GENETIC AND BIOCHEMICAL-ANALYSIS OF ERYTHROCYTE-STAGE SURFACE-ANTIGENS BELONGING TO A FAMILY OF HIGHLY CONSERVED PROTEINS OF BABESIA-EQUI AND THEILERIA SPECIES

Erythrocyte-stage Babesia equi expresses a 34-kDa immunodominant antig en recognized by antibody from persistently infected horses worldwide. This erythrocyte-stage surface protein, equi merozoite antigen-1 (EMA -1) is encoded by a single copy gene, and was previously shown to shar e 33% amino acid identity with similar sized proteins of Theileria ser genti and T. buffeli. A mean homology of 31% amino acid identity exten ds to similar sized proteins of T. parve, T. annulata and T. mutans. G enomic and cDNA copies of a second B. equi gene, ema2 were cloned. The single copy ema2 gene encodes a 30-kDa protein (EMA-2) that shares 52 % amino acid identity with EMA-1. EMA-2 also shares a mean amino acid identity of 31% with proteins of similar molecular mass from Theileria species. EMA-I and EMA-2 each contain a glycosylphosphatidylinositol anchor. These unique erythrocyte-stage surface proteins of B. equi and Theileria species lack antigenic repeats, and excluding the signal pe ptide, contain one or no cysteines. Consistent with the hypothesis tha t this family of proteins interacts with the erythrocyte surface, the T. species proteins possess a basic isoelectric point. The B. equi pro teins have acidic isoelectric points, but 24-mer peptides within them have strongly basic net charges. (C) 1997 Elsevier Science B.V.