The Dual Origin of Toxoplasma gondii N-Glycans

Toxoplasma gondii apicoplast-resident ferredoxin is an essential electron transfer protein for the MEP isoprenoid-biosynthetic pathway

Apicomplexan parasites, such as Toxoplasma gondii, are unusual in that each cell contains a single apicoplast, a plastid-like organelle that compartmentalizes enzymes involved in the essential 2C-methyl-D-erythritol 4-phosphate pathway of isoprenoid biosynthesis. The last two enzymatic steps in this organellar pathway require electrons from a redox carrier. However, the small iron-sulfur cluster-containing protein ferredoxin, a likely candidate for this function, has not been investigated in this context. We show here that inducible knockdown of T. gondii ferredoxin results in progressive inhibition of growth and eventual parasite death. Surprisingly, this phenotype is not accompanied by ultrastructural changes in the apicoplast or overall cell morphology. The knockdown of ferredoxin was instead associated with a dramatic decrease in cellular levels of the last two metabolites in isoprenoid biosynthesis, 1-hydroxy-2-methyl-2-(E)- butenyl-4-pyrophosphate, and isomeric dimethylallyl pyrophosphate/isopentenyl pyrophosphate. Ferredoxin depletion was also observed to impair gliding motility, consistent with isoprenoid metabolites being important for dolichol biosynthesis, protein prenylation, and modification of other proteins involved in motility. Significantly, pharmacological inhibition of isoprenoid synthesis of the host cell exacerbated the impact of ferredoxin depletion on parasite replication, suggesting that the slow onset of parasite death after ferredoxin depletion is because of isoprenoid scavenging from the host cell and leading to partial compensation of the depleted parasite metabolites upon ferredoxin knockdown. Overall, these findings show that ferredoxin has an essential physiological function as an electron donor for the 2C-methyl-D-erythritol 4-phosphate pathway and is a potential drug target for apicomplexan parasites.

Towards understanding the extensive diversity of protein N-glycan structures in eukaryotes

N‐glycosylation is an important post‐translational modification of proteins that has been highly conserved during evolution and is found in Eukaryota, Bacteria and Archaea. In eukaryotes, N‐glycan processing is sequential, involving multiple specific steps within the secretory pathway as proteins travel through the endoplasmic reticulum and the Golgi apparatus. In this review, we first summarize the different steps of the N‐glycan processing and further describe recent findings regarding the diversity of N‐glycan structures in eukaryotic clades. This comparison allows us to explore the different regulation mechanisms of N‐glycan processing among eukaryotic clades. Recent findings regarding the regulation of protein N‐glycosylation are highlighted, especially the regulation of the biosynthesis of complex‐type N‐glycans through manganese and calcium homeostasis and the specific role of transmembrane protein 165 (TMEM165) for which homologous sequences have been identified in several eukaryotic clades. Further research will be required to characterize the function of TMEM165 homologous sequences in different eukaryotic clades.

Toxoplasma Mechanisms for Delivery of Proteins and Uptake of Nutrients Across the Host-Pathogen Interface

Many intracellular pathogens, including the protozoan parasite Toxoplasma gondii, live inside a vacuole that resides in the host cytosol. Vacuolar residence provides these pathogens with a defined niche for replication and protection from detection by host cytosolic pattern recognition receptors. However, the limiting membrane of the vacuole, which constitutes the host-pathogen interface, is also a barrier for pathogen effectors to reach the host cytosol and for the acquisition of host-derived nutrients. This review provides an update on the specialized secretion and trafficking systems used by Toxoplasma to overcome the barrier of the parasitophorous vacuole membrane and thereby allow the delivery of proteins into the host cell and the acquisition of host-derived nutrients.

N- and O-Glycosylation Pathways in the Microalgae Polyphyletic Group

The term microalga refers to various unicellular and photosynthetic organisms representing a polyphyletic group. It gathers numerous species, which can be found in cyanobacteria (i.e., Arthrospira) as well as in distinct eukaryotic groups, such as Chlorophytes (i.e., Chlamydomonas or Chlorella) and Heterokonts (i.e., diatoms). This phylogenetic diversity results in an extraordinary variety of metabolic pathways, offering large possibilities for the production of natural compounds like pigments or lipids that can explain the ever-growing interest of industrials for these organisms since the middle of the last century. More recently, several species have received particular attention as biofactories for the production of recombinant proteins. Indeed, microalgae are easy to grow, safe and cheap making them attractive alternatives as heterologous expression systems. In this last scope of applications, the glycosylation capacity of these organisms must be considered as this post-translational modification of proteins impacts their structural and biological features. Although these mechanisms are well known in various Eukaryotes like mammals, plants or insects, only a few studies have been undertaken for the investigation of the protein glycosylation in microalgae. Recently, significant progresses have been made especially regarding protein N-glycosylation, while O-glycosylation remain poorly known. This review aims at summarizing the recent data in order to assess the state-of-the art knowledge in glycosylation processing in microalgae.

User-friendly extraction and multistage tandem mass spectrometry based analysis of lipid-linked oligosaccharides in microalgae

BACKGROUND

Protein N-glycosylation is initiated within the endoplasmic reticulum through the synthesis of a lipid-linked oligosaccharides (LLO) precursor. This precursor is then transferred en bloc on neo-synthesized proteins through the action of the oligosaccharyltransferase giving birth to glycoproteins. The N-linked glycans bore by the glycoproteins are then processed into oligomannosides prior to the exit of the glycoproteins from the endoplasmic reticulum and its entrance into the Golgi apparatus. In this compartment, the N-linked glycans are further maturated in complex type N-glycans. This process has been well studied in a lot of eukaryotes including higher plants. In contrast, little information regarding the LLO precursor and synthesis of N-linked glycans is available in microalgae.

METHODS

In this report, a user-friendly extraction method combining microsomal enrichment and solvent extractions followed by purification steps is described. This strategy is aiming to extract LLO precursor from microalgae. Then, the oligosaccharide moiety released from the extracted LLO were analyzed by multistage tandem mass spectrometry in two models of microalgae namely the green microalgae, Chlamydomonas reinhardtii and the diatom, Phaeodactylum tricornutum.

RESULTS

The validity of the developed method was confirmed by the analysis of the oligosaccharide structures released from the LLO of two xylosyltransferase mutants of C. reinhardtii confirming that this green microalga synthesizes a linear Glc3Man5GlcNAc2 identical to the one of the wild-type cells. In contrast, the analysis of the oligosaccharide released from the LLO of the diatom P. tricornutum demonstrated for the first time a Glc2Man9GlcNAc2 structure.

CONCLUSION

The method described in this article allows the fast, non-radioactive and reliable multistage tandem mass spectrometry characterization of oligosaccharides released from LLO of microalgae including the ones belonging to the Phaeodactylaceae and Chlorophyceae classes, respectively. The method is fully adaptable for extracting and characterizing the LLO oligosaccharide moiety from microalgae belonging to other phyla.

CRISPR/Cas9 and glycomics tools for Toxoplasma glycobiology

Infection with the protozoan parasite Toxoplasma gondii is a major health risk owing to birth defects, its chronic nature, ability to reactivate to cause blindness and encephalitis, and high prevalence in human populations. Unlike most eukaryotes, Toxoplasma propagates in intracellular parasitophorous vacuoles, but like nearly all other eukaryotes, Toxoplasma glycosylates many cellular proteins and lipids and assembles polysaccharides. Toxoplasma glycans resemble those of other eukaryotes, but species-specific variations have prohibited deeper investigations into their roles in parasite biology and virulence. The Toxoplasma genome encodes a suite of likely glycogenes expected to assemble N-glycans, O-glycans, a C-glycan, GPI-anchors, and polysaccharides, along with their precursors and membrane transporters. To investigate the roles of specific glycans in Toxoplasma, here we coupled genetic and glycomics approaches to map the connections between 67 glycogenes, their enzyme products, the glycans to which they contribute, and cellular functions. We applied a double-CRISPR/Cas9 strategy, in which two guide RNAs promote replacement of a candidate gene with a resistance gene; adapted MS-based glycomics workflows to test for effects on glycan formation; and infected fibroblast monolayers to assess cellular effects. By editing 17 glycogenes, we discovered novel Glc_0-2-Man₆-GlcNAc₂-type N-glycans, a novel HexNAc-GalNAc-mucin-type O-glycan, and Tn-antigen; identified the glycosyltransferases for assembling novel nuclear O-Fuc-type and cell surface Glc-Fuc-type O-glycans; and showed that they are important for in vitro growth. The guide sequences, editing constructs, and mutant strains are freely available to researchers to investigate the roles of glycans in their favorite biological processes.

Heterologous expression of the N-acetylglucosaminyltransferase I dictates a reinvestigation of the N-glycosylation pathway in Chlamydomonas reinhardtii

Eukaryotic N-glycosylation pathways are dependent of N-acetylglucosaminyltransferase I (GnTI), a key glycosyltransferase opening the door to the formation of complex-type N-glycans by transferring a N-acetylglucosamine residue onto the Man₅GlcNAc₂ intermediate. In contrast, glycans N-linked to Chlamydomonas reinhardtii proteins arise from a GnTI-independent Golgi processing of oligomannosides giving rise to Man₅GlcNAc₂ substituted eventually with one or two xylose(s). Here, complementation of C. reinhardtii with heterologous GnTI was investigated by expression of GnTI cDNAs originated from Arabidopsis and the diatom Phaeodactylum tricornutum. No modification of the N-glycans was observed in the GnTI transformed cells. Consequently, the structure of the Man₅GlcNAc₂ synthesized by C. reinhardtii was reinvestigated. Mass spectrometry analyses combined with enzyme sequencing showed that C. reinhardtii proteins carry linear Man₅GlcNAc₂ instead of the branched structure usually found in eukaryotes. Moreover, characterization of the lipid-linked oligosaccharide precursor demonstrated that C. reinhardtii exhibit a Glc₃Man₅GlcNAc₂ dolichol pyrophosphate precursor. We propose that this precursor is then trimmed into a linear Man₅GlcNAc₂ that is not substrate for GnTI. Furthermore, cells expressing GnTI exhibited an altered phenotype with large vacuoles, increase of ROS production and accumulation of starch granules, suggesting the activation of stress responses likely due to the perturbation of the Golgi apparatus.

Extracellular Toxoplasma gondii tachyzoites metabolize and incorporate unnatural sugars into cellular proteins

Toxoplasma gondii is an obligate intracellular parasite that infects all nucleated cell types in diverse warm-blooded organisms. Many of the surface antigens and effector molecules secreted by the parasite during invasion and intracellular growth are modified by glycans. Glycosylated proteins in the nucleus and cytoplasm have also been reported. Despite their prevalence, the complete inventory and biological significance of glycosylated proteins in Toxoplasma remain unknown. In this study, we aimed to globally profile parasite glycoproteins using a bioorthogonal chemical reporter strategy. This strategy involves the metabolic incorporation of unnatural functional groups (i.e., "chemical reporters") into Toxoplasma glycans, followed by covalent labeling with visual probes or affinity tags. The two-step approach enables the visualization and identification of newly biosynthesized glycoconjugates in the parasite. Using a buffer that mimics intracellular conditions, extracellular Toxoplasma tachyzoites were found to metabolize and incorporate unnatural sugars (equipped with bioorthogonal functional groups) into diverse proteins. Covalent chemistries were used to visualize and retrieve these labeled structures. Subsequent mass spectrometry analysis revealed 89 unique proteins. This survey identified novel proteins as well as previously characterized proteins from lectin affinity analyses.

Neospora caninum Recruits Host Cell Structures to Its Parasitophorous Vacuole and Salvages Lipids from Organelles

Toxoplasma gondii and Neospora caninum, which cause the diseases toxoplasmosis and neosporosis, respectively, are two closely related apicomplexan parasites. They have similar heteroxenous life cycles and conserved genomes and share many metabolic features. Despite these similarities, T. gondii and N. caninum differ in their transmission strategies and zoonotic potential. Comparative analyses of the two parasites are important to identify the unique biological features that underlie the basis of host preference and pathogenicity. T. gondii and N. caninum are obligate intravacuolar parasites; in contrast to T. gondii, events that occur during N. caninum infection remain largely uncharacterized. We examined the capability of N. caninum (Liverpool isolate) to interact with host organelles and scavenge nutrients in comparison to that of T. gondii (RH strain). N. caninum reorganizes the host microtubular cytoskeleton and attracts endoplasmic reticulum (ER), mitochondria, lysosomes, multivesicular bodies, and Golgi vesicles to its vacuole though with some notable differences from T. gondii. For example, the host ER gathers around the N. caninum parasitophorous vacuole (PV) but does not physically associate with the vacuolar membrane; the host Golgi apparatus surrounds the N. caninum PV but does not fragment into ministacks. N. caninum relies on plasma lipoproteins and scavenges cholesterol from NPC1-containing endocytic organelles. This parasite salvages sphingolipids from host Golgi Rab14 vesicles that it sequesters into its vacuole. Our data highlight a remarkable degree of conservation in the intracellular infection program of N. caninum and T. gondii. The minor differences between the two parasites related to the recruitment and rearrangement of host organelles around their vacuoles likely reflect divergent evolutionary paths.

Host Organelle Hijackers: a similar modus operandi for Toxoplasma gondii and Chlamydia trachomatis: co-infection model as a tool to investigate pathogenesis

The bacterium Chlamydia trachomatis and the protozoan parasite Toxoplasma gondii are the causative agents of chlamydiosis and toxoplasmosis in humans, respectively. Both microorganisms are obligate intracellular pathogens and notorious for extensively modifying the cytoskeletal architecture and the endomembrane system of their host cells to establish productive infections. This review highlights the similar tactics developed by these two pathogens to manipulate their host cell despite their genetic unrelatedness. Using an in vitro cell culture model whereby single fibroblasts are infected by C. trachomatis and T. gondii simultaneously, thus setting up an intracellular competition, we demonstrate that the solutions to the problem of intracellular survival deployed by the parasite and the bacterium may represent an example of convergent evolution, driven by the necessity to acquire nutrients in a hostile environment.

A nucleotide sugar transporter involved in glycosylation of the Toxoplasma tissue cyst wall is required for efficient persistence of bradyzoites

Toxoplasma gondii is an intracellular parasite that transitions from acute infection to a chronic infective state in its intermediate host via encystation, which enables the parasite to evade immune detection and clearance. It is widely accepted that the tissue cyst perimeter is highly and specifically decorated with glycan modifications; however, the role of these modifications in the establishment and persistence of chronic infection has not been investigated. Here we identify and biochemically and biologically characterize a Toxoplasma nucleotide-sugar transporter (TgNST1) that is required for cyst wall glycosylation. Toxoplasma strains deleted for the TgNST1 gene (Δnst1) form cyst-like structures in vitro but no longer interact with lectins, suggesting that Δnst1 strains are deficient in the transport and use of sugars for the biosynthesis of cyst-wall structures. In vivo infection experiments demonstrate that the lack of TgNST1 activity does not detectably impact the acute (tachyzoite) stages of an infection or tropism of the parasite for the brain but that Δnst1 parasites are severely defective in persistence during the chronic stages of the infection. These results demonstrate for the first time the critical role of parasite glycoconjugates in the persistence of Toxoplasma tissue cysts.

The Toxoplasma tissue cyst is essential to the persistence of the parasite during the chronic infection of an immunocompetent host. While significant efforts have been made to identify molecular factors that trigger and sustain parasite encystation, the role of the glycoconjugates that decorate the cyst wall has received little attention. Here we identify and characterize a bona fide nucleotide-sugar transporter, TgNST1, whose activity is required for the proper assembly of cyst wall glycoconjugates. We found that deletion of TgNST1 interferes with glycosylation during both the tachyzoite and bradyzoite stages of infection, and we observed substantial defects in the ability of Δnst1 parasites to maintain chronic infection. Surprisingly, Δnst1 parasites were not significantly defective in acute infection of mice, and showed wild type levels and migration rates to the brain. These results highlight the important role of cyst-wall glycosylation in parasite persistence during chronic infection, and suggest that drugs targeting nucleotide-sugar transporters and other enzymes required for glycosylation, perhaps in combination with drugs targeting other pathways, might be useful to prevent the establishment of chronic parasite infection.

Complicated N-linked glycans in simple organisms

Although countless genomes have now been sequenced, the glycomes of the vast majority of eukaryotes still present a series of unmapped frontiers. However, strides are being made in a few groups of invertebrate and unicellular organisms as regards their N-glycans and N-glycosylation pathways. Thereby, the traditional classification of glycan structures inevitably approaches its boundaries. Indeed, the glycomes of these organisms are rich in surprises, including a multitude of modifications of the core regions of N-glycans and unusual antennae. From the actually rather limited glycomic information we have, it is nevertheless obvious that the biotechnological, developmental and immunological relevance of these modifications, especially in insect cell lines, model organisms and parasites means that deciphering unusual glycomes is of more than just academic interest.

Structural analyses of mannose pentasaccharide of high mannose type oligosaccharides by 1D and 2D NMR spectroscopy

NMR spectroscopy is a very important and useful method for the structural analysis of oligosaccharides, despite its low sensitivity. We first applied conventional measuring methods, 2D DQF COSY, (1)H-(13)C HSQC, and (1)H-(13)C HMBC, and also the Double Pulsed Field Gradient Spin Echo (DPFGSE)-TOCSY and DPFGSE-NOESY/ROESY techniques to analyze a branched mannose pentasaccharide as a model of high mannose type N-glycans in natural abundance. The NMR spectra of the model compound are very complex and difficult to analyze owing to overlapping signals. The superior selective irradiation capability of the DPFGSE technique is useful for fine structural and conformational analyses of such complex oligosaccharides. We here introduce a novel technique called DPFGSE-Double-Selective Population Transfer (SPT)-Difference and DPFGSE-NOE/ROE-SPT-Difference spectroscopy. The DPFGSE-Double-SPT-Difference method involves irradiation of two peaks from one proton and the subtraction of higher and lower peaks from each spectrum. The DPFGSE-NOE/ROE-SPT-Difference method involves the transfer of the magnetization polarized by NOE/ROE from the nuclei to the spin-coupled nuclei through scalar spin-spin interaction using the SPT method. Even if the signals in the NMR spectra overlap, each signal can be accurately assigned. In particular, DPFGSE-NOE/ROE-SPT-Difference is very useful for identifying sugar connectivity.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2007-2008

This review is the fifth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2008. The first section of the review covers fundamental studies, fragmentation of carbohydrate ions, use of derivatives and new software developments for analysis of carbohydrate spectra. Among newer areas of method development are glycan arrays, MALDI imaging and the use of ion mobility spectrometry. The second section of the review discusses applications of MALDI MS to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, biopharmaceuticals, glycated proteins, glycolipids, glycosides and various other natural products. There is a short section on the use of MALDI mass spectrometry for the study of enzymes involved in glycan processing and a section on the use of MALDI MS to monitor products of the chemical synthesis of carbohydrates with emphasis on carbohydrate-protein complexes and glycodendrimers. Corresponding analyses by electrospray ionization now appear to outnumber those performed by MALDI and the amount of literature makes a comprehensive review on this technique impractical. However, most of the work relating to sample preparation and glycan synthesis is equally relevant to electrospray and, consequently, those proposing analyses by electrospray should also find material in this review of interest.

Analysis of the glycoproteome of Toxoplasma gondii using lectin affinity chromatography and tandem mass spectrometry

Glycoproteins are involved in many important molecular recognition processes including invasion, adhesion, differentiation, and development. To identify the glycoproteins of Toxoplasma gondii, a proteomic analysis was undertaken. T. gondii proteins were prepared and fractioned using lectin affinity chromatography. The proteins in each fraction were then separated using SDS-PAGE and identified by tryptic in gel digestion followed by tandem mass spectrometry. Utilizing these methods 132 proteins were identified. Among the identified proteins were 17 surface proteins, 9 microneme proteins, 15 rhoptry proteins, 11 heat shock proteins (HSP), and 32 hypothetical proteins. Several proteins had 1-5 transmembrane domains (TMD) with some being as large as 608.3 kDa. Both lectin-fluorescence labeling and lectin blotting were employed to confirm the presence of carbohydrates on the surface or cytoplasm of T. gondii parasites. PCR demonstrated that selected hypothetical proteins were expressed in T. gondii tachyzoites. This data provides a large-scale analysis of the T. gondii glycoproteome. Studies of the function of glycosylation of these proteins may help elucidate mechanism(s) involved in invasion improving drug therapy as well as identify glycoproteins that may prove to be useful as vaccine candidates.

Differential effects of three canonical Toxoplasma strains on gene expression in human neuroepithelial cells

Strain type is one of the key factors suspected to play a role in determining the outcome of Toxoplasma infection. In this study, we examined the transcriptional profile of human neuroepithelioma cells in response to representative strains of Toxoplasma by using microarray analysis to characterize the strain-specific host cell response. The study of neural cells is of interest in light of the ability of Toxoplasma to infect the brain and to establish persistent infection within the central nervous system. We found that the extents of the expression changes varied considerably among the three strains. Neuroepithelial cells infected with Toxoplasma type I exhibited the highest level of differential gene expression, whereas type II-infected cells had a substantially smaller number of genes which were differentially expressed. Cells infected with type III exhibited intermediate effects on gene expression. The three strains also differed in the individual genes and gene pathways which were altered following cellular infection. For example, gene ontology (GO) analysis indicated that type I infection largely affects genes related to the central nervous system, while type III infection largely alters genes which affect nucleotide metabolism; type II infection does not alter the expression of a clearly defined set of genes. Moreover, Ingenuity Pathways Analysis (IPA) suggests that the three lineages differ in the ability to manipulate their host; e.g., they employ different strategies to avoid, deflect, or subvert host defense mechanisms. These observed differences may explain some of the variation in the neurobiological effects of different strains of Toxoplasma on infected individuals.

Versatility in the acquisition of energy and carbon sources by the Apicomplexa

Members of the phylum Apicomplexa are motile and rapidly dividing intracellular parasites, able to occupy a large spectrum of niches by infecting diverse hosts and invading various cell types. As obligate intracellular parasites, most apicomplexans only survive for a short period extracellularly, and, during this time, have a high energy demand to power gliding motility and invasion into new host cells. Similarly, these fast-replicating intracellular parasites are critically dependent on host-cell nutrients as energy and carbon sources, noticeably for the extensive membrane biogenesis imposed during growth and division. To access host-cell metabolites, the apicomplexans Toxoplasma gondii and Plasmodium falciparum have evolved strategies that exquisitely reflect adaptation to their respective niches. In the present review, we summarize and compare some recent findings regarding the energetic metabolism and carbon sources used by these two genetically tractable apicomplexans during host-cell invasion and intracellular growth and replication.

Immunological reactions in response to apicomplexan glycosylphosphatidylinositols

Apicomplexan protozoa are a phylum of parasites that includes pathogens such as Plasmodium, the causative agent of the most severe form of malaria responsible for almost 1 million deaths per year and Toxoplasma gondii causing toxoplasmosis, a disease leading to cerebral meningitis in immunocompromised individuals or to abortion in farm animals or in women that are infected for the first time during pregnancy. The initial immune reactions developed by the host are similar in response to an infection with Plasmodium and Toxoplasma in the sense that the same cells of the innate immune system are stimulated to produce inflammatory cytokines. The glycosylphosphatidylinositol (GPI) anchor is the major carbohydrate modification in parasite proteins and the GPIs are essential for parasite survival. Two immediate GPI precursors with the structures ethanolamine phosphate-6(Manalpha1-2)Manalpha1-2Manalpha1-6Manalpha1-4GlcN-PI and ethanolamine phosphate-6Manalpha1-2Manalpha1-6Man-alpha1-4-GlcN-PI are synthesized by P. falciparum. Two main structures are synthesized by T. gondii: ethanolamine phosphate-6Manalpha1-2Manalpha1-6(GalNAcbeta1-4)Manalpha1-4GlcN-PI and ethanolamine phosphate-6Manalpha1-2Manalpha1-6(Glcalpha1-4GalNAcbeta1-4)Manalpha1-4GlcN-PI. This review describes the biosynthesis of the apicomplexan GPIs and their role in the activation of the host immune system.

Suggestive evidence for Darwinian Selection against asparagine-linked glycans of Plasmodium falciparum and Toxoplasma gondii

We are interested in asparagine-linked glycans (N-glycans) of Plasmodium falciparum and Toxoplasma gondii, because their N-glycan structures have been controversial and because we hypothesize that there might be selection against N-glycans in nucleus-encoded proteins that must pass through the endoplasmic reticulum (ER) prior to threading into the apicoplast. In support of our hypothesis, we observed the following. First, in protists with apicoplasts, there is extensive secondary loss of Alg enzymes that make lipid-linked precursors to N-glycans. Theileria makes no N-glycans, and Plasmodium makes a severely truncated N-glycan precursor composed of one or two GlcNAc residues. Second, secreted proteins of Toxoplasma, which uses its own 10-sugar precursor (Glc(3)Man(5)GlcNAc(2)) and the host 14-sugar precursor (Glc(3)Man(9)GlcNAc(2)) to make N-glycans, have very few sites for N glycosylation, and there is additional selection against N-glycan sites in its apicoplast-targeted proteins. Third, while the GlcNAc-binding Griffonia simplicifolia lectin II labels ER, rhoptries, and surface of plasmodia, there is no apicoplast labeling. Similarly, the antiretroviral lectin cyanovirin-N, which binds to N-glycans of Toxoplasma, labels ER and rhoptries, but there is no apicoplast labeling. We conclude that possible selection against N-glycans in protists with apicoplasts occurs by eliminating N-glycans (Theileria), reducing their length (Plasmodium), or reducing the number of N-glycan sites (Toxoplasma). In addition, occupation of N-glycan sites is markedly reduced in apicoplast proteins versus some secretory proteins in both Plasmodium and Toxoplasma.