PIP5K

PIP5K (PF3D7_0110600) is a bifunctional Plasmodium falciparum lipid kinase that catalyzes the phosphorylation of phosphatidylinositol 4-phosphate (PtdIns4P) to generate PtdIns(4,5)P2, thereby feeding the parasite's phosphoinositide biosynthetic pathway (PMID:19171150). The recombinant enzyme is a type I PIP5K whose catalytic activity is stimulated by myristoylated ARF1 but not by phosphatidic acid in vitro, linking its activity to ARF1-dependent regulation (PMID:19171150). Its C-terminal catalytic domain is fused to an unusual N-terminal domain bearing helix-loop-helix EF-hand-like motifs of the neuronal calcium sensor (NCS) family, a domain architecture that was unique among eukaryotes at the time of characterization (PMID:19171150). In the P. berghei ookinete, the PIP5K ortholog is phosphorylated in a PKG- and GCβ-dependent manner, placing the enzyme downstream of cGMP-PKG signalling in a phosphoinositide pathway that controls Ca2+ release required for gliding motility and gametocyte activation (PMID:24594931). Beyond its ARF1-stimulated PtdIns4P kinase activity, its regulatory partnerships in P. falciparum have not been characterized in the available corpus, aside from a single yeast two-hybrid report of interaction with PfHSP70-2 (PMID:32529065).

1. 2009 High

   Establishing the enzymatic identity of PF3D7_0110600 answered whether the parasite encodes a dedicated PtdIns4P 5-kinase and how its activity is regulated.

   Evidence Recombinant protein expression with in vitro lipid kinase assays on defined substrates and testing of ARF1 and phosphatidic acid as activators

   PMID:19171150

   Open questions at the time

   • Whether ARF1 stimulation operates on the enzyme in the parasite in vivo was not tested
   • No structural model of the catalytic or regulatory interface was determined
   • The physiological source and pool of the PtdIns4P substrate was not defined
2. 2009 Medium

   Domain analysis revealed an NCS-family EF-hand-like N-terminal module fused to the kinase, raising the possibility that PIP5K activity is coupled to calcium sensing.

   Evidence Sequence analysis and domain characterization of the recombinant protein

   PMID:19171150

   Open questions at the time

   • The functional link of the NCS domain to calcium-dependent regulation was proposed but not directly tested
   • Whether the EF-hand motifs bind Ca2+ was not demonstrated
   • The effect of the N-terminal domain on catalytic output was not measured
3. 2014 Medium

   Phosphoproteomics placed the PIP5K ortholog downstream of PKG/GCβ signalling, connecting phosphoinositide production to the cGMP cascade that drives motility and gametocyte activation.

   Evidence Phosphoproteomic analysis of gliding P. berghei ookinetes with PKG inhibition and gcβ genetic disruption

   PMID:24594931

   Open questions at the time

   • Shown in P. berghei ookinetes, not in P. falciparum
   • The functional consequence of the specific phosphorylation event on kinase activity was not determined
   • Whether PKG phosphorylates PIP5K directly or via an intermediate kinase is unresolved
4. 2020 Low

   A protein interaction screen linked PIP5K to PfHSP70-2, hinting at a physical connection to ARF1-regulated secretory trafficking.

   Evidence Yeast two-hybrid interaction screen with mass spectrometry mapping of affinity-purified proteins

   PMID:32529065

   Open questions at the time

   • Yeast two-hybrid only, with no in-parasite validation of the interaction
   • No reciprocal or biochemical confirmation of direct binding
   • Functional relevance of the interaction to PIP5K activity was not established

• Whether the NCS EF-hand domain confers direct Ca2+ regulation of PIP5K activity, and whether the PKG-dependent phosphorylation seen in P. berghei governs enzymatic output and PtdIns(4,5)P2 production in P. falciparum, remain unresolved.
• No direct test of calcium binding or calcium-dependent activity
• No genetic loss-of-function characterization of PF3D7_0110600 in P. falciparum
• No demonstration that PKG-dependent phosphorylation alters catalytic activity