Determination of two sites of automethylation in bovine erythrocyte protein (D-aspartyl/L-isoaspartyl) carboxyl methyltransferase

A novel automethylation reaction in the Aspergillus nidulans LaeA protein generates S-methylmethionine

The filamentous fungi in the genus Aspergillus are opportunistic plant and animal pathogens that can adapt to their environment by producing various secondary metabolites, including lovastatin, penicillin, and aflatoxin. The synthesis of these small molecules is dependent on gene clusters that are globally regulated by the LaeA protein. Null mutants of LaeA in all pathogenic fungi examined to date show decreased virulence coupled with reduced secondary metabolism. Although the amino acid sequence of LaeA contains the motifs characteristic of seven-β-strand methyltransferases, a methyl-accepting substrate of LaeA has not been identified. In this work we did not find a methyl-accepting substrate in Aspergillus nidulans with various assays, including in vivo S-adenosyl-[methyl-(3)H]methionine labeling, targeted in vitro methylation experiments using putative protein substrates, or in vitro methylation assays using whole cell extracts grown under different conditions. However, in each experiment LaeA was shown to self-methylate. Amino acid hydrolysis of radioactively labeled LaeA followed by cation exchange and reverse phase chromatography identified methionine as the modified residue. Point mutations show that the major site of modification of LaeA is on methionine 207. However, in vivo complementation showed that methionine 207 is not required for the biological function of LaeA. LaeA is the first protein to exhibit automethylation at a methionine residue. These findings not only indicate LaeA may perform novel chemistry with S-adenosylmethionine but also provide new insights into the physiological function of LaeA.

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.

Structural integrity of histone H2B in vivo requires the activity of protein L-isoaspartate O-methyltransferase, a putative protein repair enzyme

Protein L-isoaspartate O-methyltransferase (PIMT) is postulated to repair beta-aspartyl linkages (isoaspartyl (isoAsp)) that accumulate at certain Asp-Xaa and Asn-Xaa sites in association with protein aging and deamidation. To identify major targets of PIMT action we cultured rat PC12 cells with adenosine dialdehyde (AdOx), a methyltransferase inhibitor that promotes accumulation of isoAsp in vivo. Subcellular fractionation of AdOx-treated cells revealed marked accumulation of isoAsp in a 14-kDa nuclear protein. Gel electrophoresis and chromatography of nuclei (3)H-methylated in vitro by PIMT revealed this protein to be histone H2B. The isoAsp content of H2B in AdOx-treated cells was approximately 18 times that in control cells, although no isoAsp was seen in other core histones, regardless of treatment. To confirm the relevance and specificity of this effect, we measured isoAsp levels in histones from brains of PIMT knockout mice. IsoAsp was found at near stoichiometric levels in H2B extracted from knockout brains and was at least 80 times greater than that in H2B from normal mice. Little or no isoAsp was detected in H2A, H3, or H4 from mice of either genotype. Accumulation of isoAsp in histone H2B may disrupt normal gene regulation and contribute to the reduced life span that characterizes PIMT knockouts. In addition to disrupting protein function, isoAsp has been shown to trigger immunity against self-proteins. The propensity of H2B to generate isoAsp in vivo may help explain why this histone in particular is found as a major antigen in autoimmune diseases such as lupus erythematosus.

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.

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.

Protein isoaspartyl methyltransferase protects from Bax-induced apoptosis

Protein L-isoaspartyl methyltransferase (Pimt) is a highly conserved enzyme utilising S-adenosylmethionine (AdoMet) to methylate aspartate residues of proteins damaged by age-related isomerisation and deamidation. We have been particularly interested in this enzyme since addition of the compound CGP3466 to primary rat astroglia cell cultures resulted in an upregulation of Pimt at the mRNA level, as shown here by semi-quantitative RT-PCR. CGP3466 is a compound related to the anti-Parkinson's drug R-(-)-deprenyl, which has been shown to protect from neural apoptosis induced by trophic factor withdrawal [Tatton et al., 1994. J. Neurochem. 63, 1572]. The pro-apoptotic gene Bax is required in the cascade of events following withdrawal [Deckwerth et al., 1996. Neuron 17, 401]. We therefore investigated whether Pimt overexpression was able to affect Bax-induced apoptosis in primary mouse cortical neurons. Our results show that Pimt is indeed able to protect from Bax-induced apoptosis. Furthermore, this activity is not restricted to brain-specific cell types, since the same effect is also demonstrated in COS1 cells. In addition, mutational analysis suggests that the protective effect is dependent on the adenosine methionine-binding motif, which is well conserved in protein methyltransferases, and that a mutation destroying this motif crucially affects cytoskeletal structures of the cell.

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.

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.

Detection and characterization of a protein isoaspartyl methyltransferase which becomes trapped in the extracellular space during blood vessel injury

Injury to rat blood vessels in vivo was found to release intracellular pools of protein D-aspartyl/L-isoaspartyl carboxyl methyltransferase (PIMT) into the extracellular milieu, where it becomes trapped. This trapped cohort of PIMT is able to utilize radiolabeled S-adenosyl-L-methionine (AdoMet) introduced into the circulation to methylate blood vessel proteins containing altered aspartyl residues. As further shown in this study, methylated substrates are detected only at the specific site of injury. In vitro studies more fully characterized this endogenous PIMT activity in thoracic aorta and inferior vena cava. Methylation kinetics, immunoblotting, and the lability of methylated substrates at mild alkaline pH were used to demonstrate that both types of blood vessel contain an endogenous protein D-aspartyl/L-isoaspartyl carboxyl methyltransferase (PIMT). At least 50% of the PIMT activity is resistant to nonionic detergent extraction, suggesting that the enzyme activity becomes trapped within or behind the extracellular matrix (ECM). Quantities of lactate dehydrogenase (LDH), another soluble enzyme of presumed intracellular origin, were found to be similarly trapped in the extracellular space of blood vessels.