Crystal structure of the streptococcal superantigen spel and functional role of a novel loop domain in T cell activation by group V superantigens

Superantigens (SAgs) are potent microbial toxins that bind simultaneously to T cell receptors (TCRs) and class II major histocompatibility complex molecules, resulting in the activation and expansion of large T cell subsets and the onset of numerous human diseases. Within the bacterial SAg family, streptococcal pyrogenic exotoxin I (SpeI) has been classified as belonging to the group V SAg subclass, which are characterized by a unique, relatively conserved similar to 15 amino acid extension (amino acid residues 154 to 170 in SpeI; herein referred to as the alpha 3-beta 8 loop), absent in SAg groups I through IV. Here, we report the crystal structure of Spel at 1.56 A resolution. Although the alpha 3-beta 8 loop in Spel is several residues shorter than that of another group V SAg, staphylococcal enterotoxin serotype I, the C-terminal portions of these loops, which are located adjacent to the putative TCR binding site, are structurally similar. Mutagenesis and subsequent functional analysis of Spel indicates that TCR beta-chains are likely engaged in a similar general orientation as other characterized SAgs. We show, however, that the alpha 3-beta 8 loop length, and the presence of key glycine residues, are necessary for optimal activation of T cells. Based on V beta-skewing analysis of human T cells activated with Spel and structural models, we propose that the alpha 3-beta 8 loop is positioned to form productive intermolecular contacts with the TCR chain, likely in framework region 3, and that these contacts are required for optimal TCR recognition by Spel, and likely all other group V SAgs. (c) 2007 Elsevier Ltd. All rights reserved.