Single amino acid substitution altering antigen-binding specificity.

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Abstract
S107, a phosphocholine-binding, myeloma protein [from mouse S107 cells], was cloned in soft agar, and an antigen-binding variant was isolated and characterized. The variant does not bind phosphocholine attached to carrier or as free hapten in solution but does retain antigenetic determinants (idiotypes) of the parent. Chain recombination experiments suggest that the defect in binding is entirely in the H chain. Amino acid sequence analysis showed a single substitution, glutamic acid to alanine at position 35, in the 1st hypervariable or complementarity-determining region. In terms of the 3-dimensional model of the phosphocholine-binding site, glutamic acid-35 provides a H bond to tyrosine-94 of the L chain that appears to be critical for stability of this portion of the binding site. The removal of this bond and the presence of the smaller alanine side chain is thus consistent with the loss in binding activity. Small numbers of substitutions in antibodies, such as those presumably introduced by somatic mutation, may in some situations be effective in altering antigen-binding specificity.