Human βB2-Crystallin Forms a Face-en-Face Dimer in Solution: An Integrated NMR and SAXS Study

beta gamma-Crystallins are long-lived eye lens proteins that are crucial for lens transparency and refractive power. Each beta gamma-crystallin comprises two homologous domains, which are connected by a short linker. gamma-Crystallins aremonomeric, while beta-crystallins crystallize as dimers and multimers. In the crystal, human beta B2-crystallin is a domain-swapped dimer while the N-terminally truncated beta B1-crystallin forms a faceen- face dimer. Combining and integrating data from multi-angle light scattering, nuclear magnetic resonance, and small-angle X-ray scattering of full-length and terminally truncated human beta B2-crystallin in solution, we show that both these beta B2-crystallin proteins are dimeric, possess C2 symmetry, and are more compact than domain-swapped dimers. Importantly, no inter-molecular paramagnetic relaxation enhancement effects compatible with domain swapping were detected. Our collective experimental results unambiguously demonstrate that, in solution, human beta B2-crystallin is not domain swapped and exhibits a face-en-face dimer structure similar to the crystal structure of truncated beta B1-crystallin.