Dissociation of the E-Cadherin-Catenin-Complex (CCC) Is an Early and Progressive Event in Lobular Neoplasia

Recent data suggests that some lobular carcinoma in situ (LCIS) lesions may behave as precursors to invasive lobular carcinoma (ILC). Loss of E-cadherin (E-CD) mediated cell adhesion is characteristic of both LCIS and ILC and is reflected in the dis-cohesive appearance of the individual cells. Subsequent dissociation of the intracellular E-cadherin-catenin-complex (CCC) facilitates tumor progression, invasion and migration. Whether disruption of the CCC plays a role in the progression of select LCIS lesions to ILC remains unclear. The aim of this study was to evaluate the relation between loss of E-CD and dissociation of the CCC in pure LCIS and LCIS with concurrent ILC (LCIS w/ILC). Methods Thirty patients undergoing mastectomy for LCIS alone or LCIS w/ ILC were prospectively enrolled to an IRB-approved protocol. FFPE blocks were retrieved and sections prepared for IHC. 18 cases had LCIS w/ ILC, 12 cases had pure-LCIS. IHC was performed for ER, PR, E-CD, N-cadherin (N-CD), and α-, β- and phospho-β-catenin. ER/PR positivity was scored as any nuclear staining, and E-CD and N-CD by any membranous staining. a- and β-catenin expression was scored by site (membranous/cytoplasmic/nuclear) of staining compared to normal. Dissociation of the CCC was defined by loss of membranous α- and β-catenin expression.Results Median age at surgery was 51yrs (range 40-79); patients with pure LCIS were younger than those with LCIS w/ ILC (median 48yrs vs 57yrs, p=.0002). Among 18 cases of LCIS w/ ILC, the median tumor size was 2cm (range 1.4-5.7), 9 patients had N1 disease and 1 had M1 disease. All pure LCIS, LCIS w/ ILC and ILC lesions were ER/PR positive and E-CD negative. N-CD expression was also absent in all pure LCIS, LCIS w/ ILC and ILC lesions. Normal α-catenin membranous expression was confirmed in all normal epithelial cells but decreased with the transition from in-situ to invasive disease: pure-LCIS lesions 83%; LCIS w/ ILC 28%; ILC 0%. Loss of membranous α-catenin expression was accompanied by cytoplasmic α-catenin expression in all lesions. A similar trend of decreasing membranous staining from in-situ to invasive disease was observed for β-catenin, however in contrast to α-catenin, cytoplasmic β-catenin expression decreased from 67% in pure LCIS to 11% in LCIS w/ ILC and 6% in ILC. Active β-catenin (nuclear staining) was not seen in pure LCIS lesions and was only present in one case of LCIS w/ ILC. Inactive (phospho) β-catenin expression was present in all lesions.Conclusion Loss of E-CD expression is an early event in lobular neoplasia however subsequent dissociation of the intracellular CCC a a ppears to be a progressive process with complete dissociation occurring only in invasive lesions. This finding suggests that complete disruption of the CCC may be required to facilitate maintenance of the invasive phenotype; however, the absence of N-CD expression and predominance of inactive β-catenin in both in situ and invasive lesions suggests that alternate mechanisms are also required to mediate the pro-invasive effects of CCC dissociation. Further studies into the mechanisms of CCC dissociation and downstream events in lobular carcinoma are needed to define the role of this process in the transition from in-situ to invasive disease.

Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 6149.

Mixture of N-carbamoyl-L-glutamate plus L-arginine can protect rats with liver cirrhosis from acute ammonia intoxication

BACKGROUND/AIMS

We earlier reported that N-carbamoyl-L-glutamate (CG) plus L-arginine (Arg) protected normal and 70% hepatectomized rats from intoxication by a lethal or sub-lethal dose of ammonium acetate, respectively. In the present study, the protective effect of these compounds on cirrhotic rats was assessed.

METHODS

CG plus Arg were administered prior to the injection of a sub-lethal dose of ammonium acetate into dimethylnitrosamine-induced cirrhotic rats. Control rats were given phosphate-buffered saline (PBS) instead of the mixture. The behavior of the rats was monitored until the time of sacrifice. Blood ammonia level, blood urea nitrogen (BUN) and liver carbamoylphosphate synthetase I (CPS I) activity were determined.

RESULTS

Pretreatment of rats with the mixture of CG plus Arg could significantly lower the blood ammonia level (P<0.05), increase the activity of CPS I (P<0.05), improve abnormal behavior associated with ammonia intoxication (P<0.05), and increase BUN (P<0.05), as compared with the PBS-injected control group. There were significantly close correlations between (1) the increase of CPS I activity; (2) the improvement of abnormal behavior; (3) the increase of BUN; and (4) the decrease of the blood ammonia level.

CONCLUSIONS

A mixture of CG plus Arg could protect rats with liver cirrhosis from acute ammonia intoxication.

Methylation of O(6)-methylguanine-DNA methyltransferase gene is associated significantly with K-ras mutation, lymph node invasion, tumor staging, and disease free survival in patients with gastric carcinoma

BACKGROUND

O(6)-methylguanine-DNA methyltransferase (MGMT) can remove O(6)alkylG DNA adducts. If they are not removed, then the adducts mispair with T during DNA replication, resulting in G-to-A mutation. Interrelations between MGMT gene inactivation by promoter methylation, K-ras mutation, and clinicopathologic features in patients with gastric carcinoma were studied.

METHODS

Surgically removed tumor tissues from 79 patients were analyzed with MGMT methylation by genomic DNA modification and methylation specific polymerase chain reaction analysis, K-ras mutation by mutant allele specific amplification, TNM classification according to the International Union Against Cancer system, and MGMT protein expression by immunohistochemistry.

RESULTS

MGMT-promoter methylation was found in 18 of 79 tumors. Among those 18 tumors, K-ras mutations were found in 33% and 11% of tumors at codons 12 and 13, respectively, corresponding to 20 times and 7 times greater rates of mutation compared with unmethylated tumors. MGMT methylation was associated significantly with lymph node invasion (P < 0.01), tumor stage (P < 0.03) and 5-year disease free survival (P < 0.02). MGMT protein expression was detected in intestinal metaplasia and adenocarcinoma samples, whereas no expression was detected in normal foveolar cells.

CONCLUSIONS

MGMT-promoter methylation in patients with gastric carcinoma was associated significantly with point mutations of K-ras at codons 12 and 13, lymph node invasion, tumor stage, and disease free survival. These associations indicate a significant role of MGMT methylation during gastric carcinogenesis.

Purification and characterization of protein methylase II from Helicobacter pylori

Protein methylase II (AdoMet:protein-carboxyl O-methyltransferase, EC 2.1.1.24) was identified and purified 115-fold from Helicobacter pylori through Q-Sepharose ion exchange column, AdoHcy-Sepharose 4B column, and Superdex 200 HR column chromatography using FPLC. The purified preparation showed two protein bands of about 78 kDa and 29 kDa molecular mass on SDS-PAGE. On non-denaturing gel electrophoresis, the enzyme migrated as a single band with a molecular mass of 410 kDa. In addition, MALDI-TOF-MS analysis and Superdex 200 HR column chromatography of the purified enzyme showed a major mass signal with molecular mass values of 425 kDa and 430 kDa, respectively. Therefore, the above results led us to suggest that protein methylase II purified from H. pylori is composed of four heterodimers with 425 kDa (4x(78+29)=428 kDa). This magnitude of molecular mass is unusual for protein methylases II so far reported. The enzyme has an optimal pH of 6.0, a K(m) value of 5.0x10(-6) M for S-adenosyl-L-methionine and a V(max) of 205 pmol methyl-(14)C transferred min(-1) mg(-1) protein.

Phosphorylation of methylated-DNA-protein-cysteine S-methyltransferase at serine-204 significantly increases its resistance to proteolytic digestion

In a previous paper [Lim, Park, Jee, Lee and Paik (1999) J. Cancer Res. Clin. Oncol. 125, 493-499], we showed two major forms of active DNA-6-O-methylguanine:protein-L-cysteine S-methyltransferase (MGMT; EC 2.1.1.63) in the liver with N-nitrosodiethylamine (DEN)-induced carcinogenesis: these were 26 and 24 kDa species. Here we show that a 2 kDa C-terminal fragment was cleaved from the 26 kDa species in vitro by thrombin or microsomal fractions isolated from DEN-treated rat livers. When Ser(204) of the 26 kDa protein was replaced with Ala by site-directed mutagenesis, phosphorylation of the protein was completely abolished, indicating Ser(204) to be the site of phosphorylation. We also show that the phosphorylation was performed by Ca(2+)-independent protein kinase isoenzymes, and that the phosphorylated rat MGMT protein was resistant to digestion by protease(s) whose activity was increased during DEN-induced hepatocarcinogenesis and also by digestion with endopeptidase Glu-C (V8 protease).

Enzymic methylation of arginyl residues in -gly-arg-gly- peptides

N(G)-Methylation of arginine residues in many nucleic-acid-binding proteins are formed post-translationally, catalysed by S-adenosylmethionine:protein-arginine N-methyltransferase in their glycine-rich and arginine-rich motifs. The amino acid sequences of the stimulator of HIV-1 TAR (Tat-responsive element) RNA-binding protein (SRB) and fibronectin also show the presence of the internal -Gly-Arg-Gly- (-GRG-) sequence, which is potentially methylatable by the methyltransferase. To investigate the sequence requirement for methylation of these proteins, several synthetic oligopeptides with different chain lengths and sequences similar to the -GRG- regions of SRB and fibronectin were synthesized. Whereas the heptapeptide AGGRGKG (residues 16-22 in SRB) served as the methyl acceptor for the methyltransferase with a K(m) of 50 microM, the 19-mer peptide (residues 10-28 in SRB) was methylated with a K(m) of 8.3 microM, indicating that a greater peptide chain length yields a better methyl acceptor. Product analysis of the methylated [methyl-(14)C]SRB-peptide by HPLC indicated the formation of N(G)-monomethylarginine and N(G),N(G)-dimethyl(asymmetric)arginine. Synthetic peptides containing the cell attachment sequence [Arg-Gly-Asp ('RGD')] in fibronectin, GRGDSPK, GGRGDSPK and GGGRGDSPK, were also studied; whereas GRGDSPK was a poor methyl acceptor, the longer peptides were better methyl acceptors. To provide an understanding of the effect of methylation on fibronectin peptide, arginine-unmethylated and methylated GGRGDSPK were compared for their effect on the mitogenesis induced by beta-hexosaminidase A and an agonistic antibody (mAb(15)) in bovine tracheal smooth-muscle cells; whereas the former inhibited 35-67% of mitogenesis at a concentration of 5-10 microM, the latter did not block mitogenesis. This lack of inhibition by the insertion of a methyl group on the arginyl residue of the cell attachment sequence might be due to the hindrance of the binding of fibronectin peptide to integrins.

An endogenous proteinacious inhibitor in porcine liver for S-adenosyl-L-methionine dependent methylation reactions: identification as oligosaccharide-linked acyl carrier protein

A proteinacious inhibitor of S-adenosyl-L-methionine (AdoMet)-dependent transmethylation reactions was purified to homogeneity from porcine liver by size exclusion chromatography and FPLC. The molecular weight of the inhibitor was 12,222 Da. A 7400 Da polypeptide fragment of the purified inhibitor was sequenced by matrix-associated laser desorption ionization; time-of-flight MS, and was found to be identical with the known sequence of spinach acyl carrier protein (ACP). Although the remainder of the molecule was not clearly defined, 1H and H-H correlation of spectroscopy (COSY) NMR analysis revealed the presence of an oligosaccharide with alpha-glycosidic linkage. The purified oligosaccharide-linked ACP inhibited several AdoMet-dependent transmethylation reactions such as protein methylase I and II. S-farnesylcysteine O-methyltransferase, DNA methyltransferase and phospholipid methyltransferase. Protein methylase II was inhibited with a Ki value of 2.4 x 10(-3) M in a mixed inhibition pattern, whereas a well-known competitive product inhibitor S-adenosyl-L-homocysteine (AdoHcy) had Ki value of 6.3 x 10(-6) M. Commercially available active ACP fragments (65-74) and ACP from Escherichia coli had less inhibitory activity toward S-farnesylcysteine O-methyltransferase than the purified inhibitor. The biological significance of this oligosaccharide-linked ACP which has two seemingly unrelated functions (inhibitor for transmethylation and fatty acid biosynthesis) remains to be elucidated.

PRMT1 is the predominant type I protein arginine methyltransferase in mammalian cells

Type I protein arginine methyltransferases catalyze the formation of asymmetric omega-N(G),N(G)-dimethylarginine residues by transferring methyl groups from S-adenosyl-L-methionine to guanidino groups of arginine residues in a variety of eucaryotic proteins. The predominant type I enzyme activity is found in mammalian cells as a high molecular weight complex (300-400 kDa). In a previous study, this protein arginine methyltransferase activity was identified as an additional activity of 10-formyltetrahydrofolate dehydrogenase (FDH) protein. However, immunodepletion of FDH activity in RAT1 cells and in murine tissue extracts with antibody to FDH does not diminish type I methyltransferase activity toward the methyl-accepting substrates glutathione S-transferase fibrillarin glycine arginine domain fusion protein or heterogeneous nuclear ribonucleoprotein A1. Similarly, immunodepletion with anti-FDH antibody does not remove the endogenous methylating activity for hypomethylated proteins present in extracts from adenosine dialdehyde-treated RAT1 cells. In contrast, anti-PRMT1 antibody can remove PRMT1 activity from RAT1 extracts, murine tissue extracts, and purified rat liver FDH preparations. Tissue extracts from FDH(+/+), FDH(+/-), and FDH(-/-) mice have similar protein arginine methyltransferase activities but high, intermediate, and undetectable FDH activities, respectively. Recombinant glutathione S-transferase-PRMT1, but not purified FDH, can be cross-linked to the methyl-donor substrate S-adenosyl-L-methionine. We conclude that PRMT1 contributes the major type I protein arginine methyltransferase enzyme activity present in mammalian cells and tissues.

Identification of highly methylated arginine residues in an endogenous 20-kDa polypeptide in cancer cells

Enzymatic methylation of endogenous proteins in several cancer cell lines was investigated to understand a possible relationship between protein-arginine methylation and cellular proliferation. Cytosolic extracts prepared from several cancer cells (HeLa, HCT-48, A549, and HepG2) and incubated with S-adenosyl-L-[methyl-3H]methionine revealed an intensely [methyl-3H]-labeled 20-kDa polypeptide. On the other hand, cytosolic extracts prepared from normal colon cells did not show any methylation of the 20-kDa protein under identical conditions. To identify nature of the 20-kDa polypeptide, purified histones were methylated with HCT-48 cytosolic extracts and analyzed by SDS-PAGE. However, none of the histones comigrated with the methylated 20-kDa polypeptide, indicating that it is unlikely to be any of the histone subclasses. The [methyl-3H]group in the 20-kDa polypeptide was stable at pH 10-11 (37 degrees C for 30 min) and methylation was not stimulated by GTPgammaS (4 mM), thus the reaction is neither carboxyl methylesterification on isoaspartyl residues, nor on C-terminal farnesylated cysteine. The present study together with the previous identification of N(G)-methylated arginine residues in the HCT-48 cytosol fraction suggests that this novel endogenous 20-kDa arginine-methylation is a cellular proliferation-related posttranslational modification reaction.

Differential expression of O6-methylguanine-DNA methyltransferase during diethylnitrosamine-induced carcinogenesis and liver regeneration in Sprague-Dawley male rats

Differential expression of DNA-O6MeG: protein-L-cysteine S-methyltransferase (MGMT) activity and posttranslational modification of the protein during liver regeneration and carcinogenesis were compared in Sprague-Dawley male rats after partial hepatectomy and/or single i.p injection of diethylnitrosamine (DEN, 200 mg/kg). Regenerating hepatocytes after partial hepatectomy induced MGMT transiently within 3 days; however, the induction of MGMT was persistent for 2 weeks after DEN injection, and the combined treatment of DEN and partial hepatectomy maintained the elevated MGMT level for up to 4 weeks. The increased activity was transcriptionally regulated, when analyzed by Northern blot hybridization. The major active form of MGMT protein in the partially hepatectomized or DEN-treated rats was a 26-kDa or 24-kDa species respectively, which was confirmed by Western blot analysis and gel slice assay. The biological significance of the differential induction of MGMT during partial hepatectomy or DEN-induced carcinogenesis is not obvious; however, further studies on possible posttranslational modifications of MGMT protein might shed some light on the functional aspect of MGMT induction.

An endogenous proteinacious inhibitor for S-adenosyl-L-methionine-dependent transmethylation reactions; identification of S-adenosylhomocystein as an integral part

A proteinacious inhibitor with a molecular weight of 1,600 Da which inhibits S-adenosyl-L-methionine-dependent transmethylation reactions was purified from porcine liver to homogeneity by procedures including boiling, Sephadex G-25 column chromatography and repeated HPLC. Employing both Nuclear Magnetic Resonance (NMR) and Fast Atom Bombardment-Mass (FAB-Mass) spectroscopy, S-adenosylhomocysteine was conclusively identified as an integral part of the inhibitor. The purified S-adenosylhomocysteine was competitive with S-adenosyl-L-methionine with Ki value of 6.3x10(-6) M towards protein methylase II.

Recent advances in protein methylation: enzymatic methylation of nucleic acid binding proteins

Heterogeneous nuclear RNP protein A1, one of the major proteins in hnRNP particle (precursor for mRNA), is known to be posttranslationally arginine-methylated in vivo on residues 193, 205, 217 and 224 within the RGG box, the motif postulated to be an RNA binding domain. Possible effect of NG-arginine methyl-modification in the interaction of protein A1 to nucleic acid was investigated. The recombinant hnRNP protein A1 was in vitro methylated by the purified nuclear protein/histone-specific protein methylase I (S-adenosylmethionine:protein-arginine N-methyltransferase) stoichiometrically and the relative binding affinity of the methylated and the unmethylated protein A1 to nucleic acid was compared: Differences in their binding properties to ssDNA-cellulose, pI values and trypsin sensitivities in the presence and absence of MS2-RNA all indicate that the binding property of hnRNP protein A1 to single-stranded nucleic acid has been significantly reduced subsequent to the methylation. These results suggest that posttranslational methyl group insertion to the arginine residue reduces protein-RNA interaction, perhaps due to interference of H-bonding between guanidino nitrogen arginine and phosphate RNA.

Identification of protein-arginine N-methyltransferase as 10-formyltetrahydrofolate dehydrogenase

S-Adenosylmethionine:protein-arginine N-methyltransferase (EC 2.1.1. 23; protein methylase I) transfers the methyl group of S-adenosyl-L-methionine to an arginine residue of a protein substrate. The homogeneous liver protein methylase I was subjected to tryptic digestion followed by reverse phase high performance liquid chromatography (HPLC) separation and either "on-line" mass spectrometric fragmentation or "off-line" Edman sequencing of selected fractions. Data base searching of both the mass spectrometric and Edman sequencing data from several peptides identified the protein methylase as 10-formyltetrahydrofolate dehydrogenase (EC 1.5.1.6; Cook, R. J., Lloyd, R. S., and Wagner, C. (1991) J. Biol. Chem. 266, 4965-4973; Swiss accession number). This identification was confirmed by comparative HPLC tryptic peptide mapping and affinity chromatography of the methylase on the 5-formyltetrahydrofolate-Sepharose affinity gel used to purify the dehydrogenase. The purified rat liver methylase had approximately 33% of the 10-formyltetrahydrofolate dehydrogenase and 36% of the aldehyde dehydrogenase activity as compared with the recombinant dehydrogenase, which also had protein methylase I activity. Polyclonal antibodies against recombinant dehydrogenase reacted with protein methylase I purified either by polyacrylamide gel electrophoresis or 5-formyltetrahydrofolate affinity chromatography. In each instance there was only a single immunoreactive band at a molecular weight of approximately 106,000. Together, these results confirm the co-identity of protein-arginine methyltransferase and 10-formyltetrahydrofolate dehydrogenase.

Induction of growth inhibition of 293 cells by downregulation of the cyclin E and cyclin-dependent kinase 4 proteins due to overexpression of TIS21

We earlier reported that TIS21 mRNA expression was markedly decreased in A549 and NCIH69 human lung cancer cells and in thymic carcinoma tissues obtained from transgenic mice containing simian virus 40 large T antigen (J Cancer Res Clin Oncol 121:279-284, 1995). To determine how TIS21 inhibits growth, we made 293 cells that constitutively expressed TIS21 protein. The constitutive TIS21 expresser lines C9 and C11 grew to a lower saturation density than did those in the vector-transfected clones (V7 and V10) and antisense-transfected clones (AS1 and AS4), and the size of the C9 and C11 cells increased significantly after transfection with TIS21 cDNA. The serum-stimulated cell cycle was analyzed by fluorescence-activated cell sorting after double thymidine treatment; V10 progressed normally through the cell division cycle, but C9 and C11 cells accumulated continuously in G1 phase until 36 h after treatment. On the other hand, the progression of cells that had already entered to S or G2/M phase was not inhibited. When cell-cycle regulatory proteins were measured, C9 and C11 cells showed significantly reduced synthesis of cyclin E and cyclin-dependent kinase (cdk) 4 as well as a decrease in cyclin E-associated cdk activity. These observations led us to conclude that TIS21 overexpression in G1 phase decreased the amounts of cyclin E and cdk4, thereby decreasing the activity of cdks at the G1-S transition.

Enzymatic methylation of recombinant TIS21 protein-arginine residues

Recombinant TIS21 protein was overexpressed in Escherichia coli harboring the expression vector plasmid pQE-30 carrying the TIS21 cDNA coding sequence containing an extra 120 nucleotides upstream. Employing this protein consisting of 158 amino acid residues of the main chain plus 40 residues of the fusion peptide. It was found that one of the protein methylase I group [S-adenosylmethionine:nuclear protein/histone-arginine N-methyltransferase; BC 2.1.1.23; J. Biol. Chem., 269, 1075 (1994)] methylated this protein. The methylation products were identified as guanidino-N-methylated arginines. Some of the kinetics of the reaction are described.

N-carbamoyl-L-glutamate plus L-arginine can protect ammonia intoxication in rats with reduced functional liver mass

We previously reported the protective effect of N-carbamoyl-L-glutamate plus L-arginine in rats given a lethal dose (LD99.9) of ammonium acetate (Kim, S. et al., Proc. Natl. Acad. Sci. 69, 3530-3533, 1971; ref.1). The present study was undertaken to find out whether the same compounds could also be effective even after the functional mass of the liver was significantly reduced. Thus, the protective effect of these compounds in 70% partial hepatectomized rats following the injection of sublethal dose of ammonium acetate was assessed. The mixture could significantly decrease blood ammonia level compared with PBS-injected control group. In addition, abnormal behaviors observed in the control rats were significantly improved. The protective effect on the behavioral change seemed to be closely related with their effect on blood ammonia level, showing a strong correlation between the blood ammonia level and the behavioral score. The findings provide a rational basis for the clinical use of N-carbamoyl-L-glutamate plus L-arginine in the prevention and treatment of hyperammonemia encountered in liver diseases.

Methylesters of L-arginine and N-nitro-L-arginine induce nitric oxide synthase in Staphylococcus aureus

The presence of L-arginine methylester (AME), L-arginine ethylester (AEE), or N-nitro-L-arginine methylester (NAME) in the growth media of Staphylococcus aureus increased the nitric oxide synthase (NOS) activity approximately 5- to 14-fold. The increase of NOS activity was confirmed by two assay methods, namely assaying the formation of L-[3H] citrulline from L-[3H] arginine and NO formation. The increase of NOS activity was most likely due to increased de novo synthesis, demonstrated by Western immunoblot analysis. The addition of methanol to the culture medium also increased the NOS activity as much as that found with the above three compounds. Evidence is presented to show that AME, AEE, or NAME gave rise to the formation of methanol in vivo by the action of intracellular esterase(s) and that methanol is subsequently involved in the induction of NOS in this bacterial system.

Heterogeneous nuclear RNP protein A1-arginine methylation during HCT-48 cell cycle

Protein methylase I (protein-arginine N-methyltransferase) was examined in HCT-48 cells, synchronized by serum deprivation and hydroxyurea treatment. The enzyme activity to methylate the added hnRNP protein A1 increased about 2-fold from G0 to S phase, and then decreased during G2/M phase. The enzymatically [methyl-3H]-labeled hnRNP protein A1 was identified by SDS-PAGE/fluorography, and the products were identified as NG-monomethylarginine and NG,NG-dimethyl-(asymmetric)arginines by HPLC. Among endogenous proteins, the 20-kDa species in the extract was most intensely [methyl-3H]-labeled. This 20-kDa methylation was markedly inhibited by the addition of exogenous hnRNP protein A1, indicating that these two substrates compete for the same protein methylase. The possible role of this post-translational modification has been discussed.