RRF2

PF3D7_0420400 (RRF2) is a ribosome-recycling factor of Plasmodium falciparum that participates in organellar (mitochondrial) translation by disassembling post-termination ribosomes (PMID:23614815). Its leader sequence directs it to the mitochondrion, with an additional cytoplasmic pool (PMID:23614815). Recombinant RRF2 binds ribosomes and splits monosomes in vitro, an activity dependent on the mitochondrial elongation factor EF-G (PMID:23614815), and it enhances the monosome-splitting/anti-association activity of the mitochondrial initiation factor IF3a (PfIF3a) when acting together with EF-G, marking RRF2 as a functional partner in mitochondrial ribosome recycling (PMID:25689481). Distinctive among characterized recycling factors, RRF2 forms disulfide-stabilized homodimers (PMID:23614815). Despite retaining in vitro splitting activity, RRF2 does not complement a bacterial RRF temperature-sensitive mutant in vivo, indicating it cannot fully substitute for bacterial RRF (PMID:23614815). Beyond these biochemical and localization findings, no in vivo functional or structural characterization of RRF2 has been established in the available corpus.

1. 2013 Medium

   It was unknown where a candidate P. falciparum recycling factor acts in the cell; localization established RRF2 as a mitochondrion-targeted protein with an additional cytoplasmic pool, defining its organellar context.

   Evidence Leader sequence-GFP fusion localization assay in P. falciparum

   PMID:23614815

   Open questions at the time

   • Localization inferred from a leader-GFP fusion rather than endogenous tagging
   • Functional role of the cytoplasmic pool not tested
   • Native full-length protein localization not confirmed
2. 2013 High

   Whether RRF2 possessed the biochemical hallmarks of a recycling factor was unknown; recombinant protein was shown to bind ribosomes and split monosomes in the presence of mitochondrial EF-G, demonstrating bona fide recycling activity and its dependence on EF-G.

   Evidence In vitro ribosome-binding and monosome-splitting assay with recombinant protein and E. coli ribosomes

   PMID:23614815

   Open questions at the time

   • Splitting demonstrated on heterologous E. coli ribosomes rather than native mitochondrial ribosomes
   • In vivo requirement for the activity not established
   • No structural basis for the activity
3. 2013 Medium

   The oligomeric state of RRF2 was uncharacterized; biochemical analysis revealed disulfide-stabilized homodimers, a feature not previously reported for any characterized recycling factor.

   Evidence Biochemical dimer/disulfide characterization of recombinant protein

   PMID:23614815

   Open questions at the time

   • Functional consequence of dimerization for recycling activity not tested
   • Whether dimerization occurs in vivo not shown
   • Redox regulation of the disulfide not explored
4. 2013 Medium

   It was unclear whether RRF2 is functionally equivalent to bacterial RRF; failure to complement an E. coli RRF temperature-sensitive mutant showed it cannot fully substitute in vivo despite in vitro activity.

   Evidence Genetic complementation assay in E. coli LJ14frr(ts) mutant

   PMID:23614815

   Open questions at the time

   • Molecular basis for the complementation failure not identified
   • Does not address RRF2 function in its native P. falciparum mitochondrion
5. 2015 High

   How RRF2 integrates with other mitochondrial translation factors was open; reconstitution showed RRF2 with EF-G enhances PfIF3a-mediated monosome splitting, placing RRF2 as a functional partner of IF3a in mitochondrial ribosome recycling/anti-association.

   Evidence In vitro monosome-splitting assay with recombinant PfIF3a, EF-G and RRF2

   PMID:25689481

   Open questions at the time

   • Interaction demonstrated in vitro with recombinant components, not in cells
   • Stoichiometry and direct physical contact between RRF2 and IF3a not defined
   • Native mitochondrial ribosome substrate not used

• The in vivo role, essentiality, and structural mechanism of RRF2 in P. falciparum mitochondrial translation remain unresolved.
• No genetic knockout/knockdown phenotype in P. falciparum
• No structure of RRF2 or its complexes with EF-G/IF3a/ribosome
• Direct binding to native mitochondrial ribosomes not demonstrated