PP5

PP5 (PF3D7_1355500) is a TPR-repeat-containing serine/threonine protein phosphatase that couples the Plasmodium chaperone machinery to stage-specific developmental transitions (PMID:11737864, PMID:15339915). Recombinant PfPP5 is catalytically active, and its activity is stimulated by polyunsaturated fatty acids such as arachidonic and oleic acid in an N-terminus-dependent manner, but not by saturated fatty acids, and it is inhibited by okadaic acid (IC50 ~4 nM) (PMID:11737864, PMID:11897131). Through its N-terminal TPR repeats, native PfPP5 physically associates with PfHsp90 as a component of the PfHsp90 multi-chaperone complex (which also contains PfHsp70 and tubulin); geldanamycin disrupts this complex and blocks the temperature-dependent ring-to-trophozoite transition, placing PP5 within Hsp90-dependent asexual development (PMID:11737864, PMID:15339915). In vitro, PP5 is not a target of the SET/TAF-family inhibitor PfARP, which selectively inhibits PP2A, distinguishing PP5 regulation from that of PP2A (PMID:12615323). In P. falciparum, PP5 localises preferentially to the nucleus (PMID:11897131), whereas in P. berghei the ortholog localises to the cytoplasm and beneath the plasma membrane in merozoites and ookinetes; P. berghei gene deletion showed PP5 is dispensable for asexual blood-stage growth but essential for male gamete formation, fertility, and ookinete-to-oocyst conversion, a male-specific requirement confirmed by genetic crossing with sex-defective mutants (PMID:31202684).

1. 2001 Medium

   Established that PP5 is a genuine catalytic serine/threonine phosphatase whose activity is controlled by its N-terminal TPR region and lipid cofactors, and that it physically engages the Hsp90 chaperone.

   Evidence Recombinant bacterial expression with in vitro phosphatase assay, N-terminal deletion, and reciprocal co-immunoprecipitation in P. falciparum

   PMID:11737864

   Open questions at the time

   • No physiological substrate identified
   • Functional consequence of the PfHsp90 interaction not defined
   • No structural model of the TPR-Hsp90 interface
2. 2002 Medium

   Defined the pharmacological and lipid-regulatory profile of PP5 and placed it in the nucleus, framing where and how the enzyme acts in P. falciparum.

   Evidence Recombinant phosphatase assay with fatty-acid and okadaic-acid characterization plus immunofluorescence in P. falciparum

   PMID:11897131

   Open questions at the time

   • Nuclear substrates unidentified
   • Mechanism linking fatty-acid stimulation to in vivo activity unknown
3. 2003 Medium

   Distinguished PP5 regulation from PP2A by showing the SET/TAF-family inhibitor PfARP does not target PP5, clarifying that PP5 sits outside the PfARP-PP2A regulatory axis.

   Evidence In vitro phosphatase inhibition assay comparing PP5 and PP2A in P. falciparum

   PMID:12615323

   Open questions at the time

   • Does not identify any positive regulator of PP5
   • Negative result only
4. 2004 Medium

   Positioned PP5 as a functional constituent of the PfHsp90 multi-chaperone complex and linked chaperone-complex integrity to asexual stage progression.

   Evidence Co-immunoprecipitation of the PfHsp90 complex, geldanamycin disruption, and flow-cytometry stage analysis in P. falciparum

   PMID:15339915

   Open questions at the time

   • Geldanamycin acts on Hsp90, so the direct requirement for PP5 activity in stage progression is not isolated
   • Stoichiometry and substrates within the complex unknown
5. 2019 High

   Genetically defined the in vivo role of PP5, showing it is dispensable for asexual growth but essential for male gamete fertility and transmission-stage development in P. berghei.

   Evidence Targeted gene deletion, genetic epistasis by crossing with sex-defective mutants, immunofluorescence, and recombinant phosphatase-domain inhibitor assays in P. berghei

   PMID:31202684

   Open questions at the time

   • Male-fertility substrates of PP5 not identified
   • Cytoplasmic/sub-membrane localization in P. berghei differs from reported nuclear localization in P. falciparum and is unreconciled
   • Whether the Hsp90 association is required for the transmission phenotype untested

• The physiological substrates of PP5 and the mechanistic link between its Hsp90 association, its lipid/nuclear regulation, and its male-fertility requirement remain unresolved.
• No dephosphorylation substrate identified in any species
• Species divergence in localization (nuclear in P. falciparum vs cytoplasmic/sub-membrane in P. berghei) unexplained
• Whether PP5's phosphatase activity is required for gametogenesis versus a scaffolding role untested