Pfj2

Pfj2 (PF3D7_1108700) is an endoplasmic reticulum-localized chaperone of the Thioredoxin superfamily that is essential for asexual blood-stage survival of Plasmodium falciparum, where it functions at the center of an oxidative protein-folding network (PMID:33534803). Using a redox-active crosslinker that traps interactions between redox-active cysteines, Pfj2 was shown to engage the Trx-superfamily proteins PfPDI8 and PfPDI11 as redox substrates in the parasite ER (PMID:33534803). Genetic epistasis places PfPDI8 in a functional complex with Pfj2 and additional ER chaperones and shows PfPDI8 is itself required for asexual growth, whereas PfPDI11 is dispensable (PMID:33534803). The redox interactions between Pfj2 and its Trx-domain substrates are sensitive to small-molecule inhibition, establishing this Pfj2-centered oxidative folding network as a druggable target (PMID:33534803). All available evidence derives from a single study; beyond this ER redox-chaperone role, no further mechanistic detail has been characterized in the available corpus.

1. 2021 High

   Establishing that Pfj2 is an essential ER chaperone answered whether this Thioredoxin-superfamily protein has a non-redundant role in parasite biology.

   Evidence conditional knockdown mutants with asexual growth phenotype readout, plus ER localization

   PMID:33534803

   Open questions at the time

   • molecular chaperone activity not reconstituted biochemically
   • role in sexual/other life-cycle stages not tested
   • no structural model of Pfj2
2. 2021 Medium

   Identifying PfPDI8 and PfPDI11 as redox substrates defined the molecular partners through which Pfj2 acts in ER oxidative folding.

   Evidence redox-active crosslinker pulldown followed by mass spectrometry in P. falciparum

   PMID:33534803

   Open questions at the time

   • direct enzymatic redox transfer between Pfj2 and substrates not reconstituted in vitro
   • full substrate/interactome scope beyond the two PDIs unknown
   • stoichiometry and directionality of the redox reaction undefined
3. 2021 Medium

   Double conditional knockdown epistasis distinguished which Pfj2 substrates are functionally essential, placing PfPDI8 in a shared complex while showing PfPDI11 is dispensable.

   Evidence conditional knockdown of PfPDI8 and PfPDI11 in a Pfj2 conditional mutant background with asexual growth readout

   PMID:33534803

   Open questions at the time

   • complex membership inferred genetically, not biochemically reconstituted
   • identity of the 'other ER chaperones' in the proposed complex not resolved
   • architecture and stoichiometry of the Pfj2-PfPDI8 complex unknown
4. 2021 Medium

   Demonstrating that the Pfj2 redox interactions are inhibitable by small molecules established the network as a candidate antimalarial target.

   Evidence small-molecule inhibitor treatment of Pfj2 conditional mutants with parasite growth and redox-interaction readouts

   PMID:33534803

   Open questions at the time

   • molecular target/binding site of the inhibitor not defined
   • selectivity over host chaperones not established
   • no lead-optimization or in vivo efficacy data

• How Pfj2 mechanistically couples its chaperone (J-domain) activity to redox chemistry, and the full architecture of its ER folding complex, remain unresolved.
• no biochemical reconstitution of Pfj2 redox/chaperone activity
• no structural data on Pfj2 or its complexes
• all evidence from a single study/lab