Plasmodium falciparum is the causative agent of deadly malaria disease. It encodes repetitive interspersed families of polypeptides (RIFINs), which are expressed on the surface of infected erythrocytes [1]. All RIFIN sequences contain the PEXEL motif (a pentameric sequence RxLxE/Q/D, known as the Plasmodium export element) required for correct export and surface expression or host-targeting (HT) signal which plays a central role in the export of proteins into the host cell [2]. It has been reported that PEXEL is preferably located 15-20 amino acids downstream of an N-terminal hydrophobic signal sequence [3]. The RIFIN protein family can be divided into A and B types based on the presence or absence of a 25 amino acid motif located approximately 66 amino acids downstream of the PEXEL motif [3], with A- and B-types serving different roles in distinct parasite stages [2]. The specific type B RIFIN variant (PF13_0006) is expressed on the surface of free merozoites, internally in developing gametocytes and on the surface of gametes at the point of emerging from activated, mature stage V gametocytes. While type A RIFIN are expressed on the infected erythrocyte surface, potentially contributing to the antigenic variation capacity of the parasite [2].

1: RIFINs are adhesins implicated in severe Plasmodium falciparum malaria. Goel S, Palmkvist M, Moll K, Joannin N, Lara P, Akhouri RR, Moradi N, Ojemalm K, Westman M, Angeletti D, Kjellin H, Lehtio J, Blixt O, Idestrom L, Gahmberg CG, Storry JR, Hult AK, Olsson ML, von Heijne G, Nilsson I, Wahlgren M; Nat Med. 2015;21:314-317. PMID:25751816

2: Expression of a type B RIFIN in Plasmodium falciparum merozoites and gametes. Mwakalinga SB, Wang CW, Bengtsson DC, Turner L, Dinko B, Lusingu JP, Arnot DE, Sutherland CJ, Theander TG, Lavstsen T; Malar J. 2012;11:429. PMID:23259643

3: The Plasmodium export element revisited. Hiss JA, Przyborski JM, Schwarte F, Lingelbach K, Schneider G; PLoS One. 2008;3:e1560. PMID:18253504