The role of macromolecular crowding in the evolution of lens crystallins with high molecular refractive index

Abstract

Crystallins are present in the lens at extremely high concentrations in order to provide transparency and generate a high refractive power of the lens. The crystallin families prevalent in the highest density lens tissues are γ crystallins in vertebrates and S crystallins in cephalopods. In parallel evolution, both have evolved molecular refractive index increments 5 – 10 % above those of most proteins. Although this is a small increase, it is statistically very significant and can be achieved only by very unusual amino acid compositions. In contrast, such a molecular adaptation to aid in the refractive function of the lens did not occur in crystallins that are preferentially located in lower density lens tissues, such as vertebrate α crystallin and taxon specific crystallins. In the current work, we apply a model of non-interacting hard spheres to examine the thermodynamic contributions of volume exclusion at lenticular protein concentrations. We show that the small concentration decrease afforded by the higher molecular refractive index increment of crystallins can amplify nonlinearly to produce order of magnitude differences in chemical activities, and lead to reduced osmotic pressure and the reduced propensity for protein aggregation. Quantitatively, this amplification sets in only at protein concentrations as high as those found in hard lenses or the nucleus of soft lenses, in good correspondence to the observed crystalline properties in different tissues and different species. This suggests that volume exclusion effects provide the evolutionary driving force for the unusual refractive properties and the unusual amino acid compositions of γ crystallins and S crystallins.

The Molecular Refractive Function of Lens γ-Crystallins

“…Specifically, we have analyzed the thermodynamic consequences of small reductions in protein concentration that become possible from elevated crystallin refractive index increments. We found that, in the most dense lens tissues, even only 5 – 10% higher dn/dc values can, in a non-linear fashion, translate into very significant reductions in the osmotic pressure and in the propensity for aggregation 46 . We hypothesized that this can provide a driving force for the evolution of high refractive index increment crystallins.…”

“…In a related work we have explored this question from a theoretical perspective 46 . We have shown with a model of macromolecular crowding of non-interacting hard spheres, that in the extreme environment of fiber cells (which in the center of high density lenses may have total protein concentrations 500 mg/ml and above) even a small reduction of protein concentration afforded by the higher macromolecular dn/dc can have profound thermodynamic consequences.…”

“…Previously, we have applied this to the entire set of predicted proteins of different species, and found that they are normally distributed and fall in a remarkably narrow range 43 , but with exception of a few protein classes including crystallins. Second, with a biophysical model of macromolecular crowding in the lens tissue, we have explored the possible relevance of variations of molecular refractive indices 46 . Specifically, we have analyzed the thermodynamic consequences of small reductions in protein concentration that become possible from elevated crystallin refractive index increments.…”

Distribution of the Crystalline Lens Power In Vivo as a Function of Age

“…1 This gradient index originates from a difference in concentration of crystallins located in the cytoplasm of lens fiber cells, with high concentrations in the lens nucleus and lower concentrations in the lens cortex, together with a gradient in distribution of crystallin types. 2 With age, the human lens grows by a process of epithelial cell division and the formation of differentiated fiber cells that alters the dimensions of the lens, including mass, thickness, radii of curvature, and refractive index. 3,4 Since the total lenticular power depends on all these parameters, 5,6 this results in a gradual change in lenticular power as well.…”

Structure and Dynamics of the Fish Eye Lens Protein, γM7-Crystallin

“…12,13 Proteins with a high content of amino acids with high refractive index increments can achieve higher overall refractive power at relatively lower protein concentration and reduced osmotic pressure. 14 Indeed, other proteins with exceptionally high refractive index such as the S-crystallins from cephalopods 15,16 and reflectins from squid 17,18 are also highly enriched with methionine and contain a limited content of residues with low refractive index increment. 12 …”