Protein trafficking to the plastid of Plasmodium falciparum is via the secretory pathway

The plastid of Plasmodium falciparum (or 'apicoplast') is the evolutionary homolog of the plant chloroplast and represents a vestige of a photosynthet ic past. Apicoplast indispensability indicates that it still provides essen tial functions to parasites. Similar to plant chloroplasts, the apicoplast is dependent on many nucleus-encoded genes to provide these functions. The apicoplast is surrounded by four membranes, two more than plant chloroplast s. Thus, protein targeting to the apicoplast must overcome additional membr ane barriers. In P.falciparum we have analyzed apicoplast targeting using g reen fluorescent protein (GFP). We demonstrate that protein targeting is at least a two-step process mediated by bipartite N-terminal presequences tha t consist of a signal peptide for entry into the secretory pathway and a pl ant-like transit peptide for subsequent import into the apicoplast. The P.f alciparum transit peptide is exceptional compared with other known plastid transit peptides in not requiring serine or threonine residues. The presequ ence components are removed stepwise during apicoplast targeting. Targeting GFP to the apicoplast has also provided the first opportunity to examine a picoplast morphology in live P.falciparum.