Pectin methylesterase activity of plant DUF538 protein superfamily

DUF538 (domain of unknown function 538) proteins are known as a group of putative hypothetical proteins in a wide range of plant species. They have been identified from some plants challenged with various environmental stresses. However, a little is known about their functional properties. They have been newly predicted to have binding capacity and esterase-type hydrolytic activity towards bacterial lipopolysaccharides and chlorophyll molecules as carboxylic compounds in plants. In the present study, the binding ability and the methylesterase activity of DUF538 proteins towards pectin molecules were also predicted. Their similarities to pectin methylesterases and their binding ability to pectin molecule were predicted using bioinformatic tools as well as the experimental method. A probable cooperation was speculated between DUF538 and pectin methylesterase protein families in cell wall associated defense responses in plants.

Two novel methylesterases from Olea europaea contribute to the catabolism of oleoside-type secoiridoid esters

Two newly identified phytohormone cleaving esterases from Olea europaea are responsible for the glucosidase-initiated activation of the specialized metabolites ligstroside and oleuropein. Biosynthetic routes leading to the formation of plant natural products are tightly orchestrated enzymatic sequences usually involving numerous specialized catalysts. After their accumulation in plant cells and tissues, otherwise non-reactive compounds can be enzymatically activated, e.g., in response to environmental threats, like pathogen attack. In olive (Olea europaea), secoiridoid-derived phenolics, such as oleuropein or ligstroside, can be converted by glucosidases and as yet unidentified esterases to oleoside aldehydes. These are not only involved in pathogen defense, but also bear considerable promise as pharmaceuticals or neutraceuticals. Making use of the available olive genomic data, we have identified four novel methylesterases that showed significant homology to the polyneuridine aldehyde esterase (PNAE) from Rauvolfia serpentina, an enzyme acting on a distantly related metabolite group (monoterpenoid indole alkaloids, MIAs) also featuring a secoiridoid structural component. The four olive enzymes belong to the α/ß-hydrolase fold family and showed variable in vitro activity against methyl esters of selected plant hormones, namely jasmonic acid (MeJA), indole acetic acid (MeIAA), as well as salicylic acid (MeSA). None of the identified catalysts were directly active against the olive metabolites oleuropein, ligstroside, or oleoside 11-methyl ester. When employed in a sequential reaction with an appropriate glucosidase, however, two were capable of hydrolyzing these specialized compounds yielding reactive dialdehydes. This suggests that the esterases play a pivotal role in the activation of the olive secoiridoid polyphenols. Finally, we show that several of the investigated methylesterases exhibit a concomitant in vitro transesterification capacity-a novel feature, yielding ethyl esters of jasmonic acid (JA) or indole-3-acetic acid (IAA).

VvMJE1 of the grapevine (Vitis vinifera) VvMES methylesterase family encodes for methyl jasmonate esterase and has a role in stress response

The known members of plant methyl esterase (MES) family catalyze the hydrolysis of a C-O ester linkage of methyl esters of several phytohormones including indole-3-acetic acid, salicylic acid and jasmonic acid. The genome of grapevine (Vitis vinifera) was found to contain 15 MES genes, designated VvMES1-15. In this report, VvMES5 was selected for molecular, biochemical and structural studies. VvMES5 is most similar to tomato methyl jasmonate esterase. E. coli-expressed recombinant VvMES5 displayed methyl jasmonate (MeJA) esterase activity, it was renamed VvMJE1. Under steady-state conditions, VvMJE1 exhibited an apparent Km value of 92.9 μM with MeJA. VvMJE1 was also shown to have lower activity with methyl salicylate (MeSA), another known substrate of the MES family, and only at high concentrations of the substrate. To understand the structural basis of VvMJE1 in discriminating MeJA and MeSA, a homolog model of VvMJE1 was made using the X-ray structure of tobacco SABP2, which encodes for methyl salicylate esterase, as a template. Interestingly, two bulky residues at the binding site and near the surface of tobacco SABP2 are replaced by relatively small residues in VvMJE1. Such a change enables the accommodation of a larger substrate MeJA in VvMJE1. The expression of VvMJE1 was compared in control grape plants and grape plants treated with one of the three stresses: heat, cold and UV-B. While the expression of VvMJE1 was not affected by heat treatment, its expression was significantly up-regulated by cold treatment and UV-B treatment. This result suggests that VvMJE1 has a role in response of grape plants to these two abiotic stresses.