SPECIES-DEPENDENT POSTTRANSLATIONAL MODIFICATION AND POSITION-2 ALLELISM - EFFECTS ON STREPTOCOCCAL SUPERANTIGEN SSA STRUCTURE AND V-BETA SPECIFICITY

Epidemiologic and molecular population genetic analyses support a role for superantigens (SAg) in the pathogenesis of severe staphylococcal and streptococcal infections, To investigate how variations in SAg str ucture influence immunomodulatory activity, we examined the biochemica l and functional properties of two allelic variants of streptococcal S Ag SSA that differ at position 2. Mass spectrometry revealed both reco mbinant (Escherichia coli) and native (Streptococcus pyogenes) SSA all elic variants to have significantly larger molecular masses than predi cted by primary sequence alone and provided evidence that the proteins were modified by the addition of biochemical moieties, a phenomenon t hat has not been described for related SAg. Furthermore, the molecular masses of native and recombinant SSA were not the same; SSA was diffe rentially post-translationally modified by the two bacterial genera, T he substitution of E. coli-dependent processing for that of S. pyogene s altered both protease digestion and V beta specificity, suggesting t hat recombinant SAg from E. coli may not accurately represent the nati ve toxin, In addition, the observation that SSA allelic variants diffe red in V beta specificity supports a role for position 2 in SSA-TCR in teractions, That SSA position 2 contributes to V beta specificity coul d not have been predicted from functional or crystallographic studies of other SAg and suggests that SSA may adopt unique interactions with TCR and/or MHC class II molecules, Determining the structural basis fo r these differences should offer additional clues to the manner in whi ch SAg exert their effects on the immune system during infection and m ay allow the designing of SAg mutants with specific quantitative and q ualitative immunomodulatory properties.