Crystal structure of truncated human betaB1-crystallin

Abstract

Crystallins are long-lived proteins packed inside eye lens fiber cells that are essential in maintaining the transparency and refractive power of the eye lens. Members of the two-domain betagamma-crystallin family assemble into an array of oligomer sizes, forming intricate higher-order networks in the lens cell. Here we describe the 1.4 angstroms resolution crystal structure of a truncated version of human betaB1 that resembles an in vivo age-related truncation. The structure shows that unlike its close homolog, betaB2-crystallin, the homodimer is not domain swapped, but its domains are paired intramolecularly, as in more distantly related monomeric gamma-crystallins. However, the four-domain dimer resembles one half of the crystallographic bovine betaB2 tetramer and is similar to the engineered circular permuted rat betaB2. The crystal structure shows that the truncated betaB1 dimer is extremely well suited to form higher-order lattice interactions using its hydrophobic surface patches, linker regions, and sequence extensions.

Figure 1.

Ribbon diagrams showing the assembly characteristics of the domains in monomeric bovine γB-crystallin (a; PDB ID 1amm). Domain pairing is intramolecular and buries an interface defined as PQ by analogy with the interface defined in the βB2 tetramer. (b) Rat circularly permuted βB2 dimer (PDB ID 1bd7). Domain pairing buries the PQ interface and is intramolecular, whereas the QR interface, by analogy with the interface defined in the βB2 tetramer, is between monomers. (c) Human truncated βB1 dimer. Domain pairing buries the PQ interface and is intramolecular, whereas the QR interface is between monomers. (d) Bovine βB2 dimer (PDB ID 2bb2). The crystallographic dyad P is perpendicular to the page; domain pairing buries the PQ interface and is intermolecular. (e) Bovine βB2 lattice tetramer. The P-axis is perpendicular to the page, Q is vertical, and R is horizontal. The PQ interface is intermolecular, and the QR interface is between dimers. All figures were made by using MOLSCRIPT/Raster3D (Kraulis 1991; Merritt and Bacon 1997).

Figure 2.

Sequence alignment of human βB1 (hbetaB1, P53674), bovine βB2 (bbetaB2, P02522), circular permuted rat βB2 (rcpbetaB2, based on P26775 modified according to Wright et al. 1998), and bovine γB (bgammaB, P02526) with the different domains and extensions indicated. The residue numbering and secondary structure shown at the top of the alignment are of human βB1, the numbering below is of γB. Conserved glycine and serine motifs are shown in blue. Asp107, Asp108, Arg169, and Arg171 in βB2 are shown in red. Interaction sites A–D making up dimer–dimer interfaces 1 and 2 are shown in purple (residues 54–57 and 235–236), pink, dark green, and light green, respectively.

Figure 3.

Zoomed out views down the Q dyad of respective βB1 (A), cpβB2 dimers (B), and the top half of the βB2 tetramer (C). Asp168, Asp169, Arg230, and Arg 232 in trhβB1 and the equivalent Asp107, Asp108, Arg169, and Arg171 in cpβB2 and βB2 are labeled. The monomers in the βB1 dimer are shown in gray and light gray; in cpβB2, in magenta and pink, and the domains belonging to different monomers in the top half of βB2 are shown in gray, light gray, and green, and light green, respectively.

Figure 4.

(A) Close-up of interface 1. The interacting dimers are shown in yellow and blue ribbon representation. Residues from interaction site A are on the yellow dimer and are shown in dark pink. Residues from interaction site B on the blue dimer are shown in pale pink. Interaction site B is also shown in pink mapped to the semi-transparent surface of the second dimer. (B) Close up of interface 2. The interacting dimers are shown in magenta and blue ribbons, respectively. Residues from interaction site C are shown in green. Residues from interaction site D are shown in light green. Interaction site D is also shown in light green mapped to the semitransparent surface of the second dimer. Figure 4 ▶ was generated by using Aesop (M. Noble, unpubl.).