Suggestive Evidence for Darwinian Selection against Asparagine-Linked Glycans of Plasmodium falciparum and Toxoplasma gondii

Abstract

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 apicoplasttargeted 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.

Biosynthesis of GDP-fucose and Other Sugar Nucleotides in the Blood Stages of Plasmodium falciparum

“…2 and Table 2). The sugar nucleotides detected in the parasite agree well with the monosaccharide contents of known glycoconjugates in P. falciparum consisting of glucosamine (derived from GlcNAc) and Man residues present in GPI anchors and N-glycans (11,14). Furthermore, the detection of UDP-GlcNAc, UDP-Glc, GDP-Man, and GDP-Fuc is consistent with the presence of known or candidate sugar nucleotide biosynthetic enzymes encoded in the genome of P. falciparum (25).…”

“…In the particular case of the malaria parasite, glycan structures associated with the parasite itself appear to be limited to GPI anchors (11,51) and the recently described N-glycans (14). This seems to confirm the general trend of Plasmodium parasites to reduce several metabolic and biosynthetic pathways and reflects the process of evolving toward a parasitic niche (52).…”

“…Sugar Nucleotide Biosynthetic Pathways Predicted in P. falciparum-Based on the current knowledge of P. falciparum sugar biochemistry (11,14,23,32) and the predicted metabolic pathways (25), we have reconstructed the sugar nucleotide biosynthetic routes present in P. falciparum (Fig. 1).…”

“…In general, protozoan parasite N-glycosylation patterns are atypical of those described in higher eukaryotes (15,16). In P. falciparum, the secondary loss of enzymes related to the biosynthesis of N-glycosylation precursors and the quality control of glycoprotein folding in the endoplasmic reticulum (17,18) result in a very unusual N-glycosylation (14) that, if essential, could be therapeutically exploitable. To our knowledge, despite some controversy (19 -24), O-glycans have never been unequivocally described in P. falciparum.…”

Use of Radioactive Precursors for Biochemical Characterization the Biosynthesis of Isoprenoids in Intraerythrocytic Stages of Plasmodium falciparum

“…The amount of N-linked carbohydrates in whole parasite proteins is approximately 6% compared with the GPI anchors attached to proteins based on radioactive GlcN incorporated into the proteins (Gowda et al, 1997). Bushkin et al (2010) suggested that the occupation of N-glycan sites is markedly reduced in apicoplast proteins versus some secreted proteins in Plasmodium. Clearly, more work has to be done in order to elucidate the nature and purpose of N-linked glycoproteins in Plasmodium.…”