Protein damage and methylation-mediated repair in the erythrocyte

Natural isoaspartyl protein modification of ZAP70 alters T cell responses in lupus

Protein posttranslational modifications (PTMs) arise in a number of normal cellular biological pathways and in response to pathology caused by inflammation and/or infection. Indeed, a number of PTMs have been identified and linked to specific autoimmune responses and metabolic pathways. One particular PTM, termed isoaspartyl (isoAsp or isoD) modification, is among the most common spontaneous PTM occurring at physiological pH and temperature. Herein, we demonstrate that isoAsp modifications arise within the ZAP70 protein tyrosine kinase upon T-cell antigen receptor (TCR) engagement. The enzyme protein L-isoaspartate O-methyltransferase (PCMT1, or PIMT, EC 2.1.1.77) evolved to repair isoaspartyl modifications in cells. In this regard, we observe that increased levels of isoAsp modification that arise under oxidative stress are correlated with reduced PIMT activity in patients with systemic lupus erythematosus (SLE). PIMT deficiency leads to T cell hyper-proliferation and hyper-phosphorylation through ZAP70 signaling. We demonstrate that inducing the overexpression of PIMT can correct the hyper-responsive phenotype in lupus T cells. Our studies reveal a phenotypic role of isoAsp modification and phosphorylation of ZAP70 in lupus T cell autoimmunity and provide a potential therapeutic target through the repair of isoAsp modification.

Towards a better monitoring of seed ageing under ex situ seed conservation

Long-term conservation of 7.4 million ex situ seed accessions held in agricultural genebanks and botanic gardens worldwide is a challenging mission for human food security and ecosystem services. Recent advances in seed biology and genomics may have opened new opportunities for effective management of seed germplasm under long-term storage. Here, we review the current development of tools for assessing seed ageing and research advances in seed biology and genomics, with a focus on exploring their potential as better tools for monitoring of seed ageing. Seed ageing is found to be associated with the changes reflected in reactive oxygen species and mitochondria-triggered programmed cell deaths, expression of antioxidative genes and DNA and protein repair genes, chromosome telomere lengths, epigenetic regulation of related genes (microRNA and methylation) and altered organelle and nuclear genomes. Among these changes, the signals from mitochondrial and nuclear genomes may show the most promise for use in the development of tools to predict seed ageing. Non-destructive and non-invasive analyses of stored seeds through calorimetry or imaging techniques are also promising. It is clear that research into developing advanced tools for monitoring seed ageing to supplement traditional germination tests will be fruitful for effective conservation of ex situ seed germplasm.

Abnormal isoaspartyl residues in erythrocyte membranes from psoriatic patients

Spontaneous protein deamidation of labile asparagines (Asn), generating abnormal isoaspartyl residues (IsoAsp), is associated with cell aging and enhanced by an oxidative microenvironment. The presence of isopeptide bonds impairs protein structure/function and can trigger autoimmune responses. To minimize the damage, IsoAsp can be "repaired" by a specific L-isoaspartate-(D-aspartate)-protein-O-methyltransferase. The condition of chronic oxidative stress reported in psoriatic patients, and the potential etiological role of unknown self-antigens, prompted us to investigate Asn deamidation in psoriatic tissues. Erythrocytes (RBC) were selected as the model system since, lacking protein synthesis apparatus, they are unable to replace damaged proteins. Blood samples were obtained from 36 patients and 34 controls. L-isoAsp content was highly increased in RBC membrane proteins from psoriatic patients. Deamidated species included ankyrin, band 4.1, band 4.2 and the integral membrane protein band 3. A functional analysis demonstrated that this result was unrelated to a reduced efficiency of the S-adenosylmethionine-dependent repair system suggesting an increased protein instability at Asn sites, responsible for IsoAsp accumulation in psoriatic patients.

The microRNA 15a/16-1 cluster down-regulates protein repair isoaspartyl methyltransferase in hepatoma cells: implications for apoptosis regulation

Asparaginyl deamidation, a spontaneous protein post-biosynthetic modification, determines isoaspartyl formation and structure-function impairment. The isoaspartyl protein carboxyl-O-methyltransferase (PCMT1; EC 2.1.1.77) catalyzes the repair of the isopeptide bonds at isoaspartyl sites, preventing deamidation-related functional impairment. Protein deamidation affects key apoptosis mediators, such as BclxL, thus increasing susceptibility to apoptosis, whereas PCMT1 activity may effectively counteract such alterations. The aim of this work was to establish the role of RNAi as a potential mechanism for regulating PCMT1 expression and its possible implications in apoptosis. We investigated the regulatory properties of the microRNA 15a/16-1 cluster on PCMT1 expression on HepG2 cells. MicroRNA 15a or microRNA 16-1 transfection, as well as their relevant antagonists, showed that PCMT1 is effectively regulated by this microRNA cluster. The direct interaction of these two microRNAs with the seed sequence at the 3' UTR of PCMT1 transcripts was demonstrated by the luciferase assay system. The role of PCMT1 down-regulation in conditioning the susceptibility to apoptosis was investigated using various specific siRNA or shRNA approaches, to prevent non-PCMT1-specific pleiotropic effects to take place. We found that PCMT1 silencing is associated with an increase of the BclxL isoform reported to be inactivated by deamidation, thus making cells more susceptible to apoptosis induced by cisplatinum. We conclude that PCMT1 is effectively regulated by the microRNA 15a/16-1 cluster and is involved in apoptosis by preserving the structural stability and biological function of BclxL from deamidation. Control of PCMT1 expression by microRNA 15a/16-1 may thus represent a late checkpoint in apoptosis regulation.

Isoaspartate-containing amyloid precursor protein-derived peptides alter efficacy and specificity of potential beta-secretases

Neuritic plaques of Alzheimer patients are composed of multiple protein components. Among them, the amyloid beta-peptides (Abeta) 1-40/42 and further N- and C-terminally modified fragments of Abeta are highly abundant. Most prominent are the isoaspartate (isoAsp)-Abeta peptides and pyroglutamyl (pGlu)-Abeta. While pGlu-Abeta can only be formed from an N-terminal glutamate by glutaminyl cyclase, spontaneous isoAsp-isomerization cannot occur at an N-terminal aspartate of peptides. This means that isoAsp-Abeta formation must precede proteolysis of the amyloid precursor protein (APP). Abeta generation from APP by beta- and gamma-secretases initiates the amyloid peptide aggregation and deposition process. Two aspartate proteases have been identified as secretases: BACE-1 (beta-site amyloid precursor protein cleaving enzyme) and the intramembrane gamma-secretase multiprotein complex. However, recent evidence supports more than one beta-secretase initiating this cascade. Formation of Abeta1-40/42 was predominantly studied by expression of mutated human APP sequences in cell culture and transgenic animals, generating Abeta fragments that did not contain such multiple posttranslational modifications as in Alzheimer's disease. This prompted us to investigate the catalytic turnover of Asp- or isoAsp-containing APP-derived peptide sequences by BACE-1 and cathepsin B, another potential beta-secretase. While cathepsin B is more effective than BACE-1 in processing the Asp-containing peptide derivatives, only cathepsin B can cleave the isoAsp-containing peptides, which occurs with high catalytic efficiency.

Alternative pathways for production of beta-amyloid peptides of Alzheimer's disease

This highlight article describes three Alzheimer's disease (AD) studies presented at the 5th General Meeting of the International Proteolysis Society that address enzymatic mechanisms for producing neurotoxic beta-amyloid (Abeta) peptides. One group described the poor kinetics of BACE 1 for cleaving the wild-type (WT) beta-secretase site of APP found in most AD patients. They showed that cathepsin D displays BACE 1-like specificity and cathepsin D is 280-fold more abundant in human brain than BACE 1. Nevertheless, as BACE 1 and cathepsin D show poor activity towards the WT beta-secretase site, they suggested continuing the search for additional beta-secretase(s). The second group reported cathepsin B as an alternative beta-secretase possessing excellent kinetic efficiency and specificity for the WT beta-secretase site. Significantly, inhibitors of cathepsin B improved memory, with reduced amyloid plaques and decreased Abeta(40/42) in brains of AD animal models expressing amyloid precursor protein containing the WT beta-secretase site. The third group addressed isoaspartate and pyroglutamate (pGlu) posttranslational modifications of Abeta. Results showed that cathepsin B, but not BACE 1, efficiently cleaves the WT beta-secretase isoaspartate site. Furthermore, cyclization of N-terminal Glu by glutaminyl cyclase generates highly amyloidogenic pGluAbeta(3-40/42). These presentations suggest cathepsin B and glutaminyl cyclase as potential new AD therapeutic targets.

Protein isoaspartate methyltransferase prevents apoptosis induced by oxidative stress in endothelial cells: role of Bcl-Xl deamidation and methylation

Background

Natural proteins undergo in vivo spontaneous post-biosynthetic deamidation of specific asparagine residues with isoaspartyl formation. Deamidated-isomerized molecules are both structurally and functionally altered. The enzyme isoaspartyl protein carboxyl-O-methyltransferase (PCMT; EC 2.1.1.77) has peculiar substrate specificity towards these deamidated proteins. It catalyzes methyl esterification of the free α-carboxyl group at the isoaspartyl site, thus initiating the repair of these abnormal proteins through the conversion of the isopeptide bond into a normal α-peptide bond. Deamidation occurs slowly during cellular and molecular aging, being accelerated by physical-chemical stresses brought to the living cells. Previous evidence supports a role of protein deamidation in the acquisition of susceptibility to apoptosis. Aim of this work was to shed a light on the role of PCMT in apoptosis clarifying the relevant mechanism(s).

Methodology/Principal Findings

Endothelial cells transiently transfected with various constructs of PCMT, i.e. overexpressing wild type PCMT or negative dominants, were used to investigate the role of protein methylation during apoptosis induced by oxidative stress (H₂O₂; 0.1–0.5 mM range). Results show that A) Cells overexpressing “wild type” human PCMT were resistant to apoptosis, whereas overexpression of antisense PCMT induces high sensitivity to apoptosis even at low H₂O₂ concentrations. B) PCMT protective effect is specifically due to its methyltransferase activity rather than to any other non-enzymatic interactions. In fact negative dominants, overexpressing PCMT mutants devoid of catalytic activity do not prevent apoptosis. C) Cells transfected with antisense PCMT, or overexpressing a PCMT mutant, accumulate isoaspartyl-containing damaged proteins upon H₂O₂ treatment. Proteomics allowed the identification of proteins, which are both PCMT substrates and apoptosis effectors, whose deamidation occurs under oxidative stress conditions leading to programmed cell death. These proteins, including Hsp70, Hsp90, actin, and Bcl-xL, are recognized and methylated by PCMT, according to the general repair mechanism of this methyltransferase.

Conclusion/Significance

Apoptosis can be modulated by “on/off” switch partitioning the amount of specific protein effectors, which are either in their active (native) or inactive (deamidated) molecular forms. Deamidated proteins can also be functionally restored through methylation. Bcl-xL provides a case for the role of PCMT in the maintenance of functional stability of this antiapoptotic protein.

Accumulation of altered aspartyl residues in erythrocyte proteins from patients with Down's syndrome

Spontaneous protein deamidation of labile Asn residues, generating L-isoaspartates and D-aspartates, is associated with cell aging and is enhanced by an oxidative microenvironment; to minimize the damage, the isoaspartate residues can be 'repaired' by a specific L-isoaspartate (D-aspartate) protein O-methyltransferase (PIMT). As both premature aging and chronic oxidative stress are typical features of Down's syndrome (DS), we tested the hypothesis that deamidated proteins may build up in trisomic patients. Blood samples were obtained from children with karyotypically confirmed full trisomy 21 and from age-matched healthy controls. Using recombinant PIMT as a probe, we demonstrated a dramatic rise of L-isoaspartates in erythrocyte membrane proteins from DS patients. The content of D-aspartate was also significantly increased. The integrity of the repair system was checked by evaluating methionine transport, PIMT specific activity, and intracellular concentrations of adenosylmethionine and adenosylhomocysteine. The overall methylation pathway was directly monitored by incubating fresh red blood cells with methyl-labeled methionine; a three-fold increase of protein methyl esters was detected in trisomic children. Deamidated species include ankyrin, band 4.1, band 4.2 and the integral membrane protein band 3; ankyrin and band 4.1 were significantly hypermethylated in DS. When DS red blood cells were subjected to oxidative treatment in vitro, the increase of protein deamidation paralleled lipid peroxidation and free radical generation. We observed a similar pattern in Epstein-Barr virus B-lymphocytes from trisomic patients. In conclusion, our findings support the hypothesis that protein instability at asparagine sites is a biochemical feature of DS, presumably depending upon the oxidative microenvironment. The possible pathophysiological implications are discussed.

Counteracting expression deficiencies by anticipating posttranslational modification of PaHNL5-L1Q-A111G by genetic engineering

(R)-2-chloromandelic acid represents a key pharmaceutical intermediate. Its production on large scale was hampered by low turnover rates and moderate enantiomeric excess (ee) using enzyme as well as metal catalysts. The cloning and heterologous overexpression of an (R)-hydroxynitrile lyase from Prunus amygdalus opened a way to large-scale production of this compound. Especially the rationally designed mutation of alanine to glycine at amino acid position 111 of the mature protein tremendously raised the yield for enantioselective conversion of 2-chlorobenzaldehyde to (R)-2-chloromandelonitrile, which can be hydrolysed to the corresponding alpha hydroxy acid. However, expression of this mutein was less efficient than for the unmodified enzyme. Subsequent LC/MS/MS-analysis of the protein sequence revealed that mutation A111G triggered the posttranslational deamidation of the neighbouring residue asparagine (N110) to aspartic acid. This finding on the one hand could explain the decreased secretion efficiency of the mutant as compared to the wildtype enzyme, but on the other hand raised the question which of the two residues was truly accountable for the enhanced conversion. The muteins N110D, A111G and N110DA111G were constructed and compared in terms of protein productivity and performance in chemical syntheses. The expression level of the double mutein was augmented significantly and the enantioselectivity remained high. Reduced protein expression of mutein PaHNL5-L1Q-A111G was remedied by mutational anticipation of posttranslational deamidation.

Intracellular protein modification associated with altered T cell functions in autoimmunity

Posttranslational protein modifications influence a number of immunologic responses ranging from intracellular signaling to protein processing and presentation. One such modification, termed isoaspartyl (isoAsp), is the spontaneous nonenzymatic modification of aspartic acid residues occurring at physiologic pH and temperature. In this study, we have examined the intracellular levels of isoAsp residues in self-proteins from MRL(+/+), MRL/lpr, and NZB/W F(1) mouse strains compared with nonautoimmune B10.BR mice. In contrast to control B10.BR or NZB/W mice, the isoAsp content in MRL autoimmune mice increased and accumulated with age in erythrocytes, brain, kidney, and T lymphocytes. Moreover, T cells that hyperproliferate to antigenic stimulation in MRL mice also have elevated intracellular isoAsp protein content. Protein l-isoaspartate O-methyltransferase activity, a repair enzyme for isoAsp residues in vivo, remains stable with age in all strains of mice. These studies demonstrate a role for the accumulation of intracellular isoAsp proteins associated with T cell proliferative defects of MRL autoimmune mice.

Proteomic Identification of Novel Substrates of a Protein Isoaspartyl Methyltransferase Repair Enzyme

We report the use of a proteomic strategy to identify hitherto unknown substrates for mammalian protein l-isoaspartate O-methyltransferase. This methyltransferase initiates the repair of isoaspartyl residues in aged or stress-damaged proteins in vivo. Tissues from mice lacking the methyltransferase (Pcmt1(-/-)) accumulate more isoaspartyl residues than their wild-type littermates, with the most "damaged" residues arising in the brain. To identify the proteins containing these residues, brain homogenates from Pcmt1(-/-) mice were methylated by exogenous repair enzyme and the radiolabeled methyl donor S-adenosyl-[methyl-(3)H]methionine. Methylated proteins in the homogenates were resolved by both one-dimensional and two-dimensional electrophoresis, and methyltransferase substrates were identified by their increased radiolabeling when isolated from Pcmt1(-/-) animals compared with Pcmt1(+/+) littermates. Mass spectrometric analyses of these isolated brain proteins reveal for the first time that microtubule-associated protein-2, calreticulin, clathrin light chains a and b, ubiquitin carboxyl-terminal hydrolase L1, phosphatidylethanolamine-binding protein, stathmin, beta-synuclein, and alpha-synuclein, are all substrates for the l-isoaspartate methyltransferase in vivo. Our methodology for methyltransferase substrate identification was further supplemented by demonstrating that one of these methyltransferase targets, microtubule-associated protein-2, could be radiolabeled within Pcmt1(-/-) brain extracts using radioactive methyl donor and exogenous methyltransferase enzyme and then specifically immunoprecipitated with microtubule-associated protein-2 antibodies to recover co-localized protein with radioactivity. We comment on the functional significance of accumulation of relatively high levels of isoaspartate within these methyltransferase targets in the context of the histological and phenotypical changes associated with the methyltransferase knock-out mice.

Hydroxytyrosol, a natural antioxidant from olive oil, prevents protein damage induced by long-wave ultraviolet radiation in melanoma cells

Previous studies showed that long-wave ultraviolet (UVA) radiation induces severe skin damage through the generation of reactive oxygen species and the depletion of endogenous antioxidant systems. Recent results from our laboratory indicate a dramatic increase of both lipid peroxidation products (TBARS) and abnormal L-isoaspartyl residues, marker of protein damage, in UVA-irradiated human melanoma cells. In this study, the effects of hydroxytyrosol (DOPET), the major antioxidant compound present in olive oil, on UVA-induced cell damages, have been investigated, using a human melanoma cell line (M14) as a model system. In UVA-irradiated M14 cells, a protective effect of DOPET in preventing the uprise of typical markers of oxidative stress, such as TBARS and 2'7'-dichlorofluorescein (DCF) fluorescence intensity, was observed. In addition, DOPET prevents the increase of altered L-isoAsp residues induced by UVA irradiation. These protective effects are dose dependent, reaching the maximum at 400 microM DOPET. At higher concentrations, DOPET causes an arrest of M14 cell proliferation and acts as a proapoptotic stimulus by activating caspase-3 activity. In the investigated model system, DOPET is quantitatively converted into its methylated derivative, endowed with a radical scavenging ability comparable to that of its parent compound. These findings are in line with the hypothesis that the oxidative stress plays a major role in mediating the UVA-induced protein damage. Results suggest that DOPET may exerts differential effects on melanoma cells according to the dose employed and this must always be taken into account when olive oil-derived large consumer products, including cosmetics and functional foods, are employed.

Expression and activity of l-isoaspartyl methyltransferase decrease in stage progression of human astrocytic tumors

Protein l-isoaspartyl methyltransferase (PIMT) functions as a repair enzyme that acts upon damaged proteins bearing abnormal aspartyl residues. We previously reported that PIMT expression and activity are reduced by half in human epileptic hippocampus. Here we investigated PIMT regulation in astrocytic tumors, which are the most common human brain tumors. PIMT expression and enzyme activity were significantly decreased in all grades of human astrocytic tumors. More precisely, PIMT levels were significantly lower by 76% in pilocytic astrocytomas (grade I), 46% in astrocytomas (grade II), 69% in anaplastic astrocytomas (grade III), and a marked 80% in glioblastomas (grade IV) as compared to normal brains. RT-PCR analysis showed that levels of type I PIMT mRNA were up-regulated while those of type II PIMT mRNA were down-regulated in glioblastomas. Furthermore, the reduced PIMT levels correlated closely with a decrease in the number of neuron cells in astrocytic tumors as assessed by measuring the neuron-specific enolase level. Many proteins with abnormal aspartyl residues accumulated in brain tumors and some were specific to individual grades of astrocytic tumors. Similar results were obtained, either by measuring the reduction in PIMT activity and expression or by measuring the formation of abnormal proteins, in an orthotopic rat brain tumor model implanted with invasive CNS-1 glioma cells. The novelty of these findings was to provide the first evidence for a marked reduction of PIMT expression and activity during stage progression of astrocytic tumors in humans.

Olive oil hydroxytyrosol protects human erythrocytes against oxidative damages

Hydroxytyrosol, the major representative phenolic compound of virgin olive oil, is a dietary component. Its possible protective effect on hydrogen peroxide (H(2)O(2))-induced oxidative alterations was investigated in human erythrocytes. Cells were pretreated with micromolar hydroxytyrosol concentrations and then exposed to H(2)O(2) over different time intervals. Subsequently, erythrocytes were analyzed for oxidative hemolysis and lipid peroxidation. Our data demonstrate that hydroxytyrosol prevents both oxidative alterations, therefore, providing protection against peroxide-induced cytotoxicity in erythrocytes. The effect of oxidative stress on erythrocyte membrane transport systems, as well as the protective role of hydroxytyrosol, also were investigated in conditions of nonhemolytic mild H(2)O(2) treatment. Under these experimental conditions, a marked decrease in the energy-dependent methionine and leucine transport is observable; this alteration is quantitatively prevented by hydroxytyrosol pretreatment. On the other hand, the energy-independent glucose transport is not affected by the oxidative treatment. The reported data give new experimental support to the hypothesis of a protective role played by nonvitamin antioxidant components of virgin olive oil on oxidative stress in human systems.

A second protein L-isoaspartyl methyltransferase gene in Arabidopsis produces two transcripts whose products are sequestered in the nucleus

The spontaneous and deleterious conversion of l-asparaginyl and l-aspartyl protein residues to l-iso-Asp or d-Asp occurs as proteins age and is accelerated under stressful conditions. Arabidopsis (Arabidopsis L. Heynh.) contains two genes (At3g48330 and At5g50240) encoding protein-l-isoaspartate methyltransferase (EC 2.1.1.77; PIMT), an enzyme capable of correcting this damage. The gene located on chromosome 5 (PIMT2) produces two proteins differing by three amino acids through alternative 3' splice site selection in the first intron. Recombinant protein from both splicing variants has PIMT activity. Subcellular localization using cell fractionation followed by immunoblot detection, as well as confocal visualization of PIMT:GFP fusions, demonstrated that PIMT1 is cytosolic while a canonical nuclear localization signal, present in PIMT2psi and the shorter PIMT2omega, is functional. Multiplex reverse transcription-PCR was used to establish PIMT1 and PIMT2 transcript presence and abundance, relative to beta-TUBULIN, in various tissues and under a variety of stresses imposed on seeds and seedlings. PIMT1 transcript is constitutively present but can increase, along with PIMT2, in developing seeds presumably in response to increasing endogenous abscisic acid (ABA). Transcript from PIMT2 also increases in establishing seedlings due to exogenous ABA and applied stress presumably through an ABA-dependent pathway. Furthermore, cleaved amplified polymorphic sequences from PIMT2 amplicons determined that ABA preferentially enhances the production of PIMT2omega transcript in leaves and possibly in tissues other than germinating seeds.

A failure to repair self-proteins leads to T cell hyperproliferation and autoantibody production

It is clear that many factors can perturb T cell homeostasis that is critical in the maintenance of immune tolerance. Defects in the molecules that regulate homeostasis can lead to autoimmune pathology. This simple immunologic concept is complicated by the fact that many self-proteins undergo spontaneous posttranslational modifications that affect their biological functions. This is the case in the spontaneous conversion of aspartyl residues to isoaspartyl residues, a modification occurring at physiological pH and under conditions of cell stress and aging. We have examined the effect of isoaspartyl modifications on the effector functions of T lymphocytes in vivo using mice lacking the isoaspartyl repair enzyme protein carboxyl methyltransferase (PCMT). PCMT(-/-) CD4(+) T cells exhibit increased proliferation in response to mitogen and Ag receptor stimulation as compared with wild-type CD4(+) T cells. Hyperproliferation is marked by increased phosphorylation of members of both the TCR and CD28 signaling pathways. Wild-type mice reconstituted with PCMT(-/-) bone marrow develop high titers of anti-DNA autoantibodies and kidney pathology typical of that found in systemic lupus erythematosus. These observations, coupled with the fact that humans have polymorphisms in the pcmt gene, suggest that isoaspartyl self-proteins may alter the maintenance of peripheral immune tolerance.

X-ray structure of isoaspartyl dipeptidase from E.coli: a dinuclear zinc peptidase evolved from amidohydrolases

L-aspartyl and L-asparaginyl residues in proteins spontaneously undergo intra-residue rearrangements forming isoaspartyl/beta-aspartyl residues linked through their side-chain beta-carboxyl group with the following amino acid. In order to avoid accumulation of isoaspartyl dipeptides left over from protein degradation, some bacteria have developed specialized isoaspartyl/beta-aspartyl zinc dipeptidases sequentially unrelated to other peptidases, which also poorly degrade alpha-aspartyl dipeptides. We have expressed and crystallized the 390 amino acid residue isoaspartyl dipeptidase (IadA) from E.coli, and have determined its crystal structure in the absence and presence of the phosphinic inhibitor Asp-Psi[PO(2)CH(2)]-LeuOH. This structure reveals an octameric particle of 422 symmetry, with each polypeptide chain organized in a (alphabeta)(8) TIM-like barrel catalytic domain attached to a U-shaped beta-sandwich domain. At the C termini of the beta-strands of the beta-barrel, the two catalytic zinc ions are surrounded by four His, a bridging carbamylated Lys and an Asp residue, which seems to act as a proton shuttle. A large beta-hairpin loop protruding from the (alphabeta)(8) barrel is disordered in the free peptidase, but forms a flap that stoppers the barrel entrance to the active center upon binding of the dipeptide mimic. This isoaspartyl dipeptidase shows strong topological homology with the alpha-subunit of the binickel-containing ureases, the dinuclear zinc dihydroorotases, hydantoinases and phosphotriesterases, and the mononuclear adenosine and cytosine deaminases, which all are catalyzing hydrolytic reactions at carbon or phosphorous centers. Thus, nature has adapted an existing fold with catalytic tools suitable for hydrolysis of amide bonds to the binding requirements of a peptidase.

Aging as war between chemical and biochemical processes: protein methylation and the recognition of age-damaged proteins for repair

Deamidated, isomerized, and racemized aspartyl and asparaginyl residues represent a significant part of the spontaneous damage to proteins that results from the aging process. The accumulation of these altered residues can lead to the loss of protein function and the consequent loss of cellular function. However, almost all cells in nature contain a methyltransferase that can recognize the major damaged form of the L-isoaspartyl residue, and some of these enzymes can also recognize the racemized D-aspartyl residue. The methyl esterification reaction can initiate the conversion of these altered residues to the normal L-aspartyl form, although there is no evidence yet that the L-asparaginyl form can be regenerated. This enzyme, the protein L-isoaspartate (D-aspartate) O-methyltransferase (EC 2.1.1.77), thus functions as a protein repair enzyme. The importance of this enzyme in attenuating age-related protein damage can be seen by the phenotypes of organisms where the gene encoding has been disrupted, or where its expression has been augmented.

Aberrant synaptic transmission in the hippocampal CA3 region and cognitive deterioration in protein-repair enzyme-deficient mice

L-aspartate is the amino-acid residue most susceptible to spontaneous isomerization. This denaturation causes an alteration in the biological activity of the protein and is regarded as an aging process of the protein. Protein L-isoaspartyl methyltransferase (PIMT) repairs this post-translational modification and thus is implicated in retarding the aging process of proteins. PIMT is highly expressed in the brain, and its deficiency results in progressive epilepsy after 4 weeks of age, with a fatal seizure in mice. Here we report the pathophysiological role of this repair system in the hippocampal slice of PIMT-deficient mice. The hippocampal mossy fiber-CA3 synapses of PIMT-deficient mice showed hyperexcitation that was repressed by a gamma-aminobutyric acid (GABA)A receptor agonist muscimol. In addition, the mossy fiber-CA3 synapses failed to show long-term potentiation or paired-pulse facilitation. No abnormality, however, was observed in Schaffer collateral-CA1 synapses or in perforant path-dentate gyrus synapses. Electron microscopic study revealed aberrant distribution of synaptic vesicles in the mossy fiber terminals and vacuolar degeneration at the axon hillock of dentate granule cells in PIMT-deficient mice. Furthermore, the PIMT-deficient mice showed impaired spatial memory in Morris water maze test and exhibited fewer anxiety-related behaviors in the elevated-plus test. These results suggest that the mossy fiber-CA3 system is vulnerable to aspartate isomerization and that the PIMT-mediated repair system is essential for maintenance of normal functions of the hippocampus.

Protein methylation as a marker of aspartate damage in glucose-6-phosphate dehydrogenase-deficient erythrocytes: role of oxidative stress

The 'Mediterranean' variant of glucose-6-phosphate dehydrogenase (G6PD) deficiency is due to the C563CT point mutation, leading to replacement of Ser with Phe at position 188, resulting in acute haemolysis triggered by oxidants. Previous work has shown increased formation of altered aspartate residues in membrane proteins during cell ageing and in response to oxidative stress in normal erythrocytes. These abnormal residues are specifically recognized by the repair enzyme L-isoaspartate (d-aspartate) protein O-methyltransferase (PCMT; EC 2.1.1.77). The aim of this work was to study the possible involvement of protein aspartate damage in the mechanism linking the G6PD defect and erythrocyte injury, through oxidative stress. Patients affected by G6PD deficiency (Mediterranean variant) were selected. In situ methylation assays were performed by incubating intact erythrocytes in the presence of methyl-labelled methionine. Altered aspartate residues were detected in membrane proteins by methyl ester quantification. We present here evidence that, in G6PD-deficient erythrocytes, damaged residues are significantly increased in membrane proteins, in parallel with the decay of pyruvate kinase activity, used as a cell age marker. Erythrocytes from patients were subjected to oxidative stress in vitro, by treatment with t-butylhydroperoxide, monitored by a rise in concentration of both methaemoglobin and thiobarbituric acid-reactive substances. L-Isoaspartate residues increased dramatically in G6PD-deficient erythrocytes in response to such treatment, compared with baseline conditions. The increased susceptibility of G6PD-deficient erythrocytes to membrane protein aspartate damage in response to oxidative stress suggests the involvement of protein deamidation/isomerization in the mechanisms of cell injury and haemolysis.

Lupus autoantigens: their origins, forms, and presentation

The immune system has developed a number of mechanisms by which to distinguish self from foreign proteins. These mechanisms are found throughout the ontogeny of B and T cell development and include the deletion of autoreactive cells in central lymphoid organs and the induction of self-tolerance in the periphery. However, any failure of these mechanisms for self-tolerance may result in autoimmune disease. Efforts in our laboratory have been directed at understanding how autoimmunity is initiated and maintained in both the B and T cell compartment, with particular interest in the autoimmunity of systemic lupus erythematosus (SLE). This review will focus on our studies on the forms of self-antigens that may be involved in the original "antigenic sin" of SLE and in the role of B lymphocytes as autoantigen presenting cells. We will also discuss whether costimulation is a formal requirement for the induction and maintenance of autoimmunity. Finally, we have provided a model for how all of these individual elements may contribute to the autoimmune processes leading to pathology.

Protein repair methyltransferase from the hyperthermophilic archaeon Pyrococcus furiosus. Unusual methyl-accepting affinity for D-aspartyl and N-succinyl-containing peptides

Protein l-isoaspartate-(d-aspartate) O-methyltransferases (EC ), present in a wide variety of prokaryotic and eukaryotic organisms, can initiate the conversion of abnormal l-isoaspartyl residues that arise spontaneously with age to normal l-aspartyl residues. In addition, the mammalian enzyme can recognize spontaneously racemized d-aspartyl residues for conversion to l-aspartyl residues, although no such activity has been seen to date for enzymes from lower animals or prokaryotes. In this work, we characterize the enzyme from the hyperthermophilic archaebacterium Pyrococcus furiosus. Remarkably, this methyltransferase catalyzes both l-isoaspartyl and d-aspartyl methylation reactions in synthetic peptides with affinities that can be significantly higher than those of the human enzyme, previously the most catalytically efficient species known. Analysis of the common features of l-isoaspartyl and d-aspartyl residues suggested that the basic substrate recognition element for this enzyme may be mimicked by an N-terminal succinyl peptide. We tested this hypothesis with a number of synthetic peptides using both the P. furiosus and the human enzyme. We found that peptides devoid of aspartyl residues but containing the N-succinyl group were in fact methyl esterified by both enzymes. The recent structure determined for the l-isoaspartyl methyltransferase from P. furiosus complexed with an l-isoaspartyl peptide supports this mode of methyl-acceptor recognition. The combination of the thermophilicity and the high affinity binding of methyl-accepting substrates makes the P. furiosus enzyme useful both as a reagent for detecting isomerized and racemized residues in damaged proteins and for possible human therapeutic use in repairing damaged proteins in extracellular environments where the cytosolic enzyme is not normally found.

Non-enzymatic covalent modifications of proteins: mechanisms, physiological consequences and clinical applications

Given the complexity of the biosynthetic machinery and the delicate chemical composition of proteins, it is remarkable that cells manage to produce and maintain normally functioning proteins under most conditions. However, it is now well known that proteins are susceptible to various non-enzymatic covalent modifications (NECM) under physiological conditions. Such modifications can be of no or little importance to the protein or they can be absolutely detrimental. Often NECM are difficult to study due to the complex and technically demanding methods required to identify many of these modifications. Thus, the role of NECM has not yet been adequately resolved but recent research has allowed a better understanding of such modifications. The present review outlines the various forms of NECM that involve covalent modifications of proteins, and discusses their relevance, biological impact and potential applications in the study of protein turnover and diagnosis of disease.

Age-dependent deamidation of asparagine residues in proteins

Nonenzymatic deamidation of peptides and proteins represents an important degradation reaction occurring in vitro in the course of isolation or storage and in vivo during development and/or aging of cells. This review first presents a synopsis of the influence of structure on deamidation reaction proceeding via a five-membered succinimide intermediate, followed by an outline of procedures for separation and detection of deamidated forms. Selected examples for in vitro and in vivo deamidation are reviewed including the possible biological consequences of this protein degradation. Finally, the reaction of protein methyltransferase with L-isoaspartyl- and D-aspartyl residues and its possible role in protein repair is elucidated.

Post-translational protein modifications in antigen recognition and autoimmunity

It is estimated that 50-90% of the proteins in the human body are post-translationally modified. In the proper context, these modifications are necessary for the biological functions of a vast array of proteins and the effector functions of the cells in which they reside. However, it is now clear that some post-translational modifications can create new self antigens (Ags) or even mask Ags normally recognized by the immune system. In either case, they profoundly affect the recognition of Ag by bone marrow-derived cells, as well as their effector functions. How do post-translational protein modifications affect the processing of foreign and self Ags and what is their role in the origin of autoimmune responses?

Plasma proteins containing damaged L-isoaspartyl residues are increased in uremia: implications for mechanism

BACKGROUND

Several alterations of protein structure and function have been reported in uremia. Impairment of a transmethylation-dependent protein repair mechanism possibly related to a derangement in homocysteine metabolism is also present in this condition, causing erythrocyte membrane protein damage. Homocysteine may affect proteins via the accumulation of its parent compound S-adenosylhomocysteine (AdoHcy), a powerful in vivo methyltransferase inhibitor. However, since plasma homocysteine is mostly protein bound, a direct influence on protein structures cannot be ruled out. We measured the levels of L-isoaspartyl residues in plasma proteins of uremic patients on hemodialysis. These damaged residues are markers of molecular age, which accumulate when transmethylation-dependent protein repair is inhibited and/or protein instability is increased.

METHODS

L-isoaspartyl residues in plasma proteins were quantitated using human recombinant protein carboxyl methyl transferase (PCMT). Plasma concentrations of homocysteine metabolites were also measured under different experimental conditions in hemodialysis patients.

RESULTS

The concentration of damaged plasma proteins was increased almost twofold compared to control (controls 147.83 +/- 17.75, uremics 282.80 +/- 26.40 pmol of incorporated methyl groups/mg protein, P < 0.003). The major protein involved comigrated with serum albumin. Although hyperhomocysteinemia caused a redistribution of thiols bound to plasma proteins, this mechanism did not significantly contribute to the increase in isoaspartyl residues. The S-adenosylmethionine (AdoMet)/AdoHcy concentration ratio, an indicator of the flux of methyl group transfer, was altered. This ratio was partially corrected by folate treatment (0.385 +/- 0.046 vs. 0.682 +/- 0.115, P < 0.01), but protein L-isoaspartate content was not.

CONCLUSIONS

Plasma protein damage, as determined by protein L-isoaspartyl content, is increased in uremia. This alteration is to be ascribed to an increased protein structural instability, rather than the effect of hyperhomocysteinemia.

Homocysteine and transmethylations in uremia

Homocysteine is regarded as a cardiovascular risk factor in both the general population and chronic renal failure patients. Among the mechanisms for homocysteine toxicity, its interference with transmethylation reactions, through its precursor/derivative S-adenosylhomocysteine, plays a multifarious role. In uremia, inhibition of S-adenosylmethionine methyl transfer reactions has been reported by independent investigators, using multiple approaches. This has several possible consequences, which can ultimately affect the patient's relative state of health.

Pathogenesis of Hyperhomocysteinemia-New Insights

Mild to moderately elevated levels of homocysteine (Hey) in plasma, denoted as hyperhomocysteinemia, is emerging as a prevalent and strong risk factor for atherosclerotic vascular disease in coronary, cerebral and peripheral vessels, as well as for arterial and venous thromboembolism. Despite its clinical significance, the molecular mechanism of homocysteine's effects is not yet clearly defined. Most of the effects of homocysteine that have been demonstrated in vitro, affecting endothelial function have been attributed to the oxidant reactivity of this molecule, which is shown to affect the vasoregulatory and thrombotic/fibrinolytic function of endothelium. However, the relevance of these observations to the clinical situations is questionable, since excessively high concentrations of homocysteine are used in most of the experiments. We have observed that homocysteine, at physiologically relevant concentrations, specifically induces the expression of tissue factor by monocytes, and a non-specific redox effect is not involved. Tissue factor expression by monocytes is mediated by increased intracellular concentrations of the metabolic intermediate, S-adenosylhomocysteine, which is a potent inhibitor of methyl transferases. These studies suggest that tissue factor expression by circulating monocytes by intracellular perturbations may be a plausible mechanism by which homocysteine may induce thrombosis.

Expression, purification, and characterization of the protein repair l-isoaspartyl methyltransferase from Arabidopsis thaliana

Protein l-isoaspartate (d-aspartate) O-methyltransferase (EC 2.1.1. 77) is a repair enzyme that methylates abnormal l-isoaspartate residues in proteins which arise spontaneously as a result of aging. This enzyme initiates their conversion back into the normal l-aspartate form by a methyl esterification reaction. Previously, partial cDNAs of this enzyme were isolated from the higher plant Arabidopsis thaliana. In this study, we report the cloning and expression of a full-length cDNA of l-isoaspartyl methyltransferase from A. thaliana into Escherichia coli under the P(BAD) promoter, which offers a high level of expression under a tight regulatory control. The enzyme is found largely in the soluble fraction. We purified this recombinant enzyme to homogeneity using a series of steps involving DEAE-cellulose, gel filtration, and hydrophobic interaction chromatographies. The homogeneous enzyme was found to have maximum activity at 45 degrees C and a pH optimum from 7 to 8. The enzyme was found to have a wide range of affinities for l-isoaspartate-containing peptides and displayed relatively poor reactivity toward protein substrates. The best methyl-accepting substrates were KASA-l-isoAsp-LAKY (K(m) = 80 microM) and VYP-l-isoAsp-HA (K(m) = 310 microM). We also expressed the full-length form and a truncated version of this enzyme (lacking the N-terminal 26 amino acid residues) in E. coli under the T7 promoter. Both the full-length and the truncated forms were active, though overexpression of the truncated enzyme led to a complete loss of activity.

Increased methyl esterification of altered aspartyl residues in erythrocyte membrane proteins in response to oxidative stress

Protein-L-isoaspartate (D-aspartate) O-methyltransferase (PCMT; EC 2. 1.1.77) catalyses the methyl esterification of the free alpha-carboxyl group of abnormal L-isoaspartyl residues, which occur spontaneously in protein and peptide substrates as a consequence of molecular ageing. The biological function of this transmethylation reaction is related to the repair or degradation of age-damaged proteins. Methyl ester formation in erythrocyte membrane proteins has also been used as a marker reaction to tag these abnormal residues and to monitor their increase associated with erythrocyte ageing diseases, such as hereditary spherocytosis, or cell stress (thermal or osmotic) conditions. The study shows that levels of L-isoaspartyl residues rise in membrane proteins of human erythrocytes exposed to oxidative stress, induced by t-butyl hydroperoxide or H2O2. The increase in malondialdehyde content confirmed that the cell membrane is a primary target of oxidative alterations. A parallel rise in the methaemoglobin content indicates that proteins are heavily affected by the molecular alterations induced by oxidative treatments in erythrocytes. Antioxidants largely prevented the increase in membrane protein methylation, underscoring the specificity of the effect. Conversely, we found that PCMT activity, consistent with its repair function, remained remarkably stable under oxidative conditions, while damaged membrane protein substrates increased significantly. The latter include ankyrin, band 4.1 and 4.2, and the integral membrane protein band 3 (the anion exchanger). The main target was found to be particularly protein 4.1, a crucial element in the maintenance of membrane-cytoskeleton network stability. We conclude that the increased formation/exposure of L-isoaspartyl residues is one of the major structural alterations occurring in erythrocyte membrane proteins as a result of an oxidative stress event. In the light of these and previous findings, the occurrence of isoaspartyl sites in membrane proteins as a key event in erythrocyte spleen conditioning and hemocatheresis is proposed.

The carboxyl methyltransferase modifying G proteins is a metalloenzyme

The prenylated protein carboxyl methyltransferase (PPMT) catalyzes the posttranslational methylation of isoprenylated C-terminal cysteine residues found in many signaling proteins such as the small monomeric G proteins and the gamma subunits of heterotrimeric G proteins. Here we report that both membrane-bound PPMT from rat kidney and the recombinant bacterially expressed form of the enzyme required divalent cations for catalytic activity. Unlike EDTA and EGTA, the metal chelator 1,10-phenanthroline strongly inhibited the PPMT activity of kidney intracellular membranes in a dose- and time-dependent manner. 1,10-Phenanthroline was found to inhibit the methylation of the prenylcysteine analog N-acetyl-S-all-trans-geranylgeranyl-l-cysteine, a synthetic substrate for PPMT, with an IC(50) of 2.2 mM. Gel electrophoretic analysis demonstrated that 1,10-phenanthroline almost totally abolished the labeling of methylated proteins in kidney intracellular membranes. Immunoblotting analysis showed that one of the two major peaks of (3)H-methylated proteins in intracellular membranes comigrated with the small G proteins Ras, Cdc42, RhoA, and Rab1. In addition, the methylation of immunoprecipitated Ras and RhoA from kidney intracellular membranes was strongly inhibited when 1,10-phenanthroline was present. Treatment of kidney intracellular membranes with 1,10-phenanthroline increased the proteolytic degradation of PPMT by exogenous trypsin, compared to untreated membranes. We conclude from these data that metal ions are essential for the activity and the stabilization of PPMT. The finding that PPMT is a metalloenzyme may provide new insights into the functions played by this methyltransferase in signal transduction processes.

Isoaspartyl post-translational modification triggers autoimmune responses to self-proteins

The normal functioning immune system is programmed to attack foreign pathogens and other foreign proteins while maintaining tolerance to self-proteins. The mechanisms by which tolerance is broken in the initiation of autoimmunity are not completely understood. In the present study, mice immunized with the murine cytochrome c peptide 90-104 showed no response by the B or T cell compartments. However, immunization with the isoaspartyl form of this peptide, where the linkage of Asp(93) to Leu(94) occurs through the beta-carboxyl group, resulted in strong B and T cell autoimmune responses. Antibodies elicited by immunization with the isoaspartyl form of self-peptide were cross-reactive in binding to both isoforms of cytochrome c peptide and to native cytochrome c self-protein. In a similar manner, immunization of mice with the isoaspartyl form of a peptide autoantigen of human systemic lupus erythematosus (SLE) resulted in strong B and T cell responses while mice maintained tolerance to the normal aspartyl form of self-antigen. Isoaspartyl linkages within proteins are enhanced in aging and stressed cells and arise under physiological conditions. These post-translationally modified peptides may serve as an early immunologic stimulus in autoimmune disease.

Guanosine 5'-(3-O-Thio)triphosphate stimulates protein carboxyl methylation in cell membranes

Using guanosine 5'-(3-O-thio)triphosphate (GTPgammaS), we previously reported that protein carboxyl methyltransferase activities in kidney brush border membranes were increased by the GTP analog (Arch. Biochem. Biophys. 351, 149-158, 1998). Here, we investigated the distribution and characterized the effect of GTPgammaS on protein carboxyl methylation activity. The analysis of species distribution of carboxyl methylation in kidney brush border membranes showed that the GTPgammaS strongly stimulated this activity in rat (15.9-fold), mouse (14.7-fold), human (2.9-fold), and rabbit (2.7-fold). Analysis of GTPgammaS-dependent carboxyl methylation in rat tissues and cell fractions indicated that the activity was mainly localized in membranes of intestine, lung, and kidney, with the highest activity found in liver. To characterize the methyltransferase activity modulated by GTPgammaS in liver membranes, their sensitivity to the detergent 3-[(3-cholamido)dimethylammonio]-1-propanesulfonic acid (Chaps) was used. Methylation of N-acetyl-S-farnesyl cysteine, a prenylated protein methyltransferase (PPMT) substrate was strongly inhibited (86%) in the presence of Chaps, while the methylation of bovine calmodulin and ovalbumin, both of which are substrates for the protein L-isoaspartyl/d-aspartyl methyltransferase (PIMT), was slightly reduced by the detergent (0-12%). The GTPgammaS-dependent carboxyl methylation of endogenous substrates in liver membranes was decreased by 35% in the presence of Chaps, suggesting that PPMT was not the predominant methyltransferase involved in the methylation stimulated by GTPgammaS in liver membranes. Electrophoretic analysis showed that radioactive methylation of several substrates induced by GTPgammaS in liver membranes was reduced by adding calmodulin. Interestingly, addition of GTPgammaS partially inhibited the methylation of two PIMT substrates, ovalbumin (24%) and bovine calmodulin (19%), when incubated with liver membranes. Immunoprecipitation of PIMT from liver and lung membranes strongly inhibited (88-94%) the methylation stimulated by GTPgammaS. Altogether, these data support the hypothesis that GTPgammaS could regulate PIMT activity and may provide new insights into the function of the methyltransferase.

Do damaged proteins accumulate in Caenorhabditis elegans L-isoaspartate methyltransferase (pcm-1) deletion mutants?

The protein l-isoaspartate (d-aspartate) O-methyltransferase (E.C. 2. 1.1.77) can initiate the conversion of isomerized and racemized aspartyl residues to their normal l-aspartyl forms and has therefore been hypothesized to function as a repair enzyme, responsible for helping to limit the accumulation of damaged proteins in aging organisms. In this study, the effect of a disruption in the pcm-1 gene encoding the l-isoaspartyl methyltransferase was investigated in the nematode Caenorhabditis elegans. It was found that damaged proteins recognized by this enzyme accumulated to significant levels during long-term incubation of both pcm-1+ and pcm-1- nematodes in a specialized larval stage called the dauer. The l-isoaspartyl methyltransferase-deficient mutants accumulated about twice the level of damaged proteins as the control nematodes during dauer aging. The mutants also accumulated higher levels of damage when both strains were incubated at 30 degrees C for up to 3 days. However, when nonviable nematodes were removed in a Percoll separation, similar levels of damage were measured between the two strains following both dauer aging and 30 degrees C incubation. Both strains were able to effectively eliminate damaged proteins recognized by the methyltransferase after recovery from dauer. Characterization of the methyl-accepting polypeptide substrates which accumulate in aged dauers revealed that although substrates of all molecular weights are present, the majority of substrates are peptides not precipitated by acetone. These results suggest that protein degradation, rather than repair, may be the major mechanism by which C. elegans eliminates damaged proteins containing l-isoaspartyl residues.

D-amino acids in aging erythrocytes

Mature human erythrocytes are highly differentiated cells which have lost the ability to biosynthesize proteins de novo. During cell aging in circulation, erythrocyte proteins undergo spontaneous postbiosynthetic modifications, regarded as "protein fatigue" damage, which include formation of isomerized and/or racemized aspartyl residues. These damaged proteins cannot be replaced by new molecules; nevertheless, data support the notion that they can be repaired to a significant extent, through an enzymatic transmethylation reaction. This repair reaction has therefore been used as a means to monitor the increase of altered aspartyl residues in erythrocyte membrane proteins during cell aging. The relationship between protein repair and aspartyl racemization in red blood cell stress and disease is discussed.

Age-induced protein modifications and increased proteolysis in potato seed-tubers

Long-term aging of potato (Solanum tuberosum) seed-tubers resulted in a loss of patatin (40 kD) and a cysteine-proteinase inhibitor, potato multicystatin (PMC), as well as an increase in the activities of 84-, 95-, and 125-kD proteinases. Highly active, additional proteinases (75, 90, and 100 kD) appeared in the oldest tubers. Over 90% of the total proteolytic activity in aged tubers was sensitive to trans-epoxysuccinyl-L-leucylamido (4-guanidino) butane or leupeptin, whereas pepstatin was the most effective inhibitor of proteinases in young tubers. Proteinases in aged tubers were also inhibited by crude extracts or purified PMC from young tubers, suggesting that the loss of PMC was responsible for the age-induced increase in proteinase activity. Nonenzymatic oxidation, glycation, and deamidation of proteins were enhanced by aging. Aged tubers developed "daughter" tubers that contained 3-fold more protein than "mother" tubers, with a polypeptide profile consistent with that of young tubers. Although PMC and patatin were absent from the older mother tubers, both proteins were expressed in the daughter tubers, indicating that aging did not compromise the efficacy of genes encoding PMC and patatin. Unlike the mother tubers, proteinase activity in daughter tubers was undetectable. Our results indicate that tuber aging nonenzymatically modifies proteins, which enhances their susceptibility to breakdown; we also identify a role for PMC in regulating protein turnover in potato tubers.

Inhibition of GTPgammaS-dependent L-isoaspartyl protein methylation by tyrosine kinase inhibitors in kidney

Protein carboxyl methylation in rat kidney cytosol is increased by the addition of guanosine 5'-O-[gamma-thio]triphosphate (GTPgammaS), a non-hydrolysable analogue of GTP. GTPgammaS-stimulated methyl ester group incorporation takes place on isoaspartyl residues, as attested by the alkaline sensitivity of the labelling and its competitive inhibition by L-isoaspartyl-containing peptides. GTPgammaS was the most potent nucleotide tested, whereas GDPbetaS and ATPgammaS also stimulated methylation but to a lesser extent. Maximal stimulation (5-fold) of protein L-isoaspartyl methytransferase (PIMT) activity by GTPgammaS was reached at a physiological pH in the presence of 10 mM MgCl2. Other divalent cations, such as Cu2+, Zn2+ and Co2+ (100 microM), totally inhibited GTPgammaS-dependent carboxyl methylation. The phosphotyrosine phosphatase inhibitor vanadate potentiated the GTPgammaS stimulation of PIMT activity in the kidney cytosol at a concentration lower than 40 microM, but increasing the vanadate concentration to more than 40 microM resulted in a dose-dependent inhibition of the GTPgammaS effect. The tyrosine kinase inhibitors genistein (IC50 = 4 microM) and tyrphostin (IC50 = 1 microM) abolished GTPgammaS-dependent PIMT activity by different mechanisms, as was revealed by acidic gel analysis of methylated proteins. Whereas tyrphostin stabilised the methyl ester groups, genistein acted by blocking a crucial step required for the activation of PIMT activity by GTPgammaS. The results obtained with vanadate and genistein suggest that tyrosine phosphorylation regulates GTPgammaS-stimulated PIMT activity in the kidney cytosol.

Evidence for age-related deamidation reactions in erythrocyte p55 and glycophorins C and D: implications for signal transduction involving tumour-suppressor proteins in higher eukaryotes

The methylation of erythrocyte cytoskeletal proteins by methyltransferases is a strategy for the repair of deamidated asparagine residues. Evidence from extant data shows that the junction point proteins of the erythrocyte cytoskeleton, band 3, ankyrin, glycophorins C/D, band 4.1, calmodulin, and p55 are all methylated in situ. It is suggested that sequence-dependent deamidation of asparagine residues in p55, 4.1 and glycophorins C/D could affect the interactions of these junction point proteins in an age-dependent manner. Comparison of the asparagine content of 4.1 binding regions of p55 analogues dlg and hdlg acts as a caveat to the extrapolation of findings made in erythrocytes to other eukaryotic cells, and has important implications for transmembrane signalling pathways and age-dependent changes in the membrane-binding characteristics of tumour-suppressor proteins in higher cell systems.

Epitope spreading: the role of self peptides and autoantigen processing by B lymphocytes

A complex interplay of cells, soluble macromolecules, and antigen lead to a productive immune response that evolved for the survival of species. While the immune system is intended to protect from foreign agents, such as bacterial and viral infection, the presence of autoimmune diseases indicates that the system is not perfect in differentiating antigen that may cause harm from benign self constituents. The concept of epitope spreading, where many determinants on an offending antigen are the focus of immune attack, is an efficient means of clearing an infectious agent. However, the same mechanisms that lead to a diverse immune response may be harmful when the targets of attack are self tissues or self macromolecules. This review will examine the forms of self antigens that may initiate autoimmunity and the potential role of B lymphocytes, as autoantigen-presenting cells, as one mechanism by which diversification of autoimmunity may occur.

Regulation by GTPgammaS of protein carboxylmethyltransferase activity in kidney brush border membranes

The increase in carboxyl methylation induced by guanosine 5',3-O-(thio)triphosphate (GTPgammaS) in brush border membranes from rat kidney cortex was studied, and the methyltransferase activities affected by this nucleotide analog were identified. Addition of GTPgammaS to brush border membranes stimulated the carboxyl methylation in a time-dependent manner while adenosine and guanine nucleotides were ineffective. The GTPgammaS-dependent carboxyl methylation was inhibited by the chelating agents EDTA (63%) and 1,10-phenanthroline (68%), suggesting that this activity required divalent cations. The methyl ester groups induced by the addition of GTPgammaS to brush border membranes were unstable, with about 80% of them hydrolyzed following 1 h incubation at 37 degrees C. The GTPgammaS stimulation of the carboxyl methylation in brush border membranes was unaffected by the detergent 3-[(3cholamido)-dimethylammonio]-1-propanesulfonic acid up to a concentration of 0.4% (w/v). At this latter detergent concentration, the activity of prenylated protein methyltransferase (PPMT) was strongly inhibited and that of l-isoaspartyl/d-aspartylmethyltransferase (PIMT) was increased twofold, as measured with their respective exogenous substrates, N-acetyl-S-farnesyl cysteine and ovalbumin. GTPgammaS increased the methylation of several substrates in brush border membranes. The induced methylation in substrates migrating between 20 and 36 kDa was strongly decreased by the competitive inhibitor farnesylthioacetic acid, a synthetic farnesylated substrate for PPMT, while a delta-sleep-inducing peptide containing an L-isoaspartyl residue inhibited that of substrates with molecular weights above 36 kDa, suggesting that PIMT activity was also involved. This interpretation was strengthened by the observation that the increased methylation induced by GTPgammaS in these membrane substrates was completely lost following their analysis by gel electrophoresis under alkaline conditions. Taken together, these results indicate that both PPMT and PIMT activities are regulated by guanine nucleotides in brush border membranes of rat kidney.

Targeted gene disruption of the Caenorhabditis elegans L-isoaspartyl protein repair methyltransferase impairs survival of dauer stage nematodes

The methylation of abnormal L-isoaspartyl residues by protein L-isoaspartate (D-aspartate) O-methyltransferase (EC 2.1.1.77) can lead to their conversion to L-aspartyl residues. For polypeptides damaged by spontaneous reactions that generate L-isoaspartyl residues, these steps represent a protein repair pathway that can limit the accumulation of potentially detrimental proteins in the aging process. We report here the construction and the characterization of an animal model deficient in this methyltransferase. We utilized Tc1-transposon-mediated mutagenesis in the nematode Caenorhabditis elegans to construct a homozygous excision mutant lacking exons 2-5 of the pcm-1 gene encoding this enzyme. Nematodes carrying this deletion exhibited no detectable L-isoaspartyl methyltransferase activity. These worms demonstrated normal morphology and behavior and adult mutant nematodes exhibited a normal lifespan. However, the survival of dauer-phase mutants was diminished by 3.5-fold relative to wild-type dauers after 50 days in the dauer phase. The fitness of the pcm-1 deletion nematodes was reduced by about 16% relative to that of wild-type nematodes as measured by the ability of these mutants to compete reproductively against a wild-type population. We found that the absence of the functional methyltransferase gene leads to a modest accumulation of altered protein substrates in aged dauer worms. However, in the viable fraction of these dauer worms, no differences were seen in the levels of altered substrate proteins in the parent and methyltransferase-deficient worms, suggesting that the enzyme in wild-type cells does not efficiently catalyze the repair of spontaneously damaged proteins.

Analysis of deamidation of small, acid-soluble spore proteins from Bacillus subtilis in vitro and in vivo

Deamidation of one specific asparagine residue in an alpha/beta-type small, acid-soluble spore protein (SASP) of Bacillus subtilis took place readily in vitro (time for 50% deamidation [t(1/2)], approximately 1 h at 70 degrees C), and the deamidated SASP no longer bound to DNA effectively. However, DNA binding protected against this deamidation in vitro. A mutant alpha/beta-type SASP in which the reactive asparagine was changed to aspartate also failed to bind to DNA in vitro, and this protein did not restore UV radiation and heat resistance to spores lacking the majority of their alpha/beta-type SASP. When expressed in Escherichia coli, where it is bound to DNA, the alpha/beta-type SASP deamidated with a t(1/2) of 2 to 3 h at 95 degrees C. However, the alpha/beta-type SASP was extremely resistant to deamidation within spores (t(1/2), >50 h at 95 degrees C). A gamma-type SASP of B. subtilis also deamidated readily in vitro (t(1/2) for one net deamidation, approximately 1 h at 70 degrees C), but this protein (which is not associated with DNA) deamidated fairly readily in spores (t(1/2), approximately 1 h at 95 degrees C). Total spore core protein also deamidated in vivo, although the rate was two- to threefold slower than that of deamidation of total protein in heated vegetative cells. These data indicate that protein deamidation is slowed significantly in spores, presumably due to the spore's environment. However, alpha/beta-type SASP are even more strongly protected against deamidation in vivo, presumably by their binding to spore DNA. Thus, not only do alpha/beta-type SASP protect spore DNA from damage; DNA also protects alpha/beta-type SASP.