SOAP

SOAP (PF3D7_1404300) is a secreted, disulphide-bond-forming micronemal protein of Plasmodium ookinetes that is required for efficient mosquito midgut invasion and oocyst formation (PMID:12828632). In P. berghei, SOAP localizes to ookinete micronemes and assembles into high-molecular-mass complexes through disulphide bonds, and its targeted disruption markedly impairs midgut invasion and oocyst formation in vivo (PMID:12828632), a role consistent with its strong interaction with mosquito midgut laminin in yeast two-hybrid assays (PMID:12828632). Its micronemal residence was independently confirmed by proteomics of purified ookinete microneme fractions (PMID:19206106). Biochemically, SOAP is a constituent of a hetero-multimeric, >1,300 kDa chitinase-containing complex secreted by ookinetes, isolated with WARP and ookinete surface enolase via chitin-bead pull-down in P. falciparum and P. gallinaceum (PMID:33489941). Its invasion function is specific to the mosquito midgut barrier rather than to ookinete-to-oocyst transformation per se: SOAP-null ookinetes transform into oocysts at wild-type rates in a cell-free in vitro system (PMID:18489758). Beyond its complex membership, adhesion role, and micronemal targeting, no further mechanistic detail on SOAP's molecular activity has been experimentally established in the available corpus.

1. 2003 High

   Established SOAP's cellular location, biochemical behavior, and its functional necessity for the mosquito-stage transmission bottleneck, defining it as a micronemal adhesin required for midgut invasion.

   Evidence Immunofluorescence/fractionation for localization, reducing/non-reducing SDS-PAGE for disulphide-dependent complexes, and targeted gene knockout with mosquito infection readout in P. berghei

   PMID:12828632

   Open questions at the time

   • Molecular composition of the disulphide-bonded complexes not resolved in this study
   • Whether SOAP acts on the ookinete surface or within secreted material not distinguished
2. 2003 Medium

   Identified a candidate molecular basis for SOAP's midgut-invasion role by testing whether it engages a mosquito extracellular matrix component.

   Evidence Yeast two-hybrid assay showing SOAP binding to mosquito laminin (P. berghei)

   PMID:12828632

   Open questions at the time

   • Yeast two-hybrid is indirect; no direct biochemical binding or affinity measurement
   • Interaction not confirmed in the mosquito midgut context
3. 2008 Medium

   Distinguished SOAP's invasion role from a general developmental requirement by showing it is dispensable for ookinete-to-oocyst transformation itself.

   Evidence Cell-free in vitro transformation assay comparing SOAP-null and wild-type P. berghei (ANKA) ookinetes

   PMID:18489758

   Open questions at the time

   • Does not identify what specifically fails during in vivo midgut traversal
   • Single-lab in vitro readout may not capture all developmental defects
4. 2009 Medium

   Independently confirmed SOAP as a genuine micronemal constituent using an orthogonal proteomic approach.

   Evidence Gel LC-MS/MS of an EM-verified, >14-fold-enriched ookinete microneme fraction (P. berghei)

   PMID:19206106

   Open questions at the time

   • Proteomic co-fractionation does not establish direct partners
   • Does not address secretion dynamics or trafficking
5. 2015 Low

   Placed SOAP expression downstream of secretory-pathway serine protease activity, linking microneme biogenesis to SOAP accumulation.

   Evidence Ectopic expression of Neospora caninum Kazal-type protease inhibitor NcPI-S in P. berghei zygotes with IFA for SOAP and EM ultrastructure

   PMID:25803874

   Open questions at the time

   • Indirect, single-approach: SOAP loss is a downstream readout of disrupted microneme biogenesis, not a direct SOAP manipulation
   • Does not identify the responsible protease or a direct effect on SOAP
6. 2021 Medium

   Defined SOAP as a member of a large secreted chitinase-containing complex, connecting it biochemically to WARP and enolase in the peritrophic-matrix-crossing machinery.

   Evidence Size-exclusion chromatography and chitin-bead pull-down of ookinete-conditioned media with MS identification in P. falciparum and P. gallinaceum

   PMID:33489941

   Open questions at the time

   • Co-isolation does not prove direct SOAP contacts within the complex
   • Stoichiometry and functional contribution of SOAP to the complex unresolved
7. 2021 Low

   Generated an atomic-level hypothesis for how SOAP contacts chitinase and WARP within the invasion complex.

   Evidence AlphaFold structural modeling and comparative genomics identifying conserved residues (P. falciparum)

   PMID:34606368

   Open questions at the time

   • Computational prediction only, with no experimental validation of SOAP-specific interactions
   • Predicted interface residues not tested by mutagenesis

• It remains unknown how SOAP's laminin binding, its role in the secreted chitinase complex, and its requirement for midgut invasion mechanistically integrate, and whether these functions are conserved beyond the model species tested.
• No direct structural or biochemical validation of SOAP interfaces
• Function characterized largely in P. berghei; SOAP-specific loss-of-function not reported in P. falciparum
• Mechanistic contribution of SOAP within the >1,300 kDa complex not dissected