Three-Dimensional Domain Swapping in Nitrollin, a Single-Domain βγ-Crystallin from Nitrosospira multiformis, Controls Protein Conformation and Stability but Not Dimerization

The βγ-crystallin superfamily has a well-characterized protein fold, with several members found in both prokaryotic and eukaryotic worlds. A majority of them contain two βγ-crystallin domains. A few examples, such as ciona crystallin and spherulin 3a exist that represent the eukaryotic single-domain proteins of this superfamily. This study reports the high-resolution crystal structure of a single-domain βγ-crystallin protein, nitrollin, from the ammonium-oxidizing soil bacterium Nitrosospira multiformis. The structure retains the characteristic βγ-crystallin fold despite a very low sequence identity. The protein exhibits a unique case of homodimerization in βγ-crystallins by employing its N-terminal extension to undergo three-dimensional (3D) domain swapping with its partner. Removal of the swapped strand results in partial loss of structure and stability but not dimerization per se as determined using gel filtration and equilibrium unfolding studies. Overall, nitrollin represents a distinct single-domain prokaryotic member that has evolved a specialized mode of dimerization hitherto unknown in the realm of βγ-crystallins.