The apicoplast as a potential therapeutic target in Toxoplasma and other apicomplexan parasites: some additional thoughts.

Soldati (this issue) provides a thought-provoking Comment on the apicomplexan plastid, or ‘apicoplast’. It is very satisfying (if all-too-rare) when disparate areas of research converge, and this may account for the considerable interest aroused by studies on the apicoplast. Recent data have merged three lines of scientific inquiry into a single organelle: (1) the spherical body/Hohlzylinder/Golgi adjunct/organelle plurimembranaire, long noted by morphologists1xSiddall, M.E. Parasitol. Today. 1992; 8: 90–91Abstract | Full Text PDF | PubMed | Scopus (21)See all References1; (2) the 35 kb episomal DNA described by molecular biologists2xWilson, R.J.M. et al. J. Mol. Biol. 1996; 261: 155–172Crossref | PubMed | Scopus (378)See all References2; and (3) the pharmacological target of classical antibacterial antibiotics (chloramphenicol, clindamycin, thiostrepton, ciprofloxacin, etc.) in apicomplexan parasites3xBeckers, C.J.M. et al. J. Clin. Invest. 1995; 95: 367–376Crossref | PubMedSee all References, 4xFichera, M.E., Bhopale, M.K., and Roos, D.S. Antimicrob. Agents Chemother. 1995; 39: 1530–1537Crossref | PubMedSee all References, 5xMcConkey, G.A., Rogers, M.J., and McCutchan, T.F. J. Biol. Chem. 1997; 272: 2046–2049Crossref | PubMed | Scopus (125)See all References, 6xFichera, M.E. and Roos, D.S. Nature. 1997; 390: 407–409Crossref | PubMed | Scopus (381)See all References.Since the first recognition of plastid-like characteristics in the 35 kb element of Plasmodium, this field has advanced rapidly, culminating in complete sequencing of the organellar genome from both Plasmodium and Toxoplasma2xWilson, R.J.M. et al. J. Mol. Biol. 1996; 261: 155–172Crossref | PubMed | Scopus (378)See all References, 7xSee all References, in situ localization to the ‘spherical body’8xKohler, S. et al. Science. 1997; 275: 1485–1488Crossref | PubMed | Scopus (504)See all References, 9xMcFadden, G.I. et al. : 261–287See all References, and extensive phylogenetic analysis8xKohler, S. et al. Science. 1997; 275: 1485–1488Crossref | PubMed | Scopus (504)See all References8. At the recent Molecular Parasitology Meetings in Woods Hole, MA, USA (September 1998), several groups reported further studies on the apicoplast:•Barbara Clough from the Mill Hill group (London, UK) described an anti-peptide antibody that recognizes recombinant Plasmodium falciparum apicoplast elongation factor Tu (EF-Tu; encoded by the plastid tufA gene) on western blots. Antibiotics that bind to E. coli EF-Tu altered the mobility of the recombinant parasite protein in gel-shift assays. If these antibodies recognize the native protein, it will be interesting to determine whether EF-Tu synthesis in the parasite is sensitive to clindamycin, chloramphenicol, thiostrepton, etc., as previously proposed3xBeckers, C.J.M. et al. J. Clin. Invest. 1995; 95: 367–376Crossref | PubMedSee all References, 4xFichera, M.E., Bhopale, M.K., and Roos, D.S. Antimicrob. Agents Chemother. 1995; 39: 1530–1537Crossref | PubMedSee all References, 5xMcConkey, G.A., Rogers, M.J., and McCutchan, T.F. J. Biol. Chem. 1997; 272: 2046–2049Crossref | PubMed | Scopus (125)See all References, 6xFichera, M.E. and Roos, D.S. Nature. 1997; 390: 407–409Crossref | PubMed | Scopus (381)See all References.•Naomi Lang-Unnasch (University of Alabama, USA) presented a careful analysis of the plastid rpoB sequence of Toxoplasma gondii, confirming the use of UGA codons to encode tryptophan in apicoplast proteins10xDenny, P.W. et al. Protist. 1998; 149: 51–59Crossref | PubMedSee all References10. This finding is consistent with the predicted structure for the T. gondii apicoplast small subunit ribosomal RNA.•Several groups sought to identify nuclear-encoded genes destined for import into the apicoplast. Christopher Hagen (Seattle Biomedical Research Institute, USA) identified several sequences from the T. gondii EST (expressed sequence tag) database11xAjioka, J. et al. Genome Res. 1998; 8: 18–28PubMedSee all References11 as potential apicoplast genes, including elongation factor G and ribosomal protein rps9.•Ross Waller (Melbourne University, Australia) used antibody to recombinant proteins derived from the T. gondii EST database to demonstrate localization of rps9 and acyl carrier protein (ACP) to the apicoplast, and identified putative nuclear-encoded apicoplast genes in the expanding P. falciparum genome databases as well12xWaller, R.F. et al. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 12352–12357Crossref | PubMed | Scopus (529)See all References12.•Robert Donald and Boris Striepen (University of Pennsylvania, PA, USA) used recombinant fusions between ACP and the Green Fluorescent Protein13xStriepen, B. et al. Mol. Biochem. Parasitol. 1998; 92: 328–338Crossref | Scopus (132)See all References13 to show that ACP is targeted to the apicoplast in transgenic T. gondii parasites12xWaller, R.F. et al. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 12352–12357Crossref | PubMed | Scopus (529)See all References12, and went on to dissect the apicoplast targeting signal. This consists of an unusual bipartite structure that may facilitate trafficking across the four membranes surrounding the apicoplast8xKohler, S. et al. Science. 1997; 275: 1485–1488Crossref | PubMed | Scopus (504)See all References8.•Exploiting ACP–GFP as an apicoplast marker, Cynthia He (University of Pennsylvania, PA, USA) reported partial purification of the apicoplast by subcellular fractionation.Perhaps the most important question about the apicoplast still remains: ‘What essential function(s) does this intriguing organelle perform for the parasite?’ The apicoplast genome itself yields no obvious clues14xFeagin, J.E. Annu. Rev. Microbiol. 1994; 48: 81–104Crossref | PubMedSee all References, 15xWilson, R.J.M. and Williamson, D.H. Microbiol. Mol. Biol. Rev. 1997; 61: 1–16PubMedSee all References, but notable among the nuclear-encoded plastid proteins identified to date are several subunits of a type-II fatty acyl synthase complex12xWaller, R.F. et al. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 12352–12357Crossref | PubMed | Scopus (529)See all References12, suggesting that lipid biosynthesis may be one function. The distinctive nature of apicoplast targeting signals should greatly facilitate identification of additional nuclear-encoded plastid proteins among T. gondii ESTs11xAjioka, J. et al. Genome Res. 1998; 8: 18–28PubMedSee all References11 (http://www.cbil.upenn.edu/ParaDBs/Toxoplasma/index.html) and the rapidly-advancing P. falciparum genome project16xFletcher, C. Parasitol. Today. 1998; 14: 342–344Abstract | Full Text | Full Text PDF | PubMed | Scopus (10)See all References, 17xGardner, M. Science. 1998; 282: 1126–1132Crossref | PubMedSee all References (http://www.tigr.org/tdb/parasites/index.html; http://www.sanger.ac.uk/Projects/P_falciparum/; and http://sequence-www.stanford.edu/group/malaria/). By these criteria, the shikimate pathway recently identified in apicomplexan parasites18xRoberts, F. et al. Nature. 1998; 393: 801–805Crossref | PubMed | Scopus (132)See all References18 appears unlikely to be associated with the apicoplast19xSee all References19 … but plastids typically contain hundreds of nuclear-encoded proteins20xMartin, W. et al. Nature. 1998; 393: 162–165Crossref | PubMed | Scopus (474)See all References20, so much work remains to be done!