A single amino acid mutation in CDR3 of the 3-14-9 L chain abolished expression of the IDA 10-defined idiotope and antigen binding.

The ABPC 48 myeloma protein and the 3-14-9 mAb derive their V region genes from the same VH and V kappa gene families. They also share a cross-reactive idiotope defined by the anti-Id mAb IDA 10. Whereas ABPC 48 is specific for bacterial levan, 3-14-9 showed a significant Ag-binding activity to aminophenyl-beta-N-acetylglucosaminide (AZO). In order to define the molecular basis of idiotope expression and Ag-binding activity, we have cloned the genes encoding the 3-14-9 H and L chain V region genes, generated antibodies that carry mutations within the L chain genes, by site-directed mutagenesis, and investigated the effects of those mutations with respect to IDA 10 idiotope expression and binding to AZO. Our findings show that, whereas expression of the IDA 10-defined idiotope requires association of both the H and L chains, a single change (glycine to phenylalanine) at position 91 in the third complementarity-determining region of the L chain abolished both idiotope expression and Ag-binding activity. In addition, a L chain change of alanine to threonine at position 25 allowed idiotope expression to some extent but significantly reduced binding activity to AZO. These data suggest that a single amino acid change can play a crucial role in the functional activity and structural integrity of antibodies.