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To study the nature of antibody-antigen interactions, we have determined the
variable gene sequences of the anti-cytochrome c immunoglobulin G1 (IgG1)
monoclonal antibody E8, and obtained diffraction-quality crystals of the E8
antigen-binding fragment (Fab), both free and bound to its antigen, horse
cytochrome c. The FabE8 crystals belong to space group P21 with unit cell
dimensions of a = 45.0 A, b = 85.1 A, c = 63.3 A and beta = 105.5 degrees, have
one FabE8 molecule per asymmetric unit and diffract to at least 2.1 A
resolution. Crystals of the FabE8-cytochrome c complex belong to space group
P212121 with unit cell dimensions of a = 84.3 A, b = 73.3 A and c = 94.9 A,
accommodate one complex per asymmetric unit and diffract to 2.4 A resolution. In
the nucleotide-derived amino acid sequences, the light-chain variable domain
(VL) but not the heavy-chain variable domain (VH) of E8 is nearly identical to
that of the anti-lysozyme antibody D1.3, differing by only five amino acid
residues. Only one of these interacts with lysozyme in the D1.3-lysozyme crystal
structure. Six negative and four positive charges in the VH complementarity
determining regions of E8 complement four positive and three negative charges in
the E8 epitope on cytochrome c. These data suggest that only a subset of the
residues in an antibody-protein interface may be critical for binding and that
the VH may play a dominant role in antigenic recognition.
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