title: X-ray analysis of beta B2-crystallin and evolution of oligomeric lens proteins.
abstract: The beta, gamma-crystallins form a class of homologous proteins in the eye lens. Each gamma-crystallin comprises four topologically equivalent, Greek key motifs; pairs of motifs are organized around a local dyad to give domains and two similar domains are in turn related by a further local dyad. Sequence comparisons and model building predicted that hetero-oligomeric beta-crystallins also had internally quadruplicated subunits, but with extensions at the N and C termini, indicating that beta, gamma-crystallins evolved in two duplication steps from an ancestral protein folded as a Greek key. We report here the X-ray analysis at 2.1 A resolution of beta B2-crystallin homodimer which shows that the connecting peptide is extended and the two domains separated in a way quite unlike gamma-crystallin. Domain interactions analogous to those within monomeric gamma-crystallin are intermolecular and related by a crystallographic dyad in the beta B2-crystallin dimer. This shows how oligomers can evolve by conserving an interface rather than connectivity. A further interaction between dimers suggests a model for more complex aggregates of beta-crystallin in the lens.
authors.affiliation: Department of Crystallography, Birkbeck College, London University, UK.
authors.forename: B
authors.initials: B
authors.lastname: Bax
authors.forename: R
authors.initials: R
authors.lastname: Lapatto
authors.forename: V
authors.initials: V
authors.lastname: Nalini
authors.forename: H
authors.initials: H
authors.lastname: Driessen
authors.forename: P F
authors.initials: PF
authors.lastname: Lindley
authors.forename: D
authors.initials: D
authors.lastname: Mahadevan
authors.forename: T L
authors.initials: TL
authors.lastname: Blundell
authors.forename: C
authors.initials: C
authors.lastname: Slingsby
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002417:Cattle; D003459:Crystallins / Q000737:chemistry*; D046911:Macromolecular Substances; D008969:Molecular Sequence Data; D012689:Sequence Homology, Nucleic Acid; D014961:X-Ray Diffraction
chemical_list: D003459:Crystallins; D046911:Macromolecular Substances