Conformational signals in the C‐terminal domain of methionine adenosyltransferase I/III determine its nucleocytoplasmic distribution

Abstract

The methyl donor S-adenosylmethionine is synthesized in mammalian cytosol by three isoenzymes. Methionine adenosyltransferase II is ubiquitously expressed, whereas isoenzymes I (homotetramer) and III (homodimer) are considered the hepatic enzymes. In this work, we identified methionine adenosyltransferase I/III in most rat tissues, both in the cytoplasm and the nucleus. Nuclear localization was the preferred distribution observed in extrahepatic tissues, where the protein colocalizes with nuclear matrix markers. A battery of mutants used in several cell lines to decipher the determinants involved in methionine adenosyltransferase subcellular localization demonstrated, by confocal microscopy and subcellular fractionation, the presence of two partially overlapping areas at the C-terminal end of the protein involved both in cytoplasmic retention and nuclear localization. Immunoprecipitation of coexpressed FLAG and EGFP fusions and gel-filtration chromatography allowed detection of tetramers and monomers in nuclear fractions that also exhibited S-adenosylmethionine synthesis. Neither nuclear localization nor matrix binding required activity, as demonstrated with the inactive F251D mutant. Nuclear accumulation of the active enzyme only correlated with histone H3K27 trimethylation among the epigenetic modifications evaluated, therefore pointing to the necessity of methionine adenosyltransferase I/III to guarantee the supply of S-adenosylmethionine for specific methylations. However, nuclear monomers may exhibit additional roles.

MicroRNAs regulate methionine adenosyltransferase 1A expression in hepatocellular carcinoma

“…The authors speculated that presence of nuclear MAT might provide a continuous source of nuclear SAMe, since SAMe is charged and whether or not it can traverse the nuclear membrane is in debate. In support of this, nuclear accumulation of the active MAT1A protein correlated with higher levels of histone H3K27 trimethylation, an epigenetic modification associated with gene repression and DNA methylation (37). Our findings are consistent with this report.…”

“…Recently Reytor et al reported finding MATI/III in the nuclei (37). The authors speculated that presence of nuclear MAT might provide a continuous source of nuclear SAMe, since SAMe is charged and whether or not it can traverse the nuclear membrane is in debate.…”

The identification and characteristics of salinity-related microRNAs in gills of Portunus trituberculatus

“…SAM is also known as methionine adenosyltransferase (MAT), catalyzes the formation of S-adenosylmethionine by joining methionine and ATP (Horikawa et al 1990), and plays an important role in plant responses to salt, drought, alkali, and flood stresses (Kamal et al 2012;Chen et al 2014). SAM is also a methyl donor, which controls gene expression by DNA methylation (Reytor et al 2009). The mir-2788 miRNA was the only potential regulatory miRNA of SAM and was differentially expressed between LC and NC in this study, which indicated that it may play an important role in hyper-osmoregulation by regulating the expression of SAM.…”

Refolding and characterization of methionine adenosyltransferase from Euglena gracilis

“…However, the expression levels obtained were very low under all the conditions tested (growth temperature, cell density for induction, etc.). Addition of tags to other MATs, both at the N-or C-terminal ends, was previously shown to decrease the amount of soluble protein obtained [33], and hence the pT7.7-MATX plasmid that contains the MATX ORF without the His-tag was constructed. Growth and induction conditions of E. coli cells transformed with pT7.7-MATX were studied and optimized.…”