I3

PF3D7_1031700 (I3/PfI3) is a regulator of the Plasmodium falciparum protein phosphatase PfPP1 that, in contrast to its yeast and human homologs which inhibit PP1, uniquely acts as a positive regulator of PfPP1 phosphatase activity (PMID:22128182). In P. falciparum it binds PfPP1 directly and with high affinity (apparent Kd ~100 nM) through a conserved KVVRW motif, and mutation of Trp45 within this motif severely disrupts the interaction (PMID:22128182). PfI3 is an intrinsically disordered protein that localizes predominantly to the nucleus across blood-stage parasites, consistent with a role in modulating nuclear PfPP1 (PMID:22128182), and it is essential for blood-stage parasite survival (PMID:22128182). In P. berghei, the I3 ortholog is a substrate whose phosphorylation state is controlled by the PP1–LRR1 complex, becoming hypophosphorylated upon LRR1 depletion and appearing within the LRR1/PP1 interactome (PMID:35920043). Beyond its interaction with and activation of PfPP1, no further downstream substrates or effector mechanisms of PF3D7_1031700 have been characterized in the available corpus.

1. 2011 High

   Establishing whether PF3D7_1031700 engages the parasite PP1 catalytic subunit, and through what molecular determinant, defined it as a genuine PP1 partner and identified the residues driving the interaction.

   Evidence Multiple in vitro binding assays with Kd measurement and site-directed mutagenesis of the KVVRW motif, in P. falciparum

   PMID:22128182

   Open questions at the time

   • Whether the interaction occurs and is regulated in vivo in the parasite nucleus was not tested
   • The structural basis of KVVRW docking onto PfPP1 was not resolved
2. 2011 High

   Determining the functional consequence of the PfI3–PfPP1 interaction revealed that, unlike its inhibitory homologs, PF3D7_1031700 is a positive regulator that activates PfPP1, reframing the family's assumed role.

   Evidence In vitro phosphatase activity assay plus functional complementation of an I3-deficient (Ypi1 deletion) yeast strain, for P. falciparum I3

   PMID:22128182

   Open questions at the time

   • The physiological substrates whose dephosphorylation is enhanced by PfI3 were not identified
   • How activation rather than inhibition is achieved mechanistically was not defined
3. 2011 Medium

   Characterizing the biophysical state and cellular distribution of PF3D7_1031700 placed its regulatory activity in a specific compartment and structural context.

   Evidence NMR spectroscopy showing intrinsic disorder and live fluorescence microscopy of GFP-tagged PfI3 showing nuclear localization, in P. falciparum blood stages

   PMID:22128182

   Open questions at the time

   • Whether disorder is required for PfPP1 binding or activation was not tested by mutagenesis
   • Nuclear targeting signals and dynamics across the cell cycle were not mapped
4. 2011 Medium

   Testing the genetic requirement for PF3D7_1031700 established it as essential for asexual blood-stage growth, marking it as biologically indispensable.

   Evidence Reverse genetics with repeated failed knockout attempts despite locus accessibility, in P. falciparum

   PMID:22128182

   Open questions at the time

   • Essentiality is inferred from failed knockout rather than a conditional depletion phenotype
   • The essential process requiring PfI3 was not defined
5. 2022 Medium

   Probing how PF3D7_1031700 is itself regulated in vivo showed that the I3 ortholog is a phospho-target linked to the PP1–LRR1 complex, extending its role from PP1 activator to a component embedded in a regulated PP1 network.

   Evidence Phosphoproteomics in PbLRR1-depleted parasites and affinity pulldown/mass spectrometry interactome analysis, in P. berghei

   PMID:35920043

   Open questions at the time

   • Correlative regarding which kinase phosphorylates I3 and whether PP1 directly dephosphorylates it
   • Functional consequence of I3 hypophosphorylation on PP1 activation was not tested
   • Shown in P. berghei, not confirmed in P. falciparum

• The downstream substrates whose dephosphorylation PF3D7_1031700 promotes, and how its own phosphorylation state feeds back on PfPP1 activation during the asexual cycle, remain unresolved.
• No physiological PfPP1 substrate regulated by PfI3 has been identified
• The kinase acting on I3 and the functional impact of its phosphorylation are unknown
• No structure of the PfI3–PfPP1 complex has been determined