The phylum Apicomplexa comprises over 5000 intracellular
protozoan parasites, including Plasmodium and Toxoplasma,
that are clinically important pathogens affecting humans and
livestock. Malaria parasites belonging to the genus
Plasmodium possess a pellicle comprised of a plasmalemma
and inner membrane complex (IMC), which is implicated in
parasite motility and invasion. Using live cell imaging and
reverse genetics in the rodent malaria model P. berghei, we
localise two unique IMC sub-compartment proteins (ISPs)
and examine their role in defining apical polarity during
zygote (ookinete) development. We show that these proteins
localise to the anterior apical end of the parasite where IMC
organisation is initiated, and are expressed at all
developmental stages, especially those that are invasive.
Both ISP proteins are N-myristoylated, phosphorylated and
membrane-bound. Gene disruption studies suggest that ISP1
is likely essential for parasite development, whereas ISP3 is
not. However, an absence of ISP3 alters the apical localisation
of ISP1 in all invasive stages including ookinetes and
sporozoites, suggesting a coordinated function for these
proteins in the organisation of apical polarity in the parasite.