Proteins of the SNARE (Soluble N-ethylmaleimide sensitive factor attachment protein receptor) super-family have been characterized as playing an essential role in vesicle targeting and fusion in all eukaryotes. The intracellular malaria parasite Plasmodium falciparum exhibits an unusual endomembrane system that is characterized by an unstacked Golgi apparatus, a developmentally induced apical complex, and various organellar structures of parasite origin in the infected host cells. How malaria parasites target nuclear-encoded proteins to these novel compartments is a central question in Plasmodium cell biology. Ultrastructural studies elsewhere have implicated the participation of specialized vesicular elements in transport of virulence proteins, including various cytoadherance and host cell remodeling factors, into the infected erythrocyte cytoplasm. However, little is known about the machineries that define the directionality of vesicle trafficking in malaria parasites. We hypothesized that the P. falciparum SNARE proteins would exhibit novel features required for vesicle targeting to the parasite-specific compartments. We then identified for the first time and confirmed the expression of eighteen SNARE genes in P. falciparum. Members of the PfSNAREs exhibit atypical structural features (Ayong et al., 2007, Molecular & Biochemical Parasitology, 152(2), 113-122). Among the atypical PfSNAREs, PfSec22 contains an unusual insertion of the Plasmodium export element (PEXEL) within its profilin-like longin domain, preceded by an N-terminal hydrophobic segment. Localization analyses suggest that PfSec22 is predominantly a vesicle-associated SNARE of the ER/Golgi interface, but which associates partially with mobile extraparasitic vesicles in P. falciparum-infected erythrocytes at trophozoite stages. We showed that PfSec22 export into host cells occurs via a two-step model that involves extraparasitic vesicle budding from the parasite plasma membrane and fusion with the parasitophorous vacuolar membrane. Export of PfSec22 was independent of its membrane-insertion suggesting that this protein might cross the vacuolar space as a single-pass type IV membrane protein. We demonstrated that the atypical longin domain dictates the steady-state localization of PfSec22, regulating its ER/Golgi trafficking and export into host cells. Our study provides the first experimental evidence for SNARE protein export in P. falciparum, and suggests a role of PfSec22 in vesicle trafficking within the infected host cell (Ayong et al, Eukaryotic Cell, Epub Jul 17, 2009) Next, to define the physiological function of the PfSec22 protein in Plasmodium parasites, we investigated its cognate partners. Using purified recombinant proteins we showed that PfSec22 forms direct binding interactions with six other PfSNAREs in vitro. These included the PfSyn5, PfBet1, PfGS27, PfSyn6, PfSyn16 and PfSyn18 PfSNAREs. By generating GFPexpressing parasites, we successfully localized the SNARE proteins PfSyn5, PfBet1 and PfGS27 to the parasite cis-Golgi compartment. We confirmed the association of PfSec22 with PfSyn5, PfBet1 and PfGS27 in vivo by immunoprecipitation analyses. Our data indicate a conserved ERto-Golgi SNARE assembly in P. falciparum, and suggest that the malaria Sec22 protein might form novel SNARE complexes required for vesicle traffic within P. falciparum-infected erythrocytes.
Doctor of Philosophy (Ph.D.)
College of Medicine
Burnett School of Biomedical Sciences
Length of Campus-only Access
Doctoral Dissertation (Open Access)
Ayong, Lawrence Sumanjah, "Vesicle Targeting In Plasmodium Falciparum: The Identification and Molecular Characterization of Plasmodium Falciparum Family of of Snare Proteins" (2009). Electronic Theses and Dissertations. 6145.