Mutant/mutation
The mutant lacks expression of ISP1 and ISP3

Protein (function)
The inner membrane complex (IMC)-residing protein ISP (IMC subcompartment protein) is restricted to the phylum apicomplexa and plays key roles in parasite biology. Two ISP members, ISP1 and ISP3, are found in Plasmodium parasites, and display clear apical polarity of expression in zygotes, suggesting possible role in zygote protrusion or elongation. However, parasites with disruption of ISP1 display a modest decrease in zygote-to-ookinete differentiation, while ISP3 depletion has no significant effect on this process.

Phenotype
The mature ookinete conversion rate of this Δisp1/3 parasite (9%) was significantly lower than that of Δisp1 or Δisp3. Measurement of DNA content after Hoechst 33342 stain indicated the P28-positive gametes of the Δisp1/3 parasite could be fertilized and further develop from diploid to tetraploid, suggesting normal DNA replication. However, time-course (stages I–V) analysis of ookinete differentiation in vitro  revealed developmental arrest at early stages (I and II) for the Δisp1/3 cell. We also isolated parasites from infected mosquito midguts and observed a similar defect of Δisp1/3 in vivo. Consequently, the Δisp1/3 parasite showed significantly reduced number of day 7 midgut oocysts and day 14 salivary gland sporozoites compared with the WT in the infected mosquitoes. Together, these results demonstrate that loss of both ISP1 and ISP3 causes a severe defect in early ookinete differentiation and therefore mosquito transmission of the parasite.
Both Δisp1 and Δisp3 mutants (RMgm-4889, RMgm-4890) showed normal development of asexual blood stages and gametocytes in vivo as well as gamete formation and zygote-to-retort differentiation in vitro. However, the mature ookinete conversion in vitro was significantly reduced in Δisp1 (conversion rate: 34%) but not in Δisp3 (57%) compared with wild type (WT, 64%).

Additional information
From the Abstract:
'Here, we show that palmitoylation of N-terminal cysteines of two inner membrane complex (IMC) proteins (ISP1/ISP3) regulates the IMC localization of ISP1/ISP3 and zygote-to-ookinete differentiation. Palmitoylation of ISP1/ISP3 is catalyzed by an the IMC-residing palmitoyl-S-acyl-transferase (PAT) DHHC2. IMC-anchored ISP1 and ISP3 interact with microtubule component b-tubulin, serving as tethers to maintain the proper structure of subpellicular microtubules (SPMs) during zygote elongation.

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