How are plastid proteins of the apicomplexan parasites imported?
A hypothesis
Bodyl, A.
Zoological Institute, University of Wroclaw, ul. Sienkiewicza 21, 50-335 Wroclaw, Poland
Apicomplexa include parasitic protozoans which cause, among others, malaria, toxoplasmosis and coccidiosis. Unexpectedly, not only 35-kb plastid (pt) DNAs, but also modified plastids have been detected in these protozoans. Recently Köhler et al. (1) have demonstrated that the apicomplexan plastids (termed apicoplasts) are surrounded by four membranes, what indicates that their ancestor was a eukaryotic alga. The four-membrane plastids are present in most algae grouped in four phyla: Cryptophyta, Heterokonta, Haptophyta and Chlorarachniophyta. It is assumed that the two outer membranes of the plastids originate from the host's endomembrane system and the endosymbiont's plasma membrane, while the two inner membranes are derived from the envelope membranes of the endosymbiont's plastids. In cryptophytes, heterokonts and haptophytes the outermost membrane of their plastids bears ribosomes. This suggests that in these taxa the phagosomal membrane has fused with the rough endoplasmic reticulum (RER). Since in chlorarachniophytes the most external plastid membrane is smooth, it is presumably derived only from the phagosomal membrane. The electron micrographs published by Köhler et al. (1) indicate that also the outermost membrane of the apicoplasts has not fused with the RER membrane. Because the 35-kb ptDNA contains few genes, it is supposed that most apicomplexan plastid proteins are encoded by the nuclear genome. But how are they imported into the plastids? Lately I have proposed that the plastid proteins of chlorarachniophytes were first transported into the RER, then to the Golgi apparatus, and finally to the plastids (2). Considering the probable absence of the ribosomes on the outermost membrane of the apicoplasts, I suggest that the mechanism of plastid protein import in Apicomplexa may be identical. Since the transport of plastid proteins from the Golgi apparatus to the plastid must be highly specific, this stage may prove to be an Achilles' heel of the apicomplexan parasites and thus may be applied in therapy.
(1) Köhler S. et al. (1997) Science 275: 1485-1489.
(2) Bodyl A. (1997) Bot. Acta 110: 395-400.
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