Alternative proteolytic processing of mouse mammary tumor virus superantigens

Mouse mammary tumor viruses express a superantigen essential for their life cycle. It has been proposed that viral superantigens (vSags) require proce ssing by prohormone convertases (PCs) for activity. We now observe, using a panel of mutant forms of potential PC cleavage sites and in vitro cleavage assays, that only the CS1 (position 68 to 71) and CS2 (position 169 to 172 ) sites are utilized by furin and PC5, Other members of the convertase fami ly that are expressed in lymphocytes are not endowed with this activity. Fu rthermore, mutant forms of two different viral superantigens, vSag7 and vSa g9, which completely abrogated in vitro processing by convertases, were eff icient in functional presentation to responsive T-cell hybridomas, This eff ect was observed in both endogenous presentation and paracrine transfer of the vSag, Processing by convertases thus appears not to be essential for vS ag function. Finally, we have identified the purified endosomal protease ca thepsin L as another protease that is able to cleave convertase mutant vSag in vitro, yielding fragments similar to those detected in vivo, thus sugge sting that proteases other than convertases are involved in the activation of vSags.