Spectroscopic investigations of bovine lens crystallins. 1. Circular dichroism and intrinsic fluorescence

Structural insight on the liquid silk from the middle silk gland of non-mulberry silkworm Antheraea assamensis

This study highlights the preliminary characterization of liquid silk from the middle silk gland (MSG) along with the in-silico analysis of the sericin protein of a less explored non mulberry silkworm Antheraea assamensis which is endemic to the North Eastern region of India. Various biophysical methods have been applied to elucidate the conformational patterns of the liquid silk present inside the MSG without removing the sericin layer. This will help us to know the actual features of the in vivo transitional status of the silk in the MSG which travel towards the anterior silk gland (ASG) prior to spinning. The SDS PAGE analysis represented the existence of the both fibroin and sericin bands in the sample. The structural pattern of the MSG liquid silk as revealed by various methods denoted the occurrence of β-sheet component along with some random coil and β-turn components which in turn suggests the transitional state of the liquid silk attributed to the existence of both the crystalline and amorphous contents. The thermo gravimetric study and the aggregation behavior analysis results proposed the occurrence of intermolecular hydrogen bonding between the sericin and fibroin in the MSG. This study will sensitize the better understanding of the behavior of the liquid silk in the MSG of non-mulberry silkworm A. assamensis and will open avenues for various application-based studies of this silk.Communicated by Ramaswamy H. Sarma.

H101G Mutation in Rat Lens αB-Crystallin Alters Chaperone Activity and Divalent Metal Ion Binding

BACKGROUND

The molecular chaperone function of αB-crystallins is heavily involved in maintaining lens transparency and the development of cataracts.

OBJECTIVE

To study whether divalent metal ion binding improves the stability and αB-crystallin chaperone activity.

METHOD

In this study, we have developed an H101G αB-crystallin mutant and compared the surface hydrophobicity, chaperone activity, and secondary and tertiary structure with the wild type in the presence and absence of metal ions.

RESULTS

Substitution of His101 with glycine resulted in structural and functional changes. Spectral analysis and chaperone-like activity assays showed that substitution of glycine resulted in a higher percentage of random coils, increased hydrophobicity, and 22±2% higher chaperone-like activity. Whereas in the presence of the Cu²⁺ ion, H101G exhibited 32±1% less chaperone-like activity compared to the wild type.

CONCLUSION

Cu²⁺ has been reported to enhance the chaperone-like activity of lens α-crystallin. Our results indicate that H101 is the predominant Cu²⁺binding site, and the mutation resulted in a partial unfolding that impaired the binding of Cu²⁺ to H101 residue. In conclusion, this study further helps to understand the important binding site for Cu²⁺ to αB-crystallin.

Structural and functional alteration of human αA-crystallin after exposure to full spectrum solar radiation and preventive role of lens antioxidants

The chronically exposure of eye lenses to ultra violet and visible light of the solar radiation is an important risk factor for development of the senile cataract diseases. Various photosensitizer molecules including riboflavin (RF) play a significant role in photo-oxidative damages of lens proteins underlying development of opacity in the lenticular tissues. In the current study, RF-mediated photo-oxidation of human αA-crystallin (αA-Cry) was assessed using SDS-PAGE analysis, dynamic light scattering and other spectroscopic assessments. The RF-photosensitized reactions led to non-disulfide covalent cross-linking, oligomerization and significant structural changes in αA-Cry. The photo-damaging of αA-Cry under solar radiation was also accompanied by the reduction in both Trp and Tyr fluorescence intensities which followed by the formation of new photosensitizer chromophores. The solvent exposed hydrophobic patches, secondary structures and chaperone-like activity of αA-Cry were significantly altered after exposure to the solar radiation in the presence of RF. Although glutathione and ascorbate were capable to partially protect the photo-induced structural damages of human αA-Cry, they also disrupted its chaperone function when co-exposed with this protein to the solar radiation. Also, the most promising data were obtained with cysteine which its availability in the lenticular tissues is a rate limiting factor for the biosynthesis of glutathione. Overall our results suggest that glutathione and ascorbate, as the major anti-oxidant compounds within lenticular tissues, demonstrate controversial effect on structure and chaperone-like activity of human αA-Cry. Elucidation of this effect may demand further experiments.

Protective effects of carnosine on dehydroascorbate-induced structural alteration and opacity of lens crystallins: important implications of carnosine pleiotropic functions to combat cataractogenesis

The high level of dehydroascorbic acid (DHA) in the lenticular tissue is an important risk factor for the development of age-related cataracts. In this study, the effects of DHA on structure and function of lens crystallins were studied in the presence of carnosine using gel mobility shift assay, different spectroscopic techniques, and lens culture analysis. The DHA-induced unfolding and aggregation of lens proteins were largely prevented by this endogenous dipeptide. The ability of carnosine to preserve native protein structure upon exposure to DHA suggests the essential role of this dipeptide in prevention of the senile cataract development. Although the DHA-modified α-crystallin was characterized by altered chaperone activity, functionality of this protein was significantly restored in the presence of carnosine. The increased proteolytic instability of DHA-modified lens proteins was also attenuated in the presence of carnosine. Furthermore, the assessment of lens culture suggested that DHA can induce significant lens opacity which can be prevented by carnosine. These observations can be explained by the pleiotropic functions of this endogenous and pharmaceutical compound, notably by its anti-glycation and anti-aggregation properties. In summary, our study suggests that carnosine may have therapeutic potential in preventing senile cataracts linked with the increased lenticular DHA generation, particularly under pathological conditions associated with the oxidative stress.

Organization of the extracellular portion of the macrophage galactose receptor: a trimeric cluster of simple binding sites for N-acetylgalactosamine

The properties of the human macrophage galactose receptor have been investigated. Specificity for N-acetylgalactosamine (GalNAc) residues with exposed 3- and 4-hydroxyl groups explains virtually all of the results obtained from a recently expanded array of synthetic glycans and is consistent with a model for the structure of the binding site. This simple interaction is sufficient to explain the ability of the receptor to bind to tumor-cell glycans bearing Tn and sialyl-Tn antigens, but not to more elaborate O-linked glycans that predominate on normal cells. This specificity also allows for binding of parasite glycans and screening of an array of bacterial outer membrane oligosaccharides confirms that the receptor binds to a subset of these structures with appropriately exposed GalNAc residues. A key feature of the receptor is the clustering of binding sites in the extracellular portion of the protein, which retains the trimeric structure observed in the cell membrane. Chemical crosslinking, gel filtration, circular dichroism analysis and differential scanning calorimetry demonstrate that this trimeric structure of the receptor is stabilized by an α-helical coiled coil that extends from the surface of the membrane to the globular carbohydrate-recognition domains. The helical neck domains form independent trimerization domains. Taken together, these results indicate that the macrophage galactose receptor shares many of the features of serum mannose-binding protein, in which clusters of monosaccharide-binding sites serve as detectors for a simple epitope that is not common on endogenous cell surface glycans but that is abundant on the surfaces of tumor cells and certain pathogens.

Inhibition of lens photodamage by UV-absorbing contact lenses

PURPOSE

To determine whether class 1 UV-blocking contact lenses protect against UVB radiation-induced damage in a human lens epithelial cell line (HLE B-3) and postmortem human lenses using a proteomics approach.

METHODS

HLE B-3 cells were exposed to 6.4 mW/cm(2) UVB radiation at 302 nm for 2 minutes (768 mJ/cm(2)) with or without covering by senofilcon A class 1 UV-blocking contact lenses or lotrafilcon A non-UV-blocking (lotrafilcon A has some UV-blocking ability, albeit minimal) contact lenses. Control cells were not exposed to UVB radiation. Four hours after treatment, cells were analyzed by two-dimensional difference gel electrophoresis and tandem mass spectrometry, and changes in protein abundance were quantified. F-actin and microtubule cytoskeletons were examined by fluorescence staining. In addition, human donor lenses were exposed to UVB radiation at 302 nm for 4 minutes (1536 mJ/cm(2)). Cortical and epithelial cell proteins were scraped from lens surfaces and subjected to the same protein analyses.

RESULTS

Senofilcon A lenses were beneficial for protecting HLE B-3 cells against UVB radiation-induced changes in caldesmon 1 isoform, lamin A/C transcript variant 1, DEAD (Asp-Glu-Ala-Asp) box polypeptide, β-actin, glyceraldehyde 3-phosphate dehydrogenase (G3PDH), annexin A2, triose phosphate isomerase, and ubiquitin B precursor. These contact lenses also prevented actin and microtubule cytoskeleton changes typically induced by UVB radiation. Conversely, non-UV-blocking contact lenses were not protective. UVB-irradiated human lenses showed marked reductions in αA-crystallin, αB-crystallin, aldehyde dehydrogenase 1, βS-crystallin, βB2-crystallin, and G3PDH, and UV-absorbing contact lenses significantly prevented these alterations.

CONCLUSIONS

Senofilcon A class 1 UV-blocking contact lenses largely prevented UVB-induced changes in protein abundance in lens epithelial cells and in human lenses.

Binding of heparin to metals

Heparin is a major prophylactic and treatment agent for thrombosis. Structurally, this anticoagulant is a polydisperse, highly negatively charged polysaccharide mixture that contains a variable density of sulfate group substituents per molecule. Previous study has shown that heparin molecules have a high affinity for a wide range of metal ions with varying oxidation states. However, reports in literature on binding of heparin to metals have investigated only a small sampling of heparin-metal ion interactions. Since interaction of heparin with fluid phase and cell surface macromolecules in vivo is dependent on the heparin structure when bound in a metal ion complex, a survey of the physical parameters for heparin binding to metals is imperative. Atomic absorption and spectrophotometry experiments were performed for metal quantification, and in this study, the relative values for affinity constants and number of binding sites for heparin binding to several alkaline, alkaline earth, main group, and transition metals in their most common oxidation states are reported. We found an overall trend for heparin-metal affinity to be Mn(2+) > Cu(2+) > Ca(2+) > Zn(2+) > Co(2+) > Na(+) > Mg(2+) > Fe(3+) > Ni(2+) > Al(3+)> Sr(2+), with the trend in N (b) being opposite compared with the K (a).

Bovine serum albumin nanoparticle promotes the stability of quercetin in simulated intestinal fluid

Quercetin (Que) is a flavonoid widely distributed in vegetables and fruits and exhibits strong antioxidant activity, but the poor stability of Que limits its function and application. The present study developed a nanoparticle (NP) using bovine serum albumin (BSA) as a matrix to encapsulate Que. The stability of encapsulated Que by BSA NP was tracked in a simulated intestinal fluid (SIF). The antioxidant activity of encapsulated Que was evaluated by the 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical scavenging assays. Furthermore, the stabilizing mechanism of Que by BSA NP was investigated, using scanning transmisson electron microscopy (STEM), dynamic light scattering (DLS), UV-vis, fluorescence spectrometry, and circular dichroism (CD). The results revealed that Que was effectively encapsulated by BSA and formed spherical NP (<10 nm). BSA NP not only promoted the stability of encapsulated Que but also kept the antioxidant activity of encapsulated Que. The driving forces for BSA-Que association were hydrophobic interaction and hydrogen bond, and the latter was involved in the mechanism of Que stabilization. This suggested that BSA NP could be a good carrier to deliver hydrophobic flavonols.

Identification of the first fluorescent alpha-amidoboronic acids that change fluorescent properties upon sugar binding

The first amidoboronic acids were identified that show significant fluorescent property changes upon binding with various carbohydrates.

Confocal fluorescence microscopy study of interaction between lens MIP26/AQP0 and crystallins in living cells

MIP26/AQP0 is the major lens fiber membrane protein and has been reported to interact with many other lens components including crystallins, lipid, and cytoskeletal proteins. Regarding crystallins, many previous reports indicate that MIP26/AQP0 interacts with either only alpha-crystallin or some specific gamma-crystallins. Considering the possibly important role of MIP26/AQP0 in the reduction of light scattering in the lenses, we have further investigated its interaction with crystallins using confocal fluorescence resonance energy transfer (FRET) microscopy. Specifically, we used MIP26 tagged with a green fluorescence protein (GFP) as a donor and a crystallin (alphaA-, alphaB-, betaB2-, or gammaC-crystallin) tagged with a red fluorescence protein (RFP) as an acceptor. The two plasmids were cotransfected to HeLa cells. After culture, laser scattering microscopy images were taken in each of the three channels: GFP, RFP, and FRET. The net FRET images were then obtained by removing the contribution of spectral bleed-through. The pixels of net FRET were normalized with those of GFP. The results show the presence of measurable interactions between MIP26 and all crystallins, with the extent of interactions decreasing from alphaA- and alphaB-crystallin to betaB2- and gammaC-crystallin. Competitive interaction study using untagged alphaA-crystallin shows decreased net FRET, indicating specificity of the interactions between MIP26 and alphaA-crystallin. We conclude that all crystallins interact with MIP26, the physiological significance of which may be a reduction in the difference of refractive index between membrane and cytoplasm.

Synthesis, evaluation, and computational studies of naphthalimide-based long-wavelength fluorescent boronic Acid reporters

Boronic acids that change fluorescence properties upon sugar binding are very useful for the synthesis of carbohydrate sensors. Along this line, boronic acids that fluoresce beyond 500 nm are especially useful. A series of boronic acid fluorescent reporter compounds based on the 4-amino-1,8-naphthalimide structure have been synthesized (1a-d) and evaluated under near physiological conditions. These compounds showed good water solubility and significant changes in fluorescence properties after binding with sugars, with the emission wavelength being at around 570 nm. Analogues in this series with different substitutions showed similar properties. We have also examined the mechanism of the observed fluorescence changes for these compounds.

Mechanism of small heat shock protein function in vivo: a knock-in mouse model demonstrates that the R49C mutation in alpha A-crystallin enhances protein insolubility and cell death

alphaA-crystallin (Cryaa/HSPB4) is a small heat shock protein and molecular chaperone that prevents nonspecific aggregation of denaturing proteins. Several point mutations in the alphaA-crystallin gene cause congenital human cataracts by unknown mechanisms. We took a novel approach to investigate the molecular mechanism of cataract formation in vivo by creating gene knock-in mice expressing the arginine 49 to cysteine mutation (R49C) in alphaA-crystallin (alphaA-R49C). This mutation has been linked with autosomal dominant hereditary cataracts in a four-generation Caucasian family. Homologous recombination in embryonic stem cells was performed using a plasmid containing the C to T transition in exon 1 of the cryaa gene. alphaA-R49C heterozygosity led to early cataracts characterized by nuclear opacities. Unexpectedly, alphaA-R49C homozygosity led to small eye phenotype and severe cataracts at birth. Wild type littermates did not show these abnormalities. Lens fiber cells of alphaA-R49C homozygous mice displayed an increase in cell death by apoptosis mediated by a 5-fold decrease in phosphorylated Bad, an anti-apoptotic protein, but an increase in Bcl-2 expression. However, proliferation measured by in vivo bromodeoxyuridine labeling did not decline. The alphaA-R49C heterozygous and homozygous knock-in lenses demonstrated an increase in insoluble alphaA-crystallin and alphaB-crystallin and a surprising increase in expression of cytoplasmic gamma-crystallin, whereas no changes in beta-crystallin were observed. Co-immunoprecipitation analysis showed increased interaction between alphaA-crystallin and lens substrate proteins in the heterozygous knock-in lenses. To our knowledge this is the first knock-in mouse model for a crystallin mutation causing hereditary human cataract and establishes that alphaA-R49C promotes protein insolubility and cell death in vivo.

Fluorescence resonance energy transfer study of subunit exchange in human lens crystallins and congenital cataract crystallin mutants

Lens alpha-crystallin is an oligomeric protein with a molecular mass of 500-1000 kDa and a polydispersed assembly. It consists of two types of subunits, alphaA and alphaB, each with a molecular mass of 20 kDa. The subunits also form homo-oligomers in some other tissues and in vitro. Their quaternary structures, which are dynamic and characterized by subunit exchange, have been studied by many techniques, including fluorescence resonance energy transfer (FRET) and mass spectrometry analysis. The proposed mechanism of subunit exchange has been either by dissociation/association of monomeric subunits or by rapid equilibrium between oligomers and suboligomers. To explore the nature of subunit exchange further, we performed additional FRET measurements and analyses using a fluorescent dye-labeled W9F alphaA-crystallin as the acceptor probe and Trp in other crystallins (wild-type and R116C alphaA, wild-type and R120G alphaB, wild-type and Q155* betaB2) as the donor probe and calculated the transfer efficiency, Förster distance, and average distance between two probes. The results indicate only slight decreased efficiency and increased distance between two probes for the R116C alphaA and R120G alphaB mutations despite conformational changes.

Role of the Conserved SRLFDQFFG Region of α-Crystallin, a Small Heat Shock Protein

Small heat shock proteins (sHsps) are necessary for several cellular functions and in stress tolerance. Most sHsps are oligomers; intersubunit interactions leading to changes in oligomeric structure and exposure of specific regions may modulate their functioning. Many sHsps, including alpha A- and alpha B-crystallin, contain a well conserved SRLFDQFFG sequence motif in the N-terminal region. Sequence-based prediction shows that it exhibits helical propensity with amphipathic character, suggesting that it plays a critical role in the structure and function of alpha-crystallins. In order to investigate the role of this motif in the structure and function of sHsps, we have made constructs deleting this sequence from alpha A- and alpha B-crystallin, overexpressed, purified, and studied these engineered proteins. Circular dichroism spectroscopic studies show changes in tertiary and secondary structure on deletion of the sequence. Glycerol density gradient centrifugation and dynamic light scattering studies show that the multimeric size of the mutant proteins is significantly reduced, indicating a role for this motif in higher order organization of the subunits. Both deletion mutants exhibit similar oligomeric size and increased chaperone-like activity. Urea-induced denaturation study shows that the SRLFDQFFG sequence contributes significantly to the structural stability. Fluorescence resonance energy transfer studies show that the rate of exchange of the subunits in the alpha Adel-crystallin oligomer is higher compared with that in the alpha A-crystallin oligomer, suggesting that this region contributes to the oligomer dynamics in addition to the higher order assembly and structural stability. Thus, our study shows that the SRLFDQFFG sequence is one of the critical motifs in structure-function regulation of alpha A- and alpha B-crystallin.

Decreased subunit exchange of heat-treated lens alpha A-crystallin

alpha A-Crystallin high-molecular-weight (HMW) aggregates were prepared by preheating at 80-90 degrees C and studied using spectroscopic measurements. Conformational differences were suggested based on data of increased bis-ANS (4,4(')-dianilino-1,1(')-binaphthalene-5,5(')-disulfonic acid) and ThT (thioflavin T) fluorescence as well as increased far-UV and decreased near-UV circular dichroism (CD). These results indicated that HMW aggregated alpha-crystallin was more hydrophobic than the native alpha-crystallin, possibly resulting from partial unfolding of alpha-crystallin. The two cysteines in alpha A-crystallin were mostly oxidized in HMW aggregates. The effects of HMW aggregation on the dynamic structure were studied with fluorescence resonance energy transfer; subunit exchange became slower. These results strongly suggest that HMW alpha A-crystallin aggregates result from exposure of buried beta-pleated sheets and increased hydrophobic interaction.

Human lens high-molecular-weight alpha-crystallin aggregates

Alpha-crystallin high-molecular-weight (HMW) aggregates can be formed in vitro by many mechanisms, but the mechanism of in vivo aggregation has not been clearly established. HMW and LMW (low-molecular-weight) alpha-crystallins were isolated from human lenses 50-60 years of age and some spectroscopic measurements were performed. Conformational differences were suggested based on data of increased bis-ANS (4,4'-dianilino-1,1'-binaphthalene-5, 5'-disulfonic acid) and ThT (thioflavin T) fluorescence as well as increased far-UV and decreased near-UV circular dichroism (CD). These results indicated that HMW alpha-crystallin was more hydrophobic than LMW alpha-crystallin, possibly resulting from partial unfolding of alpha-crystallin. On the other hand, the increased ThT fluorescence and far-UV CD intensities indicate that an increased amount of beta-sheet conformation was involved in aggregation. These data, along with little difference in chaperone-like activity between the LMW and HMW alpha-crystallins, strongly suggest that HMW alpha-crystallin aggregates resulted from partial unfolding and disassembling-reassembling of LMW alpha-crystallin caused by posttranslational modification rather than chaperone complex formation.

Functional and structural studies of alpha-crystallin from galactosemic rat lenses

Chaperone-like activity and structural changes of lens alpha-crystallin from rats fed with galactose at various time intervals have been studied using high-performance liquid chromatograph (HPLC), circular dichroism (CD), and 1-anilinonaphthalene-8-sulfonic acid (ANS) fluorescence emission. It was found that chaperone-like activity of alpha-crystallin from galactose-fed rats toward dithiothreitol (DTT)-induced insulin B aggregation started to decrease after 3 weeks and decreased significantly after 5 weeks. Consistent results were observed in lens morphology, and lens opacity slightly developed after 3 weeks and became obvious after 5 weeks. HPLC analysis for chaperone function showed that the formation of high molecular weight aggregates (HMWA) of alpha-/gamma-crystallins decreases with the increase of galactose-feeding time, revealing that chaperone-like activity is concomitant with the formation of HMWA. Circular dichroism results showed the reduction of beta-sheet structure and loss of microenvironment of aromatic-type amino acids for opaque lenses, indicating alpha-crystallin's secondary and tertiary structure changed with the development of the lens opacity. ANS binding site estimated by Klotz equation showed it is 1.5 times higher at room temperature and is 2.4 times higher at 58 degrees C for age-matched normal alpha-crystallin than for 5-week galactose-fed lens alpha-crystallin, indicating opaque lens alpha-crystallin loses the ability to assemble into an appropriately placed hydrophobic regions. The overall results accordingly indicated that galactose-induced cataractous alpha-crystallin has disordered structure, leading to the loss of its chaperone-like activity.

Reactivity of monoclonal antibodies against a tryptophan-riboflavin adduct toward irradiated and non-irradiated bovine-eye-lens protein fractions: an indicator of in vivo visible-light-mediated phototransformations

We describe here the reactivity toward the soluble protein of bovine eye lens of anti-tryptophan-riboflavin (anti-Trp-RF) adduct monoclonal antibodies, which recognize the hapten tryptophan-riboflavin generated by irradiation of a solution of bovine serum albumin in the presence of riboflavin. It is demonstrated that five different anti-Trp-RF adduct monoclonal antibodies, all belonging to the IgG1 isotype, react with the total soluble proteins of bovine eye lens. The components of the soluble protein are separated by Sephadex G-200 chromatography and the isolated fractions analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). All the separated protein fractions also react by a direct ELISA with the monoclonal antibodies; this reaction is more intense when the isolated fractions have been previously irradiated with visible light in the presence of riboflavin under an atmosphere of oxygen or nitrogen. Irradiation of the total soluble protein with visible light in the presence of riboflavin produces the appearance of new bands, corresponding to compounds of higher molecular weight. Riboflavin-sensitized irradiation of the protein fractions with visible light under an oxygen or nitrogen atmosphere is accompanied by a concomitant decrease of the tryptophan fluorescence. It is postulated that the action of visible light in the presence of either the endogenous riboflavin or its derivatives could be partly responsible for the protein aggregation observed during aging.

The function of alpha-crystallin in vision

The alpha-crystallins account for approximately one-third of the total soluble protein in the lens, contributing to its refractive power. In addition, alpha-crystallin also has a chaperone-like function and thus can bind unfolding lens proteins. Alpha B-crystallin is also found outside the lens, having an extensive tissue distribution. It is over-expressed in response to stresses of all kinds, where it is thought to serve a general protective function. Recently, it has been shown in humans that naturally occurring point mutations in the alpha-crystallins result in a deficit in chaperone-like function, and cause cataracts as well as a desmin-related myopathy. This review summarizes much of the past and current knowledge concerning the structure and functions of alpha-crystallin.

Spectral contribution of the individual tryptophan of alphaB-crystallin: a study by site-directed mutagenesis

There are two tryptophan residues in the lens alphaB-crystallin, Trp9 and Trp60. We prepared two Trp --> Phe substituted mutants, W9F and W60F, for use in a spectroscopic study. The two tryptophan residues contribute to Trp fluorescence and near-ultraviolet circular dichroism (UV CD) differently. The major difference in the near-UV CD is the contribution of 1La of Trp: it is positive in W60F but becomes negative in W9F. Further analysis of the near-UV CD shows an increased intensity in the region of 270-280 nm for W60F, suggesting that the Tyr48 is affected by the W60F mutation. It appears that Trp60 is located in a more rigid environment than Trp9, which agrees with a recent structural model in which Trp60 is in a beta-strand.

The formation of oxidatively induced high-molecular-weight aggregate of alpha-/gamma-crystallins

alpha-/gamma-Crystallin interactions under oxidation with ascorbate-FeCl3-EDTA-H2O2 followed by dialysis have been studied. A high-molecular-weight aggregate (HMWA) composed of alpha- and gamma-crystallin was observed for the mixture of the dialyzed alpha-crystallin and the oxidized gamma-crystallin through gel-filtration chromatography. This illustrates an interaction between alpha-crystallin and partially denatured gamma-crystallin induced by oxidation. No HMWA formation was observed under the condition without dialysis and/or with the addition of catalase to the oxidized gamma-crystallin prior to the addition of alpha-crystallin. More HMWA was formed by oxidized gamma-crystallin followed by the addition of alpha-crystallin than by simultaneous oxidation of both alpha- and gamma-crystallins. Conformational changes of alpha-crystallin during oxidation analyzed by circular dichroism spectra showed that oxidized alpha-crystallin can gradually be restored to an ordered structure through dialysis. The overall results imply that structural changes of both alpha- and gamma-crystallins and dialysis are required to form HMWA. The observation of this oxidatively induced chaperone/substrate complex suggests that an efficient chaperone-like protective action against oxidative insults may exist in vivo.

Conformational change of human lens insoluble alpha-crystallin

Human lens alpha-crystallin becomes progressively insoluble with age and is the major crystallin component in the water-insoluble (WI) fraction. The mechanism that causes the originally water-soluble (WS) alpha-crystallin to become insoluble is unknown. A conformational change by chemical modification may be the cause, but the nature of insolubility renders it impossible to study protein conformation in the WI fraction by most spectroscopic measurements. In the present study, alpha-crystallin in the WI fraction was extracted by urea and reconstituted to a folded protein by dialysis. The refolded urea-soluble (US) alpha-crystallin was compared with WS alpha-crystallin. The US alpha-crystallin has a greater amount of polymeric species, but fewer degraded subunits than the WS alpha-crystallin as shown by SDS-PAGE and Western blot. Circular dichroism (CD) measurements indicate that they have the same secondary structure but a different tertiary structure, possibly a partial unfolding in the US alpha-crystallin. This is supported by fluorescence measurements: Trp residues are more exposed and protein has a more-hydrophobic surface in the US than in the WS alpha-crystallin. Blue fluorescence further indicates that the US alpha-crystallin has a greater amount of pigment than the WS alpha-crystallin. Together, these results indicate that the US alpha-crystallin is a chemically and conformationally modified protein.

The chloroplast small heat shock protein--purification and characterization of pea recombinant protein

We report here on a procedure to obtain large amounts of a chloroplast-localized heat shock protein (HSP21) with unknown structure and function, by using an Escherichia coli expression system for the pea (Pisum sativum) protein and a purification procedure based on perfusion ion-exchange chromatography. After initial precipitation steps, the sample was applied to cation- and anion-exchange on two columns connected in sequence, which allowed rapid purification of HSP21 in one equilibration and one elution step. The purified recombinant protein had an isoelectric point of 5. 0 and appeared in assembled, oligomeric form (approximately 200 kDa) composed of 21-kDa monomers, similar to the native HSP21 protein as detected by immunoblotting in plants after heat-stress treatment. This chloroplast-localized heat shock protein belongs to a special group of small heat shock proteins (sHSPs), which share an evolutionary conserved C-terminal domain with the vertebrate eye lens alpha-crystallin. The crystallins are known from both crystallographic and spectroscopic data to be all-beta proteins. In contrast, this paper presents circular dichroism spectroscopy data which shows that the purified recombinant HSP21 oligomer has a content of more than 30% alpha-helical secondary structure.

Thermodynamic and kinetic characterization of calf lens gammaF-crystallin

Gamma-crystallin is reported to be conformationally stable because of its internal structural symmetry, and gammaF (gammaIVa) is the most stable among the various gamma-crystallin gene products. However, there is no detailed report on its thermodynamic and kinetic stability. In the present study, detailed unfolding of gammaF-crystallin was investigated by equilibrium and kinetics methods with fluorescence and far-UV CD spectroscopic measurements. The GdnHCl-induced unfolding curves probed by Trp emission maximum and intensity showed a sharp single-step transition. Upon widening the unfolding transition with the use of urea in 1.5 M GdnHCl, a more proper fit for thermodynamic analysis was obtained. GammaF-Crystallin underwent a straightforward two-state process (N <==> U) without showing any measurable amount of intermediate. The conformational stability, as measured by deltaG(D)H2O (approximately 9 kcal/mol), indicates that gammaF-crystallin is a very stable protein. The high activation energy deltaG++H2O (approximately 24 kcal/mol), calculated from unfolding kinetics monitored by far-UV CD at 218 nm, also indicates that the native and unfolded states are separated by a high activation energy barrier.

Intermolecular interaction of lens crystallins: from rotationally mobile to immobile states at high protein concentrations

The conformation of lens crystallins in vivo or in a highly concentrated solution is not well established. Most studies were carried out in dilute solutions in which protein-protein interaction is minimal. In order to see whether there is conformational change (tertiary and secondary structures) when crystallin solutions are brought to high concentrations, we have performed the following molecular spectroscopic measurements: circular dichroism (CD) and Fourier transform infrared (FTIR). Near-UV CD measurements showed a more than two-fold increase in CD intensity (molar ellipticity) for the total water-soluble (WS) protein from young calf lens nucleus in a highly concentrated solution (> 300 mg/ml in a 0.01-mm cell), when compared with a dilute solution (1000-fold dilution in a 10-mm cell). The individual crystallins in concentrated solutions also showed an increase in CD intensity, but of different magnitude: alpha-crystallin > beta-crystallin > gamma-crystallin. The increased CD indicates that lens crystallins are in a more compact structure in highly concentrated solutions; they likely undergo a transition from a mobile to an immobile state. Change in near-UV CD usually is caused by restricted mobility of aromatic side groups, particularly Trp. The transition involves not only a change in protein tertiary and/or quaternary structure, but also in protein backbone structure. The change of protein backbone structure was drawn from FTIR measurements. FTIR spectra, sensitive to the secondary structure in the amide I region, could be measured for a highly concentrated solution for which far-UV CD measurement is not feasible. The secondary structure that showed prominent change for alpha-crystallin in a highly concentrated solution was beta-conformation: increase in beta-turn with a concomitant decrease of alpha-helix structure.

Intermolecular exchange and stabilization of recombinant human alphaA- and alphaB-crystallin

Lens alpha-crystallin subunits alphaA and alphaB are differentially expressed and have a 3-to-1 ratio in most mammalian lenses by intermolecular exchange. The biological significance of this composition and the mechanism of exchange are not clear. Preparations of human recombinant alphaA- and alphaB-crystallins provide a good system in which to study this phenomenon. Both recombinant alphaA- and alphaB-crystallins are folded and aggregated to the size of the native alpha-crystallin. During incubation together, they undergo an intermolecular exchange as shown by native isoelectric focusing. Circular dichroism measurements indicate that the protein with a 3-to-1 ratio of alphaA- and alphaB-crystallins has the same secondary structure but somewhat different tertiary structures after exchange: the near-UV CD increases after exchange. The resulting hybrid aggregate is more stable than the individual homogeneous aggregates: at 62 degrees C, alphaB-crystallin is more susceptible to aggregation and displays a greater light scattering than alphaA-crystallin. This heat-induced aggregation of alphaB-crystallin, however, was suppressed by intermolecular exchange with alphaA-crystallin. These phenomena are also observed by fast performance liquid chromatography gel filtration patterns. The protein structure of alphaB-crystallin is stabilized by intermolecular exchange with alphaA-crystallin.

Detection and characterization of alpha-crystallin intermediate with maximal chaperone-like activity

Lens alpha-crystallin has been reported to act like a chaperone molecule, with the chaperone-like activity enhanced by partial unfolding. The nature of the partial unfolding, however, is not fully understood. In this project, the unfolding and refolding process of alpha-crystallin was studied with guanidine hydrochloride (GdnHCl). Trp fluorescence (tertiary structure) and far-ultraviolet circular dichroism (UVCD) (secondary structure) demonstrated the presence of an intermediate in the unfolding pathway. ANS (1-anilino-8-naphthalenesulfonate) fluorescence clearly indicated a two-step transition in the unfolding-refolding process and showed that maximum hydrophobicity of the alpha-crystallin occurred at 0.8-1.0 M GdnHCl. This alpha-crystallin intermediate appears to be in a molten globule state; conformational study by near- and far-UVCD measurements indicated that alpha-crystallin intermediate exhibited tertiary structure which was significantly altered from that of the native protein, but had nearly the same secondary structure. Quaternary structure (size of aggregate) of the intermediate also remained unchanged from that of the native protein, as shown by FPLC size exclusion chromatography. The maximal hydrophobicity of the alpha-crystallin intermediate in the unfolding-refolding pathway was accompanied by maximal protection of betaH-crystallin from aggregation. However, an adverse effect of partial unfolding is that the alpha-crystallin intermediate aggregates at high concentrations. Together, these results clearly demonstrated the biological significance of the alpha-crystallin intermediate: it is a more effective chaperone than native alpha-crystallin.

Heat-induced conformational change and increased chaperone activity of lens alpha-crystallin

PURPOSE

Alpha-crystallin is the major structural protein of the eye lens known to have chaperone-like activity. Our objective is to elucidate the nature of the thermal transition that alpha-crystallin undergoes at 60 degrees C and the effect of this transition on the chaperone activity.

METHODS

FPLC size exclusion chromatography, far- and near-ultraviolet circular dichroism, and tryptophan (Trp) and 1-anilino-8-naphthalenesulfonate (ANS) fluorescence were used to study conformational change. Turbidity of dithiothreitol (DTT)-reduced insulin was used to study chaperone activity.

RESULTS

The thermal transition was identified as a conformational change in mainly tertiary (partial unfolding) and quaternary high-molecular-weight (HMW) aggregation structures, along with a loss of 10 percentage points of secondary structure (beta-sheet). Initial partial perturbation in tertiary structure increased chaperone activity, but the increase was less in the HMW aggregate. Similar results were observed in in vivo-formed HMW alpha-crystallin.

CONCLUSIONS

The conformational change and HMW aggregation of alpha-crystallin observed at 60 degrees C, as well as in vivo-formed HMW aggregates, increased chaperone activity.

Conformational and functional differences between recombinant human lens alphaA- and alphaB-crystallin

Human and other mammalian lens proteins are composed of three major crystallins: alpha-, beta-, and gamma-crystallin. alpha-Crystallin plays a prominent role in the supramolecular assembly required to maintain lens transparency. With age, the crystallins, especially alpha-crystallin, undergo posttranslational modifications that may disrupt the supramolecular assembly, and the lens becomes susceptible to other stresses resulting in cataract formation. Because these modifications occur even at a relatively young age, it is difficult to obtain pure, unmodified crystallins for in vitro experiments. alpha-Crystallin is composed of two subunits, alphaA and alphaB. Before the application of recombinant DNA technology, these two alpha-crystallin subunits were separated from calf lens in the denatured state and reconstituted by the removal of the denaturant, but they were not refolded properly. In the present studies, we applied the recombinant DNA technology to prepare native, unmodified alphaA- and alphaB-crystallins for conformational and functional studies. The expressed proteins from Escherichia coli are in the native state and can be studied directly. First, alphaA and alphaB cDNAs were isolated from a human lens epithelial cell cDNA library. The cDNAs were cloned into a pAED4 expression vector and then expressed in E. coli strain BL21(DE3). Pure recombinant alphaA- and alphaB-crystallins were obtained after purification by gel filtration and DEAE liquid chromatography. They were subjected to conformational studies involving various spectroscopic measurements and an assessment of chaperone-like activity. alphaA- and alphaB-crystallins have not only different secondary structure, but also tertiary structure. 1-Anilino-8-naphthalene sulfonate fluorescence indicates that alphaB-crystallin is more hydrophobic than alphaA-crystallin. The chaperone-like activity, as measured by the ability to protect insulin aggregation, is about 4 times greater for alphaB- than for alphaA-crystallin. The resulting data provide a base line for further studies of human lens alpha-crystallin.

Cloning, expression, and chaperone-like activity of human alphaA-crystallin

One of the major protein components of the ocular lens, alpha-crystallin, is composed of alphaA and alphaB chain subunits that have structural homology to the family of mammalian small heat shock proteins. Like other small heat shock proteins, alpha-crystallin subunits associate to form large oligomeric aggregates that express chaperone-like activity, as defined by the ability to suppress nonspecific aggregation of proteins destabilized by treatment with a variety of denaturants including heat, UV irradiation, and chemical modification. It has been proposed that age-related loss of sequences at the C terminus of the alphaA chain subunit may be a factor in the pathogenesis of cataract due to diminished capacity of the truncated crystallin to protect against nonspecific aggregation of lens proteins. To evaluate the functional consequences of alpha-crystallin modification, two mutant forms of alphaA subunits were prepared by site-directed mutagenesis. Like wild type (WT), aggregates of approximately 540 kDa were formed from a tryptophan-free alphaA mutant (W9F). When added in stoichiometric amounts, both WT and W9F subunits completely suppressed the heat-induced aggregation of aldose reductase. In contrast, subunits encoded by a truncation mutant in which the C-terminal 17 residues were deleted (R157STOP), despite having spectroscopic properties similar to WT, formed much larger aggregates with a marked reduction in chaperone-like activity. Similar results were observed when the chaperone-like activity was assessed through inhibition of gamma-crystallin aggregation induced by singlet oxygen. These results demonstrate that the structurally conservative substitution of Phe for Trp-9 has a negligible effect on the functional interaction of alphaA subunits, and that deletion of C-terminal sequences from the alphaA subunit results in substantial loss of chaperone-like activity, despite overall preservation of secondary structure.

Ascorbic acid mediated alteration of alpha-crystallin secondary structure

Glycation, the non-enzymatic addition of sugar or other carbonyl compounds to the amino groups of a protein, has been shown to occur with a variety of sugars and a diverse group of proteins. This type of alteration is believed to be an important component of aging for lens proteins and perhaps in cataractogenesis. Glycation has been shown to alter function and spectroscopic techniques have shown that in many cases conformational changes have occurred. Circular dichroism spectroscopy has documented modifications to alpha-crystallin tertiary structure induced by glucose and glucose 6-phosphate but generally no change to secondary structure. Ascorbate and is oxidized derivative dehydroascorbate have been shown to be powerful glycating agents as well as forming cross-links between peptide chains. In this study, alpha-crystallin incubated with ascorbic acid for one or two wk shows significant incorporation of ascorbate, non-reducible cross-links between the protein chains and altered CD spectra in the far UV region indicative of secondary structure modification.

Structure and modifications of the junior chaperone alpha-crystallin. From lens transparency to molecular pathology

alpha-Crystallin is a high-molecular-mass protein that for many decades was thought to be one of the rare real organ-specific proteins. This protein exists as an aggregate of about 800 kDa, but its composition is simple. Only two closely related subunits termed alpha A- and alpha B-crystallin, with molecular masses of approximately 20 kDa, form the building blocks of the aggregate. The idea of organ-specificity had to be abandoned when it was discovered that alpha-crystallin occurs in a great variety of nonlenticular tissues, notably heart, kidney, striated muscle and several tumors. Moreover alpha B-crystallin is a major component of ubiquinated inclusion bodies in human degenerative diseases. An earlier excitement arose when it was found that alpha B-crystallin, due to its very similar structural and functional properties, belongs to the heat-shock protein family. Eventually the chaperone nature of alpha-crystallin could be demonstrated unequivocally. All these unexpected findings make alpha-crystallin a subject of great interest far beyond the lens research field. A survey of structural data about alpha-crystallin is presented here. Since alpha-crystallin has resisted crystallization, only theoretical models of its three-dimensional structure are available. Due to its long life in the eye lens, alpha-crystallin is one of the best studied proteins with respect to post-translational modifications, including age-induced alterations. Because of its similarities with the small heat-shock proteins, the findings about alpha-crystallin are illuminative for the latter proteins as well. This review deals with: structural aspects, post-translational modifications (including deamidation, racemization, phosphorylation, acetylation, glycation, age-dependent truncation), the occurrence outside of the eye lens, the heat-shock relation and the chaperone activity of alpha-crystallin.

Human placental aldose reductase: role of Cys-298 in substrate and inhibitor binding

Steady-state kinetic and inhibition properties of human placental aldose reductase carboxymethylated at Cys-298 were investigated. A comparison of the primary deuterium kinetic isotope effect on the reduced and the carboxymethylated enzymes suggests that carboxymethylation did not affect the reaction sequence of substrate binding and release. Values of DV/KD-glyceraldehyde greater than DV suggest that steps in the reaction scheme subsequent to hydride transfer, particularly the release of NADP may be rate limiting. Carboxymethylation of Cys-298 was also found to affect NADPH and aldehyde binding to the enzyme. Carboxymethylation had little effect on the secondary structure of the enzyme, but a comparison of the circular dichroic spectra of the reduced and carboxymethylated enzyme, suggests a weakened interaction between the nicotinamide and 2'-monophosphoadenosine 5'-diphosphoribose of NADPH, and the carboxymethylated enzyme. Interaction between Cys-298 and NADPH appears to determine the rate of isomerization of the E:NADP binary complex and carboxymethylation-induced decrease in kcat may be due to slower isomerization of the E:NADP binary complex. The carboxymethylated enzyme was less sensitive than the reduced enzyme to most aldose reductase inhibitors including sorbinil (d-6-fluoro-spiro[chroman-4,4'-imidazolidine]-2',5'-dione), except tolrestat (N-methyl-N-[(5-trifluromethyl-6-methoxy-1-naphthalenyl)- thiomethyl]glycine) and quercetin. On the basis of these observations it is suggested that Cys-298 may form a part of the 'S'-inhibitor binding site of the enzyme and may be responsible for tight binding of NADPH.

High level expression of rat gamma-D-crystallin in Escherichia coli

Gamma-crystallins have been implicated in various kinds of cataracts. In order to facilitate studies elucidating the molecular mechanism of cataractogenesis, large quantities of rat recombinant gamma-D-crystallin were produced in E coli. A full length cDNA clone coding for gamma-D-crystallin was isolated from a rat lens lambda gt11 cDNA library using a synthetic oligonucleotide as a probe. The coding region of this cDNA was inserted into a cloning vector pKK233-2 under the control of the trc promoter. The resulting construct, pKKCR91, was transfected into E coli to produce rat gamma-D-crystallin in an amount of 10-15% of the total bacterial proteins. The crystallin produced was purified to an apparent homogeneity as judged by SDS gel electrophoresis. The sequence of the N-terminal 11 amino acids of the purified crystallin was determined, showing that it is completely identical to that predicted from the cDNA sequence. Measurements of the far-UV CD spectra also revealed that recombinant rat gamma-D-crystallin thus produced retains a native conformational structure.

Probing high order structure of proteins by fast-atom bombardment mass spectrometry

During the past decade, numerous investigations have demonstrated that the rate at which amide hydrogens located at peptide linkages undergo isotopic exchange is a sensitive probe of the high order structure and dynamics of proteins. The present investigation demonstrates that microbore high-performance liquid chromatography (HPLC) continuous-flow fast-atom bombardment mass spectrometry (FABMS) can be used to accurately quantify deuterium located at peptide linkages in short segments of large proteins. This result is important because it demonstrates the feasibility of using mass spectrometry as a tool for studying the high order structure and dynamics of large proteins. Following a period of deuterium exchange-in, a protein was placed into slow-exchange conditions and fragmented into peptides with pepsin. The digest was analyzed by continuous-flow HPLC FABMS to determine the molecular weights of the peptides, from which the number of deuterons located at the peptide linkages could be deduced. The HPLC step was used both to fractionate the peptides according to their hydrophobicities and to remove through back-exchange all deuterium except that located at peptide amide linkages. This approach has been applied to α-crystallin, a lens protein composed of two gene products with monomer molecular weights of 20 kDa and an aggregate molecular weight approaching 1000 kDa. Results from this study show that some of the peptide amide hydrogens in αA-crystallin exchange very rapidly (k > 10 h(-1)) while others exchange very slowly (k < 10(-3) h(-1)). The ability not only to detect that a conformational change has occurred, but also to identify the specific regions within the protein where the change occurred, was demonstrated by measuring changes in the exchange rates within these regions as the deuterium exchange-in temperature was increased from 10 to 80 ° C.

Estimation of the secondary structure and conformation of bovine lens crystallins by infrared spectroscopy: quantitative analysis and resolution by Fourier self-deconvolution and curve fit

The secondary structure of six bovine lens protein fractions (two alpha, three beta and one gamma-crystallin) are examined in solution and in solid forms for the first time using FTIR spectroscopy. Films of the nuclear and cortical regions of the bovine lens are also examined. The structure is quantitatively estimated from the vibrational analysis of the resolution-enhanced amide-I profile achieved by Fourier self-deconvolution and linear least-squares curve-fit algorithm. All the protein fractions fold predominantly in a beta-pleated sheet structure with little or no alpha-helical domains in solution or in lyophilized solid form. These proteins also retain their predominant beta-sheet conformation in the cellular phospholipid environment of the lens, in conformity with the structure obtained for all the mammalian species examined to date. Despite structural homology, vibrational data indicate subtle structural differences within each class of the crystallins probably due to presence of several minor substructures/subconformations. Substantial high amounts of turns (approx. 40%) observed in the beta-fractions may have a fundamental implication in stabilizing the tertiary structure of the uniquely folded-proteins vital for the transparency of the lens. These proteins in solid KBr-matrix undergo a major structural change, induced primarily by ionic interactions which refold them in a helical conformation. IR spectroscopy together with band-narrowing procedures has proven to be an effective tool to obtain structural information of proteins in solution, as solid substrates or in a complex biological tissue, such as ocular lens.

Conformational stability of bovine alpha-crystallin. Evidence for a destabilizing effect of ascorbate

Short-term incubation of bovine alpha-crystallin with ascorbate alters the protein conformational stability. The denaturation curves with urea and guanidinium-chloride show different patterns, suggesting a deviation from a two-state mechanism owing to the presence of one or more intermediates in the unfolding of ascorbate-modified alpha-crystallin. Furthermore, the latter protein profiles are shifted to lower denaturant concentrations indicating a destabilizing action of ascorbate, which is capable of facilitating protein dissociation into subunits as demonstrated by gel filtration with 1.5 M-urea. The decrease in conformational stability cannot be ascribed to any major structural alteration, but rather to localized changes in the protein molecule. In fact, no difference between native and ascorbate-treated alpha-crystallin can be detected by amino acid analysis but perturbation of the tryptophan and tyrosine environment is indicated by alterations in intrinsic fluorescence. Furthermore, turbidity and light-scattering measurements suggest an involvement of the lysine side chains, since aggregability patterns with acetylsalicylic acid are significantly altered. The ascorbate-destabilizing effect on the conformational stability of alpha-crystallin, probably exerted through oxidative modification of amino acid residues and/or the formation of covalent adducts, provokes unfavourable steric interactions between residues along the polypeptide chains, thus favouring aggregation and insolubilization of crystallins which can lead to cataract formation, as also demonstrated by proteolytic digestion patterns which show a lower rate of degradation of the ascorbate-modified alpha-crystallin.

Riboflavin-photosensitized anaerobic modification of rat lens proteins. A correlation with age-related changes

When rat lens homogenate or its soluble protein fractions are irradiated in the presence of riboflavin, a photo-adduct is obtained between this vitamin and the lens proteins. Irradiation of these proteins in the presence of riboflavin also leads to a modification in the chromatographic elution pattern with an increase in the high-molecular-weight fraction. In an aging study with rats, it was shown that the proportion of the high-molecular-weight protein fraction significantly increased with age, whereas the proportion of the low-molecular-weight protein fraction concomitantly decreased. It is postulated that aging produces an increase in the accessibility of the tryptophan residues of the lens proteins, as established by iodide fluorescence quenching experiments.

Intermolecular protein interactions in solutions of calf lens alpha-crystallin. Results from 1/T1 nuclear magnetic relaxation dispersion profiles

From analyses of the magnetic field dependence of 1/T1 (NMRD profiles) of water protons in solutions of calf lens alpha-crystallin at several concentrations, we find two regimes of solute behavior in both cortical and nuclear preparations. Below approximately 15% vol/vol protein concentration, the solute molecules appear as compact globular proteins of approximately 1,350 (cortical) and approximately 1,700 (nuclear) kD. At higher concentrations, the effective solute particle size increases, reversibly, as evidenced by the appearance of spectra-like 14N peaks in the NMRD profiles and a change in the field and temperature dependence of 1/T1. At these higher concentrations, the profiles are very similar to those of calf gamma II-crystallin, a crystallin that undergoes an analogous transition near approximately 15% protein (Koenig, S. H., C.F. Beaulieu, R. D. Brown III, and M. Spiller, 1990. Biophys. J. 57:461-469). By comparison with recent analyses of NMRD results for solutions of immobilized proteins as models for the transition from protein solutions to tissue (Koenig, S. H., and R. D. Brown III. 1991. Prog. NMR Spectr. 22:487-567), we argue that alpha-crystallin solute behaves as aggregates approximately greater than 50,000 kD as protein concentration is progressively increased above 15%. Finally, the concentration dependence of the NMRD profiles of alpha- and gamma II-crystallin can readily explain recent osmotic pressure data, in particular the intersection of the respective pressure curves at approximately 23% vol/vol (Vérétout, F., and A. Tardieu. 1989. Eur. Biophys. J. 17:61-68).

Purification and properties of the low-molecular-weight alpha-crystallin from normal goat lens: comparison with bovine lens

Low-molecular-weight alpha-crystallin (alpha L-crystallin) isolated from decapsulated lens of goat (Capra hiscus) has been purified to an apparently homogeneous population. Goat alpha L-crystallin closely resembles its bovine counterpart in size, shape, exposition of sulfhydryl groups, subunit composition and the nature of its UV-absorption profile. Like bovine alpha L-crystallin, dissociated subunits of goat alpha L-crystallin assemble upon reassociation into a particle of almost half the size of the native one. However, subunits of goat alpha L-crystallin are found to contain more aromatic amino acid residues than those of bovine subunits leading to a higher value of extinction coefficient (E1cm1%) at 280 nm for the goat protein.

Melittin-induced conformational changes in human lens protein

Circular dichroism and fluorescence measurements showed a reduced conformational order in proteins of a normal human lens when they were incubated in vitro with melittin, a bee venom peptide. Since melittin is also known to react with lipids to induce a breakdown of vesicular structure, the observed denaturation of water-soluble proteins of a human lens that developed a cataract due to multiple bee stings may be accounted for by the effects of melittin to some extent. The melittin-induced decrease of conformational order, as observed in our in-vitro studies could thus be of physiological significance.