CNA

PF3D7_0802800 (CNA) is the catalytic subunit A of Plasmodium falciparum calcineurin (PP2B), a Ca²⁺-regulated serine/threonine phosphatase that couples Ca²⁺ signaling to erythrocyte invasion by the malaria parasite (PMID:23910910, PMID:26118996). Its phosphatase activity is the direct molecular target of the cyclosporin A/cyclophilin inhibitory complex, established by CsA-resistant CNA mutants that confer CsA-resistant phosphatase activity in vitro; CNA also associates in vivo with Hsp90, an interaction disrupted by geldanamycin but not CsA (PMID:15811524). CNA activity is controlled at two levels: it is stimulated by Ca²⁺ in the full-length enzyme, and it is subject to rapid in vivo proteolytic processing that liberates a constitutively active catalytic core largely uncoupled from Ca²⁺ (PMID:15230980). Functionally, Ca²⁺-activated calcineurin dephosphorylates and depolymerizes apical actin to drive microneme exocytosis and AMA-1 secretion, a step blocked by FK506 or CsA (PMID:23910910), and it stabilizes receptor-dependent attachment of extracellular merozoites to host erythrocytes prior to entry (PMID:26118996). Beyond these roles in invasion signaling, the direct substrate repertoire and structural basis of CNA regulation have not been further characterized in the available corpus.

1. 2004 Medium

   It was unknown how CNA phosphatase activity is regulated; this work showed that proteolytic processing generates a constitutively active catalytic core, establishing a Ca²⁺-independent activation mechanism distinct from allosteric Ca²⁺ stimulation of the full-length enzyme.

   Evidence Biochemical fractionation and in vitro phosphatase assays comparing full-length and processed forms, with in vivo processing shown in parasite lysates

   PMID:15230980

   Open questions at the time

   • Protease responsible for cleavage and the cleavage site are not identified
   • Physiological trigger and timing of processing during the parasite cycle are unresolved
   • Whether processed and Ca²⁺-regulated forms act on distinct substrates is unknown
2. 2005 Medium

   The molecular target of the CsA/cyclophilin inhibitory complex was uncertain; demonstrating that CsA-resistance mutations in CNA confer CsA-resistant phosphatase activity established CNA's catalytic activity as the direct target, while co-IP revealed an Hsp90 association.

   Evidence Reciprocal co-immunoprecipitation of CNA with Hsp90, in vitro phosphatase assays with CsA-resistant mutant CNA, and sequencing of CsA-resistant clones

   PMID:15811524

   Open questions at the time

   • Functional consequence of the CNA–Hsp90 interaction for phosphatase activity is not defined
   • Whether Hsp90 modulates CNA folding, stability, or substrate targeting is untested
3. 2013 Medium

   The cellular process controlled by calcineurin was undefined; this work linked Ca²⁺-activated CNA-containing phosphatase to microneme exocytosis by showing it dephosphorylates and depolymerizes apical actin to enable AMA-1 secretion and invasion.

   Evidence Calcineurin activity assay in low-K⁺-stimulated merozoites plus FK506/CsA inhibition with AMA-1 secretion and actin phosphorylation/polymerization readouts

   PMID:23910910

   Open questions at the time

   • Direct dephosphorylation of actin (or an actin regulator) by CNA is inferred, not shown at the substrate level
   • The specific CNA phosphosite substrate driving depolymerization is not identified
4. 2015 High

   It was unclear which invasion step calcineurin governs; reverse- and chemical-genetic disruption showed calcineurin specifically stabilizes receptor-dependent attachment of merozoites to erythrocytes before entry.

   Evidence Conditional disruption/knockdown of calcineurin subunits and chemical-genetic approaches with attachment/invasion assays distinguishing receptor-dependent and -independent adhesion

   PMID:26118996

   Open questions at the time

   • The CNA substrates governing attachment stabilization are unidentified
   • The individual contribution of the CNA subunit versus the regulatory subunit to this phenotype is not resolved

• The direct physiological substrate set of CNA and the structural/regulatory basis linking Ca²⁺ sensing, proteolytic activation, and Hsp90 association remain open.
• No CNA phosphosite substrate has been biochemically defined
• No structural model of CNA or its complex is available in the corpus
• Whether processed catalytic core operates in invasion in vivo is untested