REFINED 3-DIMENSIONAL STRUCTURE OF THE FAB FRAGMENT OF A MURINE IGG1,LAMBDA ANTIBODY

We report the cDNA sequence determination and the crystal structure of the Fab fragment of a murine IgG1, lambda antibody (HC19), specific f or an influenza virus hemagglutinin. The HC19 Fab-fragment structure h as been refined; the crystallographic R factor is 19.5% at 2.3 angstro m resolution. We have compared the conformation of HC19 complementarit y determining regions (CDRs) with those of CDR loops of Fab structures available from the Protein Data Bank. These loops were chosen based o n the identity of key residues, following the canonical-structure appr oach; four CDRs have a main-chain conformation very similar to the can onical structure that had been identified. HC19 L1 CDR adopts a confor mation clearly distinct from all L1 CDRs that belong to a chain of a d ifferent class or origin; this is determined by the nature of a few re sidues at positions in the sequence different from those of key residu es in other light chains. This canonical structure should be represent ative of most murine lambda-class light chains, as inferred from the v ery high sequence homologies of these polypeptides.