Glycosylation of homologous immunodominant proteins of Ehrlichia chaffeensis and Ehrlichia canis

The glycoprotein genes of Ehrlichia chaffeensis (1,644 bp) and Ehrlichia canis (2,064 bp) encode proteins of 548 to 688 amino acids with predicted molecular masses of only 61 and 73 kDa but with electrophoretic mobilities of 120 kDa (P120) and 140 kDa (P140), respectively. The 120-kDa protein gene of E. chaffeensis contains four identical 240-bp tandem repeat units, and the 140-kDa protein gene of E. canis has 14 nearly identical, tandemly arranged 108-bp repeat units. Conserved serine-rich motifs identified in the repeat units of P120 and P140 were also found in the repeat units of the human granulocytotropic ehrlichiosis agent 130-kDa protein and of the fimbria- associated adhesin protein Fap1 of Streptococcus parasanguis. Nearly the entire (99%) E. chaffeensis P120 gene (1,616 bp), the 14-repeat region (78%) of the E. canis P140 gene (1,620 bp), and a 2-repeat region from the E. chaffeensis P120 gene (520 bp) were expressed in Escherichia coli. The recombinant proteins exhibited molecular masses ranging from 1.6 to 2 times larger than those predicted by the amino acid sequences. Antibodies against the recombinant proteins reacted with E. chaffeensis P120 and E. canis P140, respectively. Carbohydrate was detected on the E. chaffeensis and E. canis recombinant proteins, including the two-repeat polypeptide region of E. chaffeensis P120. A carbohydrate compositional analysis identified glucose, galactose, and xylose on the recombinant proteins. The presence of only one site for N-linked (Asn-Xaa-Ser/Thr) glycosylation, a lack of effect of N- glycosidase F, the presence of 70 and 126 Ser/Thr glycosylation sites in the repeat regions of P120 and P140, respectively, and a high molar ratio of carbohydrate to protein suggest that the glycans may be O linked.