Circular permutation of βB2‐crystallin changes the hierarchy of domain assembly

The βγ‐crystallins form a superfamily of eye lens proteins comprised of multiple Greek motifs that are symmetrically organized into domains and higher assemblies. In the βB2‐crystallin dimer each polypeptide folds into two similar domains that are related to monomeric γ‐crystallin by domain swapping. The crystal structure of the circularly permuted two‐domain βB2 polypeptide shows that permutation converts intermolecular domain pairing into intramolecular pairing. However, the dimeric permuted protein is, in fact, half a native tetramer. This result shows how the sequential order of domains in multi‐domain proteins can affect quaternary domain assembly.