X-ray diffraction studies of an anti-azophenylarsonate antibody and of an antigen-antibody complex.

X-ray crystallographic studies of the Fab fragments of two murine monoclonal antibodies of predefined specificity are under way. Diffracted X-ray intensities of the crystalline native Fab fragment of an anti-azophenylarsonate antibody and of three heavy atom derivatives have been measured to a resolution of 3.5 A. A preliminary 6-A resolution electron density map has been obtained. The 6-A resolution structure of an antigen-antibody (hen lysozyme-Fab) complex has been determined. There are close contacts between the antigen and the antibody over a large contact area, about 20 X 25 A. At least two segments of the polypeptide chain of lysozyme, of about 10 amino acids each (positions 19-27 and 116-129), are involved in the contacts, as well as all six complementarity-determining regions of the antibody. No gross conformational changes are observed in the antigen at this resolution, although there are some smaller local changes in areas in contact with the antibody and elsewhere. The effects of amino acid substitutions on antigen recognition by the monoclonal anti-hen lysozyme antibody were investigated using different, closely related lysozymes. These effects can be readily explained in terms of the three-dimensional model presented here. A 3.5-A resolution electron density map has been calculated and is currently under study.

Preliminary crystallographic study of the Fc fragment of a monoclonal IgG2b murine immunoglobulin.

The Fe fragment of a gamma 2b murine monoclonal anti-p-azophenylarsonate antibody (R19.9, IgG2b, kappa) has been crystallized. The crystals are tetragonal, space group P41 (or P43) with unit cell dimensions: a = b = 134.3 +/- 1.1 A, c = 144.0 +/- 0.7 A.

Crystal structure of yeast tRNAAsp.

Two independent, three-dimensional structures of yeast tRNAAsp, mainly differing by the conformation of the D loop, have been obtained from a multiple isomorphous replacement (MIR) X-ray analysis at 3.5-A resolution. The folding of the ribose-phosphate backbone is similar to that found for tRNAPhe; major differences concern the relative positioning of the acceptor and anticodon stems, and the conformation of the loops in the two molecules. Crystal packing involves self-complementary GUC anticodon interactions.