A MOUSE IG-KAPPA DOMAIN OF VERY UNUSUAL FRAMEWORK STRUCTURE LOSES FUNCTION WHEN CONVERTED TO THE CONSENSUS

Antibody gene sequences, particularly those of kappa light chains, are very well conserved in the framework region, and the variability is c oncentrated in the complementar ity-determining regions (CDR). We now found that the murine antibody 93-6 (Djavadi-Ohaniance, L., Friguet, B ., and Goldberg, M. (1984) Biochemistry 23, 97-104) whose F-ab fragmen t binds the beta-subunit of Escherichia coli tryptophan synthase with high affinity (K-d of 6.7.10(-9) M) has a highly unusual kappa light c hain framework, which is crucial for the function of this antibody. It carries an insertion of 8 amino acids in a conserved framework loop t hat faces the antigen, and its framework region 2 (FR2) which precedes CDR2 is shortened by one amino acid, normally leucine and part of an absolutely conserved beta-bulge preceding CDR2. Removal of the inserti on to restore the consensus sequence reduced the binding affinity of 9 3-6 by a factor 3, while insertion of the missing leucine into FR2 com pletely abolished binding.