Cytoskeletal Components of an Invasion Machine—The Apical Complex of Toxoplasma gondii

The apical complex of Toxoplasma gondii is widely believed to serve essential functions in both invasion of its host cells (including human cells), and in replication of the parasite. The understanding of apical complex function, the basis for its novel structure, and the mechanism for its motility are greatly impeded by lack of knowledge of its molecular composition. We have partially purified the conoid/apical complex, identified ~200 proteins that represent 70% of its cytoskeletal protein components, characterized seven novel proteins, and determined the sequence of recruitment of five of these proteins into the cytoskeleton during cell division. Our results provide new markers for the different subcompartments within the apical complex, and revealed previously unknown cellular compartments, which facilitate our understanding of how the invasion machinery is built. Surprisingly, the extreme apical and extreme basal structures of this highly polarized cell originate in the same location and at the same time very early during parasite replication

Isolation of the Conoid/Apical Complex

<div><p>(A) Combined DIC and epifluorescence image of the starting material, a suspension of extracellular YFP-α-tubulin (green) transgenic parasites.</p><p>(B) Epifluorescence image of detergent extracted, sonicated parasites. The fluorescent material, a small fraction of the total mass, is composed of small MT fragments (cyan arrows) and conoids (red arrows). The inset shows an intact “ghost,” a detergent extracted parasite, at 2× higher magnification. The total cytoskeleton prep was fractionated by differential centrifugation into the conoid-depleted (C) and conoid-enriched (D) fractions.</p></div

Localization of T. gondii IMC4, TgCAM1, and TgCAM2

<div><p>(A) LM localization of a T. gondii protein (TgIMC4) with weak similarity to articulin family members, and weaker similarity to TgIMC1. Fluorescence LM images of living transgenic parasites expressing TgIMC4 fused to the C-terminus of EGFP. Fluorescence is observed on both the mother and daughter IMC. FRAP reveals significant differences in the turnover of TgIMC1 and TgIMC4 (unpublished data).</p><p>(B and C) Fluorescence LM images of living transgenic parasites expressing GFP-CAM1 (B) and GFP-CAM2 (C). Bright fluorescence is observed in the conoid region of both mother and daughters (arrows).</p><p>(D and E) EM images of T. gondii cytoskeletons from the same two lines of transgenic parasites as in (B) and (C [immunogold-labeled with anti-GFP antibody and negatively stained with phosphotungstic acid]). Specific staining occurs over the conoid itself, and lightly over the subpellicular MTs. In (D), diagonal lines of gold particles are visible, tracing the conoid fibers.</p></div

Time-Lapse Image Imaging of Incorporation of YFP-α-Tubulin (Green) and mRFP-TgCentrin2 (Red) into Developing Daughter Parasites

<p>Combined DIC and fluorescence images are shown for the beginning (<i>t</i> = 0) and end (<i>t</i> = 250 min) of the sequence, during which a vacuole of four parasites progresses from the onset of mitosis through completion of budding out of the eight daughters. See text for a detailed description of the sequence of events.</p

EM Analysis of the Conoid-Enriched and Conoid-Depleted Fractions

<div><p>(A) EM image of a thin section through a pellet of the centrifuged conoid-enriched fraction.</p><p>(B) To give a rough visual estimate of the enrichment, material that is recognizably fragments of the conoid + polar rings has been colored red (C).</p><p>(C) EM analysis of the conoid-depleted fraction; MT are marked with cyan lines, which are much wider than the image of the MT at this magnification.</p><p>(D) Enlarged view of the region within the yellow box, showing small vesicles and MT (cyan arrows).</p></div

Image and Drawing of T. gondii

<div><p>(A) Combined DIC and epifluorescence image of human fibroblasts infected with transgenic T. gondii expressing GFP-tubulin (green). Parasitophorous vacuoles containing 1, 2, 4, or 8 parasites are seen.</p><p>(B) Drawings of T. gondii (modified from [<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.0020013#ppat-0020013-b003" target="_blank">3</a>] and [<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.0020013#ppat-0020013-b068" target="_blank">68</a>]). Left: a longitudinal section of a dividing cell. Lobes of the dividing nucleus bordered by ER, Golgi (yellow), and developing rhoptry (mauve) are surrounded by the developing daughters' scaffolds (red). Maternal and daughter conoids are shown in green, secretory organelles (rhoptries) in purple. T. gondii has three membranes: a plasma membrane (black) and two additional layers (IMC, red) formed from a patchwork of flattened vesicles. Right: semitransparent view showing subpellicular MT (green).</p><p>(C) Enlarged view of the apical complex cytoskeleton, showing the conoid (green), preconoidal, and polar rings (brown), and two intraconoid MT (green). The conoid is formed of 14 fibers of tubulin (not MT), 430 nm long, arranged in a left-handed spiral [<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.0020013#ppat-0020013-b010" target="_blank">10</a>]. Cytoskeletal elements, including the subpellicular MT (green) and a 2-dimensional lattice of intermediate filament-like proteins (not shown), are closely associated with the cytoplasmic face of the IMC.</p></div

Cartoon Diagrams of T. gondii at Various Points during Its Cell Cycle, Summarizing the Time-Course of Incorporation of the Proteins Discussed in the Text

<div><p>(Upper) The major structural reference points are labeled in a cell containing two growing daughters. The maternal IMC is shown in brown. The unlabeled daughter IMC/scaffold is shown in pink.</p><p>(Middle and Lower) The cell cycle is traversed starting with an interphase adult cell at the top left, and proceeding clockwise through early, mid, and late stages of daughter formation. The remnants of the mother are left behind as a “residual body” when the daughters bud out (lower left). The variable color scheme for the organelles is intended to emphasize colocalization (e.g<i>.</i>, green Centrin2 + blue DLC = cyan; red MORN1 + blue DLC = magenta; multiple color overlaps are shown in white). Subpellicular MTs are always present in mothers and in daughters from very early on, but are shown only for the interphase adult cell.</p></div

Distribution of T. gondii Centrin-2

<div><p>(A–C) Fluorescence LM images of a vacuole containing eight living transgenic parasites expressing mRFP-centrin-2 and YFP-α-tubulin. Bright fluorescence is observed in the extreme apical region (yellow arrows), centrioles (blue arrows), five spots around the circumference near the apical end (green arrows; only two or three are visible in one focal plane with intact parasites), and a single small patch at the basal end (magenta arrows). The left inset in (C) shows an enlarged view of the apical region. The pair of smaller insets show two projected views of a deconvolved 3D stack of images, looking from the side (left) and from the apical end (right) of a single parasite. White arrowheads point to the apical spot; five other spots are organized in a ring around the circumference of the parasite.</p><p>(D) Projection of a 3D stack of images of a vacuole containing four parasites from a transgenic parasite line expressing T. gondii EGFP-centrin-2 (green), after 24 h of oryzalin treatment. DNA (blue) is visualized with the cell-permeant dye Hoechst 33342. A host cell nucleus abutting the parasitophorous vacuole (upper right corner of the image) has been digitally attenuated for clarity. Clusters of EGFP-TgCentrin2 dots are dispersed throughout the cells. A minority of the dots occur as closely spaced pairs, presumably daughter centrioles (red arrowheads).</p></div

LM and EM Localization of T. gondii Dynein

<div><p>(A) Fluorescence LM image of a vacuole containing eight living transgenic parasites expressing EGFP-DLC. Bright fluorescence is observed in the conoid region of both mother and daughters and in an apical cap. See bracket in (B), in centrioles, and in spindle poles; arrows are identification of the centrioles, and spindle poles is based on colocalization with centrin and tubulin [<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.0020013#ppat-0020013-b007" target="_blank">7</a>,<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.0020013#ppat-0020013-b009" target="_blank">9</a>].</p><p>(B) Time-lapse imaging of live EGFP-TgDLC transgenic parasites. Elapsed time after addition of the Ca²⁺ ionophore A23187 is shown at the bottom of each frame. The bright spot of EGFP at the apical end of the parasite (arrow) elongates as the conoid protrudes through the polar ring in response to the elevated intracellular [Ca²⁺] (<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.0020013#ppat-0020013-sv001" target="_blank">Video S1</a>). After conoid protrusion, the fluorescence is seen to be concentrated in two bands of unequal width. The entire apical end of the cell narrows and elongates at the same time.</p><p>(C) Two EM images of Ca²⁺ ionophore–treated, deoxycholate-extracted EGFP–TgDLC transgenic parasites, immunogold-labeled with anti-GFP antibody and negatively stained with phosphotungstic acid. Numerous gold particles are located on the apical and basal ends of the conoid fibers as well as on the polar ring at the sites of attachment of the subpellicular MT, but are sparse along the length of the fibers and MTs, recapitulating the distribution of fluorescence in (B).</p></div

Immuno-EM Localization of T. gondii Centrin-2

<p>(A–B) Two EM images of Triton-extracted cytoskeletons from a transgenic parasite line expressing T. gondii EGFP–centrin-2, immunogold-labeled with anti-GFP antibody, and negatively stained with phosphotungstic acid. Specific staining is seen at the preconoidal rings (black arrows), and in five ~200nm annular patches (white arrows). After deoxycholate extraction (C), the annular patches disappear almost completely but the preconoidal ring staining remains. The large dark blob in the center of the parasites is primarily attributable to accumulation of the phosphotungstic acid in thicker regions, but may also include some cytoplasmic and centriolar concentrations of centrin-2.</p