   Whether PK7 controls downstream chromatin/transcriptional regulators in the melatonin pathway was unknown; knockout showed PfMORC expression is impaired after melatonin treatment.

   Evidence pfpk7- knockout with real-time PfMORC expression assay following melatonin treatment in P. falciparum 3D7

   PMID:33479315

   Open questions at the time

   • Epistasis inferred from transcription only — no direct PK7–PfMORC biochemical interaction
   • Intermediate steps between PK7 and PfMORC regulation unknown

• The physiological substrate(s) that mechanistically link PK7 to calcium mobilization, UPS/PfMORC gene regulation, merozoite production, and oocyst development remain unresolved.
• No substrate mapped to a specific downstream phenotype
• No interaction partner or scaffold identified
• How a constitutively active kinase is signal-regulated by melatonin is unknown