Characterization of Two Putative Protein Translocation Components in the Apicoplast of Plasmodium falciparum

Protein trafficking to the stroma of the apicoplast of Plasmodium falciparum requires translocation across several membranes. To further elucidate the mechanisms responsible, we investigated two proteins: P. falciparum Tic22 (PfTic22), a putative component of the translocon of the inner chloroplast membrane; and PfsDer1-1, one of two homologues of the P. falciparum symbiont-derived Der1 (sDer1) protein, a putative component of an endoplasmic reticulum-associated degradation (ERAD) complex in the periplastid membrane. We constructed parasites expressing hemagglutinin (HA)-tagged PfTic22 and PfsDer1-1 under the control of their endogenous promoters using the 3' replacement strategy. We show that both PfTic22-HA and PfsDer1-1-HA are expressed predominantly during the trophozoite stage of the asexual replication cycle, which corresponds to the most dynamic stages of apicoplast activity. Although both proteins localize to the periphery of the apicoplast, PfTic22-HA is a membrane-associated protein while PfsDer1-1-HA is an integral membrane protein. Phylogenetic analysis indicates that PfsDer1-1 is one of two Der1 paralogues predicted to localize to the apicoplast in P. falciparum and that it has orthologues in diatom algae, supporting the chromalveolate hypothesis. These observations are consistent with putative roles for PfTic22 and PfsDer1-1 in protein translocation into the apicoplast of P. falciparum.