Guanidine hydrochloride induced reversible dissociation and denaturation of duck delta2-crystallin

Lys-315 at the Interfaces of Diagonal Subunits of δ-Crystallin Plays a Critical Role in the Reversibility of Folding and Subunit Assembly

δ-Crystallin is the major structural protein in avian eye lenses and is homologous to the urea cycle enzyme argininosuccinate lyase. This protein is structurally assembled as double dimers. Lys-315 is the only residue which is arranged symmetrically at the diagonal subunit interfaces to interact with each other. This study found that wild-type protein had both dimers and monomers present in 2–4 M urea whilst only monomers of the K315A mutant were observed under the same conditions, as judged by sedimentation velocity analysis. The assembly of monomeric K315A mutant was reversible in contrast to wild-type protein. Molecular dynamics simulations showed that the dissociation of primary dimers is prior to the diagonal dimers in wild-type protein. These results suggest the critical role of Lys-315 in stabilization of the diagonal dimer structure. Guanidinium hydrochloride (GdmCl) denatured wild-type or K315A mutant protein did not fold into functional protein. However, the urea dissociated monomers of K315A mutant protein in GdmCl were reversible folding through a multiple steps mechanism as measured by tryptophan and ANS fluorescence. Two partly unfolded intermediates were detected in the pathway. Refolding of the intermediates resulted in a conformation with greater amounts of hydrophobic regions exposed which was prone to the formation of protein aggregates. The formation of aggregates was not prevented by the addition of α-crystallin. These results highlight that the conformational status of the monomers is critical for determining whether reversible oligomerization or aggregate formation occurs.

Substitution of residues at the double dimer interface affects the stability and oligomerization of goose delta-crystallin

Delta-crystallin is the major structural protein in avian and reptilian eye lenses, and confers special refractive properties. The protein is a homotetramer arranged as a dimer of dimers. In the present study, the roles of the side chains of Glu267, Lys315, and Glu327, which provide hydrogen bonds at the double dimer interface, were investigated. Hydrophobic side chain substitution led to all mutant proteins having an unstable dimer interface. The E267L/E327L mutant had the greatest sensitivity to temperature, urea and guanidinium hydrochloride denaturation, and the most extensive exposure of hydrophobic patches, as judged by 1-anilinonaphthalene-8-sulfonic acid fluorescence, CD, and tryptophan fluorescence. In contrast, the E267L/K315L/E327L mutant showed higher stability than the E267L/E327L mutant. Some level of the dissociated dimeric form was observed in the K315L mutant, but it was not observed for the K315A and E267L/K315L mutants. The E327L mutant was partially in the dissociated dimeric form, whereas the E267/E327L mutant was predominantly dissociated into dimers. In contrast, the triple mutant of E267L/K315L/E327L retained a tetrameric structure. In the presence of urea, a stable monomeric intermediate with higher stability than the wild type was identified for the K315A mutant. Disruption of interfacial interactions at Glu267 led to polymerization of partly unfolded intermediates in the presence of 3 m urea. However, these polymeric forms were not observed with combinations of the E267L mutation with other mutations. These results indicate that these hydrogen bonds, which are present at different contact surfaces in the dimer-dimer interface, perform distinct functions in double dimer assembly. The coordination of these interactions is critical for the stability and tetramer formation of delta-crystallin.

Kinetic refolding barrier of guanidinium chloride denatured goose delta-crystallin leads to regular aggregate formation

Delta-crystallin is the major soluble protein in avian eye lenses with a structural role in light scattering. Dissociation and unfolding of the tetrameric protein in guanidinium chloride (GdmCl) can be sensitively monitored by the intrinsic tryptophan fluorescence. In this study refolding of GdmCl-denatured delta-crystallin was investigated. A marked hysteresis was observed while refolding by dilution of the 5 M GdmCl-denatured delta-crystallin. The secondary structure of the refolded protein was largely restored. However, monitoring intrinsic fluorescence of single tryptophan mutants indicated that the microenvironment of domain 1 (W74) was not restored. The region containing W169, which is close to the dimer interface, remained exposed following refolding. During refolding of the wild-type protein, dimeric, tetrameric, and aggregate forms were identified. The ratio of tetramer to dimer increased with time, as judged by gel-filtration chromatography and nondenaturing gel electrophoresis. However the observed levels of tetramer did not return to the same levels as observed before GdmCl treatment. The proportion of tetramer was significantly decreased in the N-25 deletion mutant and it did not increase with time. These results suggest that there is a kinetic barrier for assembly of dimers into tetramers. The consequence of this is that dimers refold to form aggregates. Aggregation seems to follow a nucleation mechanism with an apparent reaction order of 4.7+/-0.2, suggesting four or five monomers constitute the core structure of nucleus, which propagate to form high molecular weight aggregates. Addition of alpha-crystallin during refolding prevents aggregation. Thioflavin T and Congo red assays indicated a regular structure for the protein aggregates, which appear as hollow tubules packed into helical bundles. Aggregate formation was protein concentration dependent that progressed via two stages with rate constants of 0.0039+/-0.0006 and 0.00043+/-0.00003 s(-1), respectively. We propose that the N-terminal segment of delta-crystallin plays a critical role in proper double dimer assembly and also in the assembly of nucleus to aggregate formation.

Critical role of tryptophanyl residues in the conformational stability of goose δ-crystallin

Delta-crystallin is the major structural protein in avian and reptilian eye lenses but its sequence is highly homologous with the urea cycle enzyme, argininosuccinate lyase (ASL). In previous studies the multi-step unfolding process of this protein in the presence of GdmCl was sensitively probed using tryptophan fluorescence. In this study the contribution of single tryptophan residues to the stability of the local environment was monitored by mutation of two highly conservative tryptophan residues in goose delta-crystallin, Trp 74 and Trp 169. These residues behaved differently in terms of fluorescence intensity and maxima emission wavelength, consistent with their structural location in buried or solvent accessible regions. No gross changes in the secondary structure after mutation were observed, as judged by far-UV CD. The side chains of tryptophan residues in the structure of wild-type goose delta-crystallin possess both hydrophobic and hydrogen bonding interactions. Replacement of the side chain with phenylalanine or alanine led to expose of a hydrophobic area and a reduction in thermal stability; W169A particularly has a T(m) value that was 10 degrees C lower than the wild type enzyme. In the presence of GdmCl, a sharp red shift in fluorescence wavelength due to subunit dissociation can be sensitively detected using a single tryptophan, with the region surrounding W74 undergoing the first transition with a [GdmCl](1/2) of 0.45 M. Further measurement of unfolding curves by CD revealed that the W169A mutant was most unstable with a [GdmCl](1/2) of 0.22 M. From sedimentation velocity analysis, the unstable conformation of the W169A mutant affected the assembly of the quaternary structure. Our studies demonstrate the critical role for the tryptophan residues in stabilizing protein conformations and subunit assembly in delta-crystallin.

The effect of N-terminal truncation on double-dimer assembly of goose delta-crystallin

Delta-crystallin is a soluble structural protein in avian eye lenses that confers special refractive properties. In the presence of GdmCl (guanidinium chloride), tetrameric delta-crystallin undergoes dissociation via a dimeric state to a monomeric molten globule intermediate state. The latter are denatured at higher GdmCl concentrations in a multi-state manner. In the present study, the X-ray structure of goose delta-crystallin was determined to 2.8 A (1 A=0.1 nm). In this structure the first 25 N-terminal residues interact with a hydrophobic cavity in a neighbouring molecule, stabilizing the quaternary structure of this protein. When these 25 residues were deleted this did not produce any gross structural changes, as judged by CD analysis, but slightly altered tryptophan fluorescence and ANS (8-anilino-1-naphthalenesulphonic acid) spectra. The dimeric form was significantly identified as judged by sedimentation velocity and nondenaturing gradient gel electrophoresis. This mutant had increased sensitivity to temperature denaturation and GdmCl concentrations of 0.3-1.0 M. This protein was destabilized about 3.3 kcal/mol (1 kcal=4.184 kJ) due to N-terminal truncation. After incubation at 37 degrees C N-terminal truncated proteins were prone to aggregation, suggesting the presence of the unstable dimeric conformation. An important role for the N-terminus in dimer assembly of goose delta-crystallin is proposed.

Monomeric molten globule intermediate involved in the equilibrium unfolding of tetrameric duck δ2-crystallin

Duck delta2-crystallin is a soluble tetrameric lens protein. In the presence of guanidinium hydrochloride (GdnHCl), it undergoes stepwise dissociation and unfolding. Gel-filtration chromatography and sedimentation velocity analysis has demonstrated the dissociation of the tetramer protein to a monomeric intermediate with a dissociation constant of 0.34 microM3. Dimers were also detected during the dissociation and refolding processes. The sharp enhancement of 1-anilinonaphthalene-8-sulfonic acid (ANS) fluorescence at 1 M GdnHCl strongly suggested that the dissociated monomers were in a molten globule state under these conditions. The similar binding affinity (approximately 60 microM) of ANS to protein in the presence or absence of GdnHCl suggested the potential assembly of crystallins via hydrophobic interactions, which might also produce off-pathway aggregates in higher protein concentrations. The dynamic quenching constant corresponding to GdnHCl concentration followed a multistate unfolding model implying that the solvent accessibility of tryptophans was a sensitive probe for analyzing delta2-crystallin unfolding.

Unfolding and refolding of a quinone oxidoreductase: alpha-crystallin, a molecular chaperone, assists its reactivation

alpha-Crystallin, a member of the small heat-shock protein family and present in vertebrate eye lens, is known to prevent the aggregation of other proteins under conditions of stress. However, its role in the reactivation of enzymes from their non-native inactive states has not been clearly demonstrated. We have studied the effect of alpha-crystallin on the refolding of zeta-crystallin, a quinone oxidoreductase, from its different urea-denatured states. Co-refolding zeta-crystallin from its denatured state in 2.5 M urea with either calf eye lens alpha-crystallin or recombinant human alpha B-crystallin could significantly enhance its reactivation yield. alpha B-crystallin was found to be more efficient than alpha A-crystallin in chaperoning the refolding of zeta-crystallin. In order to understand the nature of the denatured state(s) of zeta-crystallin that can interact with alpha-crystallin, we have investigated the unfolding pathway of zeta-crystallin. We find that it unfolds through three distinct intermediates: an altered tetramer, a partially unfolded dimer, which is competent to fold back to its active state, and a partially unfolded monomer. The partially unfolded monomer is inactive, exhibits highly exposed hydrophobic surfaces and has significant secondary structural elements with little or no tertiary structure. This intermediate does not refold into the active state without assistance. alpha-Crystallin provides the required assistance and improves the reactivation yield several-fold.