title: The X-ray structure of a mutant eye lens beta B2-crystallin with truncated sequence extensions.
abstract: beta-Crystallins are oligomeric eye lens proteins that are related to monomeric gamma-crystallins by domain swapping: like gamma-crystallins, they are comprised of two similar domains but they differ in having long sequence extensions. beta B2, a major component of beta-crystallin oligomers, self-associates to a homodimer in solution. In two crystal structures of native beta B2, the protein is a 222-symmetric tetramer of eight domains. It has previously been shown that a mutant of rat beta B2-crystallin, in which the bulk of the N- and C-terminal sequence extensions has been deleted, assembles into dimers and tetramers. Here we present the 3.0 A resolution X-ray structure of the tetramer, beta B2 delta NC1. The mutant tetramer has a very similar set of domain interactions to the native structure. However, the structures differ in the relative orientation of the two sets of four domains. The paired N- and C-terminal domain interface, which is at the heart of the dimer structure, is very similar to the native structure. However, the truncation of the C-terminal extension removes an important tryptophan residue, which prevents the extension from acting as a (non-covalent) linker, as it does in native beta B2. There is a knock-on structural effect that removes a contact between extension and covalent linker, and this appears to cause a small twist in the linker that is amplified into a 20 degrees rotation between sets of paired domains.
authors.affiliation: Birkbeck College, Department of Crystallography, London, United Kingdom.
authors.forename: B V
authors.initials: BV
authors.lastname: Norledge
authors.forename: S
authors.initials: S
authors.lastname: Trinkl
authors.forename: R
authors.initials: R
authors.lastname: Jaenicke
authors.forename: C
authors.initials: C
authors.lastname: Slingsby
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002417:Cattle; D003459:Crystallins / Q000737:chemistry* / Q000235:genetics; D019281:Dimerization; D008969:Molecular Sequence Data; D016296:Mutagenesis; D051381:Rats; D014961:X-Ray Diffraction
chemical_list: D003459:Crystallins