YihA1

YihA1 (PF3D7_0513400) is an apicoplast-localized GTP-binding protein of Plasmodium falciparum implicated in biogenesis of the apicoplast ribosome (PMID:32057832). Targeting experiments place the recombinant protein in the apicoplast (PMID:32057832), and it possesses intrinsic GTPase activity in vitro (PMID:32057832). YihA1 binds the large subunit (LSU) of the apicoplast ribosome in a nucleotide-independent manner, as shown by association with surrogate E. coli ribosomes in vitro and reciprocal co-immunoprecipitation of parasite organellar LSU in vivo (PMID:32057832), consistent with a role in late-stage assembly of the apicoplast ribosomal large subunit (PMID:32057832). Beyond this ribosome-association and GTPase activity, no loss-of-function phenotype, structural model, or substrate/effector mechanism for YihA1 has been characterized in the available corpus.

1. 2020 Medium

   Establishing where YihA1 acts was the first step; targeting experiments assigned the protein to the apicoplast, defining the organellar compartment in which any function must occur.

   Evidence Subcellular localization by targeting assays with recombinant protein in P. falciparum

   PMID:32057832

   Open questions at the time

   • Localization shown with recombinant targeting rather than endogenous tagging
   • Sub-organellar distribution and timing across the life cycle not resolved
2. 2020 Medium

   Whether the protein was an active enzyme was unknown; an in vitro assay demonstrated intrinsic GTPase activity, confirming it functions as a GTP-binding/hydrolyzing protein.

   Evidence In vitro GTPase activity assay with recombinant YihA1

   PMID:32057832

   Open questions at the time

   • Kinetic parameters and regulatory cofactors not defined
   • Whether GTP hydrolysis is required for its cellular role untested
3. 2020 Medium

   To link the GTPase to a process, its binding partner was identified; YihA1 was shown to associate with the apicoplast ribosomal large subunit nucleotide-independently, tying it to ribosome biology.

   Evidence In vitro binding to surrogate E. coli ribosomes plus reciprocal co-immunoprecipitation of parasite organellar LSU

   PMID:32057832

   Open questions at the time

   • Direct LSU rRNA/protein contact sites not mapped
   • Nucleotide-independence measured indirectly; functional consequence unclear
4. 2020 Low

   The pathway role was inferred: combining homology, localization, and LSU binding, YihA1 was placed in late-stage assembly of the apicoplast large ribosomal subunit, possibly with PfYihA3.

   Evidence Inference from co-IP, localization, and homology (no loss-of-function genetics)

   PMID:32057832

   Open questions at the time

   • No direct loss-of-function or genetic epistasis experiment performed
   • Cooperation with PfYihA3 not experimentally demonstrated for YihA1 itself
   • Assembly-stage placement is inferential rather than shown

• Whether YihA1 GTPase activity is essential for parasite viability and how it mechanistically drives large-subunit maturation remain unresolved.
• No conditional knockdown/knockout phenotype for YihA1
• No structure of YihA1 alone or on the ribosome
• Functional relationship to PfYihA3 not tested