Mass spectrometric analysis of lens β-crystallins

Thiolation of the gammaB-crystallins in intact bovine lens exposed to hydrogen peroxide

Oxidative damage of the lens causes disulfide bonds between cysteinyl residues of lens proteins and thiols such as glutathione and cysteine, which may lead to cataract. The effect of H2O2 oxidation was determined by comparing bovine lenses incubated with and without 30 mM H2O2. The H2O2 treatment decreased the glutathione and increased the protein-glutathione and protein-cysteine disulfides in the lens. The molecular mass of the gammaB-crystallin isolated from lenses, not treated with H2O2, agreed with the published sequence (Mr 20,966). Some lenses also had a less abundant gammaB-crystallin component 305 Da higher (Mr 21,270), suggesting the presence of a glutathione adduct. The gammaB-crystallins from H2O2 treated lenses had three components, the major one with one GSH adduct, another one with the mass of unmodified gammaB-crystallin, and a third with a mass consistent with addition of two GSH adducts. Mass spectrometric analysis of tryptic peptides of gammaB-crystallins from different lenses indicated that the +305 Da modifications were not at a specific cysteine. For the lenses incubated without H2O2, there was evidence of adducts at Cys-41 and in peptide 10-31, which includes 3 cysteines. Analysis of modified peptide 10-31 by tandem mass spectrometry showed GSH adducts at Cys-15, Cys-18, and Cys-22. In addition, gammaB-crystallins from H2O2-treated lenses had an adduct at Cys-109, partial oxidation at all 7 Met residues, and evidence for two disulfide bonds.

The sequence of human betaB1-crystallin cDNA allows mass spectrometric detection of betaB1 protein missing portions of its N-terminal extension

The sequence of human betaB1-crystallin cDNA encoded a protein of 251 amino acids in length. Mass spectrometric analysis of intact betaB1 from young human lens confirmed the deduced amino acid sequence. Lenses of human donors newborn to 27 years of age also contained partially degraded forms of betaB1 missing 15, 33, 34, 35, 36, 39, 40, and 41 amino acid residues from their N-terminal extensions. The similarity of the cleavage site between residues 15 and 16 in human betaB1 to the cleavage occurring in bovine betaB1 suggested that lenses of both species may contain a similar proteolytic activity. The remaining cleavage sites occurring in human betaB1 did not closely match those occurring in other species, possibly due to the widely divergent amino acid sequence of the N-terminal extension of betaB1 amoung species. Results from animal models suggest that cleavage of the N-terminal extension of betaB1-crystallin could enhance protein insolubilization and cataract in lens. However, the presence of partially degraded betaB1-crystallins in both water-soluble and water-insoluble fractions of lenses of young donors suggested that the rate that proteolyzed betaB1-crystallins become water-insoluble is relatively slow in humans.

Probing high order structure of proteins by fast-atom bombardment mass spectrometry

During the past decade, numerous investigations have demonstrated that the rate at which amide hydrogens located at peptide linkages undergo isotopic exchange is a sensitive probe of the high order structure and dynamics of proteins. The present investigation demonstrates that microbore high-performance liquid chromatography (HPLC) continuous-flow fast-atom bombardment mass spectrometry (FABMS) can be used to accurately quantify deuterium located at peptide linkages in short segments of large proteins. This result is important because it demonstrates the feasibility of using mass spectrometry as a tool for studying the high order structure and dynamics of large proteins. Following a period of deuterium exchange-in, a protein was placed into slow-exchange conditions and fragmented into peptides with pepsin. The digest was analyzed by continuous-flow HPLC FABMS to determine the molecular weights of the peptides, from which the number of deuterons located at the peptide linkages could be deduced. The HPLC step was used both to fractionate the peptides according to their hydrophobicities and to remove through back-exchange all deuterium except that located at peptide amide linkages. This approach has been applied to α-crystallin, a lens protein composed of two gene products with monomer molecular weights of 20 kDa and an aggregate molecular weight approaching 1000 kDa. Results from this study show that some of the peptide amide hydrogens in αA-crystallin exchange very rapidly (k > 10 h(-1)) while others exchange very slowly (k < 10(-3) h(-1)). The ability not only to detect that a conformational change has occurred, but also to identify the specific regions within the protein where the change occurred, was demonstrated by measuring changes in the exchange rates within these regions as the deuterium exchange-in temperature was increased from 10 to 80 ° C.

Identification of the posttranslational modifications of bovine lens alpha B-crystallins by mass spectrometry

A combination of mass spectrometric techniques has been used to investigate the amino acid sequence and post-translational modifications of alpha B-crystallin isolated from bovine lenses by gel filtration chromatography and reversed-phase high performance liquid chromatography. Chromatographic fractions were analyzed by electrospray ionization mass spectrometry to determine the homogeneity and molecular weights of proteins in the fractions. The alpha B-crystallin primary gene product, its mono- and diphosphorylated forms, its N- and C-terminal truncated forms, as well as other lens proteins unrelated to the alpha B-crystallins were identified by their molecular weights. Detailed information about the sites of phosphorylation, as well as evidence supporting reassignment of Asn to Asp at position 80, was obtained by analyzing proteolytic digests of these proteins by fast atom bombardment mass spectrometry. Results of this investigation indicate that alpha B-crystallin is phosphorylated in vivo at Ser 45, Ser 59, and either Ser 19 or 21. From the specificity of phosphorylation of alpha-crystallins, it appears that there may be two different kinases responsible for their phosphorylation.