Camelid heavy-chain variable domains provide efficient combining sites to haptens

Camelids can produce antibodies devoid of light chains and CHI domains (Ham ers-Casterman, C. et al. (1993) Nature 363, 446-448). Camelid heavy-chain v ariable domains (VHH) have high affinities for protein antigens and the str uctures of two of these complexes have been determined (Desmyter, A. et al. (1996) Nature Struc. Biol. 3, 803-811; Decanniere, K. et al. (1999) Struct ure 7,361-370). However, the small size of these VHHs and their monomeric n ature bring into question their capacity to bind haptens. Here, we have suc cessfully raised Ilama antibodies against the hapten ate-dye Reactive Red ( RR6) and determined the crystal structure of the complex between a dimer of this hapten and a VHH fragment. The surface of interaction between the VHH and the dimeric hapten is large, with an area of ca. 300 Angstrom(2); this correlates well with the low-dissociation constant of 22 nM measured for t he monomer. The VHH fragment provides an efficient combining site to the RR 6, using its three CDR loops. In particular, CDR1 provides a strong interac tion to the hapten through two histidine residues bound to its copper atoms . VHH fragments might, therefore, prove to be valuable tools for selecting, removing, or capturing haptens. They are likely to play a role in biotechn ology extending beyond protein recognition alone.