Proteolytic processing is required for viral superantigen activity

The mouse mammary tumor virus-7 superantigen (vSAG7) is proteolytically processed in B cells at as many as three positions. Proteolytic processing appears to be important for superantigen activity because a processed form of vSAG7 was predominant among those forms that were found to bind to major histocompatibility complex class II molecules. To determine the functional significance of proteolytic processing, a mutation was introduced in vSAG7 at one of the sites where proteolytic cleavage is thought to take place in B cells. Elimination of the putative processing site at position 171 abrogated detectable vSAG7 surface expression in B cells, indicating that proteolytic processing is required for vSAG7 function. Coexpression in insect cells of vSAG7 and furin, a proprotein-processing enzyme, also demonstrated that furin could process vSAG7 at position 171.