CNB

CNB (PF3D7_1451700) is the regulatory subunit of P. falciparum calcineurin (PP2B), a Ca²⁺/calmodulin-dependent serine/threonine phosphatase in which CNB carries the calmodulin-binding domain required for Ca²⁺ stimulation of the catalytic subunit (PMID:15230980). Mutations in CNB (together with the catalytic CnA subunit) are found in cyclosporin A-resistant parasite clones and correlate with CsA-resistant phosphatase activity in vitro, establishing that CNB participates directly in setting the catalytic properties of the enzyme; the intact CnA/CnB complex also associates with Hsp90 in vivo in a geldanamycin-sensitive, CsA-insensitive manner (PMID:15811524). The full-length complex undergoes rapid, specific in vivo proteolytic processing to yield a constitutively active catalytic core that is largely uncoupled from Ca²⁺ regulation, distinguishing it from the Ca²⁺-dependent full-length holoenzyme (PMID:15230980). Functionally, the calcineurin complex is required during erythrocyte invasion: it drives Ca²⁺-dependent microneme exocytosis in merozoites — activity rises upon low-K⁺ exposure and its inhibition blocks dephosphorylation and depolymerization of apical actin and secretion of PfAMA-1 (PMID:23910910) — and it stabilizes receptor-dependent attachment of extracellular merozoites to host erythrocytes prior to entry (PMID:26118996). In these invasion studies CNB is implicated through its membership in the calcineurin complex rather than by individual manipulation; beyond its role as the regulatory subunit and the CsA-resistance mutations, no CNB-specific mechanism has been characterized in the available corpus.

1. 2004 Medium

   It was unknown how P. falciparum calcineurin activity is controlled relative to its Ca²⁺-dependence; this work showed the holoenzyme (bearing the Ca²⁺/calmodulin-responsive CNB subunit) is proteolytically processed in vivo into a constitutively active, Ca²⁺-uncoupled core, establishing a regulatory switch for the enzyme.

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

   PMID:15230980

   Open questions at the time

   • The protease responsible for processing and the cleavage site were not identified
   • CNB's individual fate during processing (retained vs. removed) was not resolved
   • Physiological trigger and timing of processing during the life cycle not defined
2. 2005 Medium

   It was unknown whether CNB contributes to calcineurin's drug sensitivity and higher-order assembly; identifying CNB mutations in CsA-resistant clones and Hsp90 association of the complex placed CNB within a chaperoned, drug-targetable phosphatase whose catalytic properties depend on its regulatory subunit.

   Evidence In vivo co-immunoprecipitation of calcineurin with Hsp90, sequencing of CsA-resistant clones for CnA/CnB mutations, and in vitro phosphatase assays with CsA

   PMID:15811524

   Open questions at the time

   • Whether the CNB mutations alone (versus CnA mutations) confer CsA resistance was not separated
   • Structural basis of how CNB mutations alter catalytic/drug-binding properties not determined
   • Functional consequence of the Hsp90 association for CNB was not tested
3. 2013 Medium

   It was unknown what invasion step calcineurin controls; this work showed the complex is an essential regulator of Ca²⁺-dependent microneme exocytosis, acting through apical actin dephosphorylation/depolymerization to enable PfAMA-1 secretion.

   Evidence Calcineurin activity assays under low-K⁺ conditions, FK506/CsA inhibition, actin polymerization readouts, and microneme secretion (PfAMA-1 surface exposure) assays in merozoites

   PMID:23910910

   Open questions at the time

   • CNB was implicated only through the complex, not individually mutagenized or depleted
   • Direct actin/actin-regulator dephosphorylation substrate of calcineurin not identified
   • How CNB-mediated Ca²⁺ sensing is coupled to microneme secretion mechanistically unresolved
4. 2015 Medium

   It was unknown whether calcineurin acts at the attachment step of invasion; reverse- and chemical-genetic dissection showed the complex stabilizes receptor-dependent merozoite attachment to erythrocytes, distinct from a receptor-independent adhesion mode.

   Evidence Reverse-genetic and chemical-genetic approaches with invasion/attachment assays dissecting receptor-dependent vs. receptor-independent pathways

   PMID:26118996

   Open questions at the time

   • CNB was implicated as part of the calcineurin complex but not individually manipulated
   • Molecular substrates linking calcineurin activity to attachment stabilization not identified
   • Relationship between the attachment role and the microneme secretion role not integrated

• It remains unknown what CNB does independent of the calcineurin holoenzyme — no CNB-specific substrate, structural model, or individually resolved loss-of-function phenotype has been established in this corpus.
• No CNB-only genetic depletion or complementation reported
• Structural basis of CNB Ca²⁺/calmodulin sensing in P. falciparum not defined
• CNB interactome beyond CnA and Hsp90-associated complex uncharacterized