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.
