Structure of rhamnose-binding lectin CSL3: unique pseudo-tetrameric architecture of a pattern recognition protein.

The crystal structure of the L-rhamnose-binding lectin CSL3 was determined to 1.8 A resolution. This protein is a component of the germline-encoded pattern recognition proteins in innate immunity. CSL3 is a homodimer of two 20 kDa subunits with a dumbbell-like shape overall, in which the N- and C-terminal domains of different subunits form lobe structures connected with flexible linker peptides. The complex structures of the protein with specific carbohydrates demonstrated the importance of the most variable loop region among homologues for the specificity toward oligosaccharides. CSL3 and Shiga-like toxin both use Gb(3) as a cellular receptor to evoke apoptosis. They have very different overall architecture but share the separation distance between carbohydrate-binding sites. An inspection of the structure database suggested that the pseudo-tetrameric structure of CSL3 was unique among the known lectins. This architecture implies this protein might provide a unique tool for further investigations into the relationships between architecture and function of pattern recognition proteins.