CRYSTAL STRUCTURE OF THE MHC CLASS II MOLECULE HLA-DR1 IN COMPLEX WITH THE FLUOROGENIC PEPTIDE, ACPKXVKQNTLKLAT (X=3-[5-(DIMETHYLAMINO)-1,3- DIOXO-1,3-DIHYDRO-2H-ISOINDOL-2-YL]-L-ALANINE) AND THE SUPERANTIGEN, SEC3 VARIANT 3B2