Mycoplasma arthritidis-derived mitogen (MAM) is a superantigen that can activate large fractions of T cells bearing particular TCR Vbeta elements. Here we report the crystal structure of MAM complexed with a major histocompatibility complex (MHC) antigen, HLA-DR1, loaded with haemagglutinin peptide 306-318 (HA). The structure reveals that MAM has a novel fold composed of two alpha-helical domains. This fold is entirely different from that of the pyrogenic superantigens, consisting of a beta-grasped motif and a beta barrel. In the complex, the N-terminal domain of MAM binds orthogonally to the MHC alpha1 domain and the bound HA peptide, and to a lesser extent to the MHC beta1 domain. Two MAM molecules form an asymmetric dimer and cross-link two MHC antigens to form a plausible, dimerized MAM-MHC complex. These data provide the first crystallographic evidence that superantigens can dimerize MHC molecules. Based on our structure, a model of the TCR2MAM2MHC2 complex is proposed.
HLA-DR1 Antigen/chemistry , Mitogens/chemistry , Models, Molecular , Mycoplasma arthritidis/metabolism , Protein Folding , Antigens , Antigens, Bacterial , Crystallography, X-Ray , Dimerization , Major Histocompatibility Complex/physiology , Protein Structure, Tertiary , Proteins , Superantigens/chemistry
Mycoplasma arthritidis-derived mitogen (MAM), a bacterial superantigen, has been crystallized in complex with its human receptor, major histocompatibility complex (MHC) class II antigen, by the hanging-drop vapor-diffusion method. Crystals were obtained under three conditions, with ammonium sulfate, phosphate salt and PEG 8000 as the precipitant. The crystals grown under these conditions all belong to space group I222, with the same unit-cell parameters: a = 137.4, b = 178.2, c = 179.6 A. Diffraction data were collected to 3.3 and 3.4 A resolution from crystals of native and selenomethionylated MAM-MHC complexes, respectively. Self- and cross-rotation function calculations suggest the presence of two complex molecules in the asymmetric unit, resulting in a V(M) of 4.0 and a solvent content of 69%. An interpretable electron-density map was produced using a combination of molecular replacement and SAD phasing.
Major Histocompatibility Complex , Mitogens/chemistry , Mycoplasma arthritidis/metabolism , Antigens , Antigens, Bacterial/chemistry , Chromatography , Crystallography, X-Ray/methods , Diffusion , Electrons , Escherichia coli/metabolism , Glutathione Transferase/metabolism , Humans , Kinetics , Mass Spectrometry , Peptides/chemistry , Polyethylene Glycols/chemistry , Proteins , Recombinant Proteins/chemistry , Selenomethionine/chemistry , Superantigens , Time Factors
