X-ray analysis of beta B2-crystallin and evolution of oligomeric lens proteins.
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.
Bax
B
B
Department of Crystallography, Birkbeck College, London University, UK.
Lapatto
R
R
Nalini
V
V
Driessen
H
H
Lindley
P F
PF
Mahadevan
D
D
Blundell
T L
TL
Slingsby
C
C
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
D003459:Crystallins; D046911:Macromolecular Substances