Calf lens neutral proteinase activity using calf lens crystallin substrates

Glutathiolation enhances the degradation of gammaC-crystallin in lens and reticulocyte lysates, partially via the ubiquitin-proteasome pathway

PURPOSE

S-glutathiolated proteins are formed in the lens during aging and cataractogenesis. The objective of this work was to explore the role of the ubiquitin-proteasome pathway in eliminating S-glutathiolated gammaC-crystallin.

METHODS

Recombinant human gammaC-crystallin was mixed with various concentrations of glutathione (GSH) and diamide at 25 degrees C for 1 hour. The extent of glutathiolation of the gammaC-crystallin was determined by mass spectrometry. Native and S-glutathiolated gammaC-crystallins were labeled with (125)I, and proteolytic degradation was determined using both lens fiber lysate and reticulocyte lysate as sources of ubiquitinating and proteolytic enzymes. Far UV circular dichroism, tryptophan fluorescence intensity, and binding to the hydrophobic fluorescence probe 4,4'-dianilino-1,1'-binaphthalene-5,5'-disulfonic acid (Bis-ANS), were used to characterize the native and glutathiolated gammaC-crystallins.

RESULTS

On average, two and five of the eight cysteines in gammaC-crystallin were glutathiolated when molar ratios of gammaC-crystallin-GSH-diamide were 1:2:5 and 1:10:25, respectively. Native gammaC-crystallin was resistant to degradation in both lens fiber lysate and reticulocyte lysate. However, glutathiolated gammaC-crystallin showed a significant increase in proteolytic degradation in both lens fiber and reticulocyte lysates. Proteolysis was stimulated by addition of adenosine triphosphate (ATP) and Ubc4 and was substantially inhibited by the proteasome inhibitor MG132 and a dominant negative form of ubiquitin, indicating that at least part of the proteolysis was mediated by the ubiquitin-proteasome pathway. Spectroscopic analyses of glutathiolated gammaC-crystallin revealed conformational changes and partial unfolding, which may provide a signal for the ubiquitin-dependent degradation.

CONCLUSIONS

The present data demonstrate that oxidative modification by glutathiolation can render lens proteins more susceptible to degradation by the ubiquitin-proteasome pathway. Together with previous results, these data support the concept that the ubiquitin-proteasome pathway serves as a general protein quality-control mechanism.

Function of the ubiquitin proteolytic pathway in the eye

The ubiquitin pathway (UP) is involved in regulation of many essential cellular processes usually by the degradation of regulators of these processes. For example the UP is involved in regulation of cell cycle, proliferation, differentiation, organogenesis, development, and signal transduction in the lens and retina. A functional UP has also been documented in the cornea. Upon aging or exposure to stress there is an accumulation of damaged proteins, including ubiquitinated proteins, in the lens and retina. Some of these proteins may be cytotoxic. Thus, an active UP may be required to avoid such age and disease-related accumulation of damaged proteins. In this review we will explain the biochemistry of the UP and we will document the most important studies regarding UP function in the lens, retina and cornea.

Differential inhibition of three peptidase activities of the proteasome in human lens epithelium by heat and oxidation

The proteasome is a large protease complex that is thought to be responsible for proteolytic removal of damaged proteins. We have previously shown that the level of proteolytic activity due to the proteasome is lower in lens epithelium from human cataractous lenses compared to the activity in epithelium from clear donor lenses. This study aimed to characterize the three main peptidase activities of the proteasome in human lens epithelium with respect to kinetic properties and sensitivity to heat and oxidation. Human lens epithelia were obtained from cataract surgery and analysis performed on pools of epithelial cell cytoplasm. Using the fluorogenic peptide substrates Suc-Leu-Leu-Val-Tyr-AMC (LLVY), Boc-Val-Gly-Arg-AMC (VGR) and Z-Leu-Leu-Glu-betaNA (LLE), Km-values of 56, 678 and 108 micrometers were obtained. All peptidase activities were inhibited by lactacystin, a specific proteasome inhibitor, but at very different rates; with LLVY-hydrolysing activity being the most sensitive (Ki50%=0.15 micrometers). Thermostability was investigated by performing the proteolytic assay at 20 degrees, 37 degrees and 53 degrees C. The trypsin-like activity, as measured by VGR, was completely stable at 53 degrees C for at least 24 hr whereas hydrolysis of LLVY and LLE declined after a few hours at 37 degrees C. Oxidative inhibition was induced by incubation of the samples in 0.5 m m H2O2for 1 or 24 hr. One hour exposure to H2O2caused moderate inhibition of all peptidase activities. The activity could be partially restored by adding 1 m m dithiotreitol, indicating the dependency on intact SH-groups. After 24 hr, peptidase activities were decreased to 25% (LLVY), 73% (VGR) and 44% (LLE) of corresponding control. This inhibition was irreversible for VGR and LLE, but could be partly prevented by the presence of heat shock protein 90 (LLVY and VGR) or alpha-crystallin (LLVY). These data show that the peptidase activities of the human lens proteasome can be modulated by metabolites, such as reactive oxygen species, and by endogenous proteins such as alpha-crystallin and heat shock protein 90.

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

A protein fragment (M(r) approximately 9000) isolated from the cortex of nonpathological calf lenses has been structurally characterized. The polypeptide structure was well organized (39% alpha-helix, 33% beta-structure, and 28% remainder) according to the far-ultraviolet circular dichroism. The fluorescence was heterogeneous for the presence of two tryptophan classes. Structure perturbation by pH and denaturant revealed cooperative structural transitions which are characteristics of a globular organization. A single-step unfolding curve induced by Gdn-HCl (midpoint = 1.38 M Gdn-HCl) was monitored by emission maximum shift as well as by far-ultraviolet circular dichroism. This transition was analyzed as a two-state process. The standard free energy of unfolding in the absence of the denaturant, delta Go (H2O), was found to be 10.80 +/- 0.25 kJ/mol at 20 degrees C and pH 7.4. The fragment also shows an unusual thermal resistance. Its structure was unperturbed up to 90 degrees C according to the fluorescence and dichroism. This last property, its peculiar amino acid composition, and the sequence of a small segment are shared, among crystallins, only with the N-terminal region of the alpha-crystallin B chain. A search for proteolysis sites along the alpha-crystallin B chain sequence revealed that it possesses specific points for proteinase attack. These sites are particularly exposed and clustered in a very flexible region in the middle of the protein sequence. They are also well represented in the C-terminal extension of the molecule while a few are buried in the N-terminal region.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for ATP and ubiquitin dependent degradation of proteins in cultured bovine lens epithelial cells

Degradation of endogenous lens proteins has been difficult to show under physiological conditions using lens tissue preparations. In contrast, active proteolytic systems in cultured bovine lens epithelial (BLE) cells have been demonstrated previously. BLE cells also contain ubiquitin, a 76 amino-acid polypeptide which is conjugated to proteins in an ATP/Mg(2+)-dependent process prior to their cytosolic proteolysis. In this study, we show that histone H2A, alpha-crystallin, and actin are conjugated to ubiquitin, resulting in higher molecular mass species, which are detected by anti-ubiquitin antibodies. These proteins are also degraded in cell-free assays containing BLE cell supernatants under physiological conditions in an ATP/Mg(2+)-dependent manner. Observation of 125I-labeled proteolytic fragments was made after SDS polyacrylamide gel electrophoresis of the assays. Quantitation of trichloroacetic acid-soluble radiolabeled fragments generated in the presence of ATP/Mg2+ revealed that, with BLE cell supernatant, 25% of the histone H2A was degraded in 3 hr. Proteolysis of alpha-crystallin and actin amounted to 2.3% and 2.9%, respectively. The requirement of ATP/Mg2+ for proteolysis and the observation of ubiquitin conjugation to the same proteins is consistent with the presence of a ubiquitin-dependent proteolytic pathway in BLE cells. Additionally, in this study the BLE cell proteases were even more active on some substrates than the reticulocyte ubiquitin/ATP-dependent proteolytic system.

Lens proteasome shows enhanced rates of degradation of hydroxyl radical modified alpha-crystallin

Proteasome, a high molecular weight protease complex (HMP, approximately 600 kDa) was isolated from bovine eye lens epithelium tissue. In contrast with prior reports, lens proteasome degraded the major lens protein alpha-crystallin and S-carboxymethylated bovine serum albumin at 37 degrees C, mostly to trichloroacetic acid precipitable polypeptides. The proteasome, thus isolated, was labile at 55 degrees C. As indicated by the ability of p-chloromercuribenzoate and N-ethylmaleimide to block activity, a thiol group is required for activity. Alpha-crystallin was oxidized by exposure to 60Co-irradiation under an atmosphere of N2O (1-50 kilorads). This dose delivered 0.1-5.7 mol of hydroxyl radicals per mol of crystallin. Irradiation resulted in increased heterogeneity, aggregation, and fragmentation of the crystallin preparation. The proteolytic susceptibility of alpha-crystallin to the lens HMP was enhanced by the irradiation in a dose-dependent manner up to 20 kilorads (.OH concentration up to 2.3 mol per mol of alpha-crystallin). When 50 kilorads (5.7 mol .OH per mol of alpha-crystallin) was used, there was extensive aggregation and no enhancement in proteolysis over the unirradiated sample. The data indicate that the lens HMP can degrade mildly photooxidized lens proteins, but proteins which are extensively damaged are not degraded and may accumulate. This may be related to cataract formation.

The effects of aging and cataract formation on the trypsin inhibitor activity of human lens

Assays were carried out to determine the trypsin inhibitor activity present in the water-soluble and water-insoluble fractions of human lenses of various ages. Little change was seen in the inhibitor activity of the water-soluble protein fraction. When this fraction was chromatographed on an Agarose A-1.5 m column, however, the inhibitor activity was increasingly associated with the high molecular weight (HMW) protein fraction with age. A gradual increase in water-insoluble inhibitor was seen up to age 60, which correlated with the increase in protein in this fraction. After age 60, a marked increase in the water-soluble inhibitor activity was observed. In 80-90-yr old lenses, 1 mg of water-insoluble protein was able to inhibit 200 micrograms of crystallin trypsin by 50%. Similar assays on a collection of cortical and brunescent cataracts also showed very high levels of water-insoluble inhibitor activity. In most cases, these values were higher than those for the age-matched control lenses. Fractionation of the water-insoluble proteins showed that the bulk of the activity remained with the urea-insoluble fraction in cataractous lenses. A low molecular weight trypsin inhibitor was isolated from the water-soluble and water-insoluble fractions of human lenses. An age-dependent increase in this inhibitor was observed by activity measurements and electrophoretic analysis.

Characterization of a highly purified membrane proteinase from bovine lens

A highly purified bovine lens membrane proteinase has been obtained. The purification was accomplished by solubilization of the proteinase from the membrane with 2% sodium deoxycholate followed by gel-filtration chromatography. The purified proteinase showed a major protein band having molecular weight of 17,000 on sodium dodecyl sulfate-polyacrylamide-gel electrophoresis (SDS-PAGE) and an active band on a non-denaturing acrylamide gel. The proteinase existed as a tetramer having a molecular weight of 68,000 as determined by gel-filtration chromatography. The proteinase had a pH optimum of 7.8 and was unstable above 40 degrees C. It lacked any requirement for metal ions for activity and was inhibited by all serine proteinase inhibitors tested. The proteinase hydrolysed mostly arginine amide and ester bonds. Based on its properties, the newly isolated membrane proteinase seems to be distinct from any mammalian lens proteinase isolated so far.

The bovine lens neutral proteinase comprises a family of cysteine-dependent proteolytic activities

Inhibitor studies with peptide substrates demonstrate that bovine lens neutral proteinase comprises three distinct activities. Diisopropylfluorophosphate distinguishes the activity hydrolyzing carbobenzoxy-Gly-Gly-Leu-p-nitroanilide (inhibited) from that hydrolyzing carbobenzoxy-Leu-Leu-Glu-2-naphthylamide (not inhibited). Leupeptin inhibits hydrolysis of the substrate carbobenzoxy-Leu-Leu-Arg-2-naphthylamide, but not hydrolysis of carbobenzoxy-Gly-Gly-Leu-p-nitroanilide or carbobenzoxy-Leu-Leu-Glu-2-naphthylamide, demonstrating the presence of the third activity. Inhibition of the three activities by thiol reagents suggests that each activity may be dependent on an active-site cysteine residue.

Differential inhibition of two proteolytic activities in bovine lens neutral-proteinase preparations

Hydrolysis of carbobenzoxy-Leu-Leu-Glu 2-naphthylamide by bovine lens neutral-proteinase preparations is not affected by the esterase inhibitor di-isopropyl fluorophosphate, whereas hydrolysis of carbobenzoxy-Gly-Gly-Leu p-nitroanilide is completely inhibited. Hydrolysis of alpha-crystallin, a lens structural protein, can be inhibited by only 50% after prolonged treatment with di-isopropyl fluorophosphate. These data suggest that the lens neutral-proteinase preparation contains at least two enzymes, one of which may be a serine proteinase. This may account, in part, for the previously observed complex response of the preparation to inhibitors.

A synthetic endopeptidase substrate hydrolyzed by the bovine lens neutral proteinase preparation

Lens neutral proteinase is thought to exhibit primarily endopeptidase activity. We have identified a synthetic endopeptidase substrate which is hydrolyzed by the bovine lens neutral proteinase preparation. Among 11 fluoro- and chromogenic endopeptidase substrates, only carbobenzoxy-glycylglycyl-L-leucyl-p-nitroanilide is effectively hydrolyzed. The activity hydrolyzing this substrate co-elutes with neutral proteinase activity upon gel filtration and specifically attacks the leucyl-p-nitroaniline bond. Optimal hydrolysis of the synthetic substrate is at neutral pH and high temperature (53 degrees C), analogous to the alpha-crystallin protein substrate obtained from lens. The rate of hydrolysis of the synthetic substrate increased proportionally with temperature between 20 and 60 degrees C, in contrast to alpha-crystallin. The rate of hydrolysis was linear for at least 1 h at 37 degrees C and there was no evidence of enzyme activation at high temperature.

Limited proteolysis of bovine lens alpha-crystallin by calpain, a Ca2+-dependent cysteine proteinase, isolated from the same tissue

A Ca2+-dependent cysteine proteinase (calpain, EC 3.4.22.17) was found in the cystosolic fraction of bovine lens and purified to apparent homogeneity. The purified enzyme required 1 mM Ca2+ for its full activation and was composed of two subunits of Mr 80 000 and 29 000 as demonstrated by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate (SDS). This enzyme, when activated by Ca2+, degraded both A- and B-chains of alpha-crystallin, which were isolated also from bovine lens. SDS-gel electrophoresis of the digest revealed that the A-chain (Mr 19 500) was broken down to produce an 18-kDa polypeptide fragment and the B-chain (Mr 22 500) to produce a 19.5-kDa polypeptide fragment. No further cleavage occurred even upon prolonged incubation or after the second addition of the enzyme, indicating the uniquely limited proteolysis of each chain protein. The existence of calpastatin, an endogenous inhibitor protein specific for calpain, was also demonstrated in bovine lens cytosol.

Isolation and characterization of a 25K serine proteinase from bovine lens cortex

A lens serine proteinase with trypsin-like specificity has been purified to homogeneity. This is one of two serine proteinases associated with the alpha-crystallin fraction from bovine lens. The purification was accomplished by a combination of isoelectric precipitation, activation to release the proteinase, gel-filtration and affinity chromatography. The purified proteinase showed a single protein band of 25 000 daltons on SDS-PAGE. A single protein band was also seen on non-denaturing gels which correlated with the location of the proteinase activity. The proteinase had a pH optimum between 7.2 and 8.2, was stable between pH 5.8 and 8.6 but was unstable above 40 degrees C upon heating. The enzyme lacked any requirement for metal ions and hydrolyzed arginine, lysine and asparagine substrates. alpha-Crystallin, and especially the B-chain of alpha-crystalline, was rapidly hydrolyzed by the proteinase compared to other lens crystallins. Metallo- and cysteine-proteinase inhibitors had no effect upon the enzyme activity whereas three different serine-proteinase inhibitors completely abolished all activity. A number of protein and peptide trypsin inhibitors also completely inhibited the lens 25K serine proteinase.

Lens calcium activated proteinase: degradation of vimentin

The lens has been shown to contain a Ca+2 activated proteinase specific for vimentin. The proteinase is present in the soluble fraction of the cortex but not in the epithelium. It is suggested that this proteinase is expressed during terminal differentiation of the epithelial cells and may be responsible for degradation of the intermediate filaments in the fiber cells. The proteinase is inhibited by EGTA but not by several proteinase inhibitors.

Comparison of in vitro proteolysis products of the normal human lens with putative in vivo proteolysis products of the cataractous lens

Treatment of normal lens homogenates with trypsin results in the production of polypeptides of approximately 4,000 and 8,000 daltons. Peptide maps of these components are very similar to poly-peptides of comparable molecular weight found only in cataracts with brunescent and opaque nuclei. Together, these results demonstrate that putative proteolysis products produced in vivo during human cataractogenesis can be mimiced in vitro by the addition of exogeneous proteases to the normal lens homogenate.