Conformation of the hypervariable region L3 without the key proline residue

Abstract
The refined structure of the Fab fragment of the monoclonal antibody CRIS-I (IgG2ak) against the leukocyte differentiation antigen CD5, determined at 1.9 Å resolution with an agreement R-factor of 18.3%, reveals a variant of the canonical conformations proposed for the light chain complementarity determining region L3 (CDR-L3). This is the first Fab structure available with a K light chain in which the CDR-L3 lacks the key proline residue in either position 94 or 95. The conformation found could be significant for about 10% of the murine IgG molecules with k light chains without proline in their CDR-L3 sequences.