The Primary Structure of a Protein Carboxyl Methyltransferase from Bovine Brain That Selectively Methylates L-Isoaspartyl Sites

Structural and functional characterization of the trifunctional antibody catumaxomab

The Triomab family of trifunctional, bispecific antibodies that maintain an IgG-like shape are novel tumor targeting agents. These chimeras consist of two half antibodies, each with one light and one heavy chain, that originate from parental mouse IgG2a and rat IgG2b isotypes. This combination allows cost-effective biopharmaceutical manufacturing at an industrial scale since this specific mouse/rat isotype combination favors matching of corresponding antibody halves during production by means of quadroma technology. Whereas every Triomab family member is composed of an anti-CD3 rat IgG2b half antibody for T cell recognition, the antigen binding site presented by the mouse IgG2a isotype is exchangeable. Several Triomab antibodies have been generated that bind to tumor-associated antigens, e.g., EpCAM (catumaxomab), HER2/neu (ertumaxomab), CD20 (FBTA05), gangliosides GD2/GD3 (Ektomun), on appropriate tumor target cells associated with carcinomas, lymphomas or melanomas. Catumaxomab (Removab) was launched in Europe for treatment of malignant ascites in April 2009. Here, we report the structural and functional characterization of this product. Mass spectrometry revealed an intact mass of 150511 Dalton (Da) and 23717 Da, 24716 Da, 51957 Da and 52019 Da of the reduced and alkylated rat light chain, mouse light chain, rat heavy chain, mouse heavy chain chains, respectively. The observed masses were in agreement with the expected masses based on the amino acid sequence obtained from cDNA sequencing. The glycosylation profile was similar to other human IgG consisting of biantennary oligosaccharides with different numbers of terminal galactose. CD spectroscopy showed mainly beta-sheets secondary structure that is typical for IgG antibodies. Binding measurement revealed the unique trifunctional features of catumaxomab. Other analytical tools were used to evaluate characteristics of catumaxomab preparations, including the presence of isoforms and aggregates.

13 Protein L-isoaspartyl, D-aspartyl O-methyltransferases: Catalysts for protein repair

Protein L-isoaspartyl, D-aspartyl O-methyltransferases (PIMTs) are ancient enzymes distributed through all phylogenetic domains. PIMTs catalyze the methylation of L-isoaspartyl, and to a lesser extent D-aspartyl, residues arising from the spontaneous deamidation and isomerization of protein asparaginyl and aspartyl residues. PIMTs catalyze the methylation of isoaspartyl residues in a large number of primary sequence configurations, which accounts for the broad specificity of the enzyme for protein substrates both in vitro and in vivo. PIMT-catalyzed methylation of isoaspartyl substrates initiates the repair of the polypeptide backbone in its damaged substrates by a spontaneous mechanism that involves a succinimidyl intermediate. The repair process catalyzed by PEVITs is not completely efficient, however, leaving open the possibility that unidentified enzymatic activities cooperate with PIMT in the repair process. Structurally, PIMTs are members of the class I family of AdoMet-dependent methyltransferases. PIMTs have a unique topological arrangement of strands in the central β sheet that provides a signature for this class of enzymes. The regulation and physiological significance of PIMT has been studied in several model organisms. PIMTs are constitutively synthesized by cells, but they can be upregulated in response to conditions that are potentially damaging to protein structures, or when proteins are stored for prolonged periods of time. Disruption of PIMT genes in bacteria and simple eukaryotes produces subtle phenotypes that are apparent only under stress. Loss of PIMT function in transgenic mice leads to fatalepilepsy, suggesting that PIMT function is particularly important to neurons in mammals.

Catalytic Implications from theDrosophilaProteinl-Isoaspartyl Methyltransferase Structure and Site-Directed Mutagenesis†,‡

Protein L-isoaspartyl methyltransferases (PIMT; EC 2.1.1.77) catalyze the S-adenosylmethionine-dependent methylation of L-isoaspartyl residues that arise spontaneously in proteins with age, thereby initiating a repair process that restores the normal backbone configuration to the damaged polypeptide. In Drosophila melanogaster, overexpression of PIMT in transgenic flies extends the normal life span, suggesting that protein damage can be a limiting factor in longevity. To understand structural features of the Drosophila PIMT (dPIMT) important for catalysis, the crystal structure of dPIMT was determined at a resolution of 2.2 A, and site-directed mutagenesis was used to identify the role of Ser-60 in catalysis. The core structure of dPIMT is similar to the modified nucleotide-binding fold observed in PIMTs from extreme thermophiles and humans. A striking difference of the dPIMT structure is the rotation of the C-terminal residues by 90 degrees relative to the homologous structures. Effectively, this displacement generates a more open conformation that allows greater solvent access to S-adenosylhomocysteine, which is almost completely buried in other PIMT structures. The enzyme may alternate between the open conformation found for dPIMT and the more closed conformations described for other PIMTs during its catalytic cycle, thereby allowing the exchange of substrates and products. Catalysis by dPIMT requires the side chain of the conserved, active site residue Ser-60, since substitution of this residue with Thr, Gln, or Ala reduces or abolishes the methylation of both protein and isoaspartyl peptide substrates.

Crystal structure of human L-isoaspartyl methyltransferase

The enzyme l-isoaspartyl methyltransferase initiates the repair of damaged proteins by recognizing and methylating isomerized and racemized aspartyl residues in aging proteins. The crystal structure of the human enzyme containing a bound S-adenosyl-l-homocysteine cofactor is reported here at a resolution of 2.1 A. A comparison of the human enzyme to homologs from two other species reveals several significant differences among otherwise similar structures. In all three structures, we find that three conserved charged residues are buried in the protein interior near the active site. Electrostatics calculations suggest that these buried charges might make significant contributions to the energetics of binding the charged S-adenosyl-l-methionine cofactor and to catalysis. We suggest a possible structural explanation for the observed differences in reactivity toward the structurally similar l-isoaspartyl and d-aspartyl residues in the human, archael, and eubacterial enzymes. Finally, the human structure reveals that the known genetic polymorphism at residue 119 (Val/Ile) maps to an exposed region away from the active site.

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.

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.

Molecular aging of tubulin: accumulation of isoaspartyl sites in vitro and in vivo

The formation of isoaspartyl sites during aging of rat tubulin in vitro and in vivo has been studied. When incubated in vitro at pH 7.4, 37 degrees C, purified rat brain tubulin accumulated isoaspartyl sites at a rate > or = 2.4 isoaspartyl sites per 100 tubulin subunits (50 kDa) per day for 30 days. Isoaspartate levels were estimated by the transfer of radiolabeled methyl groups from S-adenosyl-L-[methyl-3H]-methionine in a reaction catalyzed by protein-L-isoaspartyl methyltransferase. isoaspartate formation occurred in parallel with, but was not dependent upon, extensive cross-linking of tubulin via formation of intermolecular disulfide bonds. When rat PC12 cells were incubated for 24 or 72 h in the presence of adenosine dialdehyde, a potent methyltransferase inhibitor, a substantial and consistent increase in the isoaspartate content of tubulin was observed. This suggests that tubulin constantly undergoes isoaspartate formation in vivo, but that the levels are normally kept low by methylation-dependent repair. These findings support the hypothesis that protein-isoaspartyl methyltransferase plays a key role in countering spontaneous damage reactions to proteins associated with cell aging. These results also suggest that tubulin is an important target for protein-isoaspartyl methyltransferase in vivo.

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

Protein (D-aspartyl/L-isoaspartyl) carboxyl methyltransferase (PCM, E.C. 2.1.1.77) was previously shown to be enzymatically methyl esterified in an autocatalytic manner at altered aspartyl residues; methyl esters are observed in a subpopulation of the enzyme termed the alpha PCM fraction [Lindquist and McFadden (1994), J. Protein Chem. 13, 23-30]. The altered aspartyl sites serving as methyl acceptors in alpha PCM have now been localized by using proteolytic enzymes and chemical cleavage techniques in combination with matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to identify fragments of the [3H]automethylated enzyme that contain a [3H]methyl ester. Methylation was positively identified at positions Asn188 and Asp217 in the enzyme sequence, a consequence of the spontaneous alteration of these sites to L-isoaspartyl or D-aspartyl sites and their methylation by active PCM molecules. The identification of more than one site of automethylation shows that alpha PCM is not a homogeneous population of damaged PCM molecules, but rather a complex population of molecules with a variety of age-altered damage sites.

S-adenosyl-L-methionine modulates firing rate of dorsal motor nucleus of the vagus neurones in vitro

We used the patch clamp technique applied to an in vitro brain slice preparation to examine the changes in firing activity of single dorsal vagal motoneurones exposed to S-adenosyl-methionine. In approximately 70% of the neurones tested, S-adenosyl-L-methionine (1-100 microM) decreased the spontaneously occurring firing in a dose dependent manner; the plateau decrease was 40 +/- 6%. The peak effect was observed approximately 5 min after the superfusion with S-adenosyl-L-methionine was started, and was usually reversible upon wash out of S-adenosyl-L-methionine from the superfusing chamber. No effect of the control salt of S-adenosyl-L-methionine, 1,4-butane-disulfonate.NA (100 microM), was observed. The frequency of discharge observed upon depolarization steps from hyperpolarized potentials was reduced to 34 +/- 17% (n = 11) of control upon S-adenosyl-L-methionine (100 microM) superfusion; no effect of S-adenosyl-L-methionine was observed on the action potential threshold. Preincubation with adenosine receptor antagonists, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 10 nM) and 3,7-dimethyl-1-propargylxanthine (DMPX, 50 microM), reversed the S-adenosyl-L-methionine-induced inhibition of firing rate, and in fact, in the presence of these adenosine antagonists, S-adenosyl-L-methionine increased the firing rate of vagal motoneurones. This excitation of vagal motoneurones was blocked by pretreatment with S-adenosyl-homocysteine (100 microM), an inhibitor of methylation reactions. It is concluded that the inhibitory activity of S-adenosyl-L-methionine on the firing rate of vagal motoneurones is due to its metabolic transformation into adenosine which then acts on adenosine receptors. The excitatory effect on firing rate appears to be due to other actions, possibly including methylation reactions of key components of signal transduction mechanisms.

Tissue-specific expression of isoaspartyl protein carboxyl methyltransferase gene in rat brain and testis

Isoaspartyl protein carboxyl methyltransferase (PIMT) is widely distributed in mammalian tissues. Using a polymerase chain reaction-generated 124-bp DNA fragment from brain cDNA as a probe, four different sizes (approximately 4.0, 2.5, 1.7, and 1.1 kb) of transcripts were detected with northern blot analysis. They were expressed predominantly in rat brain and testis. The major transcripts were 2.5 and 1.7 kb in the brain and 2.5 and 1.1 kb in the testis. One of the major transcripts specific to the testis (1.1 kb) was determined to study the structural difference of major transcripts in the two tissues. This testicular cDNA had neither the 5' (94 nucleotides) nor the 3' (594 nucleotides) end of previously reported brain cDNA corresponding to 1.7 kb. The mRNA levels and enzyme activities of different regions and developmental changes were examined in the brain. The mRNA levels and enzyme activities were concomitantly high in cerebral cortex and hippocampus. Although they increased rapidly approximately 30 days after birth in the testis and decreased in aged rats, they increased gradually after birth and remained high during the aging of the brain. Both structural and developmental studies show that the expression of the PIMT gene in brain and testis is regulated in a tissue-specific manner.

Automethylation of protein (D-aspartyl/L-isoaspartyl) carboxyl methyltransferase, a response to enzyme aging

A question that is central to understanding the mechanisms of aging and cellular deterioration is whether enzymes involved in recognition and metabolism of spontaneously damaged proteins are themselves damaged, either becoming substrates for their own activity; or being unable to act upon themselves, initiating cascades of cellular damage. We show here by in vitro experiments that protein (D-aspartyl/L-isoaspartyl) carboxyl methyltransferase (PCM) from bovine erythrocytes does methylate age-dependent amino acid damage in its own sequence. The subpopulation that is methylated, termed the alpha PCM fraction, appears to be formed through age-dependent deamidation of an asparaginly site to either an L-isoaspartyl or D-aspartyl site because (a) the stoichiometry of automethylation of purified PCM is less than 1%, a value typical of the substoichiometric methylation of many other aged protein substrates, (b) alpha PCM is slightly more acidic than the bulk of PCM, and (c) the methyl esterified site in alpha PCM has the characteristic base-lability of this type of methyl ester. Also, the methyl group is not incorporated into the enzyme as an active site intermediate because the incorporated methyl group is not chased onto substrate protein. The effect of enzyme dilution on the rate of the automethylation reaction is consistent with methylation occurring between protein molecules, showing that the pool of PCM is autocatalytic even though individual molecules may not be. The automethylation and possible self-repair of the PCM pool has implications for maintaining the in vivo efficiency of methylation-dependent protein repair.

In vitro aging of calmodulin generates isoaspartate at multiple Asn-Gly and Asp-Gly sites in calcium-binding domains II, III, and IV

We have determined the major sites responsible for isoaspartate formation during in vitro aging of bovine brain calmodulin under mild conditions. Protein L-isoaspartyl methyltransferase (EC 2.1.1.77) was used to quantify isoaspartate by the transfer of methyl-3H from S-adenosyl-L-[methyl-3H]methionine to the isoaspartyl (alpha-carboxyl) side chain. More than 1.2 mol of methyl-acceptor sites per mol of calmodulin accumulated during a 2-week incubation without calcium at pH 7.4, 37 degrees C. Analysis of proteolytic peptides of aged calmodulin revealed that > 95% of the methylation capacity is restricted to residues in the four calcium-binding domains, which are predicted to be highly flexible in the absence of calcium. We estimate that domains III, IV, and II accumulated 0.72, 0.60, and 0.13 mol of isoaspartate per mol of calmodulin, respectively. The Asn-97-Gly-98 sequence (domain III) is the greatest contributor to isoaspartate formation. Other major sites of isoaspartate formation are Asp-131-Gly-132 and Asp-133-Gly-134 in domain IV, and Asn-60-Gly-61 in domain II. Significant isoaspartate formation was also localized to Asp-20, Asp-22, and/or Asp-24 in domain I, to Asp-56 and/or Asp-58 in domain II, and to Asp-93 and/or Asp-95 in domain III. All of these residues are calcium ligands in the highly conserved EF-hand calcium-binding motif. Thus, other EF-hand proteins may also be subject to isoaspartate formation at these ligands. The results support the idea that isoaspartate formation in structured proteins is strongly influenced by both the C-flanking residue and by local flexibility.

Kinetic properties of bovine brain protein L-isoaspartyl methyltransferase determined using a synthetic isoaspartyl peptide substrate

Protein L-isoaspartyl methyltransferase, an enzyme enriched in brain, is implicated in the repair of age-damaged proteins containing atypical, isoaspartyl peptide bonds. We have investigated the kinetics of methylation using a synthetic peptide substrate having the structure Trp-Ala-Gly-Gly-isoAsp-Ala-Ser-Gly-Glu. Double-reciprocal plots of initial velocity versus concentration of S-adenosylmethionine (AdoMet) at different fixed concentrations of peptide gave straight lines converging at a positive 1/v value and a negative 1/AdoMet value. The product S-adenosylhomocysteine (AdoHcy) was a competitive inhibitor towards AdoMet and a linear mixed-type inhibitor towards peptide. These results are consistent with the rapid-equilibrium random sequential bi-bi mechanism previously proposed for the enzyme, but they also reveal the formation of the dead-end, enzyme-peptide-AdoHcy, complex. The rate constants were: Vmax = 32-34 nmol/min/mg, Kpeptide = 7.6-9.4 microM, KAdoMet = 1.9-2.2 microM, alpha = 0.43-0.53, KAdoHcy = 0.08 microM, gamma = 2.9. The interaction factors alpha and gamma indicate that binding of enzyme to peptide increases its affinity for AdoMet and decreases its affinity for AdoHcy. Methylation was linear with time throughout the transfer of 2 mol of methyl groups/mol of enzyme. This absence of burst kinetics suggests that slow release of products cannot explain the low turnover number.

Rat guanidinoacetate methyltransferase: mutation of amino acids within a common sequence motif of mammalian methyltransferase does not affect catalytic activity but alters proteolytic susceptibility

1. Manual alignment of amino acid sequences of mammalian S-adenosylmethionine-dependent methyltransferases of known sequence revealed the presence of 2 homologous regions. 2. The sequence of the region at the C-terminal side is unique to mammalian methyltransferases, and in guanidinoacetate methyltransferase this sequence occurs at residues 159-165. 3. Mutagenesis of 5 conserved residues in this sequence did not affect the catalytic activity but altered tryptic susceptibility at Arg20.

Genomic organization and tissue expression of the murine gene encoding the protein beta-aspartate methyltransferase

Two overlapping clones containing the entire 684-nucleotide (nt) sequence encoding murine protein beta-aspartate methyltransferase (EC 2.1.1.77) were isolated from a genomic library. Partial nt sequence analysis of the two clones revealed that the protein carboxyl methyltransferase (PCMT)-encoding sequence is distributed among seven exons, ranging from 32 to 339 bp in length, within 25 kb of genomic DNA. Three exons correspond to regions of primary structure which are strongly conserved among a number of eukaryotic and prokaryotic enzymes which utilize S-adenosylmethionine (AdoMet) and S-adenosylhomocysteine (AdoHcy). The 5'-flanking region of the PCMT-encoding gene (PCMT) contains an 800-bp G+C-rich region with potential binding sites for transcription factor ETF, but lacks a TATA box and binding sites for other known transcription factors. Multiple PCMT mRNAs were detected on Northern blots of RNA extracted from murine brain, testis, liver and kidney. The overall abundance of PCMT mRNAs in each tissue paralleled the measured specific activity of the PCMT. Comparison of the genomic sequence information with the 3'-untranslated regions (UTRs) of two cDNA clones from a murine testis library indicated that PCMT mRNA precursors undergo alternative splicing. The structure and widespread expression of PCMT are characteristics of vertebrate housekeeping genes.

Identification of heregulin, a specific activator of p185erbB2

The proto-oncogene designated erbB2 or HER2 encodes a 185-kilodalton transmembrane tyrosine kinase (p185erbB2), whose overexpression has been correlated with a poor prognosis in several human malignancies. A 45-kilodalton protein heregulin-alpha (HRG-alpha) that specifically induced phosphorylation of p185erbB2 was purified from the conditioned medium of a human breast tumor cell line. Several complementary DNA clones encoding related HRGs were identified, all of which are similar to proteins in the epidermal growth factor family. Scatchard analysis of the binding of recombinant HRG to a breast tumor cell line expressing p185erbB2 showed a single high affinity binding site [dissociation constant (Kd) = 105 +/- 15 picomolar]. Heregulin transcripts were identified in several normal tissues and cancer cell lines. The HRGs may represent the natural ligands for p185erbB2.

Alternative splicing of the human isoaspartyl protein carboxyl methyltransferase RNA leads to the generation of a C-terminal -RDEL sequence in isozyme II

We have isolated two cDNA clones that correspond to the mRNAs for two isozymes of the human L-isoaspartyl/D-aspartyl protein carboxyl methyltransferase (EC 2.1.1.77). The DNA sequence of one of these encodes the amino acid sequence of the C-terminal half of the human erythrocyte isozyme I. The other cDNA clone includes the complete coding region of the more acidic isozyme II. With the exception of potential polymorphic sites at amino acid residues 119 and 205, the deduced amino acid sequences differ only at the C-terminus, where the -RWK sequence of isozyme I is replaced by a -RDEL sequence in isozyme II. The latter sequence is identical to a mammalian endoplasmic reticulum retention signal. With the previous evidence for only a single gene for the L-isoaspartyl/D-aspartyl methyltransferase in humans, and with evidence for consensus sites for alternative splicing in corresponding mouse genomic clones, we suggest that alternative splicing reactions can generate the major isozymes previously identified in human erythrocytes. The presence of alternative splicing leads us to predict the existence of a third isozyme with a -R C-terminus. The calculated isoelectric point of this third form is similar to that of a previously detected but uncharacterized minor methyltransferase activity.

Amplification and detection of substrates for protein carboxyl methyltransferases in PC12 cells

A strategy that facilitates the identification of substrates for protein carboxyl methyltransferases that form "stable" methyl esters, i.e., those that remain largely intact during conventional polyacrylamide gel electrophoresis is described. Rat PC12 cells were cultured in the presence of adenosine dialdehyde (a methylation inhibitor) to promote the accumulation of hypomethylated proteins. Nonidet P-40 cell extracts were then incubated in the presence of S-[methyl-3H]adenosyl-L-methionine to label methyl-accepting sites via endogenous methyltransferases. After labeled proteins were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, gel slices were incubated in 4 N methanesulfonic acid or 6 N HCl to hydrolyze methyl esters. The resulting [3H]methanol was detected by trapping in liquid scintillation fluid. Seven carboxyl methylated proteins were observed with masses ranging from 18 to 96 kDa. Detection of five of these proteins required prior treatment of cells with adenosine dialdehyde, while methyl incorporation into one protein at 18 kDa was substantially enhanced by the treatment. The use of acidic conditions for methyl ester hydrolysis has an important advantage over assays that utilize alkaline hydrolysis conditions. In PC12 cells, and possibly other cell types where there are significant levels of arginine methylation, the methanol signal becomes obscured by high levels of volatile methylamines generated under the alkaline conditions. Carrying out diffusion assays under acidic conditions eliminates this interference. Adenosine dialdehyde, by virtue of increasing the methyl-accepting capacity of substrates for protein carboxyl methyltransferases, in combination with a more selective assay for carboxyl methylation, should prove useful in the isolation and characterization of new protein carboxyl methyltransferases and their substrates.

Distinct C-terminal sequences of isozymes I and II of the human erythrocyte L-isoaspartyl/D-aspartyl protein methyltransferase

We have purified the more acidic major isozyme (II) of the human erythrocyte L-isoaspartyl/D-aspartyl methyltransferase and compared its structure to that of the previously sequenced isozyme I. These isozymes are both monomers of 25,000 molecular weight polypeptides and have similar enzymatic properties, but have isoelectric points that differ by one pH unit. Analysis of 16 tryptic peptides of isozyme II accounting for 89% of the sequence of isozyme I revealed no differences between these enzyme forms. However, analysis of a Staphylococcal V8 protease C-terminal fragment revealed that the last two residues of these proteins differed. The Trp-Lys-COOH terminus of isozyme I is replaced by a Asp-Asp-COOH terminus in isozyme II. Southern blot analysis of genomic DNA suggests that the human genome [corrected] may contain only a single gene encoding the enzyme. We propose that the distinct C-termini of isozymes I and II can arise from the generation of multiple mRNA's by alternative splicing.

Optimal conditions for the use of protein L-isoaspartyl methyltransferase in assessing the isoaspartate content of peptides and proteins

Protein L-isoaspartyl methyltransferase provides a basis for enzymatic measurement of atypical, isoaspartyl linkages which make a major contribution to protein microheterogeneity. The low Vmax of the methyltransferase reaction and the instability of the methyl ester can hinder accurate determinations, and different laboratories using different conditions have achieved discrepant values for the isoaspartate content of the same proteins. To investigate the effects of these conditions, and to optimize the assay, isoaspartyl delta sleep-inducing peptide was methylated under a variety of conditions. We found that 1 microM methyltransferase was required to obtain stoichiometric modification of 2 microM peptide in 40-min reactions at pH 6.2 and 30 degrees C. A computer model utilizing kinetic constants obtained from studies on initial rates of methylation predicted the same requirement for enzyme concentration. Carrier protein was necessary for optimal methyltransferase activity at enzyme concentrations below 0.4 microM. Stoichiometric methylation required concentrations of S-adenosylmethionine to be in substantial excess over those of peptide; 50 microM S-adenosylmethionine is the minimum needed for complete modification of 10 microM peptide. Spontaneous demethylation was significant under all conditions tested, so that the methyl ester itself never reached a ratio of 1 mol/mol of total peptide. These results demonstrate that the most accurate measurements of isoaspartate are obtained when reactions are carried out at low peptide concentrations, high S-adenosylmethionine concentrations, and high enzyme concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of protein digests by capillary high-performance liquid chromatography and on-line fast atom bombardment mass spectrometry

An HPLC system incorporating a packed capillary C18 column has been utilized for high sensitivity peptide mapping and preparative collection for protein sequencing. This system combined with a Frit-FAB mass spectrometer interface also provides the ability to obtain molecular ions for peptides of enzymatically digested proteins in the time it takes to obtain an HPLC chromatogram. The low flow rates permit introduction of the entire column effluent into the mass spectrometer. Detection limits of 0.5-5 pmol are routine. Proteolytic digests of recombinant human methionyl growth hormone and protein carboxyl methyltransferase have been used to demonstrate the HPLC and mass spectrometer performance.

The function of protein carboxylmethyltransferase in eucaryotic cells

Protein carboxylmethyltransferase (PCM) is an enzyme whose function in eucaryotic cells remains controversial. Early studies suggested that protein carboxylmethylation subserved a regulatory, post-translational role in such diverse processes as secretion, neuronal receptor function, chemotaxis, and cellular differentiation. Later work strongly supported a totally unrelated role for this enzyme, i.e., the repair of spontaneously altered aspartate residues in cellular proteins. More recent evidence, however, suggests that a distinct, membrane-associated PCM catalyzes the methylation of alpha-carboxyl groups of C-terminal cysteines on discrete proteins. In view of these recent investigations, the data supporting a regulatory role for PCM are critically discussed and re-evaluated. There now appears to be compelling evidence that PCM(s) subserves both repair and regulatory functions in eucaryotic cells, catalyzing post-translational modifications of proteins involved in cell division, hormonal secretion, calmodulin-associated events and the interaction of guanyl nucleotide-linked proteins with the cell membrane.