title: Crystal structure of the streptococcal superantigen SpeI and functional role of a novel loop domain in T cell activation by group V superantigens.
abstract: 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 approximately 15 amino acid extension (amino acid residues 154 to 170 in SpeI; herein referred to as the alpha3-beta8 loop), absent in SAg groups I through IV. Here, we report the crystal structure of SpeI at 1.56 A resolution. Although the alpha3-beta8 loop in SpeI 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 SpeI indicates that TCR beta-chains are likely engaged in a similar general orientation as other characterized SAgs. We show, however, that the alpha3-beta8 loop length, and the presence of key glycine residues, are necessary for optimal activation of T cells. Based on Vbeta-skewing analysis of human T cells activated with SpeI and structural models, we propose that the alpha3-beta8 loop is positioned to form productive intermolecular contacts with the TCR beta-chain, likely in framework region 3, and that these contacts are required for optimal TCR recognition by SpeI, and likely all other group V SAgs.
authors.affiliation: Department of Microbiology and Immunology, The University of Western Ontario, London, ON, Canada N6A 5B8.
authors.forename: Jean-Nicholas P
authors.initials: JN
authors.lastname: Brouillard
authors.forename: Sebastian
authors.initials: S
authors.lastname: Günther
authors.forename: Ashok K
authors.initials: AK
authors.lastname: Varma
authors.forename: Irene
authors.initials: I
authors.lastname: Gryski
authors.forename: Christine A
authors.initials: CA
authors.lastname: Herfst
authors.forename: A K M Nur-ur
authors.initials: AK
authors.lastname: Rahman
authors.forename: Donald Y M
authors.initials: DY
authors.lastname: Leung
authors.forename: Patrick M
authors.initials: PM
authors.lastname: Schlievert
authors.forename: Joaquín
authors.initials: J
authors.lastname: Madrenas
authors.forename: Eric J
authors.initials: EJ
authors.lastname: Sundberg
authors.forename: John K
authors.initials: JK
authors.lastname: McCormick
mesh_terms: D000595:Amino Acid Sequence; D000942:Antigens, Bacterial / Q000737:chemistry; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000276:immunology / Q000502:physiology; D018360:Crystallography, X-Ray; D018984:Epitopes, T-Lymphocyte / Q000737:chemistry / Q000276:immunology; D019143:Evolution, Molecular; D005098:Exotoxins / Q000737:chemistry* / Q000235:genetics / Q000276:immunology / Q000502:physiology; D006801:Humans; D008213:Lymphocyte Activation / Q000276:immunology*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010802:Phylogeny; D017434:Protein Structure, Tertiary / Q000235:genetics / Q000502:physiology; D011749:Pyrogens / Q000737:chemistry / Q000145:classification; D017386:Sequence Homology, Amino Acid; D018089:Superantigens / Q000737:chemistry* / Q000235:genetics / Q000502:physiology
chemical_list: D000942:Antigens, Bacterial; D001426:Bacterial Proteins; D018984:Epitopes, T-Lymphocyte; D005098:Exotoxins; D011749:Pyrogens; D018089:Superantigens; C027951:erythrogenic toxin