Cryptosporidium parvum Cpn60 targets a relict organelle
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- Riordan, C.E., Ault, J.G., Langreth, S.G. et al. Curr Genet (2003) 44: 138. doi:10.1007/s00294-003-0432-1
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Chaperonin 60 (Cpn60) is a well-established marker protein for eukaryotic mitochondria and plastids. In order to determine whether the small double-membrane-bounded organelle posterior to the nucleus in the apicomplexan Cryptosporidium parvum is a mitochondrion, the Cpn60 gene of C. parvum sporozoites (CpCpn60) was analyzed and antibodies were generated for localization of the peptide. Sequence and phylogenetic analyses indicated that CpCpn60 is a mitochondrial isotype and that antibodies against it localize to the rough endoplasmic reticulum-enveloped remnant organelle of C. parvum sporozoites. These data show this organelle is of mitochondrial origin.
KeywordsApicomplexanRelict mitochondrionChaperonin 60 (Cpn60)
Cryptosporidium parvum belongs to the diverse group of intracellular apicomplexans that includes species of human (Babesia microti, Plasmodium falciparum, Toxoplasma gondii) and veterinary (Eimeria tenella, Neospora caninum) importance (Ellis et al. 1998; Fayer 1997). Phylogenetic analyses indicate that dinoflagellates and ciliates share a common ancestor with apicomplexans to form the group Alveolata (Cavalier-Smith 1983; Ellis et al. 1998; Sogin 1991). Within the Apicomplexa, the genus Cryptosporidium (together with the gregarines) is an early emerging branch (Barta 1989; Carreno et al. 1999; Zhu et al. 2000a), forming a monophyletic sister group to the clade comprising P. falciparum and T. gondii (Zhu et al. 2000a). While a typical single mitochondrion is present in most apicomplexans (McFadden 2003), its existence in C. parvum remained an open question (Aji et al. 1991; Tetley et al. 1998). Recently, however, we showed that sporozoites of C. parvum do contain a double-membrane-bounded organelle posterior to the nucleus that differs in its structure from typical mitochondria (Riordan et al. 1999). This organelle with an as yet unknown function is a possible candidate for a relict mitochondrion.
C. parvum appears to share with most apicomplexans extended glycolysis as the primary source of ATP, with little or no contribution by oxidative phosphorylation (Crawford et al. 2003; Entrala and Mascaro 1997; Martin et al. 2001). Unlike most eukaryotes, which contain many mitochondria, the apicomplexans P. falciparum and T. gondii contain a single mitochondrion (McFadden 2003) that retains a 6-kb organellar genome encoding only five proteins (Feagin 2000). As in all organisms, most mitochondrial proteins of the Apicomplexa are encoded by the nucleus, translated in the cytoplasm, and post-translationally imported into this mitochondrion (McFadden 2003). Both the mitochondrion of P. falciparum and the double-membrane-bounded organelle of C. parvum appear to generate a proton gradient, as indicated by the uptake of mitochondrion-specific vital fluorescent dyes (Crawford et al. 2003; Keithly, Ault, Buttle, Mannella, LaGier, Langreth, unpublished data). Unlike P. falciparum and T. gondii (Crawford et al. 2003), there is no evidence for an electron transport chain in C. parvum, but uptake of 10-N-nonyl acridine orange indicates that the C. parvum organelle contains cardiolipin, an important component of the mitochondrial inner membranes (IM).
Like eukaryotes, most apicomplexan mitochondria contain cristae with connections to the mitochondrial IM. The type and number of these cristae are differentially expressed during apicomplexan life cycles, ranging from tubular in T. gondii tachyzoites and P. falciparum gametocytes to acristate in asexual P. falciparum and P. knowlesi (Fry and Beesely 1991; Krungkrai et al. 1999, 2000). Electron tomography suggests the presence of cristae-like compartments in the C. parvum organelle (Keithly, Ault, Buttle, Mannella, LaGier, Langreth, unpublished data) but, unlike in its nearest relatives, these appeared to be unattached to the IM. Since the number of cristae tubular connections to the IM is predicted to correlate with global ATP production (Mannella et al. 2001), perhaps the distinctive cristae of the C. parvum organelle signals not only the loss of oxidative phosphorylation observed in all apicomplexan mitochondria, but also a significant divergence in organelle structure and function within the phylum.
Molecular evidence also suggests that this organelle may share a mitochondrial-type ancestry with other protists. First, the nucleus-encoded genes adenylate kinase 2 and valyl tRNA synthase [with affinities to mitochondrial homologues (Bui et al. 1996; Hashimoto et al. 1998; Sanchez and Muller 1998)], have been isolated and characterized (Riordan et al. 1999). Second, genes associated with the iron sulfur cluster (Isc) assembly [a known function of eukaryotic mitochondria (Lill and Kispal 2000)] have been isolated; and at least two of these (IscS, IscU) contain transit peptides that target green fluorescent protein (GFP) to yeast mitochondria (LaGier, Tachezy, Stejskal, Kutisova, Keithly, unpublished data).
Heat-shock protein or chaperonin 60 (Hsp/Cpn60) is the quintessential nucleus-encoded gene, indicating the endosymbiotic origin of eukaryotic mitochondria (Archibald et al. 2002; Gupta 1995; Horner and Embley 2001) from an α-proteobacterium (Bui et al. 1996; Martin et al. 2001). Previously, for example, the most convincing evidence for a mitochondrial origin of the ″amitochondriate″ Trichomonas vaginalis hydrogenosome (Bui et al. 1996) and the Entamoeba histolytica mitosome/crypton (Clark and Roger 1995; Mai et al. 1999; Tovar et al. 1999) was the presence of Cpn60 in their genomes and the subsequent localization of the chaperonin to these organelles.
Among the Apicomplexa, Cpn60 is encoded by the genomes of Toxoplasma gondii, P. falciparum, and P. yoelii (Sanchez et al. 1999; Syin and Goldman 1996; Toursel et al. 2000); and Cpn60 peptides have been localized by immunoelectron microscopy to the single mitochondrion of T. gondii tachyzoites (Toursel et al. 2000) and P. falciparum trophozoites within infected erythrocytes (Das et al. 1997). Here, we show that, like its nearest relatives, Cpn60 is encoded in the genome of C. parvum (CpCpn60) and, by sequence analysis, phylogenetic reconstruction, and localization of the CpCpn60 peptide, we show that the organelle of unknown function in C. parvum sporozoites is of mitochondrial origin.
Materials and methods
Dr. M.V. Nesterenko (Kansas State University, Manhattan, Kan.) and P. Mason (Pleasant Hill Farm, Troy, Idaho) provided oocysts of C. parvum strains KSU-1 and Iowa, respectively. Oocysts were purified by CsCl or sucrose-gradient centrifugation, followed by 5 min sterilization in 10% Clorox on ice and 5–8 washes by centrifugation in sterile water. For in vitro excystation, purified oocysts were incubated at 37 °C for 1 h in Hanks′ balanced salt solution (HBSS) containing 0.25% trypsin and 0.75% taurodeoxycholate to obtain free sporozoites, as described by Keithly et al. 1997).
Total DNA or RNA was isolated from intracellular stages of C. parvum as detailed by Zhu et al (2000b). Briefly, to isolate total RNA from intracellular stages, human ileocecal epithelial cells (HCT-8; ATCC CCL 244) cultured for 18 h at 37 °C were inoculated with 3.0×104 sterilized oocysts, so that excystation/invasion could occur. Uninfected, control HCT-8 cells were treated identically.
Cloning, gDNA isolation, libraries, and sequencing
Genomic DNA (gDNA) was isolated from free sporozoites using DNAzol genomic DNA isolation reagent (Molecular Research Center). Both pBluescript SK(+) HindIII and EcoRI gDNA libraries of C. parvum KSU-1 strain provided by Dr. N. Khramtsov (Kansas State University) were screened for mitochondrion-specific genes. A set of degenerate oligonucleotide primers corresponding to highly conserved regions of the CpCpn60 gene [sense: SK(I/V)TKGGCTV; antisense: L/P(A/P/S/T)Q(A/P/S/T)(G/R)(C/G/S/K) (Clark and Roger 1995)] were used in PCR with C. parvum gDNA to amplify a portion of the gene. The 576-bp amplicon was purified using a GeneClean Kit (Bio101) and then cloned into pCR2.1 (TA cloning kit; Invitrogen). The excised 576-bp insert was radiolabeled with [α-32P]-dATP using the Random prime DNA labeling kit (Boehringer Mannheim) before probing gDNA libraries and Southern blots. Most of the sequence was obtained from several clones by gene-walking. The 5′ end of the gene was obtained using the GeneRacer kit (Invitrogen), in which complementary DNA (cDNA) ends are rapidly amplified with an interior primer designed from the known Cpn60 sequence. Sequences of each of the isolated clones were determined twice for both strands by an automated DNA sequencer within the Wadsworth Molecular Genetics Core Facility.
Southern blots and RT-PCR
For Southern blot analysis, 1 μg sporozoite gDNA/lane was digested with EcoRI, HindIII, EcoRI/HpaII, or EcoRV, separated by electrophoresis and transferred to Zeta Probe Nylon membranes (Bio-Rad). A 576-bp DNA fragment of the CpCpn60 ORF was released from a plasmid clone by restriction digestion with EcoRI, labeled with [α-32P]-dATP, and used to probe blots under conditions of high stringency (Maniatis et al. 1982). RT-PCR used a pair of primers specific for CpCpn60, cpnRTfor (5′-ATC-ACC-GAA-CCC-TGG-CGC-TTT-TAC-TGC-3′) and cpnRTrev (5′-GTG-TTA-GCA-AGA-GCA-ATT-TTC-AAA-TCA-3′), which produced an amplicon of 576 bp. Each 50-μl reaction included 5 units of AMV reverse transcriptase, 5 units of Tfl DNA polymerase, 100 ng of total RNA isolated from C. parvum sporozoites using the RNeasy kit (Qiagen), 1 μM of each primer, and other reagents as suggested for the Access RT-PCR kit (Promega). Two negative controls containing all reagents except total RNA or reverse transcriptase, respectively, were always included. The first-strand cDNA synthesis was conducted at 48 °C for 45 min, followed by heat-inactivation of the reverse transcriptase at 94 °C for 2 min. The cDNA was then amplified for 35-cycles. All amplicons were sequenced to confirm identity.
Sequence alignments and phylogenetic analysis
Preliminary alignments were performed using the GCG Wisconsin package ver. 9.1 within the UNIX cluster at the Wadsworth Center. The alignment was improved by visual editing; and phylogenetic analyses were performed using the program packages MUST ver. 1.0 (Philippe 1993) and MOLPHY ver. 2.3 (Adachi and Hasegawa 1996). Alignments are available upon request.
Phylogenetic relationships among related sequences of CpCpn60 were inferred by the maximum likelihood (ML) method of protein phylogeny (Kishino and Hasegawa 1989). The analysis was performed with the PROTML ver. 2.3 program; and the Jones–Taylor–Thornton model (Jones et al. 1992) was assumed for substitutions. Because the number of taxonomic units analyzed was very large, a neighbor-joining (NJ) tree (Saitou and Nei 1987) was first constructed, using a distance matrix estimated by the ML method. The NJ tree was then further analyzed by ML local rearrangements.
Yeast GFP plasmid
The mitochondrion-targeted plasmid pYX223-mtGFP (Westermann and Neupert 2000) was a gift from B. Westermann (Ludwig-Maxilians-Universitat Munchen, Munich, Germany). This plasmid contains a known mitochondrial pre-sequence from protein 9 (proteolipid subunit) of the F0 part of the F1F0 ATPase of N. crassa ligated to GFP (pSu9-69-GFP) and has an inducible GAL promoter which can be regulated. The first 170 bp (57 aa) of the CpCpn60 gene were generated by PCR from gDNA by adding EcoRI and BamHI linker sequences to the 5′ and 3′ end, respectively. The N. crassa mitochondrial pre-sequence was digested from the plasmid using EcoRI and BamHI and was then replaced with the putative C. parvum pre-sequence. The new recombinant plasmid (pCpn60-57-GFP) was amplified in Ultracompetent XL-10 gold cells (Stratagene), purified, and sequenced to confirm its identity. It was then transfected into Saccharomyces cerevisiae (strain KY527) and plated onto galactose-containing S. cerevisiae-His agar to induce expression of the GFP. Only cells transformed with the recombinant plasmid grew on selective plates; and samples from these colonies were prepared for examination by attaching cells to glass slides coated with poly-l-lysine. Cells were observed with epifluorescence and phase contrast microscopy, using an Olympus BH2 phase/fluorescence microscope.
MitoTracker red vital fluorescent dye staining
MitoTracker red CMSRos (M7512) was used, as per the manufacturer′s instructions (Molecular Probes), to delineate the mitochondrial tubular network of the yeast S. cerevisiae transformed either with the recombinant plasmid pCpn60-57-GFP or with pSu9-69-GFP. This dye is well suited for multicolor-labeling experiments, because the red fluorescence (λex=579 nm, λem=599 nm) is clearly resolved from the green fluorescence of other probes. Furthermore, the reduced probes fluoresce only upon entry into an actively respiring cell, where they are oxidized to the fluorescent mitochondrion-selective probe, which is then sequestered within the mitochondrial network.
The Wadsworth Center Peptide Synthesis Core Facility synthesized peptides corresponding to amino acids (aa) 67–84 and 230–248 of the C. parvum Cpn60 sequence. The peptides were selected using antigenicity indices, hydrophilicity, surface probability, and secondary structure criteria. These values were determined using the programs PEPTIDESTRUCTURE and PLOTSTRUCTURE in the GCG Wisconsin package ver. 9.1 within the UNIX cluster at the Wadsworth Center, together with BLASTP and CN3D ver. 2.5 (a molecular modeling database) at the National Center for Biotechnology Information website (http://www.ncbi.nlm.nih.gov/blast). These programs indicated that the selected peptides had little homology to other proteins and were specific for C. parvum. The selected regions of the protein also lacked significant secondary structure, were surface-oriented, and were therefore probably available for antibody-binding. The peptides were conjugated to Mariculture keyhole limpet hemocyanin (mcKLH; Pierce) and sent to Lampire Biological Laboratories (Pipersville, Pa.), where they were injected into two female New Zealand white rabbits (2 kg each, 12 weeks old) to produce antibodies. Each 1.0-ml primary and booster injection contained approximately 200 μg of conjugated peptide (0.5 ml of conjugate solution) and 200 μg of adjuvant (0.5 ml). Injections were distributed over four sites (250 μl/site). After the initial injection, rabbits were boosted once per month for 3 months and the rabbit serum was tested for reactivity to CpCpn60 and antibody titer by ELISA. Antisera of both rabbits showed a significant signal above background; and the pooled antiserum was affinity column-purified, using the peptide conjugated to agarose beads according to Lampire protocols (http://www.lampire.com).
The antibodies were first tested for specificity by Western blot analysis, using whole-protein extracts of C. parvum sporozoites. Later, sporozoites fixed in 10% formalin were washed with PBS and then adhered to glass slides coated with poly-l-lysine . Fixed slides were washed with methanol, blocked with 2% bovine serum albumin (BSA), and incubated with varying dilutions of rabbit pre-immune serum or affinity-purified antibodies diluted in 1% BSA/PBS and incubated overnight at 4 °C. Rinsed slides were then incubated with goat anti-rabbit IgG secondary antibody conjugated to fluorescein isothiocyanate (FITC) for 1 h at room temperature. Fluorescence was observed using an Olympus BH2 phase/fluorescence microscope.
C. parvum sporozoites were excysted from oocysts and purified as detailed by Keithly et al. (1997). An aliquot of 2×107 sporozoites was lysed in standard SDS-reducing buffer at 100 °C for 10 min. Samples were centrifuged at 20,000 g for 1 min and the supernatant was loaded onto a 10% SDS-polyacrylamide gel along with appropriate ladders. The samples were run at 100 V/cm in the stacking gel and 200 V/cm in the separating gel. Separated proteins were transferred to a nitrocellulose membrane (Zetaprobe). Blots were probed with either rabbit pre-immune serum or purified antibodies at dilutions of 1:30,000 and were visualized using the Western blot chemiluminescence reagent plus kit (NEN Life Science Products) and Kodak X-Omat blue autoradiography film.
Sporozoites were fixed immediately after excystation in 4% electron microscopy (EM)-grade methanol-free formaldehyde (Polysciences) in HBSS at 4 °C for 16 h, followed by additional fixation in 4% formaldehyde/0.1% EM-grade glutaraldehyde (Polysciences) in 0.1 M Na cacodylate buffer containing 4% sucrose and 0.05 mM CaCl2 (pH 7.4) at 4 °C for 3 h. Sporozoites were given four washes in this buffer (15 min each), dehydrated in an ethanol series, and embedded in LR White resin. Semi-thin (0.08–0.20 μm) sections were cut using a Diatome diamond knife on a Reichert Ultracut E ultramicrotome. The sections were blocked for 30 min in a solution containing 50 μg reconstituted whole goat serum/ml and 20 μg BSA/ml in Tris-buffered saline (TBS; containing 20 mM Tris, 0.1% BSA, 0.05% Tween 20, 150 mM NaCl, 20 mM NaN3, pH 7.4), exposed overnight at 4 °C to a 1:50 dilution Cpn60 affinity-purified rabbit antibody (58 μg/ml) in TBS (pH 7.4), washed four times in TBS and labeled for 1 h with a 1:100 dilution of EM-grade 10-nm gold particles conjugated to goat anti-rabbit IgG antibodies (British Biocell International, via Ted Pella) in TBS containing 10 μg reconstituted whole goat serum/ml. Sections were then washed five times in TBS, exposed to 1% gluteraldehyde in water for 5 min (to covalently link the primary antibody to the secondary one), stained with 2% aqueous uranyl acetate for 20 min, and finally washed three times in water. Preincubation with a 1:50 dilution of CpCpn60-specific antibody (58 μg/ml) plus 100 μg CpCpn60 peptide/ml to block specific labeling served as the negative control. All other treatment steps were identical. Substitution of a rat polyvalent antiserum raised against purified C. parvum sporozoite membranes as the primary antibody, followed by treatment with 10-nm gold particles conjugated to goat anti-rat IgG, served as a positive control. Sections were examined at 80 kV with a Zeiss 910 transmission electron microscope.
Samples used for analysis were transmission electron micrographs of individual, longitudinal sections of whole sporozoites, including both the posterior and anterior ends. Calculations were based upon the number of gold particles bound per square micron over the area of the putative relict mitochondrion versus gold particles over the remaining area of the sporozoite. Thirty-four positive sporozoites shown in longitudinal section (treated with CpCpn60-specific antibody alone) and 24 negative sporozoites (treated with CpCpn60-specific antibody plus blocking peptide) were used for statistical analysis. The number of gold particles boundper square micron were analyzed by Student′s t-test to determine statistical significance.
Characterization of the CpCpn60 gene
Unlike the apicomplexans T. gondii (Toursel et al. 2000), P. yoelii (Sanchez et al. 1999), and one Cpn60 P. falciparum isoform (Syin and Goldman 1996), C. parvum lacks a C-terminal glycine/methionine (GGM) repeated motif commonly found in a broad spectrum of species (Hemmingsen et al.1988; McLennan et al. 1993). Instead, C. parvum and the second Cpn60 P. falciparum isoform encode either a serine-rich or glutamine/aspartate-rich C-terminal tail (Fig. 1, overlines), while most other protists and plastid-bearing eukaryotes completely lack any extended motif (Fig. 1).
Southern blot and RT-PCR analysis of gene expression
The two most likely reasons for the poor resolution among protists in this study are: (1) lack of species diversity in the Cpn60 database and (2) presence of lineages which are often fast-evolving and which can be misplaced due to long-branch attraction (LBA) artifacts (Philippe and Laurent 1998). LBA artifacts could be responsible for relationships here among entamebids, trichomonads, and diplomonads (Fig. 3). There is a weakly supported relationship between the Cpn60 of T. vaginalis and G. lamblia (BP=40%), suggesting a potential sister relationship between them that has been found in other analyses (Hashimoto et al. 1998; Roger et al. 1998; Horner and Embley 2001). The placement of C. parvum as a sister group to the apicomplexans Toxoplasma and Plasmodium (BP≥85%; Fig. 3) is essentially congruent with other studies using six proteins, small subunit (SSU) rRNA, and large (L)SU/SSU rRNA, which show a trend for the early emergence of Cryptosporidium at the base of the Apicomplexa (Zhu et al. 2000a), with variations for sister group placements depending upon the protein analyzed.
Pre-sequence targeting and antibody localization
Affinity-purified antibodies generated against CpCpn60 peptides corresponding to aa 67–84 and aa 230–248 and conjugated to mcKLH were tested by Western blot. The immune serum reacted with a band at approximately 66 kDa as predicted (Fig. 2C, left panel) and shows no cross-reactivity with pre-immune sera (right panel). FITC-conjugated antibodies to CpCpn60 localized to the posterior end of sporozoites (data not shown) but, because sporozoites are very small (1–3 μm by 6–9 μm), localization within a specific organelle could not be determined.
Here, we show by transmission electron microscopy that the apicomplexan C. parvum, like these protists, possesses a double-membrane-bounded, RER-enveloped relict organelle posterior to the nucleus in close apposition to the crystalloid body (Fig. 6). Both sequence and phylogenetic analyses suggest that CpCpn60 shares a common ancestry from an α-proteobacterium. Unlike other heat-shock proteins, which may be organellar and/or cytosolic, these data are consistent with the observation that most known protist chaperonin 60s belong to Group I, which includes those of bacteria and eukaryotic mitochondria or plastids (Van der Giezen et al. 2003). Although we cannot eliminate the possibility that a cytosolic Cpn60 might be discovered in one or more apicomplexan genome projects, to date Group II archeal or eukaryotic cytosolic Cpn60 have been confirmed only amongst the jakobid flagellates, a diverse group of mitochondriate and amitochondriate eukaryotes, the excavate taxa (Archibald et al. 2002).
Furthermore, the predicted C. parvum Cpn60 pre-sequence correctly targets the mitochondrial network of the yeast S. cerevisiae; and immunogold-labeled antibodies generated against CpCpn60 specifically localize to a structure (Fig. 5) positioned exactly where the double-membrane-bounded organelle containing cristae-like compartments is observed (Fig. 6). Although the significance of the envelopment of the relict mitochondrion by RER is as yet unknown, a prominent coating of rat liver mitochondria by ribosomes has been proposed as evidence for co-translational translocation of proteins across mitochondrial membranes (Crowley and Payne 1998). Virtually nothing is known about the compartmentalization of core energy metabolism in C. parvum.
As mentioned previously, in C. parvum, extended glycolysis is thought to be the primary source of ATP with no contribution by oxidative phosphorylation (Crawford et al. 2003; Entrala and Mascaro 1997). Furthermore, we recently showed that C. parvum encodes at least six proteins essential for iron-sulfur cluster assembly in eukaryotic mitochondria (Lill and Kispal 2000; LaGier, MJ, Tachezy J, Kutisova K, Stejskal F, Keithly JS, unpublished data). Our working hypothesis is that iron-sulfur cluster biogenesis is one function of the C. parvum relict organelle. This is consistent with previous reports that genes for assembling mitochondrial-type iron-sulfur clusters occur in the protists G. lamblia and T. vaginalis (Tachezy et al. 2001) and the microsporidian Encaphalitozoon cuniculi (Katinka et al. 2001).
Among Apicomplexa, both P. falciparum and C. parvum Cpn60 genes possess a single exon which yields one transcript (Syin and Goldman 1996; this study). These differ from the Hsp60 of T. gondii (TgHsp60) which contains five introns and six exons, the splicing of which yields two distinct mRNAs (Toursel et al. 2000). Although both T. gondii Hsp60 transcripts are detected in both slow-growing bradyzoites and rapidly replicating tachyzoites, they are differentially enriched about 2-fold in the bradyzoites. Furthermore, immunofluorescent labeling indicated differential targeting of T. gondii Hsp60 (Toursel et al. 2000). Like C. parvum sporozoites, the single mitochondrion of tachyzoites was labeled, but rather unexpectedly the chaperonin targeted two unknown vesicles in bradyzoites. Among the Alveolata, dual localization of Hsp60 has been noted for the ciliate Tetrahymena thermophila (Takeda et al. 2001). Here, the chaperonin is a bifunctional protein, localizing as a citrate synthase within mitochondria and as a 14-nm cytoskeletal filament within the microtubule organizing centers during cytokinesis. Whether C. parvum might also exhibit differential localization of CpCpn60 to structures other than the mitochondrion during its life cycle is not yet known.
The CpCpn60 sequence also has some distinctive features. One of these is the absence of a commonly shared C-terminal GGM repeated motif (Fig. 1, overlines). As mentioned previously, this motif is also missing from the Cpn60 of the protists Giardia, Entamoeba, and Leishmania (data not shown) and from plastid/chloroplast Cpn60 (Hemmingsen et al. 1988) represented by the red alga, Porphyra. Among the Apicomplexa, C. parvum lacks a GGM motif, whereas both Plasmodium falciparum and P. yoelii have one motif each (Sanchez et al. 1999) and Toxoplasma gondii possesses four C-terminal GGM repeats (Toursel et al. 2000). These data indicate that the loss of this motif may have occurred after Cryptosporidium diverged from these other Apicomplexa; and they lend further support to the hypothesis that the genus Cryptosporidium constitutes an early emerging branch within the Apicomplexa (Zhu et al. 2000a).
The significance of this repeat is as yet unknown, since Escherichia coli can replicate at normal rates using a GGM-truncated form of GroEL, the bacterial homologue of Cpn60 (McLennan 1993). Some proteins lacking the GGM repeat instead possess multiple histidines, e.g. Mycobacterium tuberculosis; and organisms with many copies of Cpn60 frequently have at least one with a GGM motif (Karlin and Brocchieri 2000). Its conservation across diverse lineages, however, does suggest an essential function. Both C. parvum and P. falciparum encode C-terminal tails rich in serine or aspartic and glutamic acid residues, respectively. The significance (if any) of these tails is also unknown.
In summary, sequence and phylogenetic analyses indicate that C. parvum Cpn60 is a mitochondrial isotype and that antibodies generated against peptides from this protein localize to a double-membrane-bounded relict organelle posterior to the nucleus. These data confirm that this organelle, like the hydrogenosome of Trichomonas vaginalis, the mitosome (crypton) of Entamoeba histolytica and other ″amitochondriate″ protists, and mitochondrial remnants from microsporidians (Katinka et al. 2001; Williams et al. 2002), arose by a symbiogenic event from a common ancestor and that it is of mitochondrial origin.
This work was supported in part by funds from National Institutes of Health grant AI40320 to J.S.K. and USUHS grants R073AM and R073IF to S.G.L. These data were submitted to the State University of New York-Albany in partial fulfillment for a PhD degree in Biomedical Sciences (C.E.R.) and have been presented in part at national and international meetings. At the Wadsworth Center, we thank the Electron Microscopy, Photo/Illustration, Molecular Genetics, and Peptide Synthesis Core Facilities, Fogarty Fellow Dr. J.R. Slapeta for assisting with GFP/MitoTracker red double-labeling yeast mitochondria, and members of the dissertation committee for many helpful comments. At Rockefeller University, we thank Dr. L.B. Sanchez for assistance with the phylogenetic analyses and Dr. M. Muller for critically reviewing the manuscript.