A BABESIA-BOVIS 225-KILODALTON SPHERICAL-BODY PROTEIN - LOCALIZATION TO THE CYTOPLASMIC FACE OF INFECTED ERYTHROCYTES AFTER MEROZOITE INVASION

A 225-kDa Babesia bovis protein occurs on the cytoplasmic side of infe cted-erythrocyte membranes. Here it is demonstrated that the 225-kDa p rotein localizes to spherical-body organelles of merozoites. Organelle s consistent in size and shape with spherical bodies were isolated bet ween 1.17 and 1.21 g/cm3 in a sucrose density gradient. Organelles con sistent with rhoptries and micronemes were also present in fractions f rom 1.17 to 1.19 g/cm(3). Antisera generated by immunizing mice with t he fraction (1.20 to 1.21 g/cm(3)) most enriched for spherical bodies reacted predominantly with spherical bodies in B. bovis merozoites. A monoclonal antibody generated from this immunization (70/97.14) recogn ized an epitope that occurs in the repeat region of the 225-kDa protei n (now referred to as SBP2). Monoclonal antibody 70/97.14 bound to mer ozoite spherical bodies, vesicles in infected-host cytoplasm, and the cytoplasmic face of the infected-erythrocyte membrane. These results i ndicate that spherical-body proteins become associated with the host m embrane via transport through the erythrocyte cytoplasm after intracel lular invasion.