Preparation and Testing of Plant Seed Meal-based Wood Adhesives

Recently, the interest in plant seed meal-based products as wood adhesives has steadily increased, as these plant raw materials are considered renewable ...

United States Department of Agriculture

The seed oil derived from the tung (Aleurites fordii Hemsl.) tree contains approximately 80% alpha-eleostearic acid (18:3delta(9cis,11trans,13trans)), an unusual conjugated fatty acid that imparts industrially important drying qualities to tung oil. Here, we describe the cloning and functional analysis of two closely related Delta(12) oleate desaturase-like enzymes that constitute consecutive steps in the biosynthetic pathway of eleostearic acid. Polymerase chain reaction screening of a tung seed cDNA library using degenerate oligonucleotide primers resulted in identification of two desaturases, FAD2 and FADX, that shared 73% amino acid identity. Both enzymes were localized to the endoplasmic reticulum of tobacco (Nicotiana tabacum cv Bright-Yellow 2) cells, and reverse transcriptase-polymerase chain reaction revealed that FADX was expressed exclusively within developing tung seeds. Expression of the cDNAs encoding these enzymes in yeast (Saccharomyces cerevisiae) revealed that FAD2 converted oleic acid (18:1delta(9cis)) into linoleic acid (18:2delta(9cis,12cis)) and that FADX converted linoleic acid into alpha-eleostearic acid. Additional characterization revealed that FADX exhibited remarkable enzymatic plasticity, capable of generating a variety of alternative conjugated and delta(12)-desaturated fatty acid products in yeast cells cultured in the presence of exogenously supplied fatty acid substrates. Unlike other desaturases reported to date, the double bond introduced by FADX during fatty acid desaturation was in the trans, rather than cis, configuration. Phylogenetic analysis revealed that tung FADX is grouped with delta(12) fatty acid desaturases and hydroxylases rather than conjugases, which is consistent with its desaturase activity. Comparison of FADX and other lipid-modifying enzymes (desaturase, hydroxylase, epoxygenase, acetylenase, and conjugase) revealed several amino acid positions near the active site that may be important determinants of enzymatic activity.

Molecular analysis of a bifunctional fatty acid conjugase/desaturase from tung. Implications for the evolution of plant fatty acid diversity.

Seeds of the tung tree (Vernicia fordii) produce large quantities of triacylglycerols (TAGs) containing approximately 80% eleostearic acid, an unusual conjugated fatty acid. We present a comparative analysis of the genetic, functional, and cellular properties of tung type 1 and type 2 diacylglycerol acyltransferases (DGAT1 and DGAT2), two unrelated enzymes that catalyze the committed step in TAG biosynthesis. We show that both enzymes are encoded by single genes and that DGAT1 is expressed at similar levels in various organs, whereas DGAT2 is strongly induced in developing seeds at the onset of oil biosynthesis. Expression of DGAT1 and DGAT2 in yeast produced different types and proportions of TAGs containing eleostearic acid, with DGAT2 possessing an enhanced propensity for the synthesis of trieleostearin, the main component of tung oil. Both DGAT1 and DGAT2 are located in distinct, dynamic regions of the endoplasmic reticulum (ER), and surprisingly, these regions do not overlap. Furthermore, although both DGAT1 and DGAT2 contain a similar C-terminal pentapeptide ER retrieval motif, this motif alone is not sufficient for their localization to specific regions of the ER. These data suggest that DGAT1 and DGAT2 have nonredundant functions in plants and that the production of storage oils, including those containing unusual fatty acids, occurs in distinct ER subdomains.

Tung tree DGAT1 and DGAT2 have nonredundant functions in triacylglycerol biosynthesis and are localized to different subdomains of the endoplasmic reticulum.

Diacylglycerol acyltransferases (DGATs) catalyze the final and rate-limiting step of triacylglycerol (TAG) biosynthesis in eukaryotic organisms. Database search has identified at least 59 DGAT1 sequences from 48 organisms, but the expression of any DGAT1 as a full-length protein in E. coli had not been reported because DGAT1s are integral membrane proteins and difficult to express and purify. The objective of this study was to establish a procedure for expressing full-length DGAT1 in E. coli.

Expression of tung tree diacylglycerol acyltransferase 1 in E. coli.

Diacylglycerol acyltransferases (DGATs) esterify sn-1,2-diacylglycerol with a long-chain fatty acyl-CoA, the last and rate-limiting step of triacylglycerol (TAG) biosynthesis in eukaryotic organisms. At least 74 DGAT2 sequences from 61 organisms have been identified, but the expression of any DGAT2 as a partial or full-length protein in Escherichia coli had not been reported. The main objective of this study was to express and purify recombinant DGAT2 (rDGAT2) from E. coli for antigen production with a minor objective to compare rDGAT2 expression in yeast. A plasmid was engineered to express tung tree DGAT2 fused to maltose binding protein and poly-histidine (His) affinity tags. Immunoblotting showed that rDGAT2 was detected in the soluble, insoluble, and membrane fractions. The rDGAT2 in the soluble fraction was partially purified by amylose resin, nickel-nitrilotriacetic agarose (Ni-NTA) beads, and tandem affinity chromatography. Multiple proteins co-purified with rDGAT2. Size exclusion chromatography estimated the size of the rDGAT2-enriched fraction to be approximately eight times the monomer size. Affinity-purified rDGAT2 fractions had a yellow tint and contained fatty acids. The rDGAT2 in the insoluble fraction was partially solubilized by seven detergents with SDS being the most effective. Recombinant DGAT2 was purified to near homogeneity by SDS solubilization and Ni-NTA affinity chromatography. Mass spectrometry identified rDGAT2 as a component in the bands corresponding to the monomer and dimer forms as observed by SDS-PAGE. Protein bands with monomer and dimer sizes were also observed in the microsomal membranes of Saccharomyces cerevisiae expressing hemagglutinin-tagged DGAT2. Nonradioactive assay showed TAG synthesis activity of DGAT2 from yeast but not E. coli. The results suggest that rDGAT2 is present as monomer and dimer forms on SDS-PAGE, associated with other proteins, lipids, and membranes, and that post-translational modification of rDGAT2 may be required for its enzymatic activity and/or the E. coli protein is misfolded.

Expression and purification of recombinant tung tree diacylglycerol acyltransferase 2.

Diacylglycerol acyltransferases (DGAT) catalyze the final and rate-limiting step of triacylglycerol (TAG) biosynthesis in eukaryotic organisms. DGAT genes have been identified in numerous organisms. Multiple isoforms of DGAT are present in eukaryotes. We previously cloned DGAT1 and DGAT2 genes of tung tree (Vernicia fordii), whose novel seed TAGs are useful in a wide range of industrial applications. The objective of this study was to understand the developmental regulation of DGAT family gene expression in tung tree. To this end, we first cloned a tung tree gene encoding DGAT3, a putatively soluble form of DGAT that possesses 11 completely conserved amino acid residues shared among 27 DGAT3s from 19 plant species. Unlike DGAT1 and DGAT2 subfamilies, DGAT3 is absent from animals. We then used TaqMan and SYBR Green quantitative real-time PCR, along with northern and western blotting, to study the expression patterns of the three DGAT genes in tung tree tissues. Expression results demonstrate that 1) all three isoforms of DGAT genes are expressed in developing seeds, leaves and flowers; 2) DGAT2 is the major DGAT mRNA in tung seeds, whose expression profile is well-coordinated with the oil profile in developing tung seeds; and 3) DGAT3 is the major form of DGAT mRNA in tung leaves, flowers and immature seeds prior to active tung oil biosynthesis. These results suggest that DGAT2 is probably the major TAG biosynthetic isoform in tung seeds and that DGAT3 gene likely plays a significant role in TAG metabolism in other tissues. Therefore, DGAT2 should be a primary target for tung oil engineering in transgenic organisms.

Dorselyn C. Chapital

Southern Regional Research Center,
Agricultural Research Service,
Southern Regional Research Center, Agricultural Research Service

United States Department of Agriculture

Preparation and Testing of Plant Seed Meal-based Wood Adhesives

Dorselyn C. Chapital

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United States Department of Agriculture

Preparation and Testing of Plant Seed Meal-based Wood Adhesives

Southern Regional Research Center,
Agricultural Research Service,
Southern Regional Research Center, Agricultural Research Service