Mutant/mutation
The mutant lacks expression of SOAP (secreted ookinete adhesive protein)

Protein (function)
SOAP, a 21-kDa protein, has the hallmarks of an extracellular soluble (secreted) protein: it contains a predicted cleavable amino-terminal signal peptide, but lacks other typical organellar targeting or membrane anchoring signals. The conserved protein contains 12, closely spaced, cysteine residues and comparison of SOAP of different Plasmodium species indicates a modular structure of two cysteine-rich domains separated by a spacer sequence of variable amino acid length and composition.
SOAP is located in the micronemes of ookinetes.

Phenotype
The phenotype analyses indicate a role in the transformation of the ookinete into the oocyst. Ookinetes appear to have a reduced capacity to invade midgut epithelial cells (see also additional information).

No evidence was found of invaded SOAP-KO ookinetes suffering increased losses after entry of the midgut epithelium, indicating that the impairment of midgut invasion is caused by a reduced ability to enter the epithelial cells. However, it should be noted that P. berghei ookinete-invaded A. stephensi midgut cells have been shown to undergo cell death and subsequent removal through a purse-string mechanism. Thus, it cannot be ruled out that invaded ookinetes killed by the mosquito might be removed by this process.

Additional information
Feeding mutant gametocytes or in vitro formed mature ookinetes resulted in a comparable reduction in formation of oocysts. These results indicate that neither the number of ookinetes developing in the midgut, nor the effects of the peritrophic membrane or digestive midgut enzymes are likely to be factors contributing to the reductions in mutant oocyst development observed. 

Ookinetes lacking expression of SOAP that are directly injected into the hemocoel, thereby by-passing the midgut wall, develop into oocysts and these ookinetes can transform in vitro into oocysts, indicating that SOAP is not essential for transformation of ookinetes into oocysts (Nacer, A. et al., Malar J.  19;7:82).

Evidence has been presented that SOAP binds to laminin γ1 in yeast cells. The interaction of PbSOAP with mosquito laminin γ1 in the yeast-two-hybrid assays suggests that the native protein may interact with the mosquito basal lamina and, hence, may play a role in ookinete-to-oocyst transition in addition to its demonstrated role in midgut invasion. This is not unprecedented: previously, the ookinete proteins P25, P28 and CTRP were shown to interact with basal lamina components as well as having functions in midgut invasion. P25 and P28 were shown to interact with laminin, while the ookinete micronemal protein CTRP has been shown to interact with both laminin and collagen IV 
 
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