Solution properties of γ‐crystallins: Compact structure and low frictional ratio are conserved properties of diverse γ‐crystallins

Abstract

c-crystallins are highly specialized proteins of the vertebrate eye lens where they survive without turnover under high molecular crowding while maintaining transparency. They share a tightly folded structural template but there are striking differences among species. Their amino acid compositions are unusual. Even in mammals, c-crystallins have high contents of sulfurcontaining methionine and cysteine, but this reaches extremes in fish cM-crystallins with up to 15% Met. In addition, fish cM-crystallins do not conserve the paired tryptophan residues found in each domain in mammalian c-crystallins and in the related b-crystallins. To gain insight into important, evolutionarily conserved properties and functionality of c-crystallins, zebrafish (Danio rerio) cM2b and cM7 were compared with mouse cS and human cD. For all four proteins, far UV CD spectra showed the expected b-sheet secondary structure. Like the mammalian proteins, cM7 was highly soluble but cM2b was much less so. The heat and denaturant stability of both fish proteins was lower than either mammalian protein. The ability of full-length and truncated versions of human aB-crystallin to retard aggregation of the heat denatured proteins also showed differences. However, when solution behavior was investigated by sedimentation velocity experiments, the diverse c-crystallins showed remarkably similar hydrodynamic properties with low frictional ratios and partial specific volumes. The solution behavior of c-crystallins, with highly compact structures suited for the densely packed environment of the lens, seems to be highly conserved and appears largely independent of amino acid composition.

Gamma crystallins of the human eye lens

“…suited for their role in the lens. The intramolecular packing involving the coming together of the N-terminal and C-terminal domains generates a compact monomeric globule of about 5 nm in size, i.e., Stokes radius of 2.13 nm [9], and a low frictional ratio of about 1.21 (only slightly above the value of 1.12 for a perfect compact smooth protein sphere), suggesting that they have low hydration and a low propensity for sticky interactions with solvent and perhaps, with other proteins [10]. These small globules, packed at such high concentrations in the lens (400-1000 mg/ml), are seen to exhibit short range order which is necessary for transparency, particularly in a crowded macromolecular environment as in the lens [11].…”

Functions of crystallins in and out of lens: Roles in elongated and post-mitotic cells

“…These four tryptophans are conserved in all human lens βγ-crystallin domains except for the quenching tryptophan in the second domain of βB2 and βA2, while in fish γM-crystallins, the pair can be absent from one or both domains (Chen et al, 2013; Zhao et al, 2013). The Ciona calcium-bound domain has both corner and quenching tryptophans, suggesting that the pair may have an ancestral function, while in βγ-domains within AIM1 and CRYBG3, the quenching tryptophan is only moderately conserved, and it is absent in the sponge domains.…”

“…In solution, all γ-crystallins exhibit low frictional ratios and partial specific volumes compared with other proteins (Chen et al, 2013). A survey of the amino acid composition of all proteins found that γ-crystallins have unusually high contents of residues with higher refractive index increments which may make a significant contribution to increased refractive power of the lens (Zhao et al, 2011).…”

“…In contrast, the major human γ-crystallins of the denser lens core, γC- and γD-crystallins, have Arg/Lys ratios of 20/3 and 21/1, respectively, while in Danio rerio γM2b- and γM7-crystallins the ratios are 21/3 and 20/2 respectively. The extremely high RI in the core of fish lenses may also benefit from γM-crystallins having very large numbers of methionines on the surface (Chen et al, 2013; Mahler et al, 2013; Zhao et al, 2013). The presence of these residues is associated with extremes in the hydrodynamic properties of compactness and low frictional ratio which are common to all γ-crystallins but particularly prominent in γM-crystallins of fish lenses.…”

“…The association of microbial superfamily βγ-crystallin members (and some sHsps) with structures such as spore coats also hints at a possible role in maintaining a low water-content, an attribute that could certainly fit some βγ-crystallins for a role in the high protein concentration lens (Chen et al, 2013; Zhao et al, 2013). However this role would seem to be dependent on the presence of high enough concentrations of βγ-crystallins to be able to influence overall water content.…”