HUMAN CLASS-II MHC MOLECULE HLA-DR1 - X-RAY STRUCTURE DETERMINED FROM 3 CRYSTAL FORMS

The three-dimensional structure of the extracellular region of a 60kDa class II major histocompatibility glycoprotein, HLA-DR1, was determined to 3.3 Angstrom by X-ray crystallography using three crystal forms, each containing two molecules per asymmetric unit. Phases were initially determined to 4.2 Angstrom using two crystal forms both containing DR1 from human lymphocytes complexed with a mixture of endogenous peptides. One of these crystal forms also contained a 28 kDa superantigen, Staphylococcus aureus enterotoxin B (SEE), bound to each DR1 molecule. Single-isomorphous replacement phasing followed by iterative two- and fourfold non-crystallographic real-space averaging between the two crystal forms resulted in 4.2 Angstrom resolution electron-density maps from which the paths of the polypeptides could be traced. Cryocrystallography and synchrotron radiation were then used to extend the resolution 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 sixfold non-crystallographic real-space averaging resulted in an electron-density map into which 340 of 371 residues could be fit unambiguously. Crystal contacts and the existence of a parallel dimer of the DR1 alpha beta heterodimer in the three crystal forms are discussed.