The Babesia bovis VESA1 virulence factor subunit 1b is encoded by the 1beta branch of the ves multigene family

Abstract

Babesia bovis, an intraerythrocytic parasite of cattle, establishes persistent infections of extreme duration. This is accomplished, at least in part, through rapid antigenic variation of a heterodimeric virulence factor, the variant erythrocyte surface antigen-1 (VESA1) protein. Previously, the VESA1a subunit was demonstrated to be encoded by a 1alpha member of the ves multigene family. Since its discovery the 1beta branch of this multigene family has been hypothesized to encode the VESA1b polypeptide, but formal evidence for this connection has been lacking. Here, we provide evidence that products of ves1beta genes are rapidly variant in antigenicity and size-polymorphic, matching known VESA1b polypeptides. Importantly, the ves1beta-encoded antigens are co-precipitated with VESA1a during immunoprecipitation with anti-VESA1a monoclonal antibodies, and antisera to ves1beta polypeptide co-precipitate VESA1a. Further, the ves1beta-encoded antigens significantly co-localize with VESA1a on the infected-erythrocyte membrane surface of live cells. These characteristics all match known properties of VESA1b, allowing us to conclude that the ves1beta gene divergently apposing the ves1beta gene within the locus of active ves transcription (LAT) encodes the 1b subunit of the VESA1 cytoadhesion ligand. However, the extent and stoichiometry of VESA1a and 1b co-localization on the surface of individual cells is quite variable, implicating competing effects on transcription, translation, or trafficking of the two subunits. These results provide essential information facilitating further investigation into this parasite virulence factor.