CRYSTALLIZATION OF 2 MONOCLONAL FEB FRAGMENTS OF SIMILAR AMINO-ACID-SEQUENCE BOUND TO THE SAME AREA OF HORSE CYTOCHROME-C AND INTERACTING BY POTENTIALLY DISTINCT MECHANISMS

The mouse monoclonal antibodies (mAb), 2E5.G10 and 1F5.D1, are specifi c for horse cytochrome c and appear to bind the same epitope, since th eir heavy (H) and light (L) chains are functionally interchangeable. C omparison of the amino-acid sequences suggests that slightly different interactions may be involved in antigen recognition. In addition, the H chains differ at only a few amino-acid residues from the H chain of a rat cytochrome c-specific mAb suggesting that specificity for one p rotein over another may be determined by these amino-acid differences. To address these possibilities, the three-dimensional structures of t he Fab portions of the mAb bound to cytochrome c are being determined by X-ray diffraction analysis. Here we describe the preparation and cr ystallization of the two complexes with horse cytochrome c. The comple x of the Fab fragment of 2E5.G10 with horse cytochrome c yielded cryst als of X-ray diffraction quality under two sets of conditions; in both the space group was P2(1). The corresponding complex of 1F5.D1 under one of these conditions crystallized in the P2(1)2(1)2(1) space group. Three-dimensional X-ray data for these two complexes have been collec ted with nominal resolutions of 2.86 and 2.48 Angstrom, respectively.