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Infect. Immun., 07 1996, 2618-2626, Vol 64, No. 7
Copyright © 1996, American Society for Microbiology

A Babesia bovis 225-kilodalton spherical-body protein: localization to the cytoplasmic face of infected erythrocytes after merozoite invasion

SC Dowling, LE Perryman and DP Jasmer
Department of Veterinary Microbiology and Pathology, College of Veterinary Medicine, Washington State University, Pullman, Washington 99164-7040, USA. dud@vetmed.wsu.edu

A 225-kDa Babesia bovis protein occurs on the cytoplasmic side of infected-erythrocyte membranes. Here it is demonstrated that the 225- kDa protein localizes to spherical-body organelles of merozoites. Organelles consistent in size and shape with spherical bodies were isolated between 1.17 and 1.21 g/cm(3) in a sucrose density gradient. Organelles consistent with rhoptries and micronemes were also present in fractions from 1.17 to 1.19 g/cm(3). Antisera generated by immunizing mice with the 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) recognized an epitope that occurs in the repeat region of the 225-kDa protein (now referred to as SBP2). Monoclonal antibody 70/97.14 bound to merozoite spherical bodies, vesicles in infected-host cytoplasm, and the cytoplasmic face of the infected- erythrocyte membrane. These results indicate that spherical-body proteins become associated with the host membrane via transport through the erythrocyte cytoplasm after intracellular invasion.