KAHsp40

KAHsp40 (PF3D7_0201800) is a PEXEL-containing Hsp40/J-domain co-chaperone that Plasmodium falciparum exports into the infected erythrocyte cytoplasm during the asexual blood stage, where it localizes to punctate peripheral structures consistent with knobs and distinct from Maurer's clefts (PMID:22970262). At these sites it co-localizes with the knob components KAHRP, PfEMP1, and PfEMP3 and with the PTEX translocon components Hsp101 and PTEX150, and biochemically forms a complex with KAHRP, PfEMP3, and Hsp101, placing it within the knob-assembly/cytoadherence machinery of the host cell (PMID:22970262). Functionally, its isolated J-domain retains an intact HPD motif and stimulates the ATPase activity of HSP70 chaperones in vitro, consistent with a canonical Type II J-protein co-chaperone activity (PMID:41618743). Consistent with a role restricted to the asexual stage, the KAHsp40 locus is epigenetically silenced during sexual differentiation, acquiring HP1 occupancy and repressive H3K36me2/3 marks in gametocytes (PMID:29503181, PMID:33794978). Beyond these findings, the specific contribution of KAHsp40's chaperone activity to knob morphogenesis and the direct nature of its complex partnerships have not been further characterized in the available corpus.

1. 2012 Medium

   Establishing that KAHsp40 is not retained in the parasite but reaches the host cell answered where this J-protein acts, defining it as an exported effector targeted to knob-like structures rather than to Maurer's clefts.

   Evidence Subcellular fractionation and immunofluorescence tracking a PEXEL-containing protein across blood-stage development

   PMID:22970262

   Open questions at the time

   • Direct demonstration that PEXEL cleavage/processing drives export was not shown
   • Whether localization is to bona fide knobs versus another peripheral compartment rests on marker co-localization
   • No knockout/knockdown to test functional requirement for export
2. 2012 Medium

   Co-localization and co-immunoprecipitation with knob and PTEX components addressed which machinery KAHsp40 associates with, embedding it in the knob-assembly complex and linking it to the export translocon.

   Evidence Double IFA with KAHRP, PfEMP1, PfEMP3, Hsp101, PTEX150 and Co-IP recovering KAHRP, PfEMP3, and Hsp101

   PMID:22970262

   Open questions at the time

   • Single Co-IP without reciprocal pull-downs; direct versus indirect interactions not resolved
   • Stoichiometry and architecture of the complex unknown
   • No functional consequence of complex disruption tested
3. 2012 Low

   Mapping the locus alongside KAHRP and PfEMP3 on chromosome 2 raised the possibility of co-regulation of knob-component genes, though it is a genomic observation only.

   Evidence Genomic sequence analysis / chromosomal mapping

   PMID:22970262

   Open questions at the time

   • Bioinformatic observation only; no experiment testing shared regulation
   • No transcriptional or chromatin data linking the clustered genes at this stage
4. 2018 Medium

   ChIP-seq showing HP1 gain at the locus in gametocytes answered how KAHsp40 expression is switched off outside the asexual stage, indicating heterochromatin-based silencing during sexual differentiation.

   Evidence HP1 ChIP-seq comparing gametocytes to asexual schizonts

   PMID:29503181

   Open questions at the time

   • Correlates chromatin state with silencing but does not directly measure KAHsp40 transcript loss
   • Mechanistic trigger for HP1 deposition at this locus unknown
5. 2021 Medium

   Detection of repressive H3K36me2/3 co-occurring with HP1 in stage II gametocytes refined the silencing model, indicating cooperative histone-mark and HP1-based repression early in gametocyte development.

   Evidence H3K36me2/H3K36me3 ChIP-seq in stage II gametocytes

   PMID:33794978

   Open questions at the time

   • Causal role of the histone marks in silencing not tested by perturbation
   • Order and interdependence of HP1 and H3K36 methylation unresolved
6. 2026 Medium

   In vitro ATPase assays with the isolated J-domain addressed whether KAHsp40 is a functional co-chaperone, showing it can stimulate parasite and human HSP70 ATPase activity as a Type II J-protein.

   Evidence In vitro ATPase activity assay using the isolated J-domain against PfHSP70-X, HsHSP70, and HsHSC70

   PMID:41618743

   Open questions at the time

   • Tested as one of three J-proteins; KAHsp40-specific results not individually highlighted
   • Isolated domain only; full-length activity and physiological HSP70 partner in the erythrocyte not established
   • No link demonstrated between chaperone activity and knob assembly

• Whether KAHsp40's co-chaperone activity is functionally required for knob morphogenesis, PfEMP1 display, and cytoadherence, and which HSP70 it partners with in vivo, remains unresolved.
• No genetic knockout/knockdown phenotype reported
• In vivo HSP70 partner at the knob unidentified
• Direct role in cytoadherence not demonstrated