Solution properties of γ-crystallins: hydration of fish and mammal γ-crystallins

3D Puzzle at the Nanoscale-How do RNA Viruses Self-Assemble their Capsids into Perfectly Ordered Structures

The phenomenon of RNA virus self-organization, first observed in the mid-20th century in tobacco mosaic virus, is the subject of extensive research. Efforts to comprehend this process intensify due to its potential for producing vaccines or antiviral compounds as well as nanocarriers and nanotemplates. However, direct observation of the self-assembly is hindered by its prevalence within infected host cells. One of the approaches involves in vitro and in silico research using model viruses featuring a ssRNA(+) genome enclosed within a capsid made up of a single type protein. While various pathways are proposed based on these studies, their relevance in vivo remains uncertain. On the other hand, the development of advanced microscopic methods provide insights into the events within living cells, where following viral infection, specialized compartments form to facilitate the creation of nascent virions. Intriguingly, a growing body of evidence indicates that the primary function of packaging signals in viral RNA is to effectively initiate the virion self-assembly. This is in contrast to earlier opinions suggesting a role in marking RNA for encapsidation. Another noteworthy observation is that many viruses undergo self-assembly within membraneless liquid organelles, which are specifically induced by viral proteins.

The Functional Significance of High Cysteine Content in Eye Lens γ-Crystallins

Cataract disease is strongly associated with progressively accumulating oxidative damage to the extremely long-lived crystallin proteins of the lens. Cysteine oxidation affects crystallin folding, interactions, and light-scattering aggregation especially strongly due to the formation of disulfide bridges. Minimizing crystallin aggregation is crucial for lifelong lens transparency, so one might expect the ubiquitous lens crystallin superfamilies (α and βγ) to contain little cysteine. Yet, the Cys content of γ-crystallins is well above the average for human proteins. We review literature relevant to this longstanding puzzle and take advantage of expanding genomic databases and improved machine learning tools for protein structure prediction to investigate it further. We observe remarkably low Cys conservation in the βγ-crystallin superfamily; however, in γ-crystallin, the spatial positioning of Cys residues is clearly fine-tuned by evolution. We propose that the requirements of long-term lens transparency and high lens optical power impose competing evolutionary pressures on lens βγ-crystallins, leading to distinct adaptations: high Cys content in γ-crystallins but low in βB-crystallins. Aquatic species need more powerful lenses than terrestrial ones, which explains the high methionine content of many fish γ- (and even β-) crystallins. Finally, we discuss synergies between sulfur-containing and aromatic residues in crystallins and suggest future experimental directions.

Transcriptomic evidence for visual adaptation during the aquatic to terrestrial metamorphosis in leopard frogs

BACKGROUND

Differences in morphology, ecology, and behavior through ontogeny can result in opposing selective pressures at different life stages. Most animals, however, transition through two or more distinct phenotypic phases, which is hypothesized to allow each life stage to adapt more freely to its ecological niche. How this applies to sensory systems, and in particular how sensory systems adapt across life stages at the molecular level, is not well understood. Here, we used whole-eye transcriptomes to investigate differences in gene expression between tadpole and juvenile southern leopard frogs (Lithobates sphenocephalus), which rely on vision in aquatic and terrestrial light environments, respectively. Because visual physiology changes with light levels, we also tested the effect of light and dark exposure.

RESULTS

We found 42% of genes were differentially expressed in the eyes of tadpoles versus juveniles and 5% for light/dark exposure. Analyses targeting a curated subset of visual genes revealed significant differential expression of genes that control aspects of visual function and development, including spectral sensitivity and lens composition. Finally, microspectrophotometry of photoreceptors confirmed shifts in spectral sensitivity predicted by the expression results, consistent with adaptation to distinct light environments.

CONCLUSIONS

Overall, we identified extensive expression-level differences in the eyes of tadpoles and juveniles related to observed morphological and physiological changes through metamorphosis and corresponding adaptive shifts to improve vision in the distinct aquatic and terrestrial light environments these frogs inhabit during their life cycle. More broadly, these results suggest that decoupling of gene expression can mediate the opposing selection pressures experienced by organisms with complex life cycles that inhabit different environmental conditions throughout ontogeny.

Acquired disorder and asymmetry in a domain-swapped model for γ-crystallin aggregation

Misfolding and aggregation of proteins occur in many pathological states. Because of the inherent disorder involved, these processes are difficult to study. We attempted to capture aggregation intermediates of γ S-crystallin, a highly stable, internally symmetrical monomeric protein, by crystallization under mildly acidic and oxidizing conditions. Here we describe novel oligomerization through strained domain-swapping and partial intermolecular disulfide formation. This forms an octamer built from asymmetric tetramers, each of which comprises an asymmetric pair of twisted, domain-swapped dimers. Each tetramer shows patterns of acquired disorder among subunits, ranging from local loss of secondary structure to regions of intrinsic disorder. The octamer ring is tied together by partial intermolecular disulfide bonds, which may contribute to strain and disorder in the octamer. Oligomerization in this structure is self-limited by the distorted octamer ring. In a more heterogeneous environment, the disordered regions could serve as seeds for cascading interactions with other proteins. Indeed, solubilized protein from crystals retain many features observed in the crystal and are prone to further oligomerization and precipitation. This structure illustrates modes of loss of organized structure and aggregation that are relevant for cataract and for other disorders involving deposition of formerly well-folded proteins.

Adenosine triphosphate energy-independently controls protein homeostasis with unique structure and diverse mechanisms

Proteins function in the crowded cellular environments with high salt concentrations, thus facing tremendous challenges of misfolding/aggregation which represents a pathological hallmark of aging and an increasing spectrum of human diseases. Recently, intrinsically disordered regions (IDRs) were recognized to drive liquid-liquid phase separation (LLPS), a common principle for organizing cellular membraneless organelles (MLOs). ATP, the universal energy currency for all living cells, mysteriously has concentrations of 2-12 mM, much higher than required for its previously-known functions. Only recently, ATP was decoded to behave as a biological hydrotrope to inhibit protein LLPS and aggregation at mM. We further revealed that ATP also acts as a bivalent binder, which not only biphasically modulates LLPS driven by IDRs of human and viral proteins, but also bind to the conserved nucleic-acid-binding surfaces of the folded proteins. Most unexpectedly, ATP appears to act as a hydration mediator to antagonize the crowding-induced destabilization as well as to enhance folding of proteins without significant binding. Here, this review focuses on summarizing the results of these biophysical studies and discussing their implications in an evolutionary context. By linking triphosphate with unique hydration property to adenosine, ATP appears to couple the ability for establishing hydrophobic, π-π, π-cation and electrostatic interactions to the capacity in mediating hydration of proteins, which is at the heart of folding, dynamics, stability, phase separation and aggregation. Consequently, ATP acquired a category of functions at ~mM to energy-independently control protein homeostasis with diverse mechanisms, thus implying a link between cellular ATP concentrations and protein-aggregation diseases.

Genetic Adaptations in Mudskipper and Tetrapod Give Insights into Their Convergent Water-to-Land Transition

Water-to-land transition has been independently evolved in multiple vertebrate lineages including the most recent common ancestor of tetrapod and multiple fish clades, and among them, mudskippers uniquely adapted to the mudflat. Even though physiological and morphological adaptation of mudskippers is thought to resemble that of the ancestral tetrapod, it is unclear if they share genome-wide evolutionary signatures. To detect potential signatures of positive selection in mudskipper and tetrapods, we analyzed 4118 singleton orthologues of terrestrial tetrapods, coelacanth, mudskipper, and fully aquatic fishes. Among positively selected genes identified in mudskipper and tetrapod lineages, genes involved in immune responses, mitochondrial oxidative phosphorylation, and kidney development were detected. On the other hand, tetrapod-specific and mudskipper-specific positively selected genes were functionally enriched for DNA repair processes, which could be associated with higher exposure to UV light. We also performed gene family analysis and discovered convergent contraction of eight gene families, including βγ-crystallin coding genes in both tetrapod and mudskipper lineages. Findings of this study suggest the similar genetic adaptation against environmental constraints between the ancient tetrapod and mudskippers for their land adaptation.

ATP differentially antagonizes the crowding-induced destabilization of human γS-crystallin and its four cataract-causing mutants

αβγ-crystallins account for ∼90% of ocular proteins in lens with concentrations ≥400 mg/ml, which has to be soluble for the whole life-span and their aggregation results in cataract. So far, four cataract-causing mutants G18V, D26G, S39C and V42 M have been identified for human γS-crystallin. Mysteriously, lens maintains ATP concentrations of 3-7 mM despite being a metabolically-quiescent organ. Here by DSF and NMR, we characterized the binding of ATP to three cataract-causing mutants of human γS-crystallin as well as its effect on the solution conformations and thermal stability. The results together decode several novel findings: 1) ATP shows no detectable binding to WT and mutants, as well as no significant alternation of their conformations even at molar ratio of 1:200.2) Cataract-causing mutants show distinctive patterns of the crowding-induced destabilization. 3) ATP differentially antagonizes their crowding-induced destabilization. Our studies suggest that the crowding-induced destabilization of human γS-crystallin is also critically dependent of the hydration shell which could be differentially altered by four mutations. Most unexpectedly, ATP acts as an effective mediator for the protein hydration shell to antagonize the crowding-induced destabilization.

Cataract-causing G18V eliminates the antagonization by ATP against the crowding-induced destabilization of human γS-crystallin

In lens, ∼90% of ocular proteins are αβγ-crystallins with concentrations ≥400 mg/ml, which need to remain soluble for the whole life-span and their aggregation leads to cataract. The G18V mutation of human γS-crystallin causes hereditary childhood-onset cortical cataract. Mysteriously, despite being a metabolically-quiescent organ, lens maintains ATP concentrations of 3-7 mM. Very recently, we found that ATP has no significant binding to γS-crystallin as well as no alternation of its conformation. Nevertheless, ATP antagonizes the crowding-induced destabilization of γS-crystallin even at 1:1, most likely by interacting with the hydration shell. Here by DSF and NMR, we characterized the effect of ATP on binding, conformation, stability of G18V γS-crystallin and its interactions with α-crystallin. The results reveal: 1) G18V significantly accelerates the crowding-induced destabilization with Tm of 67 °C reduced to 50.5 °C at 1 mM. 2) Most unexpectedly, G18V almost completely eliminates the antagonizing effect of ATP against the crowding-induced destabilization. 3) ATP shows no significant effect on the interactions of α-crystallin with both WT and G18V γS-crystallin. Results together decode for the first time that G18V causes cataract not only by accelerating the crowding-induced destabilization, but also by eliminating the antagonizing effect of ATP against the crowding-induced destabilization.

Function and Aggregation in Structural Eye Lens Crystallins

Crystallins are transparent, refractive proteins that contribute to the focusing power of the vertebrate eye lens. These proteins are extremely soluble and resist aggregation for decades, even under crowded conditions. Crystallins have evolved to avoid strong interprotein interactions and have unusual hydration properties. Crystallin aggregation resulting from mutation, damage, or aging can lead to cataract, a disease state characterized by opacity of the lens.Different aggregation mechanisms can occur, following multiple pathways and leading to aggregates with varied morphologies. Studies of variant proteins found in individuals with childhood-onset cataract have provided insight into the molecular factors underlying crystallin stability and solubility. Modulation of exposed hydrophobic surface is critical, as is preventing specific intermolecular interactions that could provide nucleation sites for aggregation. Biophysical measurements and structural biology techniques are beginning to provide a detailed picture of how crystallins crowd into the lens, providing high refractivity while avoiding excessively tight binding that would lead to aggregation.Despite the central biological importance of refractivity, relatively few experimental measurements have been made for lens crystallins. Our work and that of others have shown that hydration is important to the high refractive index of crystallin proteins, as are interactions between pairs of aromatic residues and potentially other specific structural features.This Account describes our efforts to understand both the functional and disease states of vertebrate eye lens crystallins, particularly the γ-crystallins. We use a variety of biophysical techniques, notably NMR spectroscopy, to investigate crystallin stability and solubility. In the first section, we describe efforts to understand the relative stability and aggregation propensity of different γS-crystallin variants. The second section focuses on interactions of these proteins with the holdase chaperone αB-crystallin. The third, fourth, and fifth sections explore different modes of aggregation available to crystallin proteins, and the final section highlights the importance of refractive index and the sometimes conflicting demands of selection for refractivity and solubility.

ATP antagonizes the crowding-induced destabilization of the human eye-lens protein γS-crystallin

In lens, αβγ-crystallins accounting for ∼90% of ocular proteins with concentrations >400 mg/ml need to remain soluble for the whole life-span and their aggregation can lead to cataract. Mysteriously, despite being a metabolically-quiescent organ, lens maintains ATP concentrations of 3-7 mM. Very recently, ATP was proposed to hydrotropically prevent aggregation of crystallins but the mechanism remains unexplored. Here by NMR, DLS and DSF, we characterized the association, thermal stability and conformation of the 178-residue human γS-crystallin at concentrations from 2 to 100 mg/ml in the absence and in the presence of ATP. Results together reveal for the first time that ATP does antagonize the crowding-induced destabilization, although it has no significant binding to γS-crystallin as well as no alteration of its conformation. Therefore, ATP prevents aggregation in lens by a novel mechanism, thus rationalizing the fact that declining concentrations of ATP upon being aged is related to age-related cataractogenesis. To restore the normal concentrations of ATP in lens may represent a promising therapeutic strategy to treat aggregation-causing eye diseases.

Ultrafast Dynamics of Water-Protein Coupled Motions around the Surface of Eye Crystallin

Water dynamics on the protein surface mediate both protein structure and function. However, many questions remain about the role of the protein hydration layers in protein fluctuations and how the dynamics of these layers relate to specific protein properties. The fish eye lens protein γM7-crystallin (γM7) is found in vivo at extremely high concentrations nearing the packing limit, corresponding to only a few water layers between adjacent proteins. In this study, we conducted a site-specific probing of hydration water motions and side-chain dynamics at nine selected sites around the surface of γM7 using a tryptophan scan with femtosecond spectroscopy and NMR nuclear spin relaxation (NSR). We observed correlated fluctuations between hydration water and protein side chains on the time scales of a few picoseconds and hundreds of picoseconds, corresponding to local reorientations and network restructuring, respectively. These motions are heterogeneous over the protein surface and relate to the various steric and chemical properties of the local protein environment. Overall, we found that γM7 has relatively slower water dynamics within the hydration shell than a similar β-sheet protein, which may contribute to the high packing limit of this unique protein.

Crystallins and Their Complexes

The crystallins (α, β and γ), major constituent proteins of eye lens fiber cells play their critical role in maintaining the transparency and refractive index of the lens. Under different stress factors and with aging, β- and γ-crystallins start to unfold partially leading to their aggregation. Protein aggregation in lens basically enhances light scattering and causes the vision problem, commonly known as cataract. α-crystallin as a molecular chaperone forms complexes with its substrates (β- and γ-crystallins) to prevent such aggregation. In this chapter, the structural features of β- and γ-crystallins have been discussed. Detailed structural information linked with the high stability of γC-, γD- and γS-crystallins have been incorporated. The nature of homologous and heterologous interactions among crystallins has been deciphered, which are involved in their molecular association and complex formation.

Abundant Perithecial Protein (APP) from Neurospora is a primitive functional analog of ocular crystallins

The eye arose during the Cambrian explosion from pre-existing proteins that would have been recruited for the formation of the specialized components of this organ, such as the transparent lens. Proteins suitable for the role of lens crystallins would need to possess unusual physical properties and the study of such earliest analogs of ocular crystallins would add to our understanding of the nature of recruitment of proteins as lens/corneal crystallins. We show that the Abundant Perithecial Protein (APP) of the fungi Neurospora and Sordaria fulfils the criteria for an early crystallin analog. The perithecia in these fungal species are phototropic, and APP accumulates at a high concentration in the neck of the pitcher-shaped perithecium. Spores are formed at the base of the perithecium, and light contributes to their maturation. The hydrodynamic properties of APP appear to exclude dimer formation or aggregation at high protein concentrations. APP is also deficient in Ca²⁺ binding, a property seen in its close homolog, the calcium-binding cell adhesion molecule (DdCAD-1) from Dictyostelium discoidum. Comparable to crystallins, APP occurs in high concentrations and seems to have dispensed with Ca²⁺ binding in exchange for greater stability. These crystallin-like attributes of APP lead us to demonstrate that it is a primitive form of ocular crystallins.

Structure of G57W mutant of human γS-crystallin and its involvement in cataract formation

A recently identified mutant of human γS-crystallin, G57W is associated with dominant congenital cataracts, the familial determinate of childhood blindness worldwide. To investigate the structural and functional changes that mediate the effect of this cataract-related mutant to compromise eye lens transparency and cause lens opacification in children, we recently reported complete sequence-specific resonance assignments of γS-G57W using a suite of heteronuclear NMR experiments. As a follow up, we have determined the 3D structure of γS-G57W and studied its conformational dynamics by solution NMR spectroscopy. Our structural dynamics results reveal greater flexibility of the N-terminal domain, which undergoes site-specific structural changes to accommodate W57, than its C-terminal counterpart. Our structural inferences that the unusual solvent exposure of W57 is associated with rearrangement of the N-terminal domain suggest an efficient pathway for increased aggregation in γS-G57W and illuminates the molecular dynamics underlying cataractogenic aggregation of lens crystallins in particular and aggregation of proteins in general.

Measuring Ultra-Weak Protein Self-Association by Non-ideal Sedimentation Velocity

Ultra-weak self-association can govern the macroscopic solution behavior of concentrated macromolecular solutions ranging from food products to pharmaceutical formulations and the cytosol. For example, it can promote dynamic assembly of multi-protein signaling complexes, lead to intracellular liquid–liquid phase transitions, and seed crystallization or pathological aggregates. Unfortunately, weak self-association is technically extremely difficult to study, as it requires very high protein concentrations where short intermolecular distances cause strongly correlated particle motion. Additionally, protein samples near their solubility limit in vitro frequently show some degree of polydispersity. Here we exploit the strong mass-dependent separation of assemblies in the centrifugal field to study ultra-weak binding, using a sedimentation velocity technique that allows us to determine particle size distributions while accounting for colloidal hydrodynamic interactions and thermodynamic non-ideality (Chaturvedi, S. K.; et al. Nat. Commun.2018, 9, 4415; DOI: 10.1038/s41467-018-06902-x). We show that this approach, applied to self-associating proteins, can reveal a time-average association state for rapidly reversible self-associations from which the free energy of binding can be derived. The method is label-free and allows studying mid-sized proteins at millimolar protein concentrations in a wide range of solution conditions. We examine the performance of this method with hen egg lysozyme as a model system, reproducing its well-known ionic-strength-dependent weak self-association. The application to chicken γS-crystallin reveals weak monomer–dimer self-association with K_D = 24 mM, corresponding to a standard free energy change of approximately −9 kJ/mol, which is a large contribution to the delicate balance of forces ensuring eye lens transparency.

Controlling Liquid-Liquid Phase Separation of Cold-Adapted Crystallin Proteins from the Antarctic Toothfish

Liquid-liquid phase separation (LLPS) of proteins is important to a variety of biological processes both functional and deleterious, including the formation of membraneless organelles, molecular condensations that sequester or release molecules in response to stimuli, and the early stages of disease-related protein aggregation. In the protein-rich, crowded environment of the eye lens, LLPS manifests as cold cataract. We characterize the LLPS behavior of six structural γ-crystallins from the eye lens of the Antarctic toothfish Dissostichus mawsoni, whose intact lenses resist cold cataract in subzero waters. Phase separation of these proteins is not strongly correlated with thermal stability, aggregation propensity, or cross-species chaperone protection from heat denaturation. Instead, LLPS is driven by protein-protein interactions involving charged residues. The critical temperature of the phase transition can be tuned over a wide temperature range by selective substitution of surface residues, suggesting general principles for controlling this phenomenon, even in compactly folded proteins.

Protein refractive index increment is determined by conformation as well as composition

The refractive index gradient of the eye lens is controlled by the concentration and distribution of its component crystallin proteins, which are highly enriched in polarizable amino acids. The current understanding of the refractive index increment ([Formula: see text]) of proteins is described using an additive model wherein the refractivity and specific volume of each amino acid type contributes according to abundance in the primary sequence. Here we present experimental measurements of [Formula: see text] for crystallins from the human lens and those of aquatic animals under uniform solvent conditions. In all cases, the measured values are much higher than those predicted from primary sequence alone, suggesting that structural factors also contribute to protein refractive index.

Crystal Structure of Chicken γS-Crystallin Reveals Lattice Contacts with Implications for Function in the Lens and the Evolution of the βγ-Crystallins

Previous attempts to crystallize mammalian γS-crystallin were unsuccessful. Native L16 chicken γS crystallized avidly while the Q16 mutant did not. The x-ray structure for chicken γS at 2.3Å resolution shows the canonical structure of the superfamily plus a well-ordered N-arm aligned with a β-sheet of a neighboring N-domain. L16 is also in a lattice contact, partially shielded from solvent. Unexpectedly, the major lattice contact matches a conserved interface (QR) in the multimeric β-crystallins. QR shows little conservation of residue contacts, except for one between symmetry-related tyrosines, but molecular dipoles for the proteins with QR show striking similarities while other γ-crystallins differ. In γS, QR has few hydrophobic contacts and features a thin layer of tightly bound water. The free energy of QR is slightly repulsive and AUC confirms no dimerization in solution. The lattice contacts suggest how γcrystallins allow close packing without aggregation in the crowded environment of the lens.

In Vivo Brillouin Analysis of the Aging Crystalline Lens

Purpose

To analyze the age dependence of the longitudinal modulus of the crystalline lens in vivo using Brillouin scattering data in healthy subjects.

Methods

Brillouin scans were performed along the crystalline lens in 56 eyes from 30 healthy subjects aged from 19 to 63 years. Longitudinal elastic modulus was acquired along the sagittal axis of the lens with a transverse and axial resolution of 4 and 60 μm, respectively. The relative lens stiffness was computed, and correlations with age were analyzed.

Results

Brillouin axial profiles revealed nonuniform longitudinal modulus within the lens, increasing from a softer periphery toward a stiffer central plateau at all ages. The longitudinal modulus at the central plateau showed no age dependence in a range of 19 to 45 years and a slight decrease with age from 45 to 63 years. A significant intersubject variability was observed in an age-matched analysis. Importantly, the extent of the central stiff plateau region increased steadily over age from 19 to 63 years. The slope of change in Brillouin modulus in the peripheral regions were nearly age-invariant.

Conclusions

The adult human lens showed no measurable age-related increase in the peak longitudinal modulus. The expansion of the stiff central region of the lens is likely to be the major contributing factor to age-related lens stiffening. Brillouin microscopy may be useful in characterizing the crystalline lens for the optimization of surgical or pharmacological treatments aimed at restoring accommodative power.

Stability of Protein-Specific Hydration Shell on Crowding

We demonstrate that the effect of protein crowding is critically dependent on the stability of the protein's hydration shell, which can dramatically vary between different proteins. In the human eye lens, γS-crystallin (γS-WT) forms a densely packed transparent hydrogel with a high refractive index, making it an ideal system for studying the effects of protein crowding. A single point mutation generates the cataract-related variant γS-G18V, dramatically altering the optical properties of the eye lens. This system offers an opportunity to explore fundamental questions regarding the effect of protein crowding, using γS-WT and γS-G18V: (i) how do the diffusion dynamics of hydration water change as a function of protein crowding?; and (ii) upon hydrogel formation of γS-WT, has a dynamic transition occurred generating a single population of hydration water, or do populations of bulk and hydration water coexist? Using localized spin probes, we separately probe the local translational diffusivity of both surface hydration and interstitial water of γS-WT and γS-G18V in solution. Surprisingly, we find that under the influence of hydrogel formation at highly crowded γS-WT concentrations up to 500 mg/mL, the protein hydration shell remains remarkably dynamic, slowing by less than a factor of 2, if at all, compared to that in dilute protein solutions of ∼5 mg/mL. Upon self-crowding, the population of this robust surface hydration water increases, while a significant bulk-like water population coexists even at ∼500 mg/mL protein concentrations. In contrast, surface water of γS-G18V irreversibly dehydrates with moderate concentration increases or subtle alterations to the solution conditions, demonstrating that the effect of protein crowding is highly dependent on the stability of the protein-specific hydration shell. The core function of γS-crystallin in the eye lens may be precisely its capacity to preserve a robust hydration shell, whose stability is abolished by a single G18V mutation.

Distribution of selenium in zebrafish larvae after exposure to organic and inorganic selenium forms

Selenium is found to be highly concentrated in zebrafish pigment (melanin) containing tissues especially for the organic selenium treatments, with lower concentrations in eye lens, yolk sac and heart.

Gamma crystallins of the human eye lens

BACKGROUND

Protein crystallins co me in three types (α, β and γ) and are found predominantly in the eye, and particularly in the lens, where they are packed into a compact, plastic, elastic, and transparent globule of proper refractive power range that aids in focusing incoming light on to the retina. Of these, the γ-crystallins are found largely in the nuclear region of the lens at very high concentrations (>400 mg/ml). The connection between their structure and inter-molecular interactions and lens transparency is an issue of particular interest.

SCOPE OF REVIEW

We review the origin and phylogeny of the gamma crystallins, their special structure involving the use of Greek key supersecondary structural motif, and how they aid in offering the appropriate refractive index gradient, intermolecular short range attractive interactions (aiding in packing them into a transparent ball), the role that several of the constituent amino acid residues play in this process, the thermodynamic and kinetic stability and how even single point mutations can upset this delicate balance and lead to intermolecular aggregation, forming light-scattering particles which compromise transparency. We cite several examples of this, and illustrate this by cloning, expressing, isolating and comparing the properties of the mutant protein S39C of human γS-crystallin (associated with congenital cataract-microcornea), with those of the wild type molecule. In addition, we note that human γ-crystallins are also present in other parts of the eye (e.g., retina), where their functions are yet to be understood.

MAJOR CONCLUSIONS

There are several 'crucial' residues in and around the Greek key motifs which are essential to maintain the compact architecture of the crystallin molecules. We find that a mutation that replaces even one of these residues can lead to reduction in solubility, formation of light-scattering particles and loss of transparency in the molecular assembly.

GENERAL SIGNIFICANCE

Such a molecular understanding of the process helps us construct the continuum of genotype-molecular structural phenotype-clinical (pathological) phenotype. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

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

The vertebrate lens evolved to collect light and focus it onto the retina. In development, the lens grows through massive elongation of epithelial cells possibly recapitulating the evolutionary origins of the lens. The refractive index of the lens is largely dependent on high concentrations of soluble proteins called crystallins. All vertebrate lenses share a common set of crystallins from two superfamilies (although other lineage specific crystallins exist). The α-crystallins are small heat shock proteins while the β- and γ-crystallins belong to a superfamily that contains structural proteins of uncertain function. The crystallins are expressed at very high levels in lens but are also found at lower levels in other cells, particularly in retina and brain. All these proteins have plausible connections to maintenance of cytoplasmic order and chaperoning of the complex molecular machines involved in the architecture and function of cells, particularly elongated and post-mitotic cells. They may represent a suite of proteins that help maintain homeostasis in such cells that are at risk from stress or from the accumulated insults of aging.

The βγ-crystallins: native state stability and pathways to aggregation

The βγ-crystallins are among the most stable and long-lived proteins in the human body. With increasing age, however, they transform to high molecular weight light-scattering aggregates, resulting in cataracts. This occurs despite the presence in the lens of high concentrations of the a-crystallin chaperones. Aggregation of crystallins can be induced in vitro by a variety of stresses, including acidic pH, ultraviolet light, oxidative damage, heating or freezing, and specific amino acid substitutions. Accumulating evidence points to the existence of specific biochemical pathways of protein: protein interaction and polymerization. We review the methods used for studying crystallin stability and aggregation and discuss the sometimes counterintuitive relationships between factors that favor native state stability and those that favor non-native aggregation. We discuss the behavior of βγ-crystallins in mixtures and their chaperone ability; the consequences of missense mutations and covalent damage to the side-chains; and the evolutionary strategies that have shaped these proteins. Efforts are ongoing to reveal the nature of cataractous crystallin aggregates and understand the mechanisms of aggregation in the context of key models of protein polymerization: amyloid, native-state, and domain-swapped. Such mechanistic understanding is likely to be of value for the development of therapeutic interventions and draw attention to unanswered questions about the relationship between a protein's native state stability and its transformation to an aggregated state.

Measurement of the temperature of the resting rotor in analytical ultracentrifugation

Accurate measurements of rotor temperature are critical for the interpretation of hydrodynamic parameters in analytical ultracentrifugation. We have recently developed methods for a more accurate determination of the temperature of a spinning rotor using iButton temperature loggers. Here we report that the temperature measured with the iButton on the counterbalance of a resting rotor, following thermal equilibration under high vacuum, closely corresponded to the temperature of the spinning rotor with a precision better than 0.2°C. This strategy offers an inexpensive and straightforward approach to monitor the accuracy of the temperature calibration and determine corrective temperature offsets.

Solution properties of γ-crystallins: compact structure and low frictional ratio are conserved properties of diverse γ-crystallins

γ-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, γ-crystallins have high contents of sulfur-containing methionine and cysteine, but this reaches extremes in fish γM-crystallins with up to 15% Met. In addition, fish γM-crystallins do not conserve the paired tryptophan residues found in each domain in mammalian γ-crystallins and in the related β-crystallins. To gain insight into important, evolutionarily conserved properties and functionality of γ-crystallins, zebrafish (Danio rerio) γM2b and γM7 were compared with mouse γS and human γD. For all four proteins, far UV CD spectra showed the expected β-sheet secondary structure. Like the mammalian proteins, γM7 was highly soluble but γM2b 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 αB-crystallin to retard aggregation of the heat denatured proteins also showed differences. However, when solution behavior was investigated by sedimentation velocity experiments, the diverse γ-crystallins showed remarkably similar hydrodynamic properties with low frictional ratios and partial specific volumes. The solution behavior of γ-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.