Biosynthesis of S-methylmethionine in the jack bean

S-methylmethionine reduces cell membrane damage in higher plants exposed to low-temperature stress

S-methylmethionine (SMM), an important intermediate compound in the sulphur metabolism, can be found in various quantities in majority of plants. The experiments were designed to determine the extent to which SMM is able to preserve cell membrane integrity or reduce the degree of membrane damage in the course of low-temperature stress. By measuring electrolyte leakage (EL), it was proved that SMM treatment reduced cell membrane damage, and thus EL, during low-temperature stress in both the leaves and roots of peas, maize, soy beans and eight winter wheat varieties with different levels of frost resistance. Investigations on the interaction between SMM and polyamine biosynthesis revealed that SMM increased the quantities of agmatine (Agm) and putrescine (Put) as well as that of spermidine (Spd), while it had no effect on the quantity of spermine (Spn). Using a specific inhibitor, methylglyoxal-bis-guanyl hydrazone (MGBG), it was proved that the polyamine metabolic pathway starting from methionine played no role in the synthesis of Spd or Spn, so there must be an alternative pathway for the synthesis of SMM-induced polyamines.

Role of S-methylmethionine in the plant metabolism

S-methylmethionine (SMM), a naturally occurring, biologically active compound, is a free amino acid derivative, which is increasingly recognised as playing an important part in the plant metabolism. SMM, which is synthesised from methionine, is involved in crucial processes in the S metabolism, such as the regulation of methionine and S-adenosyl methionine levels, the methylation processes taking place in cells, and the transport and storage of sulphur in certain phases of development. It is of great importance in the development of resistance to abiotic and biotic stress factors, as it is a direct precursor in the biosynthesis of the osmoprotectants and other S-containing compounds involved in defence mechanisms, while also influencing the biosynthesis of major plant hormones such as polyamines and ethylene. The present paper discusses our increasing understanding of the role played by SMM in the plant metabolism and its possible role in the improvement of traits that enable plants to overcome stress.

Transport of sulfonium compounds. Characterization of the s-adenosylmethionine and s-methylmethionine permeases from the yeast Saccharomyces cerevisiae

We report here the characterization and the molecular analysis of the two high affinity permeases that mediate the transport of S-adenosylmethionine (AdoMet) and S-methylmethionine (SMM) across the plasma membrane of yeast cells. Mutant cells unable to use AdoMet as a sulfur source were first isolated and demonstrated to lack high affinity AdoMet transport capacities. Functional complementation cloning allowed us to identify the corresponding gene (SAM3), which encodes an integral membrane protein comprising 12 putative membrane spanning regions and belonging to the amino acid permease family. Among amino acid permease members, the closest relative of Sam3p is encoded by the YLL061w open reading frame. Disruption of YLL061w was shown to specifically lead to cells unable to use SMM as a sulfur source. Accordingly, transport assays demonstrated that YLL061w disruption mutation impaired the high affinity SMM permease, and YLL061w was therefore renamed MMP1. Further study of sam3Delta and mmp1Delta mutant cells showed that in addition to high affinity permeases, both sulfonium compounds are transported into yeast cells by low affinity transport systems that appear to be carrier-facilitated diffusion.

Dissimilation of methionine in cell suspension cultures from Catharanthus roseus L

Plant cell suspension cultures from Catharanthus roseus were investigated for their capability to dissimilate methionine or its analogs in order to reutilize the sulphane group for cysteine biosynthesis. Three steps have been described as prerequisites of this process: (a) oxidative degradation by the amino-acid oxidase of methionine giving rise to methanethiol production; (b) demethylation by methyltransferases leading to homocysteine and S-methylmethionine (c) replacement of the homocysteine sulphane sulphur by alkylthiol yielding methionine and free hydrogen sulphide. A reversal of the cystathionine pathway as a source of cysteine was ruled out because the cells lack cystathionine γ-lyase. The absence of this enzyme is compensated by the S-alkyl exchange of homocysteine with methylmercaptan. Hydrogen sulphide thus liberated is used for de novo synthesis of cysteine. The complete pathway can be catalyzed by the constitutive set of enzymes present in the higher plant.