Crystal structure of a biologically inactive mutant of toxic shock syndrome toxin-1 at 2.5 A resolution (original) (raw)

Toxic shock syndrome toxin-1 (TSST-I) is one of a family of staphylococcal exotoxins recognized as microbial superantigens. The toxin plays a dominant role in the genesis of toxic shock in humans through a massive activation of the immune system. This potentially lethal illness occurs as a result of the interaction of TSST-1 with a significant proportion of the T-cell repertoire. TSST-I, like other superantigens, can bind directly to class I1 major histocompatibility (MHC class 11) molecules prior to its interaction with entire families of VP chains of the T-cell receptor (TCR). The three-dimensional structure of a mutant (His-135-Ala) TSST-1 was compared with the structure of the native (wild-type) TSST-1 at 2.5 A resolution. The replacement of His 135 of TSST-1 with an Ala residue results in the loss of T-cell mitogenicity and toxicity in experimental animals. This residue, postulated to be directly involved in the toxin-TCR interactions, is located on the major helix 012, which forms the backbone of the molecule and is exposed to the solvent. In the molecular structure of the mutant toxin, the helix a2 remains unaltered, but the His to Ala modification causes perturbations on the neighboring helix a1 by disrupting helixhelix interactions. Thus, the effects on TCR binding of the His 135 residue could actually be mediated, wholly or in part, by the a 1 helix.