Detection and biochemical characterization of the mouse mammary tumor virus 7 superantigen (Mls-1a).

Mouse mammary tumor viruses encode superantigens that bind to class II major histocompatibility complex proteins and engage T cells that bear particular V beta s. Among these superantigens is the long known, but previously uncharacterized, Mls-1a product, encoded by Mtv-7. Using a monoclonal antibody, we detect the Mtv-7 superantigen on the surface of activated B cells, but not on T cells or resting B cells. The superantigen is synthesized as a 45 kd transmembrane glycoprotein precursor, but is proteolytically processed to yield an 18.5 kd surface protein that we suggest is the functional form of the superantigen.