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

Abstract

Summary βγ-crystallins are long-lived eye lens proteins that are crucial for lens transparency and refractive power. Each βγ-crystallin comprises two homologous domains, which are connected by a short linker. γ-crystallins are monomeric, while β-crystallins crystallize as dimers and multimers. In the crystal, human βB2-crystallin is a domain-swapped dimer, while the N-terminally truncated βB1-crystallin forms a face-en-face dimer. Combining and integrating data from multi-angle light scattering, NMR and small angle X-ray scattering of full-length and terminally truncated human βB2-crystallin in solution, we show that both these βB2-crystallin proteins are dimeric, possess C2 symmetry, and are more compact than domain-swapped dimers. Importantly, no intermolecular paramagnetic relaxation enhancement effects compatible with domain-swapping were detected. Our collective experimental results unambiguously demonstrate that, in solution, human βB2-crystallin is not domain-swapped and exhibits a face-en-face dimer structure similar to the crystal structure of truncated βB1-crystallin.

Crystal Structure of Chicken γS-Crystallin Reveals Lattice Contacts with Implications for Function in the Lens and the Evolution of the βγ-Crystallins

“…In the crystal structure of βB2, dimerization occurs through domain-swapping ( Bax et al, 1990 ; Lapatto et al, 1991 ), but in the crystal lattice, dimers interact through the common QR interface to form a non-bonded lattice tetramer ( Bax et al, 1990 ; Lapatto et al, 1991 ; Smith et al, 2007 ) ( Fig 5 ). Surprisingly, a very recent study has suggested that, in solution, human βB2 forms a compact dimer thought to be a QR-based dimer without domain swapping ( Xi et al, 2017 ). As yet, there are no coordinates for this structure.…”

“…Since the domain swap in the crystal structure of βB2 complicates the calculation of monomer and dimer dipoles (because N- and C-domains of the QR interface are not parts of the same polypeptide chain) this was not considered. No coordinates are available for the recently described compact solution dimer for this protein ( Xi et al, 2017 ).…”

“…All the crystal structures for β-crystallins contain this interface, called QR, which is more or less robustly involved in same-protein interactions in different members of the family (Van Montfort et al, 2003; Xi et al, 2017). Its prevalence suggests that the QR interface is an ancestral feature of the β-crystallins and so might also be ancestral for the related γ-crystallins.…”

“…Although the large γS lattice contact illustrates ways in which γ-crystallins can interact without multimerizing, it is remarkably similar to a conserved interface involved in the multimerization of the β-crystallins in solution or in crystals. All the crystal structures for β-crystallins contain this interface, called QR, which is more or less robustly involved in same-protein interactions in different members of the family ( Van Montfort et al, 2003 ; Xi et al, 2017 ). Its prevalence suggests that the QR interface is an ancestral feature of the β-crystallins and so might also be ancestral for the related γ-crystallins.…”

Chemical Properties Determine Solubility and Stability in βγ‐Crystallins of the Eye Lens

“…For example, human βB2crystallin has been observed in different states in solution, including a domain-swapped dimer in a crystal structure (PDB ID: 1YTQ; [35] Figure 2C) and a face-en-face dimer, without domain swapping. [36] Studies of these molecules have mostly focused on two aspects: the molecular basis of their extraordinary stability and solubility, and how changes due to mutation or post-translational modification alter their biophysical properties. The γ-crystallins have shown to be more stable than βcrystallins in solution, suggesting that the inherent stability of the Greek key motif is not the only contributor to the stability of these proteins; the interdomain interface also contributes to the overall stabilization.…”