The calf gamma crystallins--a Raman spectroscopic study

In vitro unfolding, refolding, and polymerization of human gammaD crystallin, a protein involved in cataract formation

Human gammaD crystallin (HgammaD-Crys), a major protein of the human eye lens, is a primary component of cataracts. This 174-residue primarily beta-sheet protein is made up of four Greek keys separated into two domains. Mutations in the human gene sequence encoding HgammaD-Crys are implicated in early-onset cataracts in children, and the mutant protein expressed in Escherichia coli exhibits properties that reflect the in vivo pathology. We have characterized the unfolding, refolding, and competing aggregation of human wild-type HgammaD-Crys as a function of guanidinium hydrochloride (GuHCl) concentration at neutral pH and 37 degrees C, using intrinsic tryptophan fluorescence to monitor in vitro folding. Wild-type HgammaD-Crys exhibited reversible refolding above 1.0 M GuHCl. The GuHCl unfolded protein was more fluorescent than its native counterpart despite the absence of metal or ion-tryptophan interactions. Aggregation of refolding intermediates of HgammaD-Crys was observed in both equilibrium and kinetic refolding processes. The aggregation pathway competed with productive refolding at denaturant concentrations below 1.0 M GuHCl, beyond the major conformational transition region. Atomic force microscopy of samples under aggregating conditions revealed the sequential appearance of small nuclei, thin protofibrils, and fiber bundles. The HgammaD-Crys fibrous aggregate species bound bisANS appreciably, indicating the presence of exposed hydrophobic pockets. The mechanism of HgammaD-Crys aggregation may provide clues to understanding age-onset cataract formation in vivo.

Distinct cysteine sulfhydryl environments detected by analysis of Raman S-hh markers of Cys-->Ser mutant proteins

Very little is known about the character or functional relevance of hydrogen-bonded cysteine sulfhydryl (S-H) groups in proteins. The Raman S-H band is a unique and sensitive probe of the local S-H environment. Here, we report the use of Raman spectroscopy combined with site-specific mutagenesis to document the existence of five distinguishable hydrogen-bonded states of buried cysteine sulfhydryl groups in a native protein. The 666 residue subunit of the Salmonella typhimurium bacteriophage P22 tailspike contains eight cysteine residues distributed through the elongated structure. The tailspike cysteine residues display an unusual Raman S-H band complex (2500-2600 cm(-1) interval) indicative of diverse S-H hydrogen-bonding interactions in the native trimeric structure. To resolve specific Cys contributions to the complex Raman band we characterized a set of tailspike proteins with each cysteine replaced by a serine. The mutant proteins, once folded, were structurally and functionally indistinguishable from wild-type tailspikes, except for their Raman S-H signatures. Comparison of the Raman spectra of the mutant and wild-type proteins reveals the following hydrogen-bond classes for cysteine sulfhydryl groups. (i) Cys613 forms the strongest S-H...X bond of the tailspike, stronger than any heretofore observed for a protein. (ii) Cys267, Cys287 and Cys458 form robust S-H...X bonds. (iii) Moderate S-H...X bonding occurs for Cys169 and Cys635. (iv) Cys290 and Cys496 form weak hydrogen bonds. (v) It is remarkable that Cys287 contributes two Raman S-H markers, indicating the population of two distinct hydrogen-bonding states. The sum of the S-H Raman signatures of all eight mutants accurately reproduces the composite Raman band of the wild-type tailspike. The diverse cysteine states may be an outcome of the folding and assembly pathway of the tailspike, which though lacking disulfide bonds in the native state, utilizes transient disulfide bonds in the maturation pathway. This Raman study represents the first detailed assessment of local S-H hydrogen bonding in a native protein and provides information not obtainable directly by other structural probes. The method employed here should be applicable to a wide range of cysteine-containing proteins.

Estimation of the secondary structure and conformation of bovine lens crystallins by infrared spectroscopy: quantitative analysis and resolution by Fourier self-deconvolution and curve fit

The secondary structure of six bovine lens protein fractions (two alpha, three beta and one gamma-crystallin) are examined in solution and in solid forms for the first time using FTIR spectroscopy. Films of the nuclear and cortical regions of the bovine lens are also examined. The structure is quantitatively estimated from the vibrational analysis of the resolution-enhanced amide-I profile achieved by Fourier self-deconvolution and linear least-squares curve-fit algorithm. All the protein fractions fold predominantly in a beta-pleated sheet structure with little or no alpha-helical domains in solution or in lyophilized solid form. These proteins also retain their predominant beta-sheet conformation in the cellular phospholipid environment of the lens, in conformity with the structure obtained for all the mammalian species examined to date. Despite structural homology, vibrational data indicate subtle structural differences within each class of the crystallins probably due to presence of several minor substructures/subconformations. Substantial high amounts of turns (approx. 40%) observed in the beta-fractions may have a fundamental implication in stabilizing the tertiary structure of the uniquely folded-proteins vital for the transparency of the lens. These proteins in solid KBr-matrix undergo a major structural change, induced primarily by ionic interactions which refold them in a helical conformation. IR spectroscopy together with band-narrowing procedures has proven to be an effective tool to obtain structural information of proteins in solution, as solid substrates or in a complex biological tissue, such as ocular lens.

Near-infrared Fourier transform Raman and conventional Raman studies of calf gamma-crystallins in the lyophilized state and in solution

We present in this report a detailed structural study of calf gamma-crystallins both in the solid state and in solution by the newly developed technique of near-infrared (IR) Fourier transform (FT)-Raman spectroscopy as well as by the conventional Raman method. In comparison with conventional laser Raman spectroscopy, the near-IR FT-Raman approach exhibits several attractive features such as fluorescence rejection capability, frequency accuracy, and the FT's multiplex and throughput advantages. These distinct characteristics combined form the basis for the particular suitability of FT-Raman in crystallin structural analysis and elucidation. We have thus obtained evidence in support of the view that native calf gamma-II crystallin does not contain a disulfide bond either in the lyophilized state or in solution. In addition, conventional Raman spectra are examined for all four gamma-crystallin fractions. gamma-S, gamma-II, gamma-III, and gamma-IV, and the results indicate a high degree of structural similarities among them. It is also found that the sulfhydryl groups in all four gamma-crystallins are highly resistant to air oxidation and are capable of maintaining their reduced state during isolation in the absence of added reductants or such chelating agents as EDTA.