Structural organization and stability of a thermoresistant domain generated by in vivo hydrolysis of the alpha-crystallin B chain from calf lens

Post-translationally modified human lens crystallin fragments show aggregation in vitro

BACKGROUND

Crystallin fragments are known to aggregate and cross-link that lead to cataract development. This study has been focused on determination of post-translational modifications (PTMs) of human lens crystallin fragments, and their aggregation properties.

METHODS

Four crystallin fragments-containing fractions (Fraction I [∼3.5 kDa species], Fraction II [∼3.5-7 kDa species], Fraction III [∼7-10 kDa species] and Fraction IV [>10-18 kDa species]), and water soluble high molecular weight (WS-HMW) protein fraction were isolated from water soluble (WS) protein fraction of human lenses of 50-70 year old-donors. The crystallin fragments of the Fractions I-IV were separated by two-dimensional (2D)-gel electrophoresis followed by analysis of their gel-spots by mass spectrometry. The Fractions I-IV were examined for their molecular mass, particle-diameters, amyloid fibril formation, and for their aggregation by themselves and with WS-HMW proteins.

RESULTS

Crystallin fragments in Fractions I-IV were derived from α-, β- and γ-crystallins, and their 2D-gel separated spots contained multiple crystallins with PTMs such as oxidation, deamidation, methylation and acetylation. Crystallin fragments from all the four fractions exhibited self-aggregated complexes ranging in M_r from 5.5×10⁵ to 1.0×10⁸ Da, with diameters of 10-28 nm, and amyloid fibril-like formation, and aggregation with WS-HMW proteins.

CONCLUSION

The crystallin fragments exhibited several PTMs, and were capable of forming aggregated species by themselves and with WS-HMW proteins, suggesting their potential role in aggregation process during cataract development.

GENERAL SIGNIFICANCE

Crystallin fragments play a major role in human cataract development.

Negative charges in the C-terminal domain stabilize the alphaB-crystallin complex

alphaB-Crystallin is one of the six known mammalian small heat-shock proteins (sHsps). These are characterized by the presence of a conserved sequence of 80-100 residues, which constitutes the putative C-terminal domain. Like other sHsps, alphaB-crystallin forms multimeric globular complexes, often in combination with related sHsps. Here we show that in a yeast two-hybrid system, alphaB-crystallin can specifically interact with itself as well as with alphaA-crystallin and Hsp27. Analyses of the separate domains show that the conserved C-terminal domain (CalphaB) is essential for this interaction between subunits. To try and detect residues that are important in subunit interaction, the CalphaB domain was used in a two-hybrid screen as bait to select randomly mutated CalphaB mutants. In this way we obtained nine mutants that were still able to interact with wild-type CalphaB despite the presence of up to 15 replacements. Similarly, we obtained 16 mutants that were unable to bind, because of the presence of just three to nine replacements. In binding CalphaB mutants, lysine residues were most often replaced by glutamic acid residues, and in non-binding CalphaB mutants, acidic residues were often found to be replaced by non-charged residues. This indicates that negative charges are important for subunit interaction and we propose a model to explain this role of acidic residues. Furthermore, we observed that two homologs of alphaB-crystallin, alphaA-crystallin and Hsp27, generally interact similarly with the binding and non-binding CalphaB mutants as does alphaB-crystallin. This suggests that interactions involved in the complex formation of these three sHsps are largely comparable.

Interaction of H(+)-ions with alpha-crystallin: solvent accessibility of ionizable side chains and surface charge

Interaction of H(+)-ions with alpha-crystallin from goat lens has been studied at three different ionic strengths using the potentiometric titration method. Titrations have also been carried out in the presence of 1.5 M and 6 M GuHCl (guanidine hydrochloride). The isoionic pH of the protein in water and the effect of KCl on it have been determined. Titration curves have been found to be reversible between pH 3 to 9.25 at all ionic strengths. To aid in the data analyses, the reactivities of alpha-crystallin lysine residues to trinitrobenzenesulfonic acid have been determined in this work. For alpha-crystallin aggregate, 130 +/- 2 histidine side chains out of a total of 300 and about 134 +/- 4 lysine side chains out of 310 have been found to be inaccessible to the solvent in the native condition. The remaining titratable side chains determine the surface charge of the native protein. In 1.5 M GuHC1, however, the nontitratable histidine side chains are found to be available for titration as are the nontitratable lysine and tyrosine side chains in 6M GuHC1. The theoretical titration curve computed on the basis of Linderstrøm-Lang model is found to fit quite comfortably with the experimental one between pH 4.6 and 9.25. The pKint value for beta gamma-carboxyl side chains has been found to be 5.18 which is somewhat higher than usual indicating that the carboxyl groups in the protein are probably in some constrained condition which is released in the presence of a denaturant. Below pH 4.6, there begins a conformational change in the alpha-crystallin aggregate as is corroborated from the circular dichroism studies. The value of electrostatic interaction factor w which remains more or less constant between pH 4.6 and 9.25 is also found to gradually fall off below pH 4.6.

Molecular properties of glutamate dehydrogenase from the extreme thermophilic archaebacterium Sulfolobus solfataricus

This study is concerned with the structural characterization in solution of the glutamate dehydrogenase from the Archaeon Sulfolobus solfataricus. At neutral pH both alpha-helix and beta-sheet constitute the secondary structure of this enzyme, on the basis of circular dichroism. A complex, temperature dependent self-association equilibrium regulates the formation of the enzyme quaternary structure, which seems to be accompanied by a reversible structural change. At 25 degrees C the enzyme is mostly represented by monomeric subunits at concentrations lower than 0.02 mg/ml, while oligomers are predominant at concentrations higher than 0.12 mg/ml. The mid-point of the association curve shifts from 0.05 mg/ml at 25 degrees C to about 0.1 mg/ml at 45 degrees C. Only the oligomeric form appears to be temperature resistant. Monomeric and oligomeric enzyme show distinct behaviour on guanidine hydrochloride perturbation at neutral pH. The monomer denaturation, although complex, is reversible. Two fluorescent tryptophan classes are detectable in the monomer, monitoring the independent unfolding of two regions through a multistate transition. Instead, the oligomeric protein shows a complex denaturation pattern with the tendency to aggregate irreversibly at high denaturant concentration.

Structure and modifications of the junior chaperone alpha-crystallin. From lens transparency to molecular pathology

alpha-Crystallin is a high-molecular-mass protein that for many decades was thought to be one of the rare real organ-specific proteins. This protein exists as an aggregate of about 800 kDa, but its composition is simple. Only two closely related subunits termed alpha A- and alpha B-crystallin, with molecular masses of approximately 20 kDa, form the building blocks of the aggregate. The idea of organ-specificity had to be abandoned when it was discovered that alpha-crystallin occurs in a great variety of nonlenticular tissues, notably heart, kidney, striated muscle and several tumors. Moreover alpha B-crystallin is a major component of ubiquinated inclusion bodies in human degenerative diseases. An earlier excitement arose when it was found that alpha B-crystallin, due to its very similar structural and functional properties, belongs to the heat-shock protein family. Eventually the chaperone nature of alpha-crystallin could be demonstrated unequivocally. All these unexpected findings make alpha-crystallin a subject of great interest far beyond the lens research field. A survey of structural data about alpha-crystallin is presented here. Since alpha-crystallin has resisted crystallization, only theoretical models of its three-dimensional structure are available. Due to its long life in the eye lens, alpha-crystallin is one of the best studied proteins with respect to post-translational modifications, including age-induced alterations. Because of its similarities with the small heat-shock proteins, the findings about alpha-crystallin are illuminative for the latter proteins as well. This review deals with: structural aspects, post-translational modifications (including deamidation, racemization, phosphorylation, acetylation, glycation, age-dependent truncation), the occurrence outside of the eye lens, the heat-shock relation and the chaperone activity of alpha-crystallin.