HUMAN CLASS-II MHC MOLECULE HLA-DR1 - X-RAY STRUCTURE DETERMINED FROM3 CRYSTAL FORMS

The three-dimensional structure of the extracellular region of a 60kDa class II major histocompatibility glycoprotein, HLA-DR1, was determin ed to 3.3 Angstrom by X-ray crystallography using three crystal forms, each containing two molecules per asymmetric unit. Phases were initia lly determined to 4.2 Angstrom using two crystal forms both containing DR1 from human lymphocytes complexed with a mixture of endogenous pep tides. One of these crystal forms also contained a 28 kDa superantigen , Staphylococcus aureus enterotoxin B (SEE), bound to each DR1 molecul e. Single-isomorphous replacement phasing followed by iterative two- a nd fourfold non-crystallographic real-space averaging between the two crystal forms resulted in 4.2 Angstrom resolution electron-density map s from which the paths of the polypeptides could be traced. Cryocrysta llography and synchrotron radiation were then used to extend the resol ution to 3.3 Angstrom for the two lymphocyte-derived crystal forms and for a third crystal form grown from DR1 produced in insect cells and complexed in vitro with a specific antigenic peptide, Iterative sixfol d non-crystallographic real-space averaging resulted in an electron-de nsity map into which 340 of 371 residues could be fit unambiguously. C rystal contacts and the existence of a parallel dimer of the DR1 alpha beta heterodimer in the three crystal forms are discussed.