The Apicoplast as a Potential Therapeutic Target in Toxoplasma and Other Apicomplexan Parasites: Some Additional Thoughts

Abstract

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 morphologists1Siddall M.E. Parasitol. Today. 1992; 8: 90-91Abstract Full Text PDF PubMed Scopus (35) Google Scholar; (2) the 35 kb episomal DNA described by molecular biologists2Wilson R.J.M. et al.J. Mol. Biol. 1996; 261: 155-172Crossref PubMed Scopus (464) Google Scholar; and (3) the pharmacological target of classical antibacterial antibiotics (chloramphenicol, clindamycin, thiostrepton, ciprofloxacin, etc.) in apicomplexan parasites3Beckers C.J.M. et al.J. Clin. Invest. 1995; 95: 367-376Crossref PubMed Scopus (108) Google Scholar, 4Fichera M.E. Bhopale M.K. Roos D.S. Antimicrob. Agents Chemother. 1995; 39: 1530-1537Crossref PubMed Scopus (164) Google Scholar, 5McConkey G.A. Rogers M.J. McCutchan T.F. J. Biol. Chem. 1997; 272: 2046-2049Crossref PubMed Scopus (164) Google Scholar, 6Fichera M.E. Roos D.S. Nature. 1997; 390: 407-409Crossref PubMed Scopus (500) Google Scholar. 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 Toxoplasma2Wilson R.J.M. et al.J. Mol. Biol. 1996; 261: 155-172Crossref PubMed Scopus (464) Google Scholar, 7GenBank #U87135Google Scholar, in situ localization to the ‘spherical body’8Köhler S. et al.Science. 1997; 275: 1485-1488Crossref PubMed Scopus (611) Google Scholar, 9McFadden G.I. et al.Bhattacharya D. Origins of Algae and their Plastids. Springer, 1997: 261-287Google Scholar, and extensive phylogenetic analysis8Köhler S. et al.Science. 1997; 275: 1485-1488Crossref PubMed Scopus (611) Google Scholar. 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 proposed3Beckers C.J.M. et al.J. Clin. Invest. 1995; 95: 367-376Crossref PubMed Scopus (108) Google Scholar, 4Fichera M.E. Bhopale M.K. Roos D.S. Antimicrob. Agents Chemother. 1995; 39: 1530-1537Crossref PubMed Scopus (164) Google Scholar, 5McConkey G.A. Rogers M.J. McCutchan T.F. J. Biol. Chem. 1997; 272: 2046-2049Crossref PubMed Scopus (164) Google Scholar, 6Fichera M.E. Roos D.S. Nature. 1997; 390: 407-409Crossref PubMed Scopus (500) Google Scholar.•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 proteins10Denny P.W. et al.Protist. 1998; 149: 51-59Crossref PubMed Scopus (50) Google Scholar. 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) database11Ajioka J. et al.Genome Res. 1998; 8: 18-28Crossref PubMed Scopus (163) Google Scholar 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 well12Waller R.F. et al.Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 12352-12357Crossref PubMed Scopus (636) Google Scholar.•Robert Donald and Boris Striepen (University of Pennsylvania, PA, USA) used recombinant fusions between ACP and the Green Fluorescent Protein13Striepen B. et al.Mol. Biochem. Parasitol. 1998; 92: 328-338Crossref Scopus (164) Google Scholar to show that ACP is targeted to the apicoplast in transgenic T. gondii parasites12Waller R.F. et al.Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 12352-12357Crossref PubMed Scopus (636) Google Scholar, 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 apicoplast8Köhler S. et al.Science. 1997; 275: 1485-1488Crossref PubMed Scopus (611) Google Scholar.•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 clues14Feagin J.E. Annu. Rev. Microbiol. 1994; 48: 81-104Crossref PubMed Scopus (103) Google Scholar, 15Wilson R.J.M. Williamson D.H. Microbiol. Mol. Biol. Rev. 1997; 61: 1-16Crossref PubMed Scopus (186) Google Scholar, but notable among the nuclear-encoded plastid proteins identified to date are several subunits of a type-II fatty acyl synthase complex12Waller R.F. et al.Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 12352-12357Crossref PubMed Scopus (636) Google Scholar, 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 ESTs11Ajioka J. et al.Genome Res. 1998; 8: 18-28Crossref PubMed Scopus (163) Google Scholar (http://www.cbil.upenn.edu/ParaDBs/Toxoplasma/index.html) and the rapidly-advancing P. falciparum genome project16Fletcher C. Parasitol. Today. 1998; 14: 342-344Abstract Full Text Full Text PDF PubMed Scopus (11) Google Scholar, 17Gardner M. Science. 1998; 282: 1126-1132Crossref PubMed Google Scholar (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 parasites18Roberts F. et al.Nature. 1998; 393: 801-805Crossref PubMed Scopus (169) Google Scholar appears unlikely to be associated with the apicoplast19P.J. Keeling, et al., Nature ((in press)).Google Scholar … but plastids typically contain hundreds of nuclear-encoded proteins20Martin W. et al.Nature. 1998; 393: 162-165Crossref PubMed Scopus (586) Google Scholar, so much work remains to be done!

Keywords

Affiliated Institutions

• University of Pennsylvania US

Related Publications