Plasmodium falciparum STEVOR proteins are highly expressed in patient isolates and located in the surface membranes of infected red blood cells and the apical tips of merozoites

The human parasite Plasmodium falciparum has the potential to express a vast repertoire of variant proteins on the surface of the infected red blood cell (iRBC). Variation in the expression pattern of these proteins is linked to antigenic variation and thereby evasion of host antibody-mediated immunity. The genes in the stevor multigene family code for small variant antigens that are expressed in blood-stage parasites where they can be detected in membranous structures called Maurer's clefts (MC). Some studies have indicated that STEVOR protein may also be trafficked to the iRBC membrane. To address the location of STEVOR protein in more detail, we have analyzed expression in several cultured parasite lines and in parasites obtained directly from patients. We detected STEVOR expression in a higher proportion of parasites recently isolated from patients than in cultured parasite lines and show that STEVOR is trafficked in schizont-stage parasites from the MC to the RBC cytosol and the iRBC membrane. Furthermore, STEVOR protein is also detected at the apical end of merozoites. Importantly, we show that culture-adapted parasites do not require STEVOR for survival. These findings provide new insights into the role of the stevor multigene family during both the schizont and merozoite stages of the parasite and highlight the importance of studying freshly isolated parasites, rather than parasite lines maintained in culture, when investigating potential mediators of host-parasite interactions.