Plasmodium multigene families encoding variant exported blood-stage proteins are also expressed in liver-stages

All malaria parasites contain different, subtelomerically located, multigene families that encode exported proteins that are proposed to be involved in host-parasite interactions. P. falciparum-specific variant proteins encoded by the var, stevor and rifin multigene families mediate interactions between infected RBC (iRBC) and host tissue or with uninfected RBC resulting in tissue sequestration and rosetting. However, the precise function of most of other Plasmodium multigene families encoding exported proteins is unknown. To understand the role of RBC-exported proteins of rodent malaria parasites (RMP) we analysed expression and cellular location by fluorescent-tagging of members of the pir, fam-a and fam-b multigene families throughout the complete life cycle of P. berghei, including mosquito and pre-erythrocytic stages. We have also performed phylogenetic analyses of the fam-a and fam-b multigene families. We demonstrated for all three families that expression of family members is not mutually exclusive in iRBC. Most proteins were localized in the iRBC cytoplasm and not transported onto the surface membrane, indicating that these members play no direct role in interactions between iRBC and host cells. Surprisingly, and against expectation, most family members that are expressed in the host RBC are also expressed in liver-stages, where they are transported into the parasitophorous vacuole. Our observations demonstrate that these three multigene families do not function exclusively for development in RBC. Their expression in late liver-stages suggest that these proteins function to promote parasite development in both liver and blood, either by controlling merozoite formation and/or function or by manipulating the host immune response to support parasite development in the blood.