Lateral recognition of a dye hapten by a llama VHH domain

Camelids, camels and llamas, have a unique immune system able to produce he avy-chain only antibodies. Their VH domains (VHHs) are the smallest binding units produced by immune systems, and therefore suitable for biotechnologi cal applications through heterologous expression. The recognition of protei n antigens by these VHHs is rather well documented, while less is known abo ut the VHH/hapten interactions. The recently reported X-ray structure of a VHH in complex with a copper-containing azo-dye settled the ability of VHH to recognize haptens by forming a cavity between the three complementarity- determining regions (CDR). Here we report the structures of a VHH (VHH A52) free or complexed with an azo-dye, RR1, without metal ion. The structure o f the complex illustrates the involvement of CDR2, CDR3 and a framework res idue in a lateral interaction with the hapten. Such a lateral combining sit e is comparable to that found in classical antibodies, although in the abse nce of the VL. (C) 2001 Academic Press.