Rainer Jaenicke

Mutation of interfaces in domain-swapped human betaB2-crystallin.

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.