Mutation of interfaces in domain-swapped human βB2-crystallin

Authors

  • Myron A. Smith,

    1. Department of Crystallography, Institute of Structural and Molecular Biology, Birkbeck College, University of London, London WC1E 7HX, United Kingdom
    Search for more papers by this author
  • Orval A. Bateman,

    1. Department of Crystallography, Institute of Structural and Molecular Biology, Birkbeck College, University of London, London WC1E 7HX, United Kingdom
    Search for more papers by this author
  • Rainer Jaenicke,

    1. Department of Crystallography, Institute of Structural and Molecular Biology, Birkbeck College, University of London, London WC1E 7HX, United Kingdom
    Search for more papers by this author
  • Christine Slingsby

    Corresponding author
    1. Department of Crystallography, Institute of Structural and Molecular Biology, Birkbeck College, University of London, London WC1E 7HX, United Kingdom
    • Birkbeck College, Department of Crystallography, Malet Street, London WC1E 7HX, UK; fax: 44-(0)20-7631 6803.
    Search for more papers by this author

Abstract

The superfamily of eye lens βγ-crystallins is highly modularized, with Greek key motifs being used to form symmetric domains. Sequences of monomeric γ-crystallins and oligomeric β-crystallins fold into two domains that pair about a further conserved symmetric interface. Conservation of this assembly interface by domain swapping is the device adopted by family member βB2-crystallin to form a solution dimer. However, the βB1-crystallin solution dimer is formed from an interface used by the domain-swapped dimer to form a tetramer in the crystal lattice. Comparison of these two structures indicated an intriguing relationship between linker conformation, interface ion pair networks, and higher assembly. Here the X-ray structure of recombinant human βB2-crystallin showed that domain swapping was determined by the sequence and not assembly conditions. The solution characteristics of mutants that were designed to alter an ion pair network at a higher assembly interface and a mutant that changed a proline showed they remained dimeric. X-ray crystallography showed that the dimeric mutants did not reverse domain swapping. Thus, the sequence of βB2-crystallin appears well optimized for domain swapping. However, a charge-reversal mutation to the conserved domain-pairing interface showed drastic changes to solution behavior. It appears that the higher assembly of the βγ-crystallin domains has exploited symmetry to create diversity while avoiding aggregation. These are desirable attributes for proteins that have to exist at very high concentration for a very long time.

Ancillary