No abstract

Methionine adenosyltransferases (MAT) are the family of enzymes that synthesize the main biological methyl donor, S-adenosylmethionine. The high sequence conservation among catalytic subunits from bacteria and Eukarya preserves key residues that control activity and oligomerization, which is reflected in the protein structure. However, structural differences among complexes with substrates and products have led to proposals of several reaction mechanisms. In parallel, folding studies are starting to explain how the three intertwined domains of the catalytic subunit are produced, and the importance of certain intermediates in attaining the active final conformation. This review analyzes the available structural data and proposes a consensus interpretation that facilitates an understanding of the pathological problems derived from impairment of MAT function. In addition, new research opportunities directed toward clarification of aspects that remain obscure are also identified.Keywords methionine adenosyltransferase; S-adenosylmethionine synthetase; crystal structure; reaction mechanism; folding; mutants; hepatic disease Methionine is a non-polar amino acid characterized by the presence of a methyl group attached to a sulfur atom located in its side chain. In addition to its role in protein synthesis, large amounts of this amino acid are used for the synthesis of S-adenosylmethionine (AdoMet) by methionine adenosyltransferases (MAT) in a reaction that is the rate-limiting step of the methionine cycle ( Figure 1) [1,2]. The MAT catalyzed reaction combines methionine, ATP and water to produce AdoMet, pyrophosphate and inorganic phosphate; the enzyme requires both Mg 2+ and K + ions for maximal activity [1,2]. AdoMet participates in a large number of reactions, due to its ability to donate all the groups surrounding the sulfur atom. SAM radical proteins use the 5′-deoxyadenosyl moiety of AdoMet to synthesize biotin, among other compounds. Also, after decarboxylation AdoMet participates in polyamine synthesis, rendering 5′-deoxy-5′ methylthioadenosine (MTA) [3] that is recycled for methionine synthesis through the methionine salvage pathway [4]. Methyltransferases use AdoMet to obtain the methyl groups used to synthesize a large number of compounds, such as phospholipids and neurotransmitters, and in each case S-adenosylhomocysteine (AdoHcy) is formed. AdoHcy can act in many cases as a potent inhibitor of these enzymes. The AdoMet/AdoHcy ratio is known as the methylation index, with its normal value in mammalian cells being approximately three [1]. Maintenance * Author to whom correspondence should be addressed at: Instituto de Investigaciones Biomédicas "Alberto Sols" (CSIC-UAM), Arturo Duperier 4, 28029 Madrid, Spain. (Phone: 34-915854414; FAX: 34-915854010; email: mapajares@iib.uam.es) NIH Public Access Author ManuscriptCell Mol Life Sci. Author manuscript; available in PMC 2010 February 1. Published in final edited form as:Cell Mol Life Sci. 2009 February ; 66(4): 636-648. doi:10.1007/s00018-008-8516-1....

The intermediate filament protein vimentin constitutes a critical sensor for electrophilic and oxidative stress, which induce extensive reorganization of the vimentin cytoskeletal network. Here, we have investigated the mechanisms underlying these effects. In vitro, electrophilic lipids, including 15-deoxy-Δ12,14-prostaglandin J2 (15d-PGJ2) and 4-hydroxynonenal (HNE), directly bind to vimentin, whereas the oxidant diamide induces disulfide bond formation. Mutation of the single vimentin cysteine residue (Cys328) blunts disulfide formation and reduces lipoxidation by 15d-PGJ2, but not HNE. Preincubation with these agents differentially hinders NaCl-induced filament formation by wild-type vimentin, with effects ranging from delayed elongation and increased filament diameter to severe impairment of assembly or aggregation. Conversely, the morphology of vimentin Cys328Ser filaments is mildly or not affected. Interestingly, preformed vimentin filaments are more resistant to electrophile-induced disruption, although chemical modification is not diminished, showing that vimentin (lip)oxidation prior to assembly is more deleterious. In cells, electrophiles, particularly diamide, induce a fast and drastic disruption of existing filaments, which requires the presence of Cys328. As the cellular vimentin network is under continuous remodeling, we hypothesized that vimentin exchange on filaments would be necessary for diamide-induced disruption. We confirmed that strategies reducing vimentin dynamics, as monitored by FRAP, including cysteine crosslinking and ATP synthesis inhibition, prevent diamide effect. In turn, phosphorylation may promote vimentin disassembly. Indeed, treatment with the phosphatase inhibitor calyculin A to prevent dephosphorylation intensifies electrophile-induced wild-type vimentin filament disruption. However, whereas a phosphorylation-deficient vimentin mutant is only partially protected from disorganization, Cys328Ser vimentin is virtually resistant, even in the presence of calyculin A. Together, these results indicate that modification of Cys328 and vimentin exchange are critical for electrophile-induced network disruption.