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Superantigens (SAGs) aberrantly alter immune system function through simultaneous interaction with lateral surfaces of MHC class II molecules on APCs and with particular variable regions of the TCR β-chain (Vβ). To further define the interface between the bacterial SAG toxic shock syndrome toxin-1 (TSST-1) and the TCR, we performed alanine scanning mutagenesis within the putative TCR binding region of TSST-1 along the central α helix adjacent to the N-terminal α helix and the β7-β9 loop as well as with two universally conserved SAG residues (Leu 137 and Tyr 144 in TSST-1). Mutants were analyzed for multiple functional activities, and various residues appeared to play minor or insignificant roles in the TCR interaction. The locations of six residues (Gly 16, Trp 116, Glu 132, His 135, Gln 136, and Gln 139), each individually critical for functional activity as well as direct interaction with the human TCR Vβ2. 1 …