PF3D7_1453700

PF3D7_1453700 (p23) is a cytoplasmic co-chaperone of the P. falciparum Hsp90 chaperone machine that couples an intrinsic holdase activity with direct regulation of PfHsp90 (PMID:20140477, PMID:29191421). As a purified recombinant protein it suppresses the thermal aggregation of model client proteins, establishing an ATP-independent holdase function, and it binds PfHsp90 directly to suppress its ATPase activity (PMID:20140477, PMID:29191421). This interaction is mediated by residues K91, H93, W94, and K96 of p23, identified by mutagenesis of a modeled p23–PfHsp90 complex (PMID:20140477). The binding interface overlaps with that of the co-chaperone PfAha1, with which p23 competes for PfHsp90 under MgCl2/ATP-dependent conditions (PMID:22100910). p23 mRNA and protein are expressed from the early ring through the trophozoite stage and decline at the onset of schizogony (PMID:12756555). The P. falciparum genome additionally encodes a second, structurally distinct p23 paralogue (Pfp23B) sharing only ~13% identity with PF3D7_1453700 but displaying similar PfHsp90-inhibitory and holdase activity, while PF3D7_1453700 (Pfp23A) is the more chemically and thermally stable of the two (PMID:29191421). Beyond its role as a validated PfHsp90 co-chaperone, no in vivo function or client repertoire for PF3D7_1453700 has been characterized in the available corpus.

1. 2003 Medium

   Establishing that P. falciparum encodes a p23 homologue and defining when it is expressed set the stage for asking whether the parasite operates a canonical Hsp90 co-chaperone system.

   Evidence Gene sequencing, immunoblotting for stage-specific expression, and mRNA analysis in P. falciparum

   PMID:12756555

   Open questions at the time

   • No functional reconstitution or interaction with PfHsp90 demonstrated
   • Subcellular localization inferred from abundance, not directly imaged
   • Functional role of the developmental expression decline at schizogony not tested
2. 2010 High

   Reconstituting the recombinant protein answered whether p23 is functionally a chaperone and how it engages PfHsp90, defining both intrinsic holdase activity and the binding residues required for ATPase suppression.

   Evidence In vitro citrate synthase aggregation assay, ATPase inhibition assay, homology modeling and site-directed mutagenesis of the p23–PfHsp90 complex

   PMID:20140477

   Open questions at the time

   • Binding residues identified from a modeled complex, not an experimental structure
   • Physiological clients of the holdase activity unknown
   • Function not tested in the parasite
3. 2010 Low

   Detection of a PfHsp90 acetylation site overlapping the Aha1/p23 binding domain raised the possibility that post-translational modification of the partner regulates assembly of the p23-containing complex.

   Evidence Mass spectrometric mapping of PfHsp90 acetylation and histone deacetylase inhibitor treatment with a complex-dissociation readout

   PMID:20837488

   Open questions at the time

   • Modification is on PfHsp90, not on p23 itself
   • Effect on p23 recruitment specifically not directly demonstrated
   • Functional consequence inferred rather than shown
4. 2011 Medium

   Testing p23 against another co-chaperone answered how co-chaperones are arranged on PfHsp90, showing that p23 and PfAha1 compete for an overlapping interface rather than binding simultaneously.

   Evidence GST pull-down competition assay between PfAha1 and p23 for PfHsp90 under MgCl2/ATP-dependent conditions

   PMID:22100910

   Open questions at the time

   • Single-lab in vitro competition without structural mapping of the shared interface
   • Cellular relevance of the competition not tested
   • Order and dynamics of co-chaperone exchange on PfHsp90 unresolved
5. 2014 Low

   A synthesis of experimentally validated PfHsp90 co-chaperones confirmed p23's place within a defined set of five bona fide interactors, situating it in the parasite's chaperone network.

   Evidence Literature review consolidating prior co-immunoprecipitation and pull-down studies

   PMID:24560171

   Open questions at the time

   • No new experimental data
   • Does not address in vivo essentiality of p23
   • Client specificity within the network not resolved
6. 2017 High

   Characterizing both parasite p23 proteins answered whether the second paralogue is functionally redundant, showing that despite low sequence identity Pfp23B shares holdase and PfHsp90-inhibitory activity while PF3D7_1453700 is the more stable isoform.

   Evidence Recombinant expression, SAXS, thermal/chemical stability assays, aggregation suppression, and ATPase inhibition assays for both p23 proteins

   PMID:29191421

   Open questions at the time

   • Biological rationale for two divergent p23 paralogues unknown
   • Whether the two paralogues occupy PfHsp90 in distinct contexts not tested
   • No in-parasite functional comparison

• The in vivo function, essentiality, and client repertoire of PF3D7_1453700 in the parasite remain unresolved.
• No genetic knockout or knockdown phenotype reported
• No experimental structure of p23 or the p23–PfHsp90 complex
• Physiological clients handled by p23 holdase activity unidentified