Interactions and chaperone function of alphaA-crystallin with T5P gammaC-crystallin mutant

The Y46D Mutation Destabilizes Dense Packing of the Second Greek Key Pair of Human γC-Crystallin Causing Congenital Nuclear Cataracts

The γ-crystallins are highly expressed structural lens proteins comprising four Greek key motifs arranged in two domains. Their globular structure and short-range spatial ordering are essential for lens transparency. Aromatic residues play a vital role in stabilizing Greek key folds by forming Greek key or non-Greek key pairs or tyrosine corners. We investigated the effects of the cataractogenic Y46D mutation in the second Greek key pair (Y46-Y51) of human γC-crystallin on its stability and aggregation. Wild-type and Y46D mutant human γC-crystallin were overexpressed in E. coli BL-21(DE3) PLysS cells, purified using ion-exchange and size-exclusion chromatography, and analyzed by fluorescence spectroscopy and circular dichroism spectroscopy. The Y46D mutation does not affect the γC-crystallin backbone conformation under benign conditions but alters the tryptophan microenvironment, exposing hydrophobic residues to the surface. The Y46D mutant undergoes a three-state transition under thermal stress with midpoints of 54.6 and 67.7 °C while the wild type shows a two-state transition with a midpoint of 77.6 °C. The Y46D mutant also shows a three-state transition under GuHCl stress with Cm values of 0.9 and 2.1 M while the wild type shows a two-state transition with a Cm of 2.4 M GuHCl. Mutant but not wild-type γC-crystallin forms light scattering particles upon heating at 65 °C. Overall, the Y46D CRYGS mutation leaves the protein fold intact under benign conditions but destabilizes the molecule by altering the tryptophan microenvironment and exposing hydrophobic residues to its surface, thus increasing its susceptibility to thermal and chemical stress with resultant self-aggregation, light scattering, and cataract.

Biochemical characterization of G64W mutant of acidic beta-crystallin 4

Crystallins are structural proteins in the lens that last a lifetime with little turnover. Deviant in crystallins can cause rare but severe visual impairment, namely, congenital cataracts. It is reported that several mutations in the acidic β-crystallin 4 (CRYBA4) are related to congenital cataracts. However, the pathogenesis of these mutants is not well understood at molecular level. Here we evaluate the biochemical properties of wild type CRYBA4 (CRYBA4^WT) and a pathogenic G64W mutant (CRYBA4^G64W) including protein folding, polymerization state and protein stability. Furthermore, we explore the differences in their interactions with α-crystallin A (CRYAA) and basic β-crystallin 1 (CRYBB1) via yeast two-hybrid and pull-down assay in vitro, through which we find that G64W mutation leads to protein misfolding, decreases protein stability, blocks its interaction with CRYBB1 but maintains its interaction with CRYAA. Our results deepen our understanding of the pathogenesis of congenital cataracts.

Multidimensional significance of crystallin protein-protein interactions and their implications in various human diseases

BACKGROUND

Crystallins are the important structural and functional proteins in the eye lens responsible for refractive index. Post-translational modifications (PTMs) and mutations are major causative factors that affect crystallin structural conformation and functional characteristics thus playing a vital role in the etiology of cataractogenesis.

SCOPE OF REVIEW

The significance of crystallin protein-protein interactions (PPIs) in the lens and non-lenticular tissues is summarized.

MAJOR CONCLUSIONS

Aberrancy of PPIs between crystallin, its associated protein and metal ions has been accomplished in various human diseases including cataract. A detailed account on multidimensional structural and functional significance of crystallin PPI in humans must be brought into limelight, in order to understand the biochemical and molecular basis augmenting the aberrancies of such interaction. In this scenario, the present review is focused to shed light on studies which will aid to expand our present understanding on disease pathogenesis related to loss of PPI thereby paving the way for putative future therapeutic targets to curb such diseases.

GENERAL SIGNIFICANCE

The interactions with α-crystallins always aid to protect their structural and functional characteristics. The up-regulation of αB-crystallin in the non-lenticular tissues always decodes as biomarker for various stress related disorders. For better understanding and treatment of various diseases, PPI studies provide overall outline about the structural and functional characteristics of the proteins. This information not only helps to find out the route of cataractogenesis but also aid to identify potential molecules to inhibit/prevent the further development of such complicated phenomenon. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

Looking the cow in the eye: deletion in the NID1 gene is associated with recessive inherited cataract in Romagnola cattle

Cataract is a known condition leading to opacification of the eye lens causing partial or total blindness. Mutations are known to cause autosomal dominant or recessive inherited forms of cataracts in humans, mice, rats, guinea pigs and dogs. The use of large-sized animal models instead of those using mice for the study of this condition has been discussed due to the small size of rodent lenses. Four juvenile-onset cases of bilateral incomplete immature nuclear cataract were recently observed in Romagnola cattle. Pedigree analysis suggested a monogenic autosomal recessive inheritance. In addition to the cataract, one of the cases displayed abnormal head movements. Genome-wide association and homozygosity mapping and subsequent whole genome sequencing of a single case identified two perfectly associated sequence variants in a critical interval of 7.2 Mb on cattle chromosome 28: a missense point mutation located in an uncharacterized locus and an 855 bp deletion across the exon 19/intron 19 border of the bovine nidogen 1 (NID1) gene (c.3579_3604+829del). RT-PCR showed that NID1 is expressed in bovine lenses while the transcript of the second locus was absent. The NID1 deletion leads to the skipping of exon 19 during transcription and is therefore predicted to cause a frameshift and premature stop codon (p.1164fs27X). The truncated protein lacks a C-terminal domain essential for binding with matrix assembly complexes. Nidogen 1 deficient mice show neurological abnormalities and highly irregular crystal lens alterations. This study adds NID1 to the list of candidate genes for inherited cataract in humans and is the first report of a naturally occurring mutation leading to non-syndromic catarct in cattle provides a potential large animal model for human cataract.

Crystalline cataract caused by a heterozygous missense mutation in γD-crystallin (CRYGD)

PURPOSE

To describe phenotypic characteristics of two pedigrees manifesting early onset crystalline cataract with mutations in the γD-crystallin gene (CRYGD).

METHODS

A detailed medical history was obtained from two Caucasian pedigrees manifesting autosomal dominant congenital cataracts. Genomic DNA was extracted from saliva (DNA Genotek). Single Nucleotide Polymorphism (SNP) based genome analysis of the larger pedigree revealed linkage to an 8.2 MB region on chromosome 2q33-q35 which encompassed the crystallin-gamma gene cluster (CRYG). Exons and flanking introns of CRYGA, CRYGB, CRYGC and CRYGD were amplified and sequenced to identify disease-causing mutations.

RESULTS

A morphologically unique cataract with extensive refractile "crystals" scattered throughout the nucleus and perinuclear cortex was found in the probands from both pedigrees. A heterozygous C→A mutation was identified at position 109 of the coding sequence (R36S of the processed protein) in exon 2 of CRYGD and this missense mutation was found to cosegregate with the disease in the larger family; this mutation was then identified in affected individuals of pedigree 2 as well.

CONCLUSIONS

The heterozygous 109C→A CRYGD missense mutation is associated with a distinct crystalline cataract in two US Caucasian pedigrees. This confirms crystalline cataract formation with this mutation, as previously reported in sporadic childhood case from the Czech Republic and in members of a Chinese family.

Molecular characteristics of inherited congenital cataracts

Congenital cataracts are a major cause of induced blindness in children, and inherited cataracts are the major cause of congenital cataracts. Inherited congenital cataracts have been associated with mutations in specific genes, including those of crystallins, gap junction proteins, membrane transport and channel proteins, the cytoskeleton, and growth and transcription factors. Locating and identifying the genes and mutations involved in cataractogenesis are essential to gaining an understanding of the molecular defects and pathophysiologic characteristics of inherited congenital cataracts. In this review, we summarize the current research in this field.

Mechanism of the very efficient quenching of tryptophan fluorescence in human gamma D- and gamma S-crystallins: the gamma-crystallin fold may have evolved to protect tryptophan residues from ultraviolet photodamage

Proteins exposed to UV radiation are subject to irreversible photodamage through covalent modification of tryptophans (Trps) and other UV-absorbing amino acids. Crystallins, the major protein components of the vertebrate eye lens that maintain lens transparency, are exposed to ambient UV radiation throughout life. The duplicated beta-sheet Greek key domains of beta- and gamma-crystallins in humans and all other vertebrates each have two conserved buried Trps. Experiments and computation showed that the fluorescence of these Trps in human gammaD-crystallin is very efficiently quenched in the native state by electrostatically enabled electron transfer to a backbone amide [Chen et al. (2006) Biochemistry 45, 11552-11563]. This dispersal of the excited state energy would be expected to minimize protein damage from covalent scission of the excited Trp ring. We report here both experiments and computation showing that the same fast electron transfer mechanism is operating in a different crystallin, human gammaS-crystallin. Examination of solved structures of other crystallins reveals that the Trp conformation, as well as favorably oriented bound waters, and the proximity of the backbone carbonyl oxygen of the n - 3 residues before the quenched Trps (residue n), are conserved in most crystallins. These results indicate that fast charge transfer quenching is an evolved property of this protein fold, probably protecting it from UV-induced photodamage. This UV resistance may have contributed to the selection of the Greek key fold as the major lens protein in all vertebrates.

Protein-protein interactions involving congenital cataract T5P gammaC-crystallin mutant: a confocal fluorescence microscopy study

The human lens crystallin gene CRYGC T5P is associated with Coppock-like cataract and has a phenotype of a dust-like opacity of the fetal lens nucleus and deep cortical region. Previous in vitro mutation studies indicate that the protein has changed conformation, solubility, and stability, which may make it susceptible to aggregation, as seen in cataractous lens and cell culture expression. To investigate the mechanisms leading to these events, we studied protein-protein interactions using confocal fluorescence resonance energy transfer (FRET) microscopy. The method detects protein-protein interactions in the natural environment of living cells. Crystallin genes (CRYGC T5P, CRYGC, and CRYAA) were fused to either the green fluorescence protein (GFP) or red fluorescence protein (DsRED or RFP) vector. Each of the following GFP-RFP (donor-acceptor) plasmid pairs was cotransfected into HeLa cells: gammaC-gammaC, gammaC-gammaCT5P, gammaCT5P-gammaCT5P, alphaA-gammaC, and alphaA-gammaCT5P. After culture, confocal fluorescence cell images were taken. Protein-protein interactions in the form of net FRET were evaluated. The confocal fluorescence images show that cells expressing T5P gammaC-crystallin contain many protein aggregates, but cells co-expressing with either gammaC- or alphaA-crystallin reduce the aggregation considerably. FRET determination indicates that gammaCT5P-gammaCT5P shows less protein-protein interaction than either gammaC-gammaC or gammaC-gammaCT5P. Cotransfection with alphaA-crystallin (alphaA-gammaC or alphaA-T5PgammaC) increases nFRET compared with gammaC-gammaC or gammaC-T5PgammaC. Our results demonstrate that T5P gammaC-crystallin shows more protein aggregates and less protein-protein interaction than WT gammaC-crystallin. Chaperone alphaA-crystallin can rescue T5P gammaC-crystallin from aggregation through increased protein interaction. The formation of congenital cataract may be due to reduced protein-protein interactions and increased aggregation from an insufficient amount of alpha-crystallin for protection.

Mechanism of the efficient tryptophan fluorescence quenching in human gammaD-crystallin studied by time-resolved fluorescence

Human γD-crystallin (HγD-Crys) is a two-domain, β-sheet eye lens protein found in the lens nucleus. Its long-term solubility and stability are important to maintain lens transparency throughout life. HγD-Crys has four highly conserved buried tryptophans (Trps), with two in each of the homologous β-sheet domains. In situ, these Trps will be absorbing ambient UV radiation that reaches the lens. The dispersal of the excited-state energy to avoid covalent damage is likely to be physiologically relevant for the lens crystallins. Trp fluorescence is efficiently quenched in native HγD-Crys. Previous steady-state fluorescence measurements provide strong evidence for energy transfer from Trp42 to Trp68 in the N-terminal domain and from Trp130 to Trp156 in the C-terminal domain [Chen, J., et al. (2006) Biochemistry 45, 11552−11563]. Hybrid quantum mechanical−molecular mechanical (QM-MM) simulations indicated that the fluorescence of Trp68 and Trp156 is quenched by fast electron transfer to the amide backbone. Here we report additional information obtained using time-resolved fluorescence spectroscopy. In the single-Trp-containing proteins (Trp42-only, Trp68-only, Trp130-only, and Trp156-only), the highly quenched Trp68 and Trp156 have very short lifetimes, τ ∼0.1 ns, whereas the moderately fluorescent Trp42 and Trp130 have longer lifetimes, τ ∼3 ns. In the presence of the energy acceptor (Trp68 or Trp156), the lifetime of the energy donor (Trp42 or Trp130) decreased from ∼3 to ∼1 ns. The intradomain energy transfer efficiency is 56% in the N-terminal domain and is 71% in the C-terminal domain. The experimental values of energy transfer efficiency are in good agreement with those calculated theoretically. The absence of a time-dependent red shift in the time-resolved emission spectra of Trp130 proves that its local environment is very rigid. Time-resolved fluorescence anisotropy measurements with the single-Trp-containing proteins, Trp42-only and Trp130-only, indicate that the protein rotates as a rigid body and no segmental motion is detected. A combination of energy transfer with electron transfer results in short excited-state lifetimes of all Trps, which, together with the high rigidity of the protein matrix around Trps, could protect HγD-Crys from excited-state reactions causing permanent covalent damage.

A nonsense mutation in CRYGC associated with autosomal dominant congenital nuclear cataract in a Chinese family

PURPOSE

To identify the genetic defect associated with autosomal dominant congenital nuclear cataract in a Chinese family.

METHODS

Family history and phenotypic data were recorded, and the phenotypes were documented by slit lamp photography. The genomic DNA was extracted from peripheral blood leukocytes. All the exons and flanking intronic sequences of CRYGC and CRYGD were amplified by polymerase chain reaction (PCR) and screened for mutation by direct DNA sequencing. Structural models of the wild type and mutant gammaC-crystallin were generated and analyzed by SWISS-MODEL.

RESULTS

Sequencing of the coding regions of CRYGC and CRYGD showed the presence of a heterozygous C>A transversion at c.327 of the coding sequence in exon 3 of CRYGC (c.327C>A), which results in the substitution of a wild type cysteine to a nonsense codon (C109X). One and a half Greek key motifs at the COOH-terminus were found to be absent in the structural model of the mutant truncated gammaC-crystallin.

CONCLUSIONS

A novel nonsense mutation in CRYGC was detected in a Chinese family with consistent autosomal dominant congenital nuclear cataract, providing clear evidence of a relationship between the genotype and the corresponding cataract phenotype.

Confocal fluorescence resonance energy transfer microscopy study of protein-protein interactions of lens crystallins in living cells

PURPOSE

To determine protein-protein interactions among lens crystallins in living cells.

METHODS

Fluorescence resonance energy transfer (FRET) microscopy was used to visualize interactions in living cells directly. Two genes, one (alphaA-crystallin) fused with green fluorescence protein (GFP) and the other (each of the following genes: alphaB-, betaB2-, gammaC-crystallin, and R120G alphaB-crystallin mutant) fused with GFP variant red fluorescence protein (RED), were cotransfected into HeLa cells. After culture, confocal microscopy images were taken and FRET values were calculated.

RESULTS

FRET occurs when the two proteins interact. The data show strong interactions between alphaA- and alphaB-crystallin and weak interactions between alphaA- and betaB2- or gammaC-crystallin, which is consistent with our previous two-hybrid system study. The R120G alphaB-crystallin mutant, however, showed significantly less FRET than wild-type alphaB-crystallin. There are also more R120G alphaB-crystallin transfected cells with protein aggregates than wild-type alphaB-crystallin transfected cells. Cotransfection with alphaA-crystallin could not rescue R120G alphaB-crystallin from aggregation.

CONCLUSIONS

FRET microscopy gave excellent results on the protein-protein interactions among crystallins. It supports many previous studies and provides a novel technique for further study of protein-protein interactions among lens proteins including membrane and cytoskeletal proteins.

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.

Interdomain side-chain interactions in human gammaD crystallin influencing folding and stability

Human gammaD crystallin (HgammaD-Crys) is a two domain, beta-sheet eye lens protein that must remain soluble throughout life for lens transparency. Single amino acid substitutions of HgammaD-Crys are associated with juvenile-onset cataracts. Features of the interface between the two domains conserved among gamma-crystallins are a central six-residue hydrophobic cluster, and two pairs of interacting residues flanking the cluster. In HgammaD-Crys these pairs are Gln54/Gln143 and Arg79/Met147. We previously reported contributions of the hydrophobic cluster residues to protein stability. In this study alanine substitutions of the flanking residue pairs were constructed and analyzed. Equilibrium unfolding/refolding experiments at 37 degrees C revealed a plateau in the unfolding/refolding transitions, suggesting population of a partially folded intermediate with a folded C-terminal domain (C-td) and unfolded N-terminal domain (N-td). The N-td was destabilized by substituting residues from both domains. In contrast, the C-td was not significantly affected by substitutions of either domain. Refolding rates of the N-td were significantly decreased for mutants of either domain. In contrast, refolding rates of the C-td were similar to wild type for mutants of either domain. Therefore, domain interface residues of the folded C-td probably nucleate refolding of the N-td. We suggest that these residues stabilize the native state by shielding the central hydrophobic cluster from solvent. Glutamine and methionine side chains are among the residues covalently damaged in aged and cataractous lenses. Such damage may generate partially unfolded, aggregation- prone conformations of HgammaD-Crys that could be significant in cataract.

Genetic disorders involving molecular-chaperone genes: a perspective

Molecular chaperones are important for maintaining a functional set of proteins in all cellular compartments. Together with protein degradation machineries (e.g., the ubiquitin-proteasome system), chaperones form the core of the cellular protein-quality control mechanism. Chaperones are proteins, and as such, they can be affected by mutations. At least 15 disorders have been identified that are associated with mutations in genes encoding chaperones, or molecules with features suggesting that they function as chaperones. These chaperonopathies and a few other candidates are presented in this article. In most cases, the mechanisms by which the defective genes contribute to the observed phenotypes are still uncharacterized. However, the reported observations definitely point to the possibility that abnormal chaperones participate in pathogenesis. The available data open novel perspectives and should encourage searches for new genetic chaperonopathies, as well as further analyses of the disorders discussed in this article, including detection of new cases.

Contributions of hydrophobic domain interface interactions to the folding and stability of human gammaD-crystallin

Human gammaD-crystallin (HgammaD-Crys) is a monomeric eye lens protein composed of two highly homologous beta-sheet domains. The domains interact through interdomain side chain contacts forming two structurally distinct regions, a central hydrophobic cluster and peripheral residues. The hydrophobic cluster contains Met43, Phe56, and Ile81 from the N-terminal domain (N-td) and Val132, Leu145, and Val170 from the C-terminal domain (C-td). Equilibrium unfolding/refolding of wild-type HgammaD-Crys in guanidine hydrochloride (GuHCl) was best fit to a three-state model with transition midpoints of 2.2 and 2.8 M GuHCl. The two transitions likely corresponded to sequential unfolding/refolding of the N-td and the C-td. Previous kinetic experiments revealed that the C-td refolds more rapidly than the N-td. We constructed alanine substitutions of the hydrophobic interface residues to analyze their roles in folding and stability. After purification from E. coli, all mutant proteins adopted a native-like structure similar to wild type. The mutants F56A, I81A, V132A, and L145A had a destabilized N-td, causing greater population of the single folded domain intermediate. Compared to wild type, these mutants also had reduced rates for productive refolding of the N-td but not the C-td. These data suggest a refolding pathway where the domain interface residues of the refolded C-td act as a nucleating center for refolding of the N-td. Specificity of domain interface interactions is likely important for preventing incorrect associations in the high protein concentrations of the lens nucleus.

Crystallin {gamma}B-I4F mutant protein binds to {alpha}-crystallin and affects lens transparency

A new mouse mutant line, Clapper, identified from N-ethyl-N-nitrosurea (ENU)-mutagenized mice, develops a dominant lamellar cataract. The cataract blocks the image of retinal fundus and transmits a fuzzy fluorescein image of retinal vasculature during angiography. The cataractous lens opacity decreases as the mice age. The Clapper mutation has been identified to be a missense mutation of the gammaB-crystallin gene that replaces the 4th isoleucine residue with a phenylalanine (gammaB-I4F). Unlike wild type gammaB, the gammaB-I4F mutant protein binds to alpha-crystallin to form high molecular weight complexes in vivo and in vitro. Circular dichroism measurements indicate that gammaB-I4F protein is less stable than wild type gammaB at high temperature. Darkly stained aggregates, enlarged interfiber spaces, and disorganized and smaller inner mature fibers were found in the regions of the cataract in homozygous Clapper mutant lenses. Thus, the lamellar cataract is likely due to the light-scattering effects of the enlarged interfiber spaces and protein aggregates caused by gammaB-I4F mutant proteins interacting with alpha-crystallin in the lens.