The influence of the N-terminal region proximal to the core domain on the assembly and chaperone activity of αB-crystallin

αB-Crystallin (HSPB5) is a small heat-shock protein that is composed of dimers that then assemble into a polydisperse ensemble of oligomers. Oligomerisation is mediated by heterologous interactions between the C-terminal tail of one dimer and the core "α-crystallin" domain of another and stabilised by interactions made by the N-terminal region. Comparatively little is known about the latter contribution, but previous studies have suggested that residues in the region 54-60 form contacts that stabilise the assembly. We have generated mutations in this region (P58A, S59A, S59K, R56S/S59R and an inversion of residues 54-60) to examine their impact on oligomerisation and chaperone activity in vitro. By using native mass spectrometry, we found that all the αB-crystallin mutants were assembly competent, populating similar oligomeric distributions to wild-type, ranging from 16-mers to 30-mers. However, circular dichroism spectroscopy, intrinsic tryptophan and bis-ANS fluorescence studies demonstrated that the secondary structure differs to wild type, the 54-60 inversion mutation having the greatest impact. All the mutants exhibited a dramatic decrease in exposed hydrophobicity. We also found that the mutants in general were equally active as the wild-type protein in inhibiting the amorphous aggregation of insulin and seeded amyloid fibrillation of α-synuclein in vitro, except for the 54-60 inversion mutant, which was significantly less effective at inhibiting insulin aggregation. Our data indicate that alterations in the part of the N-terminal region proximal to the core domain do not drastically affect the oligomerisation of αB-crystallin, reinforcing the robustness of αB-crystallin in functioning as a molecular chaperone.

Evaluating the Effect of Phosphorylation on the Structure and Dynamics of Hsp27 Dimers by Means of Ion Mobility Mass Spectrometry

The quaternary structure and dynamics of the human small heat-shock protein Hsp27 are linked to its molecular chaperone function and influenced by post-translational modifications, including phosphorylation. Phosphorylation of Hsp27 promotes oligomer dissociation and can enhance chaperone activity. This study explored the impact of phosphorylation on the quaternary structure and dynamics of Hsp27. Using mutations that mimic phosphorylation, and ion mobility mass spectrometry, we show that successive substitutions result in an increase in the conformational heterogeneity of Hsp27 dimers. In contrast, we did not detect any changes in the structure of an Hsp27 12-mer, representative of larger Hsp27 oligomers. Our data suggest that oligomer dissociation and increased flexibility of the dimer contribute to the enhanced chaperone activity of phosphorylated Hsp27. Thus, post-translational modifications such as phosphorylation play a crucial role in modulating both the tertiary and quaternary structure of Hsp27, which is pivotal to its function as a key component of the proteostasis network in cells. Our data demonstrate the utility of ion mobility mass spectrometry for probing the structure and dynamics of heterogeneous proteins.

Susceptibility of Mutant SOD1 to Form a Destabilized Monomer Predicts Cellular Aggregation and Toxicity but Not In vitro Aggregation Propensity

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the rapid and progressive degeneration of upper and lower motor neurons in the spinal cord, brain stem and motor cortex. The first gene linked to ALS was the gene encoding the free radical scavenging enzyme superoxide dismutase-1 (SOD1) that currently has over 180, mostly missense, ALS-associated mutations identified. SOD1-associated fALS patients show remarkably broad mean survival times (<1 year to ~17 years death post-diagnosis) that are mutation dependent. A hallmark of SOD1-associated ALS is the deposition of SOD1 into large insoluble aggregates in motor neurons. This is thought to be a consequence of mutation induced structural destabilization and/or oxidative damage leading to the misfolding and aggregation of SOD1 into a neurotoxic species. Here we aim to understand the relationship between SOD1 variant toxicity, structural stability, and aggregation propensity using a combination of cell culture and purified protein assays. Cell based assays indicated that aggregation of SOD1 variants correlate closely to cellular toxicity. However, the relationship between cellular toxicity and disease severity was less clear. We next utilized mass spectrometry to interrogate the structural consequences of metal loss and disulfide reduction on fALS-associated SOD1 variant structure. All variants showed evidence of unfolded, intermediate, and compact conformations, with SOD1^G37R, SOD1^G93A and SOD1^V148G having the greatest abundance of intermediate and unfolded SOD1. SOD1^G37R was an informative outlier as it had a high propensity to unfold and form oligomeric aggregates, but it did not aggregate to the same extent as SOD1^G93A and SOD1^V148G in in vitro aggregation assays. Furthermore, seeding the aggregation of DTT/EDTA-treated SOD1^G37R with preformed SOD1^G93A fibrils elicited minimal aggregation response, suggesting that the arginine substitution at position-37 blocks the templating of SOD1 onto preformed fibrils. We propose that this difference may be explained by multiple strains of SOD1 aggregate and this may also help explain the slow disease progression observed in patients with SOD1^G37R.

Mass spectrometry data and size exclusion chromatography profiles of Australian taipan venom toxins

The compositions of paradoxin and taipoxin (PDx and TPx, respectively) were investigated using size exclusion chromatography (SEC) and nano-electrospray ionization mass spectrometry (nano-ESI-MS). The elution profiles from size exclusion chromatography of the venoms from Oxyuranus microlepidotus and Oxyuranus scutellatus were similar. Fractions corresponding to the trimeric toxins were treated with guanidinium hydrochloride and the individual subunits were separated by HPLC. In this report we present the size exclusion chromatography profiles for these toxins, and the nano-ESI mass spectra of the subunits after separation by HPLC: the first such comparative study of these toxins at the protein level. Data in this article are associated with the research article published in Toxicon: “Insight into the subunit arrangement and diversity of paradoxin and taipoxin” (J.A. Harrison, J.A. Aquilina, 2016) [1].

Phosphomimics destabilize Hsp27 oligomeric assemblies and enhance chaperone activity

Serine phosphorylation of the mammalian small heat-shock protein Hsp27 at residues 15, 78, and 82 is thought to regulate its structure and chaperone function; however, the site-specific impact has not been established. We used mass spectrometry to assess the combinatorial effect of mutations that mimic phosphorylation upon the oligomeric state of Hsp27. Comprehensive dimerization yielded a relatively uncrowded spectrum, composed solely of even-sized oligomers. Modification at one or two serines decreased the average oligomeric size, while the triple mutant was predominantly a dimer. These changes were reflected in a greater propensity for oligomers to dissociate upon increased modification. The ability of Hsp27 to prevent amorphous or fibrillar aggregation of target proteins was enhanced and correlated with the amount of dissociated species present. We propose that, in vivo, phosphorylation promotes oligomer dissociation, thereby enhancing chaperone activity. Our data support a model in which dimers are the chaperone-active component of Hsp27.

Transcriptional and posttranscriptional regulation of Bacillus sp. CDB3 arsenic-resistance operon ars1

Bacillus sp. CDB3 possesses a novel eight-gene ars cluster (ars1, arsRYCDATorf7orf8) with some unusual features in regard to expression regulation. This study demonstrated that the cluster is a single operon but can also produce a short three-gene arsRYC transcript. A hairpin structure formed by internal inverted repeats between arsC and arsD was shown to diminish the expression of the full operon, thereby probably acting as a transcription attenuator. A degradation product of the arsRYC transcript was also identified. Electrophoretic mobility shift analysis demonstrated that ArsR interacts with the ars1 promoter forming a protein-DNA complex that could be impaired by arsenite. However, no interaction was detected between ArsD and the ars1 promoter, suggesting that the CDB3 ArsD protein may not play a regulatory role. Compared to other ars gene clusters, regulation of the Bacillus sp. CDB3 ars1 operon is more complex. It represents another example of specific mRNA degradation in the transporter gene region and possibly the first case of attenuator-mediated regulation of ars operons.

Age-related cleavages of crystallins in human lens cortical fiber cells generate a plethora of endogenous peptides and high molecular weight complexes

Low molecular weight peptides derived from the breakdown of crystallins have been reported in adult human lenses. The proliferation of these LMW peptides coincides with the earliest stages of cataract formation, suggesting that the protein cleavages involved may contribute to the aggregation and insolubilization of crystallins. This study reports the identification of 238 endogenous LMW crystallin peptides from the cortical extracts of four human lenses representing young, middle and old-age human lenses. Analysis of the peptide terminal amino acids showed that Lys and Arg were situated at the C-terminus with significantly higher frequency compared to other residues, suggesting that trypsin-like proteolysis may be active in the lens cortical fiber cells. Selected reaction monitoring analysis of an endogenous αA-crystallin peptide (αA(57-65)) showed that the concentration of this peptide in the human lens increased gradually to middle age, after which the rate of αA(57-65) formation escalated significantly. Using 2D gel electrophoresis/nanoLC-ESI-MS/MS, 12 protein complexes of 40-150 kDa consisting of multiple crystallin components were characterized from the water soluble cortical extracts of an adult human lens. The detection of these protein complexes suggested the possibility of crystallin cross-linking, with these complexes potentially acting to stabilize degraded crystallins by sequestration into water soluble complexes.

Stability of the octameric structure affects plasminogen-binding capacity of streptococcal enolase

Group A Streptococcus (GAS) is a human pathogen that has the potential to cause invasive disease by binding and activating human plasmin(ogen). Streptococcal surface enolase (SEN) is an octameric α-enolase that is localized at the GAS cell surface. In addition to its glycolytic role inside the cell, SEN functions as a receptor for plasmin(ogen) on the bacterial surface, but the understanding of the molecular basis of plasmin(ogen) binding is limited. In this study, we determined the crystal and solution structures of GAS SEN and characterized the increased plasminogen binding by two SEN mutants. The plasminogen binding ability of SEN^K312A and SEN^K362A is ~2- and ~3.4-fold greater than for the wild-type protein. A combination of thermal stability assays, native mass spectrometry and X-ray crystallography approaches shows that increased plasminogen binding ability correlates with decreased stability of the octamer. We propose that decreased stability of the octameric structure facilitates the access of plasmin(ogen) to its binding sites, leading to more efficient plasmin(ogen) binding and activation.

Glutathionylation potentiates benign superoxide dismutase 1 variants to the toxic forms associated with amyotrophic lateral sclerosis

Dissociation of superoxide dismutase 1 dimers is enhanced by glutathionylation, although the dissociation constants reported to date are imprecise. We have quantified the discreet dissociation constants for wild-type superoxide dismutase 1 and six naturally occurring sequence variants, in their unmodified and glutathionylated forms, at the ratios expressed. Unmodified superoxide dismutase 1 variants that shared similar dissociation constants with SOD1^WT had disproportionately increased dissociation constants when glutathionylated. This defines a key role for glutathionylation in superoxide dismutase 1 associated familial amyotrophic lateral sclerosis.

Structural and functional aspects of hetero-oligomers formed by the small heat shock proteins αB-crystallin and HSP27

BACKGROUND

αB-crystallin and HSP27 are mammalian intracellular small heat shock proteins.

RESULTS

These proteins exchange subunits in a rapid and temperature-dependent manner.

CONCLUSION

This facile subunit exchange suggests that differential expression could be used by the cell to regulate the response to stress.

SIGNIFICANCE

A robust technique defines parameters for the dynamic interaction between the major mammalian small heat shock proteins. Small heat shock proteins (sHSPs) exist as large polydisperse species in which there is constant dynamic subunit exchange between oligomeric and dissociated forms. Their primary role in vivo is to bind destabilized proteins and prevent their misfolding and aggregation. αB-Crystallin (αB) and HSP27 are the two most widely distributed and most studied sHSPs in the human body. They are coexpressed in different tissues, where they are known to associate with each other to form hetero-oligomeric complexes. In this study, we aimed to determine how these two sHSPs interact to form hetero-oligomers in vitro and whether, by doing so, there is an increase in their chaperone activity and stability compared with their homo-oligomeric forms. Our results demonstrate that HSP27 and αB formed polydisperse hetero-oligomers in vitro, which had an average molecular mass that was intermediate of each of the homo-oligomers and which were more thermostable than αB, but less so than HSP27. The hetero-oligomer chaperone function was found to be equivalent to that of αB, with each being significantly better in preventing the amorphous aggregation of α-lactalbumin and the amyloid fibril formation of α-synuclein in comparison with HSP27. Using mass spectrometry to monitor subunit exchange over time, we found that HSP27 and αB exchanged subunits 23% faster than the reported rate for HSP27 and αA and almost twice that for αA and αB. This represents the first quantitative evaluation of αB/HSP27 subunit exchange, and the results are discussed in the broader context of regulation of function and cellular proteostasis.

Truncation, cross-linking and interaction of crystallins and intermediate filament proteins in the aging human lens

The optical properties of the lens are dependent upon the integrity of proteins within the fiber cells. During aging, crystallins, the major intra-cellular structural proteins of the lens, aggregate and become water-insoluble. Modifications to crystallins and the lens intermediate filaments have been implicated in this phenomenon. In this study, we examined changes to, and interactions between, human lens crystallins and intermediate filament proteins in lenses from a variety of age groups (0-86years). Among the lens-specific intermediate filament proteins, filensin was extensively cleaved in all postnatal lenses, with truncated products of various sizes being found in both the lens cortical and nuclear extracts. Phakinin was also truncated and was not detected in the lens nucleus. The third major intermediate filament protein, vimentin, remained intact in lens cortical fiber cells across the age range except for an 86year lens, where a single ~49kDa breakdown product was observed. An αB-crystallin fusion protein (maltose-binding protein-αB-crystallin) was found to readily exchange subunits with endogenous α-crystallin, and following mild heat stress, to bind to filensin, phakinin and vimentin and to several of their truncated products. Tryptic digestion of a truncated form of filensin suggested that the binding site for α-crystallin may be in the N-terminal region. The presence of significant amounts of small peptides derived from γS- and βB1-crystallins in the water-insoluble fraction of the lens indicates that these interact tightly with cytoskeletal or membrane components. Interestingly, water-soluble complexes (~40kDa) contained predominantly γS- and βB1-crystallins, suggesting that cross-linking is an alternative pathway for modified β- and γ-crystallins in the lens.

The quaternary organization and dynamics of the molecular chaperone HSP26 are thermally regulated

The function of ScHSP26 is thermally controlled: the heat shock that causes the destabilization of target proteins leads to its activation as a molecular chaperone. We investigate the structural and dynamical properties of ScHSP26 oligomers through a combination of multiangle light scattering, fluorescence spectroscopy, NMR spectroscopy, and mass spectrometry. We show that ScHSP26 exists as a heterogeneous oligomeric ensemble at room temperature. At heat-shock temperatures, two shifts in equilibria are observed: toward dissociation and to larger oligomers. We examine the quaternary dynamics of these oligomers by investigating the rate of exchange of subunits between them and find that this not only increases with temperature but proceeds via two separate processes. This is consistent with a conformational change of the oligomers at elevated temperatures which regulates the disassembly rates of this thermally activated protein.

Evidence for specific subunit distribution and interactions in the quaternary structure of alpha-crystallin

The quaternary structure of alpha-crystallin is dynamic, a property which has thwarted crystallographic efforts towards structural characterization. In this study, we have used collision-induced dissociation mass spectrometry to examine the architecture of the polydisperse assemblies of alpha-crystallin. For total alpha-crystallin isolated directly from fetal calf lens using size-based chromatography, the alphaB-crystallin subunit was found to be preferentially dissociated from the oligomers, despite being significantly less abundant overall than the alphaA-crystallin subunits. Furthermore, upon mixing molar equivalents of purified alphaA- and alphaB-crystallin, the levels of their dissociation were found to decrease and increase, respectively, with time. Interestingly though, dissociation of subunits from the alphaA- and alphaB-crystallin homo-oligomers was comparable, indicating that strength of the alphaA:alphaA, and alphaB:alphaB subunit interactions are similar. Taken together, these data suggest that the differences in the number of subunit contacts in the mixed assemblies give rise to the disproportionate dissociation of alphaB-crystallin subunits. Limited proteolysis mass spectrometry was also used to examine changes in protease accessibility during subunit exchange. The C-terminus of alphaA-crystallin was more susceptible to proteolytic attack in homo-oligomers than that of alphaB-crystallin. As subunit exchange proceeded, proteolysis of the alphaA-crystallin C-terminus increased, indicating that in the hetero-oligomeric form this tertiary motif is more exposed to solvent. These data were used to propose a refined arrangement for the interactions of the alpha-crystallin domains and C-terminal extensions of subunits within the alpha-crystallin assembly. In particular, we propose that the palindromic IPI motif of alphaB-crystallin gives rise to two orientations of the C-terminus.

Defining the structural basis of human plasminogen binding by streptococcal surface enolase

The flesh-eating bacterium group A Streptococcus (GAS) binds and activates human plasminogen, promoting invasive disease. Streptococcal surface enolase (SEN), a glycolytic pathway enzyme, is an identified plasminogen receptor of GAS. Here we used mass spectrometry (MS) to confirm that GAS SEN is octameric, thereby validating in silico modeling based on the crystal structure of Streptococcus pneumoniae alpha-enolase. Site-directed mutagenesis of surface-located lysine residues (SEN(K252 + 255A), SEN(K304A), SEN(K334A), SEN(K344E), SEN(K435L), and SEN(Delta434-435)) was used to examine their roles in maintaining structural integrity, enzymatic function, and plasminogen binding. Structural integrity of the GAS SEN octamer was retained for all mutants except SEN(K344E), as determined by circular dichroism spectroscopy and MS. However, ion mobility MS revealed distinct differences in the stability of several mutant octamers in comparison with wild type. Enzymatic analysis indicated that SEN(K344E) had lost alpha-enolase activity, which was also reduced in SEN(K334A) and SEN(Delta434-435). Surface plasmon resonance demonstrated that the capacity to bind human plasminogen was abolished in SEN(K252 + 255A), SEN(K435L), and SEN(Delta434-435). The lysine residues at positions 252, 255, 434, and 435 therefore play a concerted role in plasminogen acquisition. This study demonstrates the ability of combining in silico structural modeling with ion mobility-MS validation for undertaking functional studies on complex protein structures.

Glutamic acid residues in the C-terminal extension of small heat shock protein 25 are critical for structural and functional integrity

Small heat shock proteins (sHsps) are intracellular molecular chaperones that prevent the aggregation and precipitation of partially folded and destabilized proteins. sHsps comprise an evolutionarily conserved region of 80-100 amino acids, denoted the alpha-crystallin domain, which is flanked by regions of variable sequence and length: the N-terminal domain and the C-terminal extension. Although the two domains are known to be involved in the organization of the quaternary structure of sHsps and interaction with their target proteins, the role of the C-terminal extension is enigmatic. Despite the lack of sequence similarity, the C-terminal extension of mammalian sHsps is typically a short, polar segment which is unstructured and highly flexible and protrudes from the oligomeric structure. Both the polarity and flexibility of the C-terminal extension are important for the maintenance of sHsp solubility and for complexation with its target protein. In this study, mutants of murine Hsp25 were prepared in which the glutamic acid residues in the C-terminal extension at positions 190, 199 and 204 were each replaced with alanine. The mutants were found to be structurally altered and functionally impaired. Although there were no significant differences in the environment of tryptophan residues in the N-terminal domain or in the overall secondary structure, an increase in exposed hydrophobicity was observed for the mutants compared with wild-type Hsp25. The average molecular masses of the E199A and E204A mutants were comparable with that of the wild-type protein, whereas the E190A mutant was marginally smaller. All mutants displayed markedly reduced thermostability and chaperone activity compared with the wild-type. It is concluded that each of the glutamic acid residues in the C-terminal extension is important for Hsp25 to act as an effective molecular chaperone.

Small heat shock protein activity is regulated by variable oligomeric substructure

The alpha-crystallins are members of the small heat shock protein family of molecular chaperones that have evolved to minimize intracellular protein aggregation; however, they are also implicated in a number of protein deposition diseases. In this study, we employed novel mass spectrometry techniques to investigate the changes in quaternary structure associated with this switch from chaperone to adjuvant of aggregation. We replicated the oligomeric rearrangements observed for post-translationally modified alpha-crystallins, without altering the protein sequence, by refolding the alpha-crystallins in vitro. This refolding resulted in a loss of dimeric substructure concomitant with an augmentation of substrate affinity. We show that packaging of small heat shock proteins into dimeric units is used to control the level of chaperone function by regulating the exposure of hydrophobic surfaces. We propose that a bias toward monomeric substructure is responsible for the aberrant chaperone behavior associated with the alpha-crystallins in protein deposition diseases.

Chemical cross-linking of the chloroplast localized small heat-shock protein, Hsp21, and the model substrate citrate synthase

The molecular mechanism whereby the small heat-shock protein (sHsp) chaperones interact with and prevent aggregation of other proteins is not fully understood. We have characterized the sHsp-substrate protein interaction at normal and increased temperatures utilizing a model substrate protein, citrate synthase (CS), widely used in chaperone assays, and a dodecameric plant sHsp, Hsp21, by chemical cross-linking with 3,3'-Dithiobis[sulfosuccinimidylpropionate] (DTSSP) and mass spectrometric peptide mapping. In the absence of CS, the cross-linker captured Hsp21 in dodecameric form, even at increased temperature (47 degrees C). In the presence of equimolar amounts of CS, no Hsp21 dodecamer was captured, indicating a substrate-induced Hsp21 dodecamer dissociation by equimolar amounts of CS. Cross-linked Hsp21-Hsp21 dipeptides indicated an exposure of the Hsp21 C-terminal tails and substrate-binding sites normally covered by the C terminus. Cross-linked Hsp21-CS dipeptides mapped to several sites on the surface of the CS dimer, indicating that there are numerous weak and short-lived interactions between Hsp21 and CS, even at normal temperatures. The N-terminal arms especially interacted with a motif in the CS dimer, which is absent in thermostable forms of CS. The cross-linking data suggest that the presence of substrate rather than temperature influences the conformation of Hsp21.

Protein-bound UV filters in normal human lenses: the concentration of bound UV filters equals that of free UV filters in the center of older lenses

PURPOSE

To survey the levels of protein-bound UV filters in the cortices and nuclei of normal human lenses as a function of age and to relate this to the concentration of free UV filters.

METHODS

Levels of each of the three kynurenine (Kyn) UV filters, 3-hydroxykynurenine glucoside (3OHKG), Kyn, and 3-hydroxykynurenine (3OHKyn), covalently attached to proteins, were determined by using a newly developed method of reductive capture, after base treatment of the intact lens proteins.

RESULTS

The data show that, in the normal lens, each of the three UV filters became bound to proteins to a significant extent only after age 50 and, further, that the levels in the nucleus were much higher than in the cortex. These findings are consistent with the lens barrier that forms in middle age. 3OHKG was present at the highest levels followed by Kyn, with 3OHKyn being attached in the lowest amount. The ratio was 145:4:1 (3OHKG-Kyn-3OHKyn), with a total protein-bound UV filter concentration in the lens nucleus after age 50 of approximately 1300 picomoles/mg protein. This ratio is in agreement with 3OHKG being the most abundant free UV filter in the human lens and 3OHKyn being present in the lowest concentration with free Kyn present in intermediate amounts.

CONCLUSIONS

The three Kyn UV filters are bound to the nuclear proteins of all normal lenses over the age of 50. Indeed in the center of older normal lenses, the concentration of UV filters bound to proteins is approximately equal to that of the free filters. Since bound UV filters promote oxidation of proteins after exposure to wavelengths of light that penetrate the cornea, lenses in middle-aged and older individuals may be more prone to photooxidation than those of young people.

Protein-bound and free UV filters in cataract lenses. The concentration of UV filters is much lower than in normal lenses

In human cataract lenses the UV filters, 3-hydroxykynurenine glucoside (3OHKG) and kynurenine (Kyn) were found to be covalently bound to proteins and the levels in the nucleus were much higher than in the cortex. The levels of the bound UV filters in cataract nuclei were much lower than those in age-matched normal lenses. 3-Hydroxykynurenine could not be detected in cataract lenses. As with normal lenses, protein-bound 3OHKG in cataract lenses was found at the highest levels followed by Kyn. Free UV filter concentrations were also markedly reduced in cataract lenses. This feature may well contribute to the lower protein-bound levels; however, there was no clear relationship between free and bound UV filter contents when individual lenses were examined. We propose that since cysteine is a major site for UV filter binding, the well-documented oxidation of protein sulfhydryl groups during the progression of nuclear cataract may account, in part, for the pronounced decrease in bound UV filters in cataract lenses.

The N-terminal domain of αB-crystallin is protected from proteolysis by bound substrate

Alpha-crystallin, a major structural protein of the lens can also function as a molecular chaperone by binding to unfolding substrate proteins. We have used a combination of limited proteolysis at low temperature, and mass spectrometry to identify the regions of alpha-crystallin directly involved in binding to the structurally compromised substrate, reduced alpha-lactalbumin. In the presence of trypsin, alpha-crystallin which had been pre-incubated with substrate showed markedly reduced proteolysis at the C-terminus compared with a control, indicating that the bound substrate restricted access of trypsin to R157, the main cleavage site. Chymotrypsin was able to cleave at residues in both the N- and C-terminal domains. In the presence of substrate, alpha-crystallin showed markedly reduced proteolysis at four sites in the N-terminal domain when compared with the control. Minor differences in cleavage were observed within the C-terminal domain suggesting that the N-terminal region of alpha-crystallin contains the major substrate interaction sites.

Effects of Glycosylation on the Structure and Function of the Extracellular Chaperone Clusterin†

Clusterin is the first well characterized, constitutively secreted extracellular chaperone that binds to exposed regions of hydrophobicity on non-native proteins. It may help control the folding state of extracellular proteins by targeting them for receptor-mediated endocytosis and intracellular lysosomal degradation. A notable feature of secreted clusterin is its heavy glycosylation. Although carbohydrate comprises approximately 20-25% of the total mass of the mature molecule, its function is unknown. Results from the current study demonstrate that deglycosylation of human serum clusterin had little effect on its overall secondary structure content but produced a small increase in solvent-exposed hydrophobicity and enhanced the propensity of the molecule to aggregate in solution. These changes were associated with increased binding to a variety of ligands but did not substantially impact the ability of clusterin to inhibit heat-induced precipitation of citrate synthase. Evidence suggesting that the normally conjugated sugars are important in the interaction of secreted clusterin with a lectin-type receptor on liver cells is also presented. Bulk expression of fully processed, glycosylated clusterin in mammalian cells is difficult, often producing inappropriately disulfide-bonded high molecular weight aggregates; this has hampered previous studies aimed at identifying those regions of the molecule important in its chaperone action. The current results suggest that it may be possible in the future to study the structure and chaperone function of clusterin using recombinant protein (lacking sugars) conveniently bulk-expressed in bacteria.

Tandem Mass Spectrometry Reveals the Quaternary Organization of Macromolecular Assemblies

The application of mass spectrometry (MS) to the study of progressively larger and more complex macromolecular assemblies is proving increasingly useful for structural biologists. The scope of this approach has recently been widened through the application of a tandem MS procedure. This two-step technique involves the selection of specific assemblies in the gas phase and inducing their dissociation through collisions with argon atoms. Here, we investigate the mechanism of this process and show that dissociation of subunits from a macromolecular assembly follows a sequential pathway, with the partitioning of charge between the dissociation products governed primarily by their relative surface areas. Using this basis of understanding, we highlight differences in the dissociation pathways of three related macromolecular assemblies and show how these are a direct consequence of changes in both local and global oligomeric organization.

Targeting C-reactive protein for the treatment of cardiovascular disease

Complement-mediated inflammation exacerbates the tissue injury of ischaemic necrosis in heart attacks and strokes, the most common causes of death in developed countries. Large infarct size increases immediate morbidity and mortality and, in survivors of the acute event, larger non-functional scars adversely affect long-term prognosis. There is thus an important unmet medical need for new cardioprotective and neuroprotective treatments. We have previously shown that human C-reactive protein (CRP), the classical acute-phase protein that binds to ligands exposed in damaged tissue and then activates complement, increases myocardial and cerebral infarct size in rats subjected to coronary or cerebral artery ligation, respectively. Rat CRP does not activate rat complement, whereas human CRP activates both rat and human complement. Administration of human CRP to rats is thus an excellent model for the actions of endogenous human CRP. Here we report the design, synthesis and efficacy of 1,6-bis(phosphocholine)-hexane as a specific small-molecule inhibitor of CRP. Five molecules of this palindromic compound are bound by two pentameric CRP molecules, crosslinking and occluding the ligand-binding B-face of CRP and blocking its functions. Administration of 1,6-bis(phosphocholine)-hexane to rats undergoing acute myocardial infarction abrogated the increase in infarct size and cardiac dysfunction produced by injection of human CRP. Therapeutic inhibition of CRP is thus a promising new approach to cardioprotection in acute myocardial infarction, and may also provide neuroprotection in stroke. Potential wider applications include other inflammatory, infective and tissue-damaging conditions characterized by increased CRP production, in which binding of CRP to exposed ligands in damaged cells may lead to complement-mediated exacerbation of tissue injury.

3-Hydroxykynurenine Oxidizes α-Crystallin: Potential Role in Cataractogenesis†

The alpha-, beta-, and gamma-crystallins are the major structural proteins of mammalian lenses. The human lens also contains tryptophan-derived UV filters, which are known to spontaneously deaminate at physiological pH and covalently attach to lens proteins. 3-Hydroxykynurenine (3OHKyn) is the third most abundant of the kynurenine UV filters in the lens, and previous studies have shown this compound to be unstable and to be oxidized under physiological conditions, producing H2O2. In this study, we show that methionine and tryptophan amino acid residues are oxidized when bovine alpha-crystallin is incubated with 3-hydroxykynurenine. We observed almost complete oxidation of methionines 1 and 138 in alphaA-crystallin and a similar extent of oxidation of methionines 1 and 68 in alphaB-crystallin after 48 h. Tryptophans 9 and 60 in alphaB-crystallin were oxidized to a lesser extent. AlphaA-crystallin was also found to have 3OHKyn bound to its single cysteine residue. Examination of normal aged human lenses revealed no evidence of oxidation of alpha-crystallin; however, oxidation was detected at methionine 1 in both alphaA- and alphaB-crystallin from human cataractous lenses. Age-related nuclear cataract is associated with coloration and insolubilization of lens proteins and extensive oxidation of cysteine and methionine residues. Our findings demonstrate that 3-hydroxykynurenine can readily catalyze the oxidation of methionine residues in both alphaB- and alphaA-crystallin, and it has been reported that alpha-crystallin modified in this way is a poorer chaperone. Thus, 3-hydroxykynurenine promotes the oxidation and modification of crystallins and may contribute to oxidative stress in the human lens.

Subunit Exchange of Polydisperse Proteins

The small heat shock protein, alpha-crystallin, plays a key role in maintaining lens transparency by chaperoning structurally compromised proteins. This is of particular importance in the human lens, where proteins are exposed to post-translational modifications over the life-time of an individual. Here, we examine the structural and functional consequences of one particular modification of alphaA-crystallin involving the truncation of 5 C-terminal residues (alphaA(1-168)). Using novel mass spectrometry approaches and established biophysical techniques, we show that alphaA(1-168) forms oligomeric assemblies with a lower average molecular mass than wild-type alphaA-crystallin (alphaA(WT)). Also apparent from the mass spectra of both alphaA(WT) and alphaA(1-168) assemblies is the predominance of oligomers containing even numbers of subunits; interestingly, this preference is more marked for alphaA(1-168). To examine the rate of exchange of subunits between assemblies, we mixed alphaB crystallin with either alphaA(WT) or alphaA(1-168) and monitored in a real-time mass spectrometry experiment the formation of heteroligomers. The results show that there is a significant decrease in the rate of exchange when alphaA(1-168) is involved. These reduced exchange kinetics, however, have no effect upon chaperone efficiency, which is found to be closely similar for both alphaA(WT) and alphaA(1-168). Overall, therefore, our results allow us to conclude that, in contrast to mechanisms established for analogous proteins from plants, yeast, and bacteria, the rate of subunit exchange is not the critical parameter in determining efficient chaperone behavior for mammalian alphaA-crystallin.

R120G alphaB-crystallin promotes the unfolding of reduced alpha-lactalbumin and is inherently unstable

alpha-Crystallin is the principal lens protein which, in addition to its structural role, also acts as a molecular chaperone, to prevent aggregation and precipitation of other lens proteins. One of its two subunits, alphaB-crystallin, is also expressed in many nonlenticular tissues, and a natural missense mutation, R120G, has been associated with cataract and desmin-related myopathy, a disorder of skeletal muscles [Vicart P, Caron A, Guicheney P, Li Z, Prevost MC, Faure A, Chateau D, Chapon F, Tome F, Dupret JM, Paulin D & Fardeau M (1998) Nat Genet20, 92-95]. In the present study, real-time 1H-NMR spectroscopy showed that the ability of R120G alphaB-crystallin to stabilize the partially folded, molten globule state of alpha-lactalbumin was significantly reduced in comparison with wild-type alphaB-crystallin. The mutant showed enhanced interaction with, and promoted unfolding of, reduced alpha-lactalbumin, but showed limited chaperone activity for other target proteins. Using NMR spectroscopy, gel electrophoresis, and MS, we observed that, unlike the wild-type protein, R120G alphaB-crystallin is intrinsically unstable in solution, with unfolding of the protein over time leading to aggregation and progressive truncation from the C-terminus. Light scattering, MS, and size-exclusion chromatography data indicated that R120G alphaB-crystallin exists as a larger oligomer than wild-type alphaB-crystallin, and its size increases with time. It is likely that removal of the positive charge from R120 of alphaB-crystallin causes partial unfolding, increased exposure of hydrophobic regions, and enhances its susceptibility to proteolysis, thus reducing its solubility and promoting its aggregation and complexation with other proteins. These characteristics may explain the involvement of R120G alphaB-crystallin with human disease states.

Tsp36, a tapeworm small heat-shock protein with a duplicated α-crystallin domain, forms dimers and tetramers with good chaperone-like activity

Small heat shock proteins (sHSPs), which range in monomer size between 12 and 42 kDa, are characterized by a conserved C-terminal alpha-crystallin domain of 80-100 residues. They generally form large homo- or heteromeric complexes, and typically have in vitro chaperone-like activity, keeping unfolding proteins in solution. A special type of sHSP, with a duplicated alpha-crystallin domain, is present in parasitic flatworms (Platyhelminthes). Considering that an alpha-crystallin domain is essential for the oligomerization and chaperone-like properties of sHSPs, we characterized Tsp36 from the tapeworm Taenia saginata. Both wild-type Tsp36 and a mutant (Tsp36C-->R) in which the single cysteine has been replaced by arginine were expressed and purified. Far-UV CD measurements of Tsp36 were in agreement with secondary structure predictions, which indicated alpha-helical structure in the N-terminal region and the expected beta-sandwich structure for the two alpha-crystallin domains. Gel permeation chromatography and nano-ESI-MS showed that wild type Tsp36 forms dimers in a reducing environment, and tetramers in a non-reducing environment. The tetramers are stabilized by disulfide bridges involving a large proportion of the Tsp36 monomers. Tsp36C-->R exclusively occurs as dimers according to gel permeation chromatography, while the nondisulfide bonded fraction of wild type Tsp36 dissociates from tetramers into dimers under nonreducing conditions at increased temperature (43 degrees C). The tetrameric form of Tsp36 has a greater chaperone-like activity than the dimeric form.

Phosphorylation of αB-Crystallin Alters Chaperone Function through Loss of Dimeric Substructure

Phosphorylation is the most common posttranslational modification of the alpha-crystallins in the human lens. These phosphorylated forms are not only important because of their abundance in aging lenses and the implications for cataract but also because they have been identified in patients with degenerative brain disease. By using mimics corresponding to the reported in vivo phosphorylation sites in the human lens, we have examined the effects of phosphorylation upon the chaperone-like properties and structure of alphaB-crystallin. Here we show that phosphorylation of alphaB-crystallin at Ser-45 results in uncontrolled aggregation. By using an innovative tandem mass spectrometry approach, we demonstrate how this alteration in behavior stems from disruption of dimeric substructure within the polydisperse alphaB-crystallin assembly. This structural perturbation appears to disturb the housekeeping role of alphaB-crystallin and consequently has important implications for the disease states caused by protein aggregation in the lens and deposition in non-lenticular tissue.

Interaction of the Molecular Chaperone αB-Crystallin with α-Synuclein: Effects on Amyloid Fibril Formation and Chaperone Activity

alpha-Synuclein is a pre-synaptic protein, the function of which is not completely understood, but its pathological form is involved in neurodegenerative diseases. In vitro, alpha-synuclein spontaneously forms amyloid fibrils. Here, we report that alphaB-crystallin, a molecular chaperone found in Lewy bodies that are characteristic of Parkinson's disease (PD), is a potent in vitro inhibitor of alpha-synuclein fibrillization, both of wild-type and the two mutant forms (A30P and A53T) that cause familial, early onset PD. In doing so, large irregular aggregates of alpha-synuclein and alphaB-crystallin are formed implying that alphaB-crystallin redirects alpha-synuclein from a fibril-formation pathway towards an amorphous aggregation pathway, thus reducing the amount of physiologically stable amyloid deposits in favor of easily degradable amorphous aggregates. alpha-Synuclein acts as a molecular chaperone to prevent the stress-induced, amorphous aggregation of target proteins. Compared to wild-type alpha-synuclein, both mutant forms have decreased chaperone activity in vitro against the aggregation of reduced insulin at 37 degrees C and the thermally induced aggregation of betaL-crystallin at 60 degrees C. Wild-type alpha-synuclein abrogates the chaperone activity of alphaB-crystallin to prevent the precipitation of reduced insulin. Interaction between these two chaperones and formation of a complex are also indicated by NMR spectroscopy, size-exclusion chromatography and mass spectrometry. In summary, alpha-synuclein and alphaB-crystallin interact readily with each other and affect each other's properties, in particular alpha-synuclein fibril formation and alphaB-crystallin chaperone action.

Temperature and concentration-controlled dynamics of rhizobial small heat shock proteins

A hallmark of alpha-crystallin-type small heat shock proteins (sHsps) is their highly dynamic oligomeric structure which promotes intermolecular interactions involved in subunit exchange and substrate binding (chaperone-like activity). We studied the oligomeric features of two classes of bacterial sHsps by size exclusion chromatography and nanoelectrospray mass spectrometry. Proteins of both classes formed large complexes that rapidly dissociated upon dilution and at physiologically relevant heat shock temperatures. As the secondary structure was not perturbed, temperature- and concentration-dependent dissociations were fully reversible. Complexes formed between sHsps and the model substrate citrate synthase were stable and exceeded the size of sHsp oligomers. Small Hsps, mutated in a highly conserved glycine residue at the C-terminal end of the alpha-crystallin domain, formed labile complexes that disassembled more readily than the corresponding wild-type proteins. Reduced complex stability coincided with reduced chaperone activity.

Investigating interactions of the pentraxins serum amyloid P component and C-reactive protein by mass spectrometry

The oligomeric state of human SAP (serum amyloid P component) in the absence and presence of known ligands has been investigated using nanoelectrospray ionization MS. At pH 8.0, in the absence of Ca2+, SAP has been shown to consist of pentameric and decameric forms. In the presence of physiological levels of Ca2+, SAP was observed to exist primarily as a pentamer, reflecting its in vivo state. dAMP was shown not only to promote decamerization, but also to lead to decamer stacking involving up to 30 monomers. A mechanism for this finding is proposed. CRP (C-reactive protein), a pentraxin closely related to SAP, exists as a pentamer in the presence or absence of Ca2+. Pentamers of CRP and SAP were shown to form mixed decamers in Ca2+-free buffer; however, in the presence of Ca2+, this interaction was not observed. Furthermore, no exchange of monomeric subunits was observed between the SAP and CRP oligomers, suggesting a remarkable stability of the individual pentameric complexes.

Polydispersity of a mammalian chaperone: mass spectrometry reveals the population of oligomers in alphaB-crystallin

The quaternary structure of the polydisperse mammalian chaperone alphaB-crystallin, a member of the small heat-shock protein family, has been investigated by using electrospray mass spectrometry. The intact assemblies give rise to mass spectra that are complicated by the overlapping of charge states from the different constituent oligomers. Therefore, to determine which oligomers are formed by this protein, tandem mass spectrometry experiments were performed. The spectra reveal a distribution, primarily of oligomers containing 24-33 subunits, the relative populations of which were quantified, to reveal a dominant species being composed of 28 subunits. Additionally, low levels of oligomers as small as 10-mers and as large as 40-mers were observed. Interpretation of the tandem mass spectral data was confirmed by simulating and summing spectra arising from the major individual oligomers. The ability of mass spectrometry to quantify the relative populations of particular oligomeric states also revealed that, contrary to the dimeric associations observed in other small heat-shock proteins, there is no evidence for any stable substructures of bovine alphaB-crystallin isolated from the lens.

UV filter instability: consequences for the human lens

Human lenses appear to become coloured with age primarily due to the covalent binding of UV filter compounds to lens proteins. These crystallin modifications result from the inherent instability of the kynurenine UV filters. Here we investigate this decomposition, the role this may have in the formation of other primate UV filters, and the interaction of the intermediates (alpha,beta-ketoalkenes) with lens components. The UV filters kynurenine, 3-hydroxykynurenine and 3-hydroxykynurenine glucoside were incubated at neutral pH in the presence or absence of NADH or NADPH. The three UV filters underwent spontaneous deamination, such that at pH 7 less than half of the starting materials (kynurenine (42%), 3-hydroxykynurenine glucoside (30%) and 3-hydroxykynurenine (21%)) remained after 7 days. In the presence of NAD(P)H, the double bond of the UV filter-derived deamination compounds, were reduced. Deamination of 3-hydroxykynurenine glucoside, followed by reduction with NAD(P)H, could thus account for the formation of the major lens UV filter 4-(2-amino-3-hydroxyphenyl)-4-oxobutanoic acid glucoside. beta-Benzoylacrylic acid, which possesses the same alpha,beta-ketoalkene sidechain as the deaminated kynurenine UV filters, underwent Michael addition with glutathione, was reduced (hydrogenated) by NAD(P)H, however was unreactive with ascorbate. Surprisingly, at pH 7 the UV filter-derived alpha,beta-ketoalkene intermediates, do not readily undergo intramolecular cyclization. This feature makes the double bond more available for reaction with protein nucleophilic residues and other lens components such as glutathione. On the basis of these data it is likely that glutathione and NAD(P)H, but not ascorbate, protect proteins in the lens from modification by UV filters.

Identifying sites of attachment of UV filters to proteins in older human lenses

Recent results indicate that covalent modification of proteins by tryptophan-derived UV filters may explain the age-dependent coloration of human lenses, and play a role in age-related cataract. The sites of attachment of the UV filters to the lens crystallins, however, have not been determined. This study utilized a database of predicted masses of UV filter-modified tryptic peptides to target sites of UV filter attachment. Proteins were isolated from old normal lenses and digested with trypsin at pH 6, in order to preserve the integrity of the sites of modification. Peptides were separated by high-performance liquid chromatography and characterized by mass spectrometry. Major colored and fluorescent peaks in the digest were found to correspond to cysteine-containing peptides in which the sulfur atom of the sidechain was linked to the major UV filter compound, 3-hydroxykynurenine glucoside. Three of the peptides originated from gammaS-crystallin and one from betaB1-crystallin. These results show that a predicted mass database can be used to facilitate the identification of sites of UV filter modification in human lens crystallins. Furthermore, this work represents the first evidence that UV filters bind to specific residues on lens proteins in vivo, and suggests that sulfhydryl groups may be important sites for the attachment of UV filters.

Glutathione and NADH, but not ascorbate, protect lens proteins from modification by UV filters

Age-dependent human lens colouration and fluorescence may stem primarily from the covalent binding of UV filters to crystallins. The tendency of the kynurenine (Kyn) UV filters to deaminate at neutral pH, with the generation of reactive alpha,beta-ketoalkenes, underlies this phenomenon. In this study the authors examined the ability of small molecular weight antioxidants, which are known to be present in the lens, to inhibit this process. Crystallins were incubated with Kyn at pH 7 in the presence of glutathione (GSH), ascorbate or NADH. Ascorbate, even at high (15 m M) levels, was not found to significantly retard the time-dependent covalent binding of Kyn to the proteins. GSH, and to a lesser extent NADH, however, had a major impact in preventing this modification. The increase in protein UV absorbance and fluorescence was inhibited by GSH intercepting the reactive ketone intermediate, to form a GSH-Kyn adduct. NADH seemed to protect by both reduction of the reactive ketone intermediate and by competing with Kyn for presumably hydrophobic sites on the crystallins. This may indicate that the covalent attachment of aromatic Kyn molecules could be facilitated by initial hydrophobic interactions. Since GSH is present at far greater concentrations than NADH, these results show that in primate lenses, GSH is the key agent responsible for protecting the crystallins from covalent modification.

Novel protein modification by kynurenine in human lenses

It is known that human lenses increase in color and fluorescence with age, but the molecular basis for this is not well understood. We demonstrate here that proteins isolated from human lenses contain significant levels of the UV filter kynurenine covalently bound to histidine and lysine residues. Identification was confirmed by synthesis of the kynurenine amino acid adducts and comparison of the chromatographic retention times and mass spectra of these authentic standards with those of corresponding adducts isolated from human lenses following acid hydrolysis. Using calf lens proteins as a model, covalent binding of kynurenine to lens proteins has been shown to proceed via side chain deamination in a manner analogous to that observed for the related UV filter, 3-hydroxykynurenine O-beta-D-glucoside. Levels of histidylkynurenine and lysylkynurenine were low in human lenses in subjects younger than 30, but thereafter increased in concentration with the age of the individual. Post-translational modification of lens proteins by tryptophan metabolites therefore appears to be responsible, at least in part, for the age-dependent increase in coloration and fluorescence of the human lens, and this process may also be important in other tissues in which up-regulation of tryptophan catabolism occurs.

Identification of a new human lens UV filter compound

A new UV filter compound, 4-(2-amino-3-hydroxyphenyl)-4-oxobutanoic acid O-diglucoside, has been identified in human lenses. The structure suggests that it is a further metabolic product of the second most abundant UV filter compound, 4-(2-amino-3-hydroxyphenyl)-4-oxobutanoic acid O-glucoside. Quantification studies on the new compound show that it decreases towards zero in both the nucleus and cortex as a function of age. The discovery of this novel disaccharide completes the identification of the major UV filter compounds present in the human lens.

Kynurenine binds to the peptide binding region of the chaperone alphaB-crystallin

UV filters, such as kynurenine, are present in the human lens. They are spontaneously unstable at neutral pH and deaminate to form reactive alpha, beta unsaturated ketones. This process becomes more prominent after the lens barrier develops in middle age. Here we show that deaminated kynurenine reacts primarily with histidine residues in alphaB-crystallin: a major lens protein that lacks cysteine. Five of the nine histidines in alphaB-crystallin were found to be conjugated with kynurenine. Furthermore, a major site of covalent modification was at histidine 83, which is found in the putative peptide binding region of alphaB-crystallin; a site crucial for its role as a chaperone. We propose that modification of alphaB-crystallin by UV filters may compromise the chaperone action of this protein.

Polypeptide modification and cross-linking by oxidized 3-hydroxykynurenine

3-Hydroxykynurenine (3OHKyn) is present in the mammalian lens as a UV filter and is formed from kynurenine in the tryptophan metabolic pathway. 3OHKyn is a readily autoxidized o-aminophenol which binds to proteins in vitro. The lens, particularly its central region, the nucleus, becomes increasingly oxidized with age. Under such conditions, the oxidation products of 3OHKyn may bind to lens proteins and contribute to nuclear cataract formation. The purpose of this study was to determine the structures of in vitro reaction products of 3OHKyn with model peptides as a general model for 3OHKyn modification of proteins. 3OHKyn was incubated with the dipeptide glycylglycine (GG) and the tetrapeptide tuftsin (sequence TKPR) under oxidizing conditions, and the reaction products were characterized by a variety of spectroscopic techniques. The major 3OHKyn-GG reaction product involves formation of a benzimidazole moiety between the GG N-terminus and the oxidized amino and/or phenol groups of 3OHKyn. In contrast, tuftsin, which has an N-terminal threonine, forms predominantly a cross-linked dimer with oxidized 3OHKyn. This product is analogous in structure to the dimeric reaction product, quinilinobenzoxamine, formed between oxidized 3OHKyn and glycyllysine [Aquilina, J. A., et al. (1999) Biochemistry 38, 11455-11464], which contains a benzoxazole moiety. The identification of a tuftsin dimer suggests that 3OHKyn can react with any peptide having a free alpha-amino group, via a general side chain elimination mechanism. The identification of both benzimidazole and benzoxazole adducts in peptides with a free N-terminus suggests that peptide amino groups can react initially at either the aromatic amino or hydroxyl group of oxidized 3OHKyn. The proportion of each adduct may change, however, depending on the amino acid sequence at the N-terminus.

Cysteine is the initial site of modification of alpha-crystallin by kynurenine

Tryptophan metabolites, such as kynurenine, are spontaneously unstable at neutral pH. They undergo side-chain deamination yielding reactive alpha, beta unsaturated ketones. In the lens, where these compounds act as UV filters, reaction of the breakdown products with lens proteins (crystallins) may be largely responsible for age-dependent colouration of this tissue. In previous research, where high pH (pH 9) was used to promote deamination and conjugation with lens protein, histidine, lysine, and cysteine residues were found to be modified. In this study we show that, at pH 7, site of reaction with the major lens chaperone alpha-crystallin, is the single cysteine residue of the alphaA subunit. This apparent selectivity has important ramifications because the cysteine-kynurenine adduct is itself unstable under physiological conditions.

Elucidation of a novel polypeptide cross-link involving 3-hydroxykynurenine

3-Hydroxykynurenine, a metabolite of tryptophan, is a powerful antioxidant and neurotoxin. The neurotoxicity results from the oxidation of 3-hydroxykynurenine, and hydroxyl radicals, formed via H(2)O(2), may also be implicated [Okuda, S., Nishiyama, N., Saito, H. , and Katsuki, H. (1996) Proc. Natl. Acad. Sci. U.S.A. 93, 12553-12558]. Oxidation of o-aminophenols, such as 3-hydroxykynurenine, also results in the formation of highly reactive quinonimines. Thus, one possible consequence of 3-hydroxykynurenine oxidation may be covalent modification of cellular macromolecules. Such a process could contribute to the neurotoxicity and may potentially be important in other tissues, such as the human lens, where 3-hydroxykynurenine functions as a UV filter. In this work, we demonstrate that 3-hydroxykynurenine can bind to protein amino groups and, further, that under oxidative conditions, 3-hydroxykynurenine can function to cross-link polypeptide chains. The structure of the cross-linked moiety, using the peptide glycyllysine, has been elucidated. The cross-link, which is both colored and fluorescent, involves the peptide alpha-amino groups. Proteins modified by 3-hydroxykynurenine become colored and fluorescent as well as cross-linked. LC-MS studies indicate that the cross-link is also present in gamma-crystallin, following incubation of this lens protein in the presence of 3-hydroxykynurenine. Similar posttranslational modifications of lens proteins accompany cataract formation, and knowledge of the precise mode of reaction of 3-hydroxykynurenine with proteins will assist in determining if 3-hydroxykynurenine is involved in degenerative conditions in which oxidation of such aminophenols is implicated.

Oxidation products of 3-hydroxykynurenine bind to lens proteins: relevance for nuclear cataract

3-Hydroxykynurenine (3OHKyn), present as a human lens UV filter, has also been implicated as a carcinogen and neurotoxin. It has been suggested that oxidation of 3OHKyn is involved in each of these effects. In the presence of oxygen, 3OHKyn has been found to react with bovine crystallins, to give brown-coloured products (Stutchbury and Truscott, 1993). In this study the roles of UV-light, pH, glutathione and oxygen were examined, with the objective of determining how these factors may affect the binding of 3OHKyn to crystallins under the conditions found within the lens itself. The presence of oxygen was found to be an important parameter for determining the extent to which 3OHKyn reacts with protein, and when it was totally excluded, little modification was observed. UV-light was not required for activation, but was found to augment the extent of modification and cross-linking, while an elevated pH, which is known to accelerate the rate of 3OHKyn oxidation, did not markedly increase the extent of reaction with the crystallins. 3OHKyn binding was accompanied by crystallin aggregation, pigmentation, and development of non-tryptophan fluorescence, all of which have been associated with cataract formation. The inclusion of glutathione, a ubiquitous antioxidant, in reaction mixtures resulted in a delayed onset of crystallin modification. This effect was apparent at concentrations of glutathione greater than 1 mM. When glutathione levels fell below 1 mM, crystallins became modified by 3OHKyn. Since lens glutathione concentrations decrease with age, and are known to be lower in the lens nucleus than the cortex, this region appears particularly vulnerable to modification by this UV filter. Thus, whilst the other human lens UV filters, kynurenine (Kyn) and 3-hydroxykynurenine glucoside (3HKG), appear to require activation by UV-light in order to react with proteins, 3OHKyn can modify crystallins in the absence of light, under conditions of low oxygen tension, and in the presence of glutathione concentrations found in the nucleus of an aged lens. Its reactivity is increased in the presence of both light and oxygen. The contributions of these parameters to the reactivity of 3OHKyn are discussed, with respect to the aetiology of senile nuclear cataract.

Age-related changes in bovine alpha-crystallin and high-molecular-weight protein

The high-molecular-weight (HMW) protein from the lens is composed mostly of alpha-crystallin in a highly aggregated state. Bovine HMW protein was carefully separated from alpha-crystallin by size-exclusion chromatography. alpha-Crystallin has chaperone-like ability whereby it stabilizes other proteins under conditions of stress (e.g. heat). Comparison of bovine HMW protein and alpha-crystallin shows that the HMW protein has a markedly reduced chaperone ability compared to alpha-crystallin. However, in contrast to the results of other workers, we observe no alteration with age in the ability of alpha-crystallin to act as a chaperone. Using electrospray ionisation mass spectrometry, changes in the phosphorylation of the alpha-crystallin subunits with age have been quantified. Phosphorylation of alpha-crystallin occurs early in life but does not alter in proportion after about three years of age. In addition, phosphorylation of the A subunit of alpha-crystallin has little effect on its chaperone ability. As is found in the artificially prepared HMW complex of alpha- and gamma-crystallin, NMR spectroscopy shows that in the naturally occurring HMW protein, the short C-terminal extension of the alpha B subunit has lost its flexibility whereas the alpha A subunit extension is still flexible. Post-translational modifications therefore seem to have little effect on the chaperone action of alpha-crystallin, but alterations in the quaternary structure of alpha-crystallin via incorporation into the HMW aggregate, lead to major changes in the chaperone ability of the protein. The results are consistent with the notion that one of the contributing factors to cataract formation in the lens is the depletion of alpha-crystallin with age as it is converted into the HMW protein.

Supramolecular order within the lens: 1H NMR spectroscopic evidence for specific crystallin-crystallin interactions

alpha-, beta- and gamma-crystallins from bovine lens contain flexible terminal extensions which are readily observed by NMR spectroscopy. By monitoring these resonances, NMR spectroscopy therefore offers a means of examining specific protein-protein interactions in crystallin mixtures. In this paper, a 1H NMR spectroscopic study of bovine lens nuclear and cortical homogenates and various crystallin mixtures is presented. In both homogenates, resonances from the flexible C-terminal extensions of alpha-crystallin and the N-terminal extension of beta B2-crystallin are readily observed suggesting that these regions are not involved in crystallin-crystallin interactions. In the cortical homogenate, resonances from the short N-terminal extension of gamma S-crystallin are also present. The cortical homogenate gives rise to more intense resonances than the nuclear homogenate, suggesting that the cortical region has many more mobile crystallin regions. In both homogenates, the C-terminal extension of beta B2-crystallin and the very short C-terminal extension of gamma B-crystallin are not observed. Thus, the C-terminal regions of these proteins are involved in interactions with other crystallins. Similar effects are observed upon mixing of the individual crystallins, e.g. the C-terminal extension of gamma B-crystallin is absent in spectra of mixtures of total gamma-crystallin and high-molecular-weight beta-crystallin aggregates (beta H). Overall, the results are consistent with a short-range order for the crystallins within the lens.