THE THREE DIMENSIONAL STRUCTURE OF STAPHYLOCOCCAL ENTEROTOXIN C2 FROM TWO CRYSTAL FORMS. S. Swaminathan, W. Furey, J. Pletcher and M. Sax, Biocrystallography Laboratory, VA Medical Center, University Drive C, PO Box 12055, Pittsburgh, PA 15240 & Department of Crystallography, University of Pittsburgh, Pittsburgh, PA 15260 USA.

Bacterial superantigens induce massive T cell proliferation when presented by major histocompatibility complex class II (MHCII) molecules. These superantigens induce all T cells bearing particular types of Vb elements irrespective of other variable elements present in the T cell receptor by forming a ternary complex with MHCII and T cell receptor (TCR). Staphylococcal enterotoxins produced by Staphylococcus aureus are both toxins and superantigens. As toxins they cause vomiting and diarrhea in humans. There are five distinct serotypes of staphylococcal enterotoxins which are labeled A through E. SEC is further subdivided into SEC1-3 due to minor epitope variations. Even though all staph enterotoxins possess a common SE-fold the mode of association of these with MHCII molecule appear to be different. Further, in spite of very high sequence homology the Vb specificity of these also differ, though there is some overlap. The crystal structure of SEC2 was determined to better understand the reasons for the differences in the mode of association and Vb specificities.

SEC2 crystallizes in two forms. The monoclinic form is in space group P21 with cell dimensions a = 43.43, b = 69.92, c = 42.22Å and [[beta]] = 90.1deg. and has two molecules in the unit cell . The tetragonal form is in space group P43₂12 and has cell dimensions a = b = 42.98 and c = 289.3Å . The crystal structure determination of these two forms by the molecular replacement method will be presented. The differences in the structures of SEB and SEC2 will also be discussed.