Identification of the S-adenosyl-L-methionine binding site of protein-carboxyl O-methyltransferase using 8-azido-S-adenosyl-L-methionine

Photoaffinity Labeling and Mutational Analysis of 24‐C‐Sterol Methyltransferase Defines the AdoMet Binding Site

Photolabeling and site-directed mutagenesis were performed on recombinant Saccharomyces cerevisiae 24-C-sterol methyltransferase (SMT) to elucidate the location and role of active site residues involved in AdoMet binding and catalysis. Bioinformatic analysis of the SMT revealed a ten amino acid segment, conserved between L124 and P133, associated with the Rossmann-like fold belonging to AdoMet-dependent methyltransferases. Irradiation of the SMT in the presence of [methyl-3H3]AdoMet directly photolabeled the protein. The specificity of photolabeling was demonstrated by inactivation experiments with structural analogs of AdoMet, including sinefungin. Trypsin digestion of the [methyl-3H3]AdoMet photolabeled Erg6p afforded a single radioactive band in SDS-PAGE gel of 4 kDa. HPLC purification of this material generated a single radioactive fraction. The corresponding 3H-AdoMet-peptide adduct was subjected to Edman sequencing and the first fifteen residues gave a sequence Gly120-Asp-Leu-Val-Leu-Asp-Val-Gly-Cys-Gly-Val-Gly-Gly-Pro-Ala134 that contained the predicted AdoMet binding site. Amino acid residues in the tryptic digest fragment considered to bind covalently with the AdoMet at Asp125, Cys128, Pro133 and Tyr153 were replaced with leucine and analyzed kinetically and by photolabeling inactivation experiments. The results indicate that one or both of Cys128 and Pro133 are covalently bound to AdoMet.

Efficient synthesis of azide-bearing cofactor mimics

8-Azido-5'-aziridino-5'-deoxyadenosine (6), a novel cofactor mimic, was synthesized in nine steps from commercially available 2',3'-isopropylideneadenosine in approximately 4% overall yield. Crucial to this success was a very unorthodox phthalimide cleavage procedure, C8 azidation prior to aziridination and late stage alkylation of the 5' amino group with iodoethanol necessitated by the high degree of lability endowed by the aryl azide moiety. Aziridine 6 is envisioned as a useful biochemical tool by which to probe DNA and protein methylation patterns.

Daphnetin methylation by a novel O-methyltransferase is associated with cold acclimation and photosystem II excitation pressure in rye

In plants, O-methylation of phenolic compounds plays an important role in such processes as lignin synthesis, flower pigmentation, chemical defense, and signaling. However, apart from phenylpropanoids and flavonoids, very few enzymes involved in coumarin biosynthesis have been identified. We report here the molecular and biochemical characterization of a gene encoding a novel O-methyltransferase that catalyzes the methylation of 7,8-dihydroxycoumarin, daphnetin. The recombinant protein displayed an exclusive methylation of position 8 of daphnetin. The identity of the methylated product was unambiguously identified as 7-hydroxy-8-methoxycoumarin by co-chromatography on cellulose TLC and coelution from high performance liquid chromatography, with authentic synthetic samples, as well as by UV, mass spectroscopy, (1)H NMR spectral analysis, and NOE correlation signals of the relevant protons. Northern blot analysis and enzyme activity assays revealed that the transcript and corresponding enzyme activity are up-regulated by both low temperature and photosystem II excitation pressure. Using various phenylpropanoid and flavonoid substrates, we demonstrate that cold acclimation of rye leaves increases O-methyltransferase activity not only for daphnetin but also for the lignin precursors, caffeic acid, and 5-hydroxyferulic acid. The significance of this novel enzyme and daphnetin O-methylation is discussed in relation to its putative role in modulating cold acclimation and photosystem II excitation pressure.

mraW, an essential gene at the dcw cluster of Escherichia coli codes for a cytoplasmic protein with methyltransferase activity

Three new open reading frames, mraZ, mraW and mraR (also called ftsL), were revealed by DNA sequencing immediately upstream of gene pbpB in the dcw cluster of Escherichia coli. We have found that mraW and mraZ are active genes, coding for two proteins with relative molecular masses of 34 800 and 17 300, respectively. MraW is a cytoplasmic protein that under overproduction condition is also loosely bound to the membrane. Soluble MraW was purified up to 90% by a single high performance electrophoresis (HPEC) step from an extract of an overproducing strain. The protein exhibits a S-adenosyl-dependent methyltransferase activity on membrane-located substrates.

Organization and characterization of the ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit epsilon N-methyltransferase gene in tobacco

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) large subunit (LS) epsilon N-methyltransferase (Rubisco LSMT) catalyzes the posttranslational methylation of the epsilon-amino group of Lys-14 in the LS of Rubisco in many higher plant species including tobacco. The tobacco Rubisco LSMT gene (rbcMT-T) and its cDNA were isolated, sequenced, and characterized. The gene contains 6 exons and spans about 6 kb. Primer extension analysis indicated one transcription start site located 93 nt upstream of the translation initiation site. Sequence analysis of the 5'-flanking region suggests several potential binding sites for transcription factors, including 7 GT-1 elements and an HSP-70.5 element. Gene dosage analysis by Southern hybridization demonstrated that the tobacco rbcMT-T gene is present as a single copy in the tobacco haploid genome. The full-length cDNA for tobacco rbcMT-T is 1974 nt in length excluding the 3' poly(A)15 tail, and encodes a 491 amino acid polypeptide with a molecular mass of ca. 56kDa. The deduced amino acid sequence of tobacco Rubisco LSMT has 64.5% identity and 75.3% similarity with the sequence of pea Rubisco LSMT, and both proteins contain several copies of a conserved imperfect leucine-rich repeat motif.

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.

Protein damage and methylation-mediated repair in the erythrocyte

Images

A putative O-methyltransferase from barley is induced by fungal pathogens and UV light

A cDNA clone, pBH72-F1 (F1), was isolated from a cDNA library prepared from barley leaves 72 h after inoculation with Erysiphe graminis f.sp. hordei. The 1388 bp nucleotide sequence of pBH72-F1 contains an open reading frame encoding a 42.3 kDa polypeptide of 390 amino acids which shows sequence similarity to O-methyltransferases (OMTs) from different plant species; the highest identity (41%) was observed with a putative OMT expressed in roots of maize. A phylogenetic analysis shows that the barley and maize sequences are distinctly different from the ortho-diphenol-OMTs involved in lignin formation. A putative S-adenosyl-L-methionine-binding motif (KELVDDSITN) determined for a rabbit protein-carboxyl OMT is partially conserved in the encoded amino acid sequence. Genomic Southern blot hybridization shows that pBH72-F1 probably represents a single copy gene. The F1 clone corresponds to a gene transcript exhibiting a relatively late accumulation in mildew-infected barley leaves compared to other pathogen-induced transcripts, such as transcripts encoding PR proteins, a peroxidase, and transcripts homologous to a maize caffeic acid OMT. No transcript was detected in plants exhibiting papilla resistance at time points when resistance is thought to be manifested. The atypical transcript accumulation pattern for F1 was also observed after infection by other pathogens and after UV-light treatment.

Spontaneous methylation of hemoglobin by S-adenosyl-methionine by a specific and saturable mechanism

The methyl group from S-adenosylmethionine (AdoMet) is transferred into hemoglobin without any evident involvement of an enzyme. There are multiple sites for incorporation of the methyl group into hemoglobin, since both alpha and beta chains are methylated. The methyl linkages formed in hemoglobin are stable at both alkaline and acidic pH, and the reaction occurs optimally at slightly below neutral pH. Only a small fraction (approximately 2%) of hemoglobin tetramers are methylated under the conditions tested. Acid hydrolysis of [3H-methyl]-labeled hemoglobin and determination of phenylisothiocynate derivatives yields N epsilon-methyl lysine, which accounts for about one-half of the incorporated [3H-methyl] radioactivity. Other amino acids are methylated as well, with much of the remaining radioactivity being distributed among one or more of the side chains of histidine, cysteine, and arginine. Methyl group transfer to hemoglobin from AdoMet is slow and inefficient (kcat/Km approximately 5 x 10(-2), but the reaction velocity tends toward a plateau with increasing AdoMet concentration in a manner suggesting that saturable binding of AdoMet onto hemoglobin is involved in methyl transfer. The velocity of hemoglobin methylation is inhibited by S-adenosylhomocysteine, the known end-product inhibitor of methyltransferases, a further indication that methyl group transfer involves binding and catalysis by a specific site (or sites) in the hemoglobin molecule. These observations may help to explain the known existence of methylated hemoglobins in erythrocyte.

Comparative studies on S-adenosyl-L-methionine binding sites of protein N-methyltransferases, using 8-azido-S-adenosyl-L-methionine as photoaffinity probe

Employing a photoaffinity labeling procedure with 8-azido-S-adenosyl-L-[methyl-3H]methionine (8-N3-Ado[methyl-3H]Met), the binding sites for S-adenosyl-L-methionine(AdoMet) of three protein N-methyltransferases [AdoMet:myelin basic protein-arginine N-methyltransferase (EC 2.1.1.23); AdoMet:histone-arginine N-methyltransferase (EC2.1.1.23); and AdoMet:cytochrome c-lysine N-methyltransferase (EC2.1.1.59)] have been investigated. The incorporation of the photoaffinity label into the enzymes upon UV irradiation was highly specific. In order to define the AdoMet binding sites, the photolabeled enzymes were sequentially digested with trypsin, chymotrypsin, and endoproteinase Glu-C. After each proteolytic digestion, radiolabeled peptide from each enzyme was resolved on HPLC first by gradient elution and further purified by an isocratic elution. Retention times of the purified radiolabeled peptides from the three enzymes from the corresponding proteolysis were significantly different, indicating that their sizes and compositions were different. Amino acid composition analysis of these peptides confirmed further that the AdoMet binding sites of these protein N-methyltransferases are quite different.