CONFORMATION OF THE HYPERVARIABLE REGION L3 WITHOUT THE KEY PROLINE RESIDUE

The refined structure of the Fab fragment of the monoclonal antibody C RIS-1 (IgG2a kappa) against the leukocyte differentiation antigen CD5, determined at 1.9 Angstrom resolution with an agreement R-factor of 1 8.3%, reveals a variant of the canonical conformations proposed for th e light chain complementarity determining region L3 (CDR-L3). This is the first Fab structure available with a kappa light chain in which th e CDR-L3 lacks the key proline residue in either position 94 or 95. Th e conformation found could be significant for about 10% of the murine IgG molecules with kappa light chains without proline in their CDR-L3 sequences.