NifU

PF3D7_0921400 (NifU) is a NifU-like [Fe-S] cluster carrier of Plasmodium that operates primarily within the apicoplast SUF pathway of iron-sulfur cluster biogenesis (PMID:28695709, PMID:23805304). Acting as a homodimer, it accepts [4Fe-4S] clusters from the PfSufB-C2-D scaffold complex and transfers them to downstream target [4Fe-4S] proteins in vitro, doing so more efficiently than the A-type carrier PfSufA (PMID:28695709). It can also function, with lower efficiency than PfIscA2, as a carrier that delivers [4Fe-4S] clusters to apo-aconitase B in the mitochondrial ISC pathway (PMID:34032321). Consistent with a carrier role that is functionally backed up by parallel carriers, NifU is individually dispensable for parasite survival: its ortholog can be ablated in P. berghei with completion of the full life cycle, though nfu(-) parasites show reduced merosome formation, and individual deletion of NfuApi in P. falciparum blood stages is tolerated in an apicoplast bypass line (PMID:23805304, PMID:35164549). Its essentiality is revealed only in combination: double deletion of NfuApi with SufA is synthetically lethal, establishing that the two carriers act redundantly to deliver [Fe-S] clusters to ferredoxin and other essential apicoplast [Fe-S] proteins (PMID:35164549). Beyond these cluster-transfer and genetic-redundancy findings, no further mechanistic detail on NifU has been characterized in the available corpus.

1. 2013 Medium

   Establishing where NifU acts and whether it is essential was the first question; genetic ablation of the ortholog showed the carrier is dispensable for the life cycle yet contributes to the liver-to-blood transition.

   Evidence Genetic knockout in P. berghei with in vitro merosome formation assay

   PMID:23805304

   Open questions at the time

   • Does not identify the molecular basis of the merosome-formation defect
   • Redundancy with other carriers not tested at the genetic level here
   • Phenotype shown in P. berghei, not P. falciparum
2. 2013 Medium

   To place NifU in a subcellular pathway, its localization was resolved and shown to be apicoplast-dependent, tying its function to that organelle.

   Evidence Live fluorescence imaging of GFP/epitope-tagged fusion in P. berghei with azithromycin and fosmidomycin treatment

   PMID:23805304

   Open questions at the time

   • Import/targeting mechanism not defined
   • Localization determined in P. berghei ortholog, not P. falciparum protein directly
3. 2017 Medium

   The biochemical function was defined by showing NifU is a homodimeric carrier that receives [4Fe-4S] clusters from the SUF scaffold and passes them to a target protein, quantitatively outperforming PfSufA.

   Evidence In vitro [Fe-S] cluster transfer assay and biochemical characterization in P. falciparum

   PMID:28695709

   Open questions at the time

   • Physiological target proteins beyond a model substrate not identified
   • Single lab in vitro reconstitution
   • Interaction with SufB-C2-D not structurally resolved
4. 2021 Medium

   Whether NifU also contributes to mitochondrial cluster assembly was tested; it can carry [4Fe-4S] to apo-aconitase B but less efficiently than PfIscA2, indicating an auxiliary, lower-priority role in the ISC pathway.

   Evidence In vitro [Fe-S] cluster transfer assay comparing carrier efficiency in P. falciparum

   PMID:34032321

   Open questions at the time

   • In vivo relevance of mitochondrial activity not demonstrated
   • Whether the protein is dually localized in cells not shown here
   • Single in vitro comparison
5. 2022 High

   The apparent dispensability was resolved as functional redundancy: individual deletion is tolerated but co-deletion with SufA is synthetically lethal, proving both carriers redundantly supply clusters to ferredoxin and other essential apicoplast proteins.

   Evidence Individual and combinatorial genetic deletion in the PfMev apicoplast bypass line with synthetic lethality assay

   PMID:35164549

   Open questions at the time

   • Full inventory of essential [Fe-S] client proteins not defined
   • Molecular basis of carrier selectivity between NifU and SufA unresolved

• The complete set of physiological [Fe-S] client proteins NifU serves, and the structural/kinetic basis by which cluster hand-off is partitioned between NifU, SufA, and IscA2, remain open.
• No structural model of NifU or its scaffold/target complexes
• Endogenous apicoplast targets beyond ferredoxin not enumerated
• In vivo significance of the mitochondrial ISC activity untested