Redundant proteolytic activation of a viral superantigen

Proteolytic activation of viral superantigens (vSAgs)4 expressed in Chinese hamster ovary (CHO) cells is required for T cell stimulation, and is mediated primarily by the protein convertase (PC) furin. Three PC recognition sites are highly conserved in vSAgs, but it was not known which sites are required for PC dependent vSAg activation. Moreover, because the PC recognition sites are not conserved in all functional vSAgs it was possible that activation could occur by processing at any of several sites. To identify the location(s) where processing of vSAg7 generates an active superantigen, each of two PC recognition sites, and a third related site were altered by in vitro mutagenesis, and the mutant proteins were tested for their abilities to activate T cells. Mutation of the PC recognition site at position 68-71 in vSAg7 had no effect on its ability to activate T cells. Mutation of the processing site at position 169-172 completely abolished T cell activation, and indicated that cleavage at this position was obligatory for proteolytic activation of vSAg7. However, introduction of a PC recognition site at position 192-195, a position that in many other vSAgs encodes a PC recognition site, restored activity to a vSAg7 protein that lacked a recognition site at position 169-172. The data revealed that processing of vSAgs at either position 169-172 or 192-195 was sufficient for vSAg7 activation, and explain how vSAgs that lack some PC recognition sites can be activated by proteolytic processing.