B9

B9 (PF3D7_0317100) is a 6-cysteine family protein that governs the transition of Plasmodium sporozoites from migratory behavior to productive hepatocyte infection and the subsequent establishment of the intracellular liver stage (PMID:24509910, PMID:36403748). B9 transcript is translationally repressed in sporozoites and the protein is expressed after hepatocyte invasion, localizing to the parasite plasma membrane, where loss of B9 in P. berghei, P. yoelii, and P. falciparum causes developmental arrest associated with a compromised parasitophorous vacuole membrane, establishing B9 as required for establishment and maintenance of the parasitophorous vacuole (PMID:24509910). In P. falciparum, B9 is secreted from sporozoite micronemes and is required for productive invasion; its N-terminal beta-propeller domain is essential for infectivity and mediates interaction with the 6-cysteine proteins P36 and P52 in a heterologous system (PMID:36761022). Consistent with a role in triggering infection commitment, B9 contributes to the sporozoite's ability to stop migrating and commit to invasion, implicating it in recognition of hepatocyte surface ligands (PMID:36403748). Deletion of b9 yields sporozoites that invade but completely abort liver-stage development in human hepatocytes in vitro and in humanized mice, confirming an independent and critical function in liver-stage progression (PMID:25407681).

1. 2014 High

   Established that B9 is a stage-specific liver-stage effector rather than a constitutive sporozoite protein, and that it is essential for building the parasitophorous vacuole that shelters the developing parasite.

   Evidence RT-PCR and immunofluorescence for expression/localization plus gene knockout across three Plasmodium species with in vivo and in vitro hepatocyte readouts

   PMID:24509910

   Open questions at the time

   • Molecular mechanism by which B9 stabilizes the PVM is undefined
   • No binding partners identified in this study
   • How translational repression is imposed and released is not addressed
2. 2014 High

   Showed B9's function is post-invasion and independent, since b9-deleted sporozoites retain hepatocyte infectivity but abort development, distinguishing invasion competence from liver-stage progression.

   Evidence Double knockout of b9 and slarp in P. falciparum with infectivity tested in human hepatocytes in vitro and in humanized mice

   PMID:25407681

   Open questions at the time

   • Individual contribution of b9 versus slarp to arrest not fully resolved
   • Precise developmental stage of arrest not mechanistically dissected
   • No molecular effectors of the arrested phenotype identified
3. 2022 Medium

   Placed B9 in the decision to convert from cell-traversal to infection mode, addressing how sporozoites sense that they have reached a suitable hepatocyte.

   Evidence Double knockout of B9 and SPECT2 in Plasmodium with a behavioral assay of migration versus infection commitment

   PMID:36403748

   Open questions at the time

   • Hepatocyte-specific ligand recognized by B9 not identified
   • Single lab, single behavioral method
   • Direct receptor-binding evidence lacking
4. 2023 Medium

   Defined the structural basis and binding partners of B9, revealing an N-terminal beta-propeller domain structurally related to CyRPA that engages the 6-cysteine proteins P36 and P52 and is required for infectivity.

   Evidence Structural determination of the B9 N-terminal domain, heterologous-expression interaction assays with P36 and P52, microneme localization, and genetic loss-of-function in P. falciparum

   PMID:36761022

   Open questions at the time

   • Interactions shown only in heterologous system, not by native parasite co-IP
   • Stoichiometry and architecture of any B9-P36-P52 complex unresolved
   • Host receptor engaged by the beta-propeller not identified

• The molecular mechanism linking B9's beta-propeller/P36-P52 interactions to PVM establishment and the identity of the hepatocyte surface ligand triggering infection commitment remain unresolved.
• No native-parasite validation of the B9-P36-P52 complex
• Host receptor for B9 unidentified
• Mechanistic link between invasion-triggering and PVM biogenesis not established