Biomedical subjects

George B Benedek

Publications and source records attributed to George B Benedek.

9 recordsLinked to original sources

PubMed2005

Decrease in protein solubility and cataract formation caused by the Pro23 to Thr mutation in human gamma D-crystallin.

The P23T mutation in the human gammaD-crystallin gene has in recent years been associated with a number of well known cataract phenotypes. To understand the molecular mechanism of lens opacity caused by this mutation, we expressed human gammaD-crystallin (HGD), the P23T mutant, and other related mutant proteins in Escherichia coli and compared the structures and thermodynamic properties of these proteins in vitro. The results show that the cataract-causing mutation P23T does not exhibit any significant structural change relative to the native protein. However, in marked contrast to the native protein, the mutant shows a dramatically lowered solubility. The reduced solubility results from the association of the P23T mutant to form a new condensed phase that contains clusters of the mutant protein. The monomer-cluster equilibrium is represented by a solubility curve in the phase diagram. When the solubility limit is exceeded, the mutant protein forms the condensed phase after a nucleation time of 10-20 min. We found that the solubility of the P23T mutant exhibits an inverse dependence on temperature, i.e., the protein clusters are increasingly soluble as the temperature of the solution decreases. The solubility of P23T can be substantially altered by the introduction of specific mutations at or in the immediate vicinity of residue 23. We examined the mutants P23S, P23V, P23TInsP24, and P23TN24K and found that the latter two mutations can restore the solubility of the P23T mutant. These findings may help develop a strategy for the rational design of small molecule inhibitors of this type of condensed phase.

Amino Acid Sequence↗

PubMed2005

Oligomerization and phase transitions in aqueous solutions of native and truncated human beta B1-crystallin.

Human betaB1-crystallin is a major eye-lens protein that undergoes in vivo truncation at the N-terminus with aging. By studying native betaB1 and truncated betaB1DeltaN41, which mimics an age-related in vivo truncation, we have determined quantitatively the effect of truncation on the oligomerization and phase transition properties of betaB1 aqueous solutions. The oligomerization studies show that the energy of attraction between the betaB1DeltaN41 proteins is about 10% greater than that of the betaB1 proteins. We have found that betaB1DeltaN41 aqueous solutions undergo two distinct types of phase transitions. The first phase transition involves an initial formation of thin rodlike assemblies, which then evolve to form crystals. The induction time for the formation of rodlike assemblies is sensitive to oligomerization. The second phase transition can be described as liquid-liquid phase separation (LLPS) accompanied by gelation within the protein-rich phase. We refer to this process as heterogeneous gelation. These two phase transitions are not observed in the case of betaB1 aqueous solutions. However, upon the addition of poly(ethylene glycol) (PEG), we observe heterogeneous gelation also for betaB1. Our PEG experiments allow us to estimate the difference in phase separation temperatures between betaB1 and betaB1DeltaN41. This difference is consistent with the increase in energy of attraction found in our oligomerization studies. Our work suggests that truncation is a cataractogenic modification since it favors protein condensation and the consequent formation of light scattering elements, and highlights the importance of the N-terminus of betaB1 in maintaining lens transparency.

Circular Dichroism↗

PubMed2005

Quasielastic light scattering for protein assembly studies.

Quasielastic light scattering (QLS) spectroscopy is an optical method for the determination of diffusion coefficients of particles in solution. In this chapter, we discuss the principles and practice of QLS with respect to protein assembly reactions. Particles undergoing Brownian motion produce fluctuations in scattered light intensity. We describe how the temporal correlation function of these fluctuations can be measured and how this correlation function provides information about the distribution of diffusion coefficients of the particles in solution. We discuss the intricacies of deconvolution of the correlation function and the assumptions incorporated into data analysis procedures. We explain how the Stokes-Einstein relationship can be used to convert distributions of diffusion coefficients into distributions of particle size. Noninvasive observation of the temporal evolution of particles sizes provides a powerful tool for studying protein aggregation and self-assembly. We use examples from studies of Abeta fibrillogenesis to illustrate QLS application for understanding the molecular mechanisms of the nucleation and growth of amyloid fibrils.

Chemistry Techniques, Analytical↗

PubMed2003

Liquid-solid transition in nuclei of protein crystals.

It is generally assumed that crystallization begins with a small, crystalline nucleus. For proteins this paradigm may not be valid. Our numerical simulations show that under conditions typically used to produce protein crystals, small clusters of model proteins (particles with short-range, attractive interactions) cannot maintain a crystalline structure. Protein crystal nucleation is therefore an indirect, two-step process. A nucleus first forms and grows as a disordered, liquid-like aggregate. Once the aggregate grows beyond a critical size (about a few hundred particles) crystal nucleation becomes possible.

Computer Simulation↗

PubMed2003

High-resolution X-ray crystal structures of human gammaD crystallin (1.25 A) and the R58H mutant (1.15 A) associated with aculeiform cataract.

Several human cataracts have been linked to mutations in the gamma crystallin gene. One of these is the aculeiform cataract, which is caused by an R58H mutation in gammaD crystallin. We have shown previously that this cataract is caused by crystallization of the mutant protein, which is an order of magnitude less soluble than the wild-type. Here, we report the very high-resolution crystal structures of the mutant and wild-type proteins. Both proteins crystallize in the same space group and lattice. Thus, a strict comparison of the protein-protein and protein-water intermolecular interactions in the two crystal lattices is possible. Overall, the differences between the mutant and wild-type structures are small. At position 58, the mutant protein loses the direct ion-pair intermolecular interaction present in the wild-type, due to the differences between histidine and arginine at the atomic level; the interaction in the mutant is mediated by water molecules. Away from the mutation site, the mutant and wild-type lattice structures differ in the identity of side-chains that occupy alternate conformations. Since the interactions in the crystal phase are very similar for the two proteins, we conclude that the reduction in the solubility of the mutant is mainly due to the effect of the R58H mutation in the solution phase. The results presented here are also important as they are the first high-resolution X-ray structures of human gamma crystallins.

Amino Acid Substitution↗

PubMed2003

Observation of liquid-liquid phase separation for eye lens gammaS-crystallin.

gammaS-crystallin (gammaS) is an important human and bovine eye lens protein involved in maintaining the transparency of the eye. By adding small amounts of polyethylene glycol (PEG) to the binary aqueous bovine gammaS solutions, we have observed liquid-liquid phase separation (LLPS) at -8 degrees C and revealed that, in the binary gammaS-water system, this phase transition would occur at -28 degrees C. We have measured both the effect of PEG concentration on the LLPS temperature and proteinPEG partitioning between the two liquid coexisting phases. We use our measurements of proteinPEG partitioning to determine the nature and the magnitude of the gammaS-PEG interactions and to quantitatively assess the effectiveness of PEG as a crystallizing agent for gammaS. We use our measurements of LLPS temperature as a function of protein and PEG concentration to successfully determine the location of the critical point for the binary gammaS-water system. This phase transition cannot be observed in the absence of PEG because it is inaccessible due to the freezing of the system. Our findings indicate that the effective interactions between gammaS molecules in the binary gammaS-water system are attractive. We compare the magnitude of the attraction found for gammaS with the results obtained for the other gamma-crystallins for which the critical temperature is located above the freezing point of the system. This work suggests that PEG can be used to reveal the existence of LLPS for a much wider range of binary protein-water systems than known previously.

Animals↗

PubMed2002

Amyloid beta -protein (Abeta) assembly: Abeta 40 and Abeta 42 oligomerize through distinct pathways.

Amyloid beta-protein (Abeta) is linked to neuronal injury and death in Alzheimer's disease (AD). Of particular relevance for elucidating the role of Abeta in AD is new evidence that oligomeric forms of Abeta are potent neurotoxins that play a major role in neurodegeneration and the strong association of the 42-residue form of Abeta, Abeta42, with the disease. Detailed knowledge of the structure and assembly dynamics of Abeta thus is important for the development of properly targeted AD therapeutics. Recently, we have shown that Abeta oligomers can be cross-linked efficiently, and their relative abundances quantified, by using the technique of photo-induced cross-linking of unmodified proteins (PICUP). Here, PICUP, size-exclusion chromatography, dynamic light scattering, circular dichroism spectroscopy, and electron microscopy have been combined to elucidate fundamental features of the early assembly of Abeta40 and Abeta42. Carefully prepared aggregate-free Abeta40 existed as monomers, dimers, trimers, and tetramers, in rapid equilibrium. In contrast, Abeta42 preferentially formed pentamerhexamer units (paranuclei) that assembled further to form beaded superstructures similar to early protofibrils. Addition of Ile-41 to Abeta40 was sufficient to induce formation of paranuclei, but the presence of Ala-42 was required for their further association. These data demonstrate that Abeta42 assembly involves formation of several distinct transient structures that gradually rearrange into protofibrils. The strong etiologic association of Abeta42 with AD may thus be a result of assemblies formed at the earliest stages of peptide oligomerization.

Alzheimer Disease↗

PubMed2002

Effect of polyethylene glycol on the liquid-liquid phase transition in aqueous protein solutions.

We have studied the effect of polyethylene glycol (PEG) on the liquid-liquid phase separation (LLPS) of aqueous solutions of bovine gammaD-crystallin (gammaD), a protein in the eye lens. We observe that the phase separation temperature increases with both PEG concentration and PEG molecular weight. PEG partitioning, which is the difference between the PEG concentration in the two coexisting phases, has been measured experimentally and observed to increase with PEG molecular weight. The measurements of both LLPS temperature and PEG partitioning in the ternary gammaD-PEG-water systems are used to successfully predict the location of the liquid-liquid phase boundary of the binary gammaD-water system. We show that our LLPS measurements can be also used to estimate the protein solubility as a function of the concentration of crystallizing agents. Moreover, the slope of the tie-lines and the dependence of LLPS temperature on polymer concentration provide a powerful and sensitive check of the validity of excluded volume models. Finally, we show that the increase of the LLPS temperature with PEG concentration is due to attractive protein-protein interactions.

Animals↗

PubMed2001

Structure determination of micelle-like intermediates in amyloid beta -protein fibril assembly by using small angle neutron scattering.

Increasing evidence supports the hypothesis that amyloid beta-protein (Abeta) assembly is a key pathogenic feature of Alzheimer's disease. Thus, understanding the assembly process offers opportunities for the development of strategies for treating this devastating disease. In prior studies, Abeta was found to form micelle-like aggregates under acidic conditions. These structures exhibited an average observed hydrodynamic radius of 7 nm. They were found to be in rapid equilibrium with Abeta monomers or low molecular weight oligomers, and were centers of fibril nucleation. Here the technique of small angle neutron scattering has been used to determine the structure of these Abeta micelles. The data reveal that the micellar assemblies comprise 30-50 Abeta monomers and have elongated geometries. The best fit of the data to a uniform spherocylinder yields a radius approximately 2.4 nm and cylinder length approximately 11 nm. These structure parameters remain constant over more than a decade in concentration range. The concentration independence of the length of the cylindrical aggregate indicates the presence of an internal nonrepetitive structure that spans the entire length of the Abeta assembly.

Amyloid beta-Peptides↗