3D structure of the native α-crystallin from bovine eye lens.

α-Crystallin is the major eye lens protein that has been shown to support lens transparency by preventing the aggregation of lens proteins. The 3D structure of α-crystallin is largely unknown. Electron microscopy, single-particle 3D reconstruction, size exclusion chromatography, dynamic light scattering, and analytical ultracentrifugation were used to study the structure of the native α-crystallin. Native α-crystallin has a wide distribution in size. The shape of mass distribution is temperature-dependent, but the oligomers with a sedimentation coefficient of ~22 S (750-830 kDa) strongly prevailed at all temperatures used. A 3D model of native α-crystallin with resolution of ~2 nm was created. The model is asymmetrical, has an elongated bean-like shape 13 × 19 nm with a dense core and filamentous "kernel". It does not contain a central cavity. The majority of α-crystallin particles regardless of experimental conditions are 13 × 19 nm, which corresponds to 22S sedimentation coefficient, hydrodynamic diameter 20 nm and mass of 750-830 kD. These particles are in dynamic equilibrium with particles of smaller and larger sizes.

Interaction of α-crystallin with some small molecules and its effect on its structure and function.

BACKGROUND: α-Crystallin acts like a molecular chaperone by interacting with its substrate proteins and thus prevents their aggregation. It also interacts with various kinds of small molecules that affect its structure and function. SCOPE OF REVIEW: In this article we will present a review of work done with respect to the interaction of ATP, peptide generated from lens crystallin and other proteins and some bivalent metal ions with α-crystallin and discuss the role of these interactions on its structure and function and cataract formation. We will also discuss the interaction of some hydrophobic fluorescence probes and surface active agents with α-crystallin. MAJOR CONCLUSIONS: Small molecule interaction controls the structure and function of α-crystallin. ATP and Zn+2 stabilize its structure and enhance chaperone function. Therefore the depletion of these small molecules can be detrimental to maintenance of lens transparency. However, the accumulation of small peptides due to protease activity in the lens can also be harmful as the interaction of these peptides with α-crystallin and other crystallin proteins in the lens promotes aggregation and loss of lens transparency. The use of hydrophobic probe has led to a wealth of information regarding the location of substrate binding site and nature of chaperone-substrate interaction. Interaction of surface active agents with α-crystallin has helped us to understand the structural stability and oligomeric dissociation in α-crystallin. GENERAL SIGNIFICANCE: These interactions are very helpful in understanding the mechanistic details of the structural changes and chaperone function of α-crystallin. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

Structure and function of α-crystallins: Traversing from in vitro to in vivo.

BACKGROUND: The two α-crystallins (αA- and αB-crystallin) are major components of our eye lenses. Their key function there is to preserve lens transparency which is a challenging task as the protein turnover in the lens is low necessitating the stability and longevity of the constituent proteins. α-Crystallins are members of the small heat shock protein family. αB-crystallin is also expressed in other cell types. SCOPE OF THE REVIEW: The review summarizes the current concepts on the polydisperse structure of the α-crystallin oligomer and its chaperone function with a focus on the inherent complexity and highlighting gaps between in vitro and in vivo studies. MAJOR CONCLUSIONS: Both α-crystallins protect proteins from irreversible aggregation in a promiscuous manner. In maintaining eye lens transparency, they reduce the formation of light scattering particles and balance the interactions between lens crystallins. Important for these functions is their structural dynamics and heterogeneity as well as the regulation of these processes which we are beginning to understand. However, currently, it still remains elusive to which extent the in vitro observed properties of α-crystallins reflect the highly crowded situation in the lens. GENERAL SIGNIFICANCE: Since α-crystallins play an important role in preventing cataract in the eye lens and in the development of diverse diseases, understanding their mechanism and substrate spectra is of importance. To bridge the gap between the concepts established in vitro and the in vivo function of α-crystallins, the joining of forces between different scientific disciplines and the combination of diverse techniques in hybrid approaches are necessary. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

The eye lens chaperone alpha-crystallin forms defined globular assemblies.

Alpha-crystallins are molecular chaperones that protect vertebrate eye lens proteins from detrimental protein aggregation. alphaB-Crystallin, 1 of the 2 alpha-crystallin isoforms, is also associated with myopathies and neuropathological diseases. Despite the importance of alpha-crystallins in protein homeostasis, only little is known about their quaternary structures because of their seemingly polydisperse nature. Here, we analyzed the structures of recombinant alpha-crystallins using biophysical methods. In contrast to previous reports, we show that alphaB-crystallin assembles into defined oligomers consisting of 24 subunits. The 3-dimensional (3D) reconstruction of alphaB-crystallin by electron microscopy reveals a sphere-like structure with large openings to the interior of the protein. alphaA-Crystallin forms, in addition to complexes of 24 subunits, also smaller oligomers and large clusters consisting of individual oligomers. This propensity might explain the previously reported polydisperse nature of alpha-crystallin.

Dodecameric structure of the small heat shock protein Acr1 from Mycobacterium tuberculosis.

Small heat shock proteins are a ubiquitous and diverse family of stress proteins that have in common an alpha-crystallin domain. Mycobacterium tuberculosis has two small heat shock proteins, Acr1 (alpha-crystallin-related protein 1, or Hsp16.3/16-kDa antigen) and Acr2 (HrpA), both of which are highly expressed under different stress conditions. Small heat shock proteins form large oligomeric assemblies and are commonly polydisperse. Nanoelectrospray mass spectrometry showed that Acr2 formed a range of oligomers composed of dimers and tetramers, whereas Acr1 was a dodecamer. Electron microscopy of Acr2 showed a variety of particle sizes. Using three-dimensional analysis of negative stain electron microscope images, we have shown that Acr1 forms a tetrahedral assembly with 12 polypeptide chains. The atomic structure of a related alpha-crystallin domain dimer was docked into the density to build a molecular structure of the dodecameric Acr1 complex. Along with the differential regulation of these two proteins, the differences in their quaternary structures demonstrated here supports their distinct functional roles.

A backbone-reversed form of an all-beta alpha-crystallin domain from a small heat-shock protein (retro-HSP12.6) folds and assembles into structured multimers.

The structural consequences of polypeptide backbone reversal ("retro" modification) remain largely unexplored, in particular, for the retro forms of globular all-beta-sheet proteins. To examine whether the backbone-reversed form of a model all-beta-sheet protein can fold and adopt secondary and tertiary structure, we created and examined the recombinant retro form of a 110-residue-long polypeptide, an alpha-crystallin-like small heat-shock protein, HSP12.6, from C. elegans. Following intracellular overexpression in fusion with a histidine affinity tag in Escherichia coli, purification under denaturing conditions, and removal of denaturant through dialysis, retro-HSP12.6 was found to fold to a soluble state. The folded protein was examined using fluorescence and CD spectroscopy, gel filtration chromatography, non-denaturing electrophoresis, differential scanning calorimetry, and electron microscopy and confirmed to have adopted secondary structure and assembled into a multimer. Interestingly, like its parent polypeptide, retro-HSP12.6 did not aggregate upon heating; rather, heating led to a dramatic increase in structural content and the adoption of what would appear to be a very well folded state at high temperatures. However, this was essentially reversed upon cooling with some hysteresis being observed resulting in greater structural content in the heated-cooled protein than in the unheated protein. The heated-cooled samples displayed CD spectra indicative of structural content comparable to that of any naturally occurring globular protein. Attempts are being made to refine crystallization conditions for the folded protein.