Refined three-dimensional structure of the Fab fragment of a murine IgGl,λ antibody

We report the cDNA sequence determination and the crystal structure of the Fab fragment of a murine IgG1,λ antibody (HC19), specific for an influenza virus hemagglutinin. The HC19 Fab-fragment structure has been refined; the crystallographic R-factor is 19.5% at 2.3 Å resolution. We have compared the conformation of HC19 complementarity determining regions (CDRs) with those of CDR loops of Fab structures available from the Protein Data Bank. These loops were chosen based on the identity of key residues, following the canonical-structure approach; four CDRs have a main-chain conformation very similar to the canonical structure that had been identified. HC19 L1 CDR adopts a conformation clearly distinct from all L1 CDRs that belong to a chain of a different class or origin; this is determined by the nature of a few residues at positions in the sequence different from those of key residues in other light chains. This canonical structure should be representative of most murine λ-class light chains, as inferred from the very high sequence homologies of these polypeptides.