Structural requirements for catalysis and dimerization of human methionine adenosyltransferase I/III

Genomic analysis of 9 infants with hypermethioninemia by whole-exome sequencing among in Henan, China

BACKGROUND

Hypermethioninemia is an inborn error of metabolism with elevated plasma methionine (Met) caused by methionine adenosyltransferase deficiency. Methionine adenosyltransferase (MAT) I/III deficiency is the most common cause of hypermethioninemia. Except for increased blood Met, most patients have no symptoms, but a small number have nervous system complications, including cognitive impairment and mental retardation.

OBJECTIVE

To investigate the gene variation of patients with hypermethioninemia in newborns in Henan province.

METHODS

9 cases of hypermethioninemia were screened for amino acids profile and acyl carnitine by tandem mass spectrometric (MS/MS) among 245 054 newborns. We performed whole-exome sequencing on 9 families of infants with hypermethioninemia. We identified mutated genes under different models of inheritance and further assessed these mutations through Sanger sequencing and association analysis.

RESULTS

The incidence of neonatal hypermethioninemia was 1:27 228 in Henan province. A total of ten mutations in the MAT1A gene in the 9 patients were identified, including nine reported mutations (c.1070C>T, c.895C>T, c.100T>A, c.315C>A, c.529C>T, c.623A>C, c.407G>T, c.1066C>T, 867G>T) and one novel mutations (c.772G>C). c.772G>C was detected in 2 families and is the most common variant. 7 infants (7/9) with hypermethioninemia were genetically autosomal dominant, and 2 infants (2/9) with hypermethioninemia were genetically autosomal recessive.

CONCLUSION

Our findings expand the mutational spectrum of hypermethioninemia, with the description of one new mutation. They improve the understanding of the genetic background and clinical manifestation of MAT1A in Chinese patients.

Methionine adenosyltransferase II serves as a transcriptional corepressor of Maf oncoprotein

Protein methylation pathways comprise methionine adenosyltransferase (MAT), which produces S-adenosylmethionine (SAM) and SAM-dependent substrate-specific methyltransferases. However, the function of MAT in the nucleus is largely unknown. MafK represses or activates expression of heme oxygenase-1 (HO-1) gene, depending on its heterodimer partners. Proteomics analysis of MafK revealed its interaction with MATIIα, a MAT isozyme. MATIIα was localized in nuclei and found to form a dense network with chromatin-related proteins including Swi/Snf and NuRD complexes. MATIIα was recruited to Maf recognition element (MARE) at HO-1 gene. When MATIIα was knocked down in murine hepatoma cell line, expression of HO-1 was derepressed at both basal and induced levels. The catalytic activity of MATIIα, as well as its interacting factors such as MATIIβ, BAF53a, CHD4, and PARP1, was required for HO-1 repression. MATII serves as a transcriptional corepressor of MafK by interacting with chromatin regulators and supplying SAM for methyltransferases.

GAP promoter library for fine-tuning of gene expression in Pichia pastoris

A library of engineered promoters of various strengths is a useful genetic tool that enables the fine-tuning and precise control of gene expression across a continuum of broad expression levels. The methylotrophic yeast Pichia pastoris is a well-established expression host with a large academic and industrial user base. To facilitate manipulation of gene expression spanning a wide dynamic range in P. pastoris, we created a functional promoter library through mutagenesis of the constitutive GAP promoter. Using yeast-enhanced green fluorescent protein (yEGFP) as the reporter, 33 mutants were chosen to form the functional promoter library. The 33 mutants spanned an activity range between ∼0.6% and 19.6-fold of the wild-type promoter activity with an almost linear fluorescence intensity distribution. After an extensive characterization of the library, the broader applicability of the results obtained with the yEGFP reporter was confirmed using two additional reporters (β-galactosidase and methionine adenosyltransferase [MAT]) at the transcription and enzyme activity levels. Furthermore, the utility of the promoter library was tested by investigating the influence of heterologous MAT gene expression levels on cell growth and S-adenosylmethionine (SAM) production. The extensive characterization of the promoter strength enabled identification of the optimal MAT activity (around 1.05 U/mg of protein) to obtain maximal volumetric SAM production. The promoter library permits precise control of gene expression and quantitative assessment that correlates gene expression level with physiologic parameters. Thus, it is a useful toolbox for both basic and applied research in P. pastoris.

Biochemical basis for the dominant inheritance of hypermethioninemia associated with the R264H mutation of the MAT1A gene. A monomeric methionine adenosyltransferase with tripolyphosphatase activity

Methionine adenosyltransferase (MAT) catalyzes the synthesis of S-adenosylmethionine (AdoMet), the main alkylating agent in living cells. Additionally, in the liver, MAT is also responsible for up to 50% of methionine catabolism. Humans with mutations in the gene MAT1A, the gene that encodes the catalytic subunit of MAT I and III, have decreased MAT activity in liver, which results in a persistent hypermethioninemia without homocystinuria. The hypermethioninemic phenotype associated with these mutations is inherited as an autosomal recessive trait. The only exception is the dominant mild hypermethioninemia associated with a G-A transition at nucleotide 791 of exon VII. This change yields a MAT1A-encoded subunit in which arginine 264 is replaced by histidine. Our results indicate that in the homologous rat enzyme, replacement of the equivalent arginine 265 by histidine (R265H) results in a monomeric MAT with only 0.37% of the AdoMet synthetic activity. However the tripolyphosphatase activity is similar to that found in the wild type (WT) MAT and is inhibited by PP(i). Our in vivo studies demonstrate that the R265H MAT I/III mutant associates with the WT subunit resulting in a dimeric R265H-WT MAT unable to synthesize AdoMet. Tripolyphosphatase activity is maintained in the hybrid MAT, but is not stimulated by methionine and ATP, indicating a deficient binding of the substrates. Our data indicate that the active site for tripolyphosphatase activity is functionally active in the monomeric R265H MAT I/III mutant. Moreover, our results provide a molecular mechanism that might explain the dominant inheritance of the hypermethioninemia associated with the R264H mutation of human MAT I/III.