The N-terminal segment of Plasmodium falciparum SURFIN4.1 is required for its trafficking to the red blood cell cytosol through the endoplasmic reticulum

Abstract

Plasmodium falciparum SURFIN is a type I transmembrane protein that shares domains with molecules expressed on the surface of the red blood cells (RBCs) infected with a variety of malaria parasite species, such as P. falciparum PfEMP1, Plasmodium vivax VIR proteins, and Plasmodium knowlesi SICAvar. Thus, understanding the export mechanism of SURFIN to the RBC may provide fundamental insights into how malaria parasites export their proteins to RBC cytosol in general. We re-evaluate SURFIN4.1 for its exon-intron boundaries, location, and the function of each region by expressing recombinant SURFIN4.1 in P. falciparum. We found that, in two 3D7 lines and one Thai isolate, SURFIN4.1 possesses only 19 amino acids after the predicted transmembrane region, whereas in the FCR3 line, it possesses two tryptophan-rich domains in its intracellular region. Recombinant SURFIN4.1 based on the 3D7 sequence was detected in the Maurer's clefts of infected RBCs, suggesting that endogenous SURFIN4.1 is also exported to Maurer's clefts. Brefeldin A-sensitive export of SURFIN4.1 indicates that its export is endoplasmic reticulum (ER)/Golgi-dependent. By sequential deletion and replacement with unrelated protein sequences, we find that the SURFIN4.1 transmembrane region is essential for the initial recruitment of the protein to the ER, and the following sorting step to the parasitophorous vacuole is determined by two independent signals located in the N-terminus 50 amino acids. TM region with the adjacent cytoplasmic region also contains information for the efficient recruitment to the ER and/or for the efficient translocation across the parasitophorous vacuole membrane. We also found that SURFIN4.1 might form a homomeric complex during the trafficking using cysteine rich domain and/or variable region.