title: Domain swapping of a llama VHH domain builds a crystal-wide beta-sheet structure.
abstract: Among mammals, camelids have a unique immunological system since they produce functional antibodies devoid of light chains and CH1 domains. To bind antigens, whether they are proteins or haptens, camelids use the single domain VH from their heavy chain (VHH). We report here on such a llama VHH domain (VHH-R9) which was raised against a hapten, the RR6 red dye. This VHH possesses the shortest complementarity determining region 3 (CDR3) among all the known VHH sequences and nevertheless binds RR6 efficiently with a K(d) value of 83 nM. However, the crystal structure of VHH-R9 exhibits a striking feature: its CDR3 and its last beta-strand (beta9) do not follow the immunoglobulin VH domain fold, but instead extend out of the VHH molecular boundary and associate with a symmetry-related molecule. The two monomers thus form a domain-swapped dimer which establishes further contacts with symmetry-related molecules and build a crystal-wide beta-sheet structure. The driving force of the dimer formation is probably the strain induced by the short CDR3 together with the cleavage of the first seven residues.
authors.affiliation: Architecture et Fonction des Macromolécules Biologiques, UMR-6098, CNRS and Universités d'Aix-Marseille I and II, 31 Chemin Joseph Aiguier, 13402 Marseille Cedex 20, France.
authors.forename: Silvia
authors.initials: S
authors.lastname: Spinelli
authors.forename: Aline
authors.initials: A
authors.lastname: Desmyter
authors.forename: Leon
authors.initials: L
authors.lastname: Frenken
authors.forename: Theo
authors.initials: T
authors.lastname: Verrips
authors.forename: Mariella
authors.initials: M
authors.lastname: Tegoni
authors.forename: Christian
authors.initials: C
authors.lastname: Cambillau
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002161:Camelids, New World; D022801:Complementarity Determining Regions / Q000737:chemistry; D003460:Crystallization; D018360:Crystallography, X-Ray; D019281:Dimerization; D006241:Haptens / Q000276:immunology; D007143:Immunoglobulin Heavy Chains / Q000737:chemistry* / Q000276:immunology; D008297:Male; D008969:Molecular Sequence Data; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D016415:Sequence Alignment
chemical_list: D022801:Complementarity Determining Regions; D006241:Haptens; D007143:Immunoglobulin Heavy Chains