Authors

Discussion

C-mannosylation is a poorly known posttranslational modification of proteins which differs from other types of glycosylation by the carbon-carbon bond that links the anomeric carbon of the mannose residue to the indole C2 carbon of tryptophan. This modification is characteristically found in WXXW/C motifs, present in thrombospondin type-1 repeat domains (TSR) and type 1 cytokine receptors in metazoans. This modification is catalyzed by C-mannosyltransferases of the DPY19 family located in the endoplasmic reticulum and it affects the folding and secretion of several proteins. Interestingly, orthologues of the encoding gene were found in the genome of apicomplexan parasites. Considering that apicomplexans share the same recognition motif as mammals, over 30 C-mannosylated proteins might be present in these parasites. Recently, the micronemal adhesion thrombospondin-related anonymous protein (TRAP) was shown to be C-hexosylated in Plasmodium falciparum sporozoites. Here, we demonstrate that also the micronemal protein MIC2 secreted by Toxoplasma gondii tachyzoites is C-hexosylated. When expressed in a cell line deficient in C-mannosylation, P. falciparum and T. gondii DPY19 homologues are able to modify TSR domains of the micronemal adhesins TRAP/MIC2 family, known to be integral components of the glideosome and therefore of paramount importance for the parasite motility and invasion. Furthermore, we observed a decreased amount of recombinant MIC2 secretion in absence of C-mannosylation, suggesting this modification might play an important role for the proper folding of this protein. In vitro, the apicomplexan enzymes can transfer mannose to a WXXWXXC peptide. Since one or more TSR domains are commonly found in several surface proteins of apicomplexan parasites, C‑mannosylation may be a common modification in this phylum. Since this protein is predicted to be expressed at many parasite stages, we suggest it plays an important role in infection.