Glycosylation of Anaplasma marginale major surface protein 1a and its putative role in adhesion to tick cells.

Anaplasma marginale, the causative agent of bovine anaplasmosis, is a tick-borne rickettsial pathogen of cattle that multiplies in erythrocytes and tick cells. Major surface protein 1a (MSP1a) and MSP1b form the MSP1 complex of A. marginale, which is involved in adhesion of the pathogen to host cells. In this study we tested the hypothesis that MSP1a and MSP1b were glycosylated, because the observed molecular weights of both proteins were greater than the deduced molecular masses. We further hypothesized that the glycosylation of MSP1a plays a role in adhesion of A. marginale to tick cells. Native and Escherichia coli-derived recombinant MSP1a and MSP1b proteins were shown by gas chromatography to be glycosylated and to contain neutral sugars. Glycosylation of MSP1a appeared to be mainly O-linked to Ser/Thr residues in the N-terminal repeated peptides. Glycosylation may play a role in adhesion of A. marginale to tick cells because chemical deglycosylation of MSP1a significantly reduced its adhesive properties. Although the MSP1a polypeptide backbone alone was adherent to tick cell extract, the glycans in the N-terminal repeats appeared to enhance binding and may cooperatively interact with one or more surface molecules on host cells. These results demonstrated that MSP1a and MSP1b are glycosylated and suggest that the glycosylation of MSP1a plays a role in the adhesion of A. marginale to tick cells.