α-Crystallins in the Vertebrate Eye Lens: Complex Oligomers and Molecular Chaperones

Abstract

α-Crystallins are small heat-shock proteins that act as holdase chaperones. In humans, αA-crystallin is expressed only in the eye lens, while αB-crystallin is found in many tissues. α-Crystallins have a central domain flanked by flexible extensions and form dynamic, heterogeneous oligomers. Structural models show that both the C- and N-terminal extensions are important for controlling oligomerization through domain swapping. α-Crystallin prevents aggregation of damaged β- and γ-crystallins by binding to the client protein using a variety of binding modes. α-Crystallin chaperone activity can be compromised by mutation or posttranslational modifications, leading to protein aggregation and cataract. Because of their high solubility and their ability to form large, functional oligomers, α-crystallins are particularly amenable to structure determination by solid-state nuclear magnetic resonance (NMR) and solution NMR, as well as cryo-electron microscopy.

Keywords: intermolecular interactions; molecular chaperone; protein oligomer; protein solubility; vertebrate lens protein; α-crystallin.

Figure 1. α-crystallin binding to client protein.

(A) The initial recognition of the client protein sometimes occurs at the N- terminal extension of the α-crystallin (116). (B) Mini αA-crystallin contains many hydrophobic residues that bind to the client protein through hydrophobic interactions (110). (C) αA-crystallin tightly binds the client protein, keeping it in solution.

Figure 2

Oligomeric states of α-crystallins. (A) The Jehle (top) (31, 42) and Braun (bottom, PDB: 2YGD) (43) αB 24mer pseudoatomic models. Homodimers (left) are formed through ACD (yellow) contacts. Dimers combine to form a hexameric species (middle) through NTR (green) and ACD contacts. The C-terminal domain is highlighted in blue. Hexamers combine to form the dominant 24-mer structures (right). (B) Pseudoatomic models of a 16-mer of wild-type reduced human αA-crystallin(49). Top: αA-crystallin monomers (blue and red) forming the β7-interface dimer. Dimers via interactions between N-terminal extensions, to form z-shaped tetramers that stack to make up the pillars of the hollow oligomer. Bottom: Monomers (red) binding through N-terminal interactions. (C) Cryo-EM density maps from the Electron Microscopy Database (EMD) (49) of the (top) 12-mer (EMD-4895), (middle) 16-mer (EMD-4894), and (bottom) 20-mer (EMD 4896) with three, four, and five-fold symmetry, respectively, from the apical axis (right). All oligomers are formed from a z-shaped tetrameric building block. Abbreviations: ACD, α-crystallin domain; cryo-EM, cryo-electron microscopy; EMD, Electron Microscopy Database; NTR, N-terminal region; PDB, Protein Data Bank.

Figure 3

A A schematic of the interaction modes of αB-crystallin with client proteins. There are several different possible paths for client recognition and chaperone activity. Both the N- (green) and C-terminal extensions (blue) can act as site for initial recognition or even as the active holdase region. In the case of initial recognition, the client initially binds the flexible extension and then becomes bound to the α-crystallin domain. The α-crystallin domain itself can also directly interact with client proteins for holdase activity, without intermediate binding. B. Many specific residues and sequence regions are implicated in αB-crystallin holdase activity. The mini-αB peptide is highlighted in red, while the other key residues throughout the protein are labeled. These residues are D2, F24, F27 (107), R56 (157), D109 (59), R120(158), R157 (159), K82, K90, K92, K121, K166, K174, K175 (160). Abbreviation: PDB, Protein Data Bank.