SURFIN(4.1), a schizont-merozoite associated protein in the SURFIN family of Plasmodium falciparum

BACKGROUND: In its effort to survive the human immune system, Plasmodium falciparum uses several parasite-derived antigens most of which are expressed at the surface of the parasitized red blood cells (pRBCs). Recently SURFINs, a new family of antigens encoded by the surf multi-gene family, has been reported. One member of the family, SURFIN(4.2), was found present both at the pRBC-surface and at the merozoite apex. METHODS: The presence of a second SURFIN member, SURFIN(4.1 )(PFD0100c, PFD0105c) is reported here. Bioinformatic tools were used to study the structure of the surf(4.1 )gene. To investigate the expression of surf genes PCR and real-time quantitative PCR (Rt-QPCR) were employed and Northern and Western blots were used to confirm the size of the surf(4.1 )gene and the SURFIN(4.1 )protein respectively. Localization of SURFIN(4.1 )was determined using immunofluorescence assays. RESULTS: The surf(4.1 )gene was found present in one copy by Rt-QPCR in some parasites (3D7AH1, 3D7S8, 7G8) whereas six copies of the gene were identified in FCR3 and FCR3S1.2. surf(4.1 )was found transcribed in the late asexual stages of the parasite beginning ≈32 hours post invasion and throughout the schizont stages with the level of transcription peaking at late schizogony. The levels of transcript correlated with the number of gene copies in FCR3 and 3D7S8. surf(4.1 )was found to encode a polypeptide of ≈Mw 258 kDa (SURFIN(4.1)) present within the parasitophorous vacuole (PV), around free merozoites as merozoite-associated material, but not at the pRBC-surface. Despite multiple surf(4.1 )gene copies in some parasites this was not reflected in the levels of SURFIN(4.1 )polypeptide. CONCLUSION: SURFIN(4.1 )is a member of the SURFINs, present in the PV and on the released merozoite. The results suggest different SURFINs to be expressed at different locations in the parasite and at distinct time-points during the intra-erythrocytic cycle.