Surface interactions of gamma-crystallins in the crystal medium in relation to their association in the eye lens

Model for screened, charge-regulated electrostatics of an eye lens protein: Bovine gammaB-crystallin

We model screened, site-specific charge regulation of the eye lens protein bovine gammaB-crystallin (γB) and study the probability distributions of its proton occupancy patterns. Using a simplified dielectric model, we solve the linearized Poisson-Boltzmann equation to calculate a 54×54 work-of-charging matrix, each entry being the modeled voltage at a given titratable site, due to an elementary charge at another site. The matrix quantifies interactions within patches of sites, including γB charge pairs. We model intrinsic pK values that would occur hypothetically in the absence of other charges, with use of experimental data on the dependence of pK values on aqueous solution conditions, the dielectric model, and literature values. We use Monte Carlo simulations to calculate a model grand-canonical partition function that incorporates both the work-of-charging and the intrinsic pK values for isolated γB molecules and we calculate the probabilities of leading proton occupancy configurations, for 4<pH<8 and Debye screening lengths from 6 to 20 Å. We select the interior dielectric value to model γB titration data. At pH 7.1 and Debye length 6.0 Å, on a given γB molecule the predicted top occupancy pattern is present nearly 20% of the time, and 90% of the time one or another of the first 100 patterns will be present. Many of these occupancy patterns differ in net charge sign as well as in surface voltage profile. We illustrate how charge pattern probabilities deviate from the multinomial distribution that would result from use of effective pK values alone and estimate the extents to which γB charge pattern distributions broaden at lower pH and narrow as ionic strength is lowered. These results suggest that for accurate modeling of orientation-dependent γB-γB interactions, consideration of numerous pairs of proton occupancy patterns will be needed.

Thermodynamic analysis of weak protein interactions using sedimentation equilibrium

Proteins self-associate to form dimers and tetramers. Purified proteins are used to study the thermodynamics of protein interactions using the analytical ultracentrifuge. In this approach, monomer-dimer equilibrium constants are directly measured at various temperatures. Data analysis is used to derive thermodynamic parameters, such as Gibbs free energy, enthalpy, and entropy, which can predict which major forces are involved in protein association.

β-Crystallin association

Beta-crystallins are major protein constituents of the mammalian lens, where their stability and association into higher order complexes are critical for lens clarity and refraction. Dimerization is an initial step in formation of beta-crystallin complexes. Beta-crystallin association into dimers is energetically highly favoured, but rapidly reversible under physiological conditions. Beta-crystallin dimers can exchange monomers, probably through a transient and energetically unfavoured monomer intermediate state. As predicted by molecular modelling, the fraction of beta-crystallin present as dimers increases with increasing temperature, implying that beta-crystallin association is entropically driven.

Progressive juvenile-onset punctate cataracts caused by mutation of the gammaD-crystallin gene

Cataracts are a significant public health problem. Here, we describe the genetic alteration responsible for a progressive form of cataract, segregating as an autosomal dominant trait in a three-generation pedigree. Unlike most autosomal dominant cataracts, these are not clinically apparent at birth but are initially observed in the first year or two of life. The opacification evolves relatively slowly, generally necessitating removal of the lens in childhood or early adolescence. A genome-wide search in our kindred revealed linkage at 2q33-35 where the gamma-crystallin gene cluster resides. A single base alteration resulting in an Arg- 14 --> Cys (R14C) substitution in gammaD-crystallin was subsequently identified. Protein modeling suggests that the effect of this mutation is a subtle one, affecting the surface properties of the crystallin molecule rather than its tertiary structure, consistent with the fact that the patients' lenses are normal at birth. This is the first gene defect shown to be responsible for a noncongenital progressive cataract, and studying the defective protein should teach us more about the mechanisms underlying cataract formation.

A small-angle neutron scattering study of gamma-crystallins near their isoelectric point

In this paper, a small-angle neutron scattering study of gammaII-crystallins near their isoelectric point is presented. The experiments were carried out using protein concentrations of 5.7-85.7 mg/ml at temperatures in the range 11 -60 degrees C. The experimental data were analyzed using an ellipsoidal model for intraparticle correlations and the mean spherical approximation for interparticle correlations. Our studies revealed that gammaII-crystallins have a thick hydration layer, which is possibly due to the special arrangement of polar and ionic groups on their surface. The temperature scan shows that, as a result of relatively strong attractive forces, clusters of two, three, or higher oligomers are present below 20 degrees C. Our results suggest that protein clusters, with a distinctive hydration layer, form a protein-rich phase that separates from a protein-lean phase as the temperature is decreased below some threshold value.

Simulations of reversible protein aggregate and crystal structure

We simulated the structure of reversible protein aggregates as a function of protein surface characteristics, protein-protein interaction energies, and the entropic penalty accompanying the immobilization of protein in a solid phase. These simulations represent an extension of our previous work on kinetically irreversible protein aggregate structure and are based on an explicit accounting of the specific protein-protein interactions that occur within reversible aggregates and crystals. We considered protein monomers with a mixture of hydrophobic and hydrophilic surface regions suspended in a polar solvent; the energetic driving force for aggregation is provided by the burial of solvent-exposed hydrophobic surface area. We analyzed the physical properties of the generated aggregates, including density, protein-protein contact distributions, solvent accessible surface area, porosity, and order, and compared our results with the protein crystallization literature as well as with the kinetically irreversible case. The physical properties of reversible aggregates were consonant with those observed for the irreversible aggregates, although in general, reversible aggregates were more stable energetically and were more crystal-like in their order content than their irreversible counterparts. The reversible aggregates were less dense than the irreversible aggregates, indicating that the increased energetic stability is derived primarily from the optimality rather than the density of the packing in the solid phase. The extent of hydrophobic protein-protein contacts and solvent-exposed surface area within the aggregate phase depended on the aggregation pathway: reversible aggregates tended to have a greater proportion of hydrophobic-hydrophobic contacts and a smaller fraction of hydrophobic solvent-exposed surface area. Furthermore, the arrangement of hydrophobic patches on the protein surface played a major role in the distribution of protein contacts and solvent content. This was readily reflected in the order of the aggregates: the greater the contiguity of the hydrophobic patches on the monomer surface, the less ordered the aggregates became, despite the opportunities for rearrangement offered by a reversible pathway. These simulations have enhanced our understanding of the impact of protein structural motifs on aggregate properties and on the demarcation between aggregation and crystallization.

Alpha-crystallin: molecular chaperone and protein surfactant

Bovine lens alpha-crystallin has recently been shown to function as a molecular chaperone by stabilizing proteins against heat denaturation (Horwitz, J. (1992) Proc. Natl. Acad. Sci. USA, 89, 10449-10453). An investigation, using a variety of physico-chemical methods, is presented into the mechanism of stabilization. alpha-Crystallin exhibits properties of a surfactant. Firstly, a plot of conductivity of alpha-crystallin versus concentration shows a distinct inflection in its profile, i.e., a critical micelle concentration (cmc), over a concentration range from 0.15 to 0.17 mM. Gel chromatographic and 1H-NMR spectroscopic studies spanning the cmc indicate no change in the aggregated state of alpha-crystallin implying that a change in conformation of the aggregate occurs at the cmc. Secondly, spectrophotometric studies of the rate of heat-induced aggregation and precipitation of alcohol dehydrogenase (ADH), beta L- and gamma-crystallin in the presence of alpha-crystallin and a variety of synthetic surfactants show that stabilization against precipitation results from hydrophobic interactions with alpha-crystallin and monomeric anionic surfactants. Per mole of subunit or monomer, alpha-crystallin is the most efficient at stabilization. alpha-Crystallin, however, does not preserve the activity of ADH after heating. After heat inactivation, gel permeation HPLC indicates that ADH and alpha-crystallin form a high molecular weight aggregate. Similar results are obtained following incubation of beta L- and gamma-crystallin with alpha-crystallin. 1H-NMR spectroscopy of mixtures of alpha- and beta L-crystallin, in their native states, reveals that the C-terminus of beta B2-crystallin is involved in interaction with alpha-crystallin. In the case of gamma- and alpha-crystallin mixtures, a specific interaction occurs between alpha-crystallin and the C-terminal region of gamma B-crystallin, an area which is known from the crystal structure to be relatively hydrophobic and to be involved in intermolecular interactions. The short, flexible C-terminal extensions of alpha-crystallin are not involved in specific interactions with these proteins. It is concluded that alpha-crystallin interacts with native proteins in a weak manner. Once a protein has become denatured, however, the soluble complex with alpha-crystallin cannot be readily dissociated. In the aging lens this finding may have relevance to the formation of high molecular weight crystallin aggregates.

Protein interactions in the calf eye lens: interactions between beta-crystallins are repulsive whereas in gamma-crystallins they are attractive

Non-specific interactions in beta- and gamma-crystallins have been studied by solution X-ray scattering and osmotic pressure experiments. Measurements were carried out as a function of protein concentration at two ionic strengths. The effect of temperature was tested between 7 degrees C and 31 degrees C. Two types of interactions were observed. With beta-crystallin solutions, a repulsive coulombic interaction could be inferred from the decrease of the normalized X-ray scattering intensity near the origin with increasing protein concentration and from the fact that the osmotic pressure increases much more rapidly than in the ideal case. As was previously observed with alpha-crystallins, such behaviour is dependent upon ionic strength but is hardly affected by temperature. In contrast, with gamma-crystallin solutions, the normalized X-ray scattering intensity near the origin increases with increasing protein concentration and the osmotic pressure increases less rapidly than in the ideal case. Such behaviour indicates that attractive forces are predominant, although we do not yet know their molecular origin. Under our experimental conditions, the effect of temperature was striking whereas no obvious contribution of the ionic strength could be seen, perhaps owing to masking by the large temperature effect. The relevance of the different types of non-specific interactions for lens function is discussed.

Solid-liquid phase boundaries of lens protein solutions

We report measurement of the solid-liquid phase boundary, or liquidus line, for aqueous solutions of three pure calf gamma-crystallin proteins: gamma II, gamma IIIa, and gamma IIIb. We also studied the liquidus line for solutions of native gamma IV-crystallin calf lens protein, which consists of 85% gamma IVa/15% gamma IVb. In all four proteins the liquidus phase boundaries lie higher in temperature than the previously determined liquid-liquid coexistence curves. Thus, over the range of concentration and temperature for which liquid-liquid phase separation occurs, the coexistence of a protein crystal phase with a protein liquid solution phase is thermodynamically stable relative to the metastable separated liquid phases. The location of the liquidus lines clearly divides these four crystallin proteins into two groups: those in which liquidus lines flatten at temperatures greater than 70 degrees C: gamma IIIa and gamma IV, and those in which liquidus lines flatten at temperatures less than 50 degrees C: gamma II and gamma IIIb. We have analyzed the form of the liquidus lines by using specific choices for the structures of the Gibbs free energy in solution and solid phases. By applying the thermodynamic conditions for equilibrium between the two phases to the resulting chemical potentials, we can estimate the temperature-dependent free energy change upon binding of protein and water into the solid phase.

The interaction of gamma-crystallins with model surfaces

Three biophysical techniques were employed to study the structure and thermal stability of a series of homologous bovine lens gamma-crystallins upon binding to three model surfaces. The surfaces in order of increasing hydrophobicity were silica, methyl silica, and diphenyl silica. Secondary structure was analyzed by deconvolution Fourier transform infrared spectroscopy, while tertiary structure alterations were probed by front surface fluorescence spectroscopy. The effect of surface binding on protein thermal stability was analyzed by fluorescence and differential scanning calorimetry. The comparison of free and surface-bound protein with variations in the electrostatic and hydrophobic character of both the protein and the adsorbent surface with these techniques demonstrated that: (i) destabilization on hydrophobic surfaces is greater than on a more hydrophilic interface, (ii) detectable conformational changes tend to increase as the hydrophobicity of the surface increases, and (iii) subtle structural differences among proteins can play an important role in determining differences in protein stability and structure upon surface adsorption.

Crystal structure of calf eye lens gamma-crystallin IIIb at 2.5 A resolution: its relation to function

The crystal structure of gamma-crystallin IIIb (gamma C) from calf eye lens has been refined at 2.5 A resolution. The molecule of about 21 kDa consists of two similar domains. Each domain is composed of two motifs with the 'Greek key' topology which form a pair of four-stranded beta-sheets with an antiparallel packing. The molecule has three hydrophobic cores: one within each domain and one between them. Six of the eight functionally important cysteines are located within the N-domain, and only two in the C-domain. Several large clusters of charged residues are at the surface of the molecule. Surface residues Val 101, Met 103 and Leu 155 are important for packing of molecules in crystal medium and possibly in the lens. Features of the gamma-crystallin IIIb molecule which may be related to its function in the vertebrate eye lens are briefly discussed. An attempt has been made to correlate molecular characteristics with some general properties of the eye lens such as high density and refractive index gradients and strong stability of the lens during an organism's lifetime.

Binary-liquid phase separation of lens protein solutions

We have determined the coexistence curves (plots of phase-separation temperature T versus protein concentration C) for aqueous solutions of purified calf lens proteins. The proteins studied, calf gamma IIIa-, gamma IIIb-, and gamma IVa-crystallin, have very similar amino acid sequences and three-dimensional structures. Both ascending and descending limbs of the coexistence curves were measured. We find that the coexistence curves for each of these proteins and for gamma II-crystallin can be fit, near the critical point, to the function /(Cc-C)/Cc/ = A [(Tc - T)/Tc]beta, where beta = 0.325, Cc is the critical protein concentration in mg/ml, Tc is the critical temperature for phase separation in K, and A is a parameter that characterizes the width of the coexistence curve. We find that A and Cc are approximately the same for all four coexistence curves (A = 2.6 +/- 0.1, Cc = 289 +/- 20 mg/ml), but that Tc is not the same. For gamma II- and gamma IIIb-crystallin, Tc approximately 5 degrees C, whereas for gamma IIIa- and gamma IVa-crystallin, Tc approximately 38 degrees C. By comparing the published protein sequences for calf, rat, and human gamma-crystallins, we postulate that a few key amino acid residues account for the division of gamma-crystallins into low-Tc and high-Tc groups.

The calf gamma crystallins--a Raman spectroscopic study

The solution structures of the four major components of bovine lens gamma-crystallin, gamma s, gamma II, gamma III and gamma IV are compared using Raman spectroscopy. The spectral region sensitive to the vibrational frequencies of aromatic and sulfur containing residues and to the backbone skeletal stretching modes (500-1000 cm-1), and that reflecting secondary structure (1,000-1,700 cm-1) are strikingly similar in all four gamma-crystallin fractions. These similarities are indicative of the dominant anti-parallel beta sheet structure common to all the gamma-crystallins. A comparison of the ratios of the Raman intensities at 850 cm-1 and 830 cm-1 (I850/I830), an empirical measure of the degree of hydrogen bonding of phenolic hydroxyl groups, suggests that the tyrosine residues in all the gamma-crystallin fractions are moderately hydrogen bonded. Distinct differences in the solution structures of the gamma-crystallins were observed in the higher energy end of the vibrational Raman spectra. The sulfhydryl stretching frequencies for the gamma-crystallins exhibit complex splitting patterns in the 2,500-2,600 cm-1 region. These patterns are due to the competing effects of hydrogen bonding and S-pi interactions with neighboring aromatic residues. All five proteins exhibit multiple, but distinct, thiol frequencies, suggesting that the microenvironments of the cysteine residues in these proteins are significantly different.

Oligomerization and conformation change in solutions of calf lens gamma II-crystallin. Results from 1/T1 nuclear magnetic relaxation dispersion profiles

From analyses of the magnetic field dependence of 1/T1 (nuclear magnetic relaxation dispersion [NMRD] profiles) of water protons in solutions of highly purified calf lens gamma II-crystallin, we find that monomers form oligomers at relatively low concentrations, which increase in size with increasing concentration and decreasing temperature. At approximately 16% by volume and -4 degrees C, the mean oligomeric molecular weight is approximately 120-fold greater than the monomeric value of 20 kD. Below this concentration, there is no indication of any substantive change in conformation of the monomeric subunits. At higher concentrations, the tertiary structure of the monomer appears to reconfigure rather abruptly, but reversibly, as evidenced by the appearance of spectra-like 14N peaks in the NMRD profiles. The magnitudes of these peaks, known to arise from cross-relaxation of water protons through access to amide (NH) moieties of the protein backbone, indicate that the high concentration conformation is not compact, but open and extended in a manner that allows enhanced interaction with solvent. The data are analogous to those found for homogenates of calf and chicken lens (Beaulieu, C. F., J. I. Clark, R. D. Brown III, M. Spiller, and S. H. Koenig. 1988. Magn. Reson. Med. 8:47-57; Beaulieu, C. F., R. D. Brown III, J. I. Clark, M. Spiller, and S. H. Koenig. 1989. Magn. Reson. Med. 10:62-72). This unusually large dependence of oligomeric size and conformation on concentration in the physiological range is suggested as the mechanism by which osmotic equilibrium is maintained, at minimal metabolic expense, in the presence of large gradients of protein concentration in the lens in vivo (cf Vérétout and Tardieu, 1989. Eur. Biophys. J. 17:61-68). Finally, the results of the NMRD data provide a ready explanation of the low temperature phase transition, and "cold-cataract" separation of phases, observed in gamma II-crystallin solutions; we suggest that the phases that separate are the two major conformers detected by NMRD.

Packing interactions in the eye-lens. Structural analysis, internal symmetry and lattice interactions of bovine gamma IVa-crystallin

gamma-Crystallins are a family of low molecular weight proteins found in high concentration in the densely packed regions of high refractive index in vertebrate lenses. Certain members have the characteristic property of a high critical temperature (tc) for phase separation. We report the three-dimensional structure determination of such a protein, bovine lens gamma IVa-crystallin, which has been refined to give an X-ray R-factor of 0.143. Its high tc contrasts with the low tc gamma II-crystallin, whose structure we have already published. The root mean square difference between the alpha-carbon atoms of these two proteins is 0.70 A and gamma IVa has an internal symmetry even higher than that of gamma II. The presence of a protein that exhibits the phenomenon of phase separation at body temperature renders the lens very susceptible to a transformation from transparent to an opaque state due to irregularities in the refractive index. Protein interactions of gamma IVa-crystallin have implications for the mechanism of cataract formation. Modes of self-association behaviour of gamma IVa-crystallin have been inferred from an analysis of the lattice interactions in the crystalline state, where the protein packing density is similar to that of the intact lens. It appears that the point mutation at position 103 from a serine residue in gamma II to a valine in gamma IVa gives rise to a lattice contact formed by two four-stranded beta-sheets in gamma IVa. A group-specific mutation at position 118 from leucine to phenylalanine induces subtle differences in core packing, leading to a reorganization around residue 103. However, the final phase separation determinant may be a complex combination of many side-chain functions.