Structural and dynamic basis of substrate permissiveness in hydroxycinnamoyltransferase (HCT)

Mechanistic basis for the emergence of EPS1 as a catalyst in salicylic acid biosynthesis of Brassicaceae

Salicylic acid (SA) production in Brassicaceae plants is uniquely accelerated from isochorismate by EPS1, a newly identified enzyme in the BAHD acyltransferase family. We present crystal structures of EPS1 from Arabidopsis thaliana in both its apo and substrate-analog-bound forms. Integrating microsecond-scale molecular dynamics simulations with quantum mechanical cluster modeling, we propose a pericyclic rearrangement lyase mechanism for EPS1. We further reconstitute the isochorismate-derived SA biosynthesis pathway in Saccharomyces cerevisiae, establishing an in vivo platform to examine the impact of active-site residues on EPS1 functionality. Moreover, stable transgenic expression of EPS1 in soybean increases basal SA levels, highlighting the enzyme's potential to enhance defense mechanisms in non-Brassicaceae plants lacking an EPS1 ortholog. Our findings illustrate the evolutionary adaptation of an ancestral enzyme's active site to enable a novel catalytic mechanism that boosts SA production in Brassicaceae plants.

Genome-wide association identifies a BAHD acyltransferase activity that assembles an ester of glucuronosylglycerol and phenylacetic acid

Genome-wide association studies (GWAS) are an effective approach to identify new specialized metabolites and the genes involved in their biosynthesis and regulation. In this study, GWAS of Arabidopsis thaliana soluble leaf and stem metabolites identified alleles of an uncharacterized BAHD-family acyltransferase (AT5G57840) associated with natural variation in three structurally related metabolites. These metabolites were esters of glucuronosylglycerol, with one metabolite containing phenylacetic acid as the acyl component of the ester. Knockout and overexpression of AT5G57840 in Arabidopsis and heterologous overexpression in Nicotiana benthamiana and Escherichia coli demonstrated that it is capable of utilizing phenylacetyl-CoA as an acyl donor and glucuronosylglycerol as an acyl acceptor. We, thus, named the protein Glucuronosylglycerol Ester Synthase (GGES). Additionally, phenylacetyl glucuronosylglycerol increased in Arabidopsis CYP79A2 mutants that overproduce phenylacetic acid and was lost in knockout mutants of UDP-sulfoquinovosyl: diacylglycerol sulfoquinovosyl transferase, an enzyme required for glucuronosylglycerol biosynthesis and associated with glycerolipid metabolism under phosphate-starvation stress. GGES is a member of a well-supported clade of BAHD family acyltransferases that arose by duplication and neofunctionalized during the evolution of the Brassicales within a larger clade that includes HCT as well as enzymes that synthesize other plant-specialized metabolites. Together, this work extends our understanding of the catalytic diversity of BAHD acyltransferases and uncovers a pathway that involves contributions from both phenylalanine and lipid metabolism.

Uncovering the Role of Hydroxycinnamoyl Transferase in Boosting Chlorogenic Acid Accumulation in Carthamus tinctorius Cells under Methyl Jasmonate Elicitation

Chlorogenic acids (CGAs) are bioactive compounds widely used in the food, pharmaceutical, and cosmetic industries. Carthamus tinctorius is an important economic crop, and its suspension cells are rich in CGAs. However, little is known about the biosynthesis and regulation of CGAs in Carthamus tinctorius cells. This study first elucidated the regulatory mechanism of CGA biosynthesis in methyl jasmonate (MeJA)-treated Carthamus tinctorius cells and the role of the MeJA-responsive hydroxycinnamoyl transferase (HCT) gene in enhancing their CGA accumulation. Firstly, temporal changes in intracellular metabolites showed that MeJA increased the intracellular CGA content up to 1.61-fold to 100.23 mg·g-1. Meanwhile, 31 primary metabolites showed significant differences, with 6 precursors related to increasing CGA biosynthesis. Secondly, the transcriptome data revealed 3637 new genes previously unannotated in the Carthamus tinctorius genome and 3653 differentially expressed genes. The genes involved in the plant signaling pathway and the biosynthesis of CGAs and their precursors showed a general up-regulation, especially the HCT gene family, which ultimately promoted CGA biosynthesis. Thirdly, the expression of a newly annotated and MeJA-responsive HCT gene (CtHCT, CtNewGene_3476) was demonstrated to be positively correlated with CGA accumulation in the cells, and transient overexpression of CtHCT enhanced CGA accumulation in tobacco. Finally, in vitro catalysis kinetics and molecular docking simulations revealed the ability and mechanism of the CtHCT protein to bind to various substrates and catalyze the formation of four hydroxycinnamic esters, including CGAs. These findings strengthened our understanding of the regulatory mechanism of CGA biosynthesis, thereby providing theoretical support for the efficient production of CGAs.

Analysing a Group of Homologous BAHD Enzymes Provides Insights into the Evolutionary Transition of Rosmarinic Acid Synthases from Hydroxycinnamoyl-CoA:Shikimate/Quinate Hydroxycinnamoyl Transferases

The interplay of various enzymes and compounds gives rise to the intricate secondary metabolic networks observed today. However, the current understanding of their formation and expansion remains limited. BAHD acyltransferases play important roles in the biosynthesis of numerous significant secondary metabolites. In plants, they are widely distributed and exhibit a diverse range of activities. Among them, rosmarinic acid synthase (RAS) and hydroxycinnamoyl-CoA:shikimate/quinate hydroxycinnamoyl transferase (HCT) have gained significant recognition and have been extensively investigated as prominent members of the BAHD acyltransferase family. Here, we conducted a comprehensive study on a unique group of RAS homologous enzymes in Mentha longifolia that display both catalytic activities and molecular features similar to HCT and Lamiaceae RAS. Subsequent phylogenetic and comparative genome analyses revealed their derivation from expansion events within the HCT gene family, indicating their potential as collateral branches along the evolutionary trajectory, leading to Lamiaceae RAS while still retaining certain ancestral vestiges. This discovery provides more detailed insights into the evolution from HCT to RAS. Our collective findings indicate that gene duplication is the driving force behind the observed evolutionary pattern in plant-specialized enzymes, which probably originated from ancestral enzyme promiscuity and were subsequently shaped by principles of biological adaptation.

Family characteristics, phylogenetic reconstruction, and potential applications of the plant BAHD acyltransferase family

The BAHD acyltransferase family is a class of proteins in plants that can acylate a variety of primary and specialized secondary metabolites. The typically acylated products have greatly improved stability, lipid solubility, and bioavailability and thus show significant differences in their physicochemical properties and pharmacological activities. Here, we review the protein structure, catalytic mechanism, and phylogenetic reconstruction of plant BAHD acyltransferases to describe their family characteristics, acylation reactions, and the processes of potential functional differentiation. Moreover, the potential applications of the BAHD family in human activities are discussed from the perspectives of improving the quality of economic plants, enhancing the efficacy of medicinal plants, improving plant biomass for use in biofuel, and promoting stress resistance of land plants. This review provides a reference for the research and production of plant BAHD acyltransferases.

BAHD acyltransferase induced by histone deacetylase inhibitor catalyzes 3-O-hydroxycinnamoylquinic acid formation in bamboo cells

Suspension-cultured cells of a bamboo species (Bambusa multiplex; Bm) produce 3-O-feruloylquinic acid (3-FQA) and 3-O-p-coumaroylquinic acid (3-pCQA) by treatment with the histone deacetylase inhibitor suberoyl bis-hydroxamic acid (SBHA). Acyltransferases catalyzing the formation of 5-O-hydroxycinnamoylquinic acid esters by transesterification from hydroxycinnamoyl-CoAs to the C-5 hydroxy group of quinic acid (hydroxycinnamoyl-CoA:quinate hydroxycinnamoyltransferase, HQT) have been identified in the biosynthesis of chlorogenic acids and monolignols; however, an HQT that catalyzes the acylation of the C-3 hydroxy group of quinic acid has not been identified previously. In the present study, we purified a native HQT from SBHA-treated Bm cells. The purified enzyme preferentially accepted feruloyl-/p-coumaroyl-CoAs as acyl-donors and quinic acid as the acyl-acceptor, and the enzyme specifically formed 3-FQA and 3-pCQA but not 5-O-hydroxycinnamoylquinic acid esters or esters with shikimic acid. A cDNA (BmHQT1) encoding this HQT was isolated. Although BmHQT1 is a phylogenetically unique member of the BAHD acyltransferase superfamily that does not cluster with other HQTs, functional characterization of the recombinant enzyme verified that BmHQT1 catalyzes the regiospecific formation of 3-O-hydroxycinnamoylquinic acid esters. Transcript levels of BmHQT1 markedly increased in Bm cells cultured in the presence of SBHA. Moreover, elevated acetylation levels of histone H3 were observed in the coding region of BmHQT1 in the presence of SBHA, indicating that the induced accumulation of 3-FQA/3-pCQA by SBHA is caused by transcriptional activation of BmHQT1 by the action of SBHA as a histone deacetylase inhibitor. The results demonstrate the utility of HDAC inhibitors for discovery of cryptic secondary metabolites and unknown biosynthetic enzymes.

Hydroxycinnamoyltransferase and CYP98 in phenolic metabolism in the rosmarinic acid-producing hornwort Anthoceros agrestis

MAIN CONCLUSION

Anthoceros agrestis hydroxycinnamoyltransferase accepts shikimic and 3-hydroxyanthranilic acids while hydroxycinnamoylester/amide 3-hydroxylase (CYP98A147) preferred p-coumaroyl-(3-hydroxy)anthranilic acid compared to the shikimic acid derivative. Alternative pathways towards rosmarinic acid have to be considered. Rosmarinic acid (RA) is a well-known ester of caffeic acid and 3,4-dihydroxyphenyllactic acid. In the search for enzymes involved in RA biosynthesis in the hornwort Anthoceros agrestis, the hydroxycinnamoyltransferase sequence with the highest similarity to rosmarinic acid synthase from Lamiaceae has been amplified and heterologously expressed in Escherichia coli. In parallel, the single cytochrome P450 sequence belonging to the CYP98 group in Anthoceros agrestis was isolated and expressed in Saccharomyces cerevisiae which did not result in protein formation. Codon optimization and co-expression with NADPH:cytochrome P450 reductase (CPR) from Coleus blumei resulted in the formation of active enzymes. Both, the hydroxycinnamoyltransferase and CYP98 were characterized with respect to their temperature and pH optimum as well as their substrate acceptance. The hydroxycinnamoyltransferase (AaHCT6) readily accepted p-coumaroyl- and caffeoyl-CoA with a slightly higher affinity towards p-coumaroyl-CoA. The best acceptor substrate was shikimic acid (Km 25 µM with p-coumaroyl-CoA) followed by 3-hydroxyanthranilic acid (Km 153 µM with p-coumaroyl-CoA). Another accepted substrate was 2,3-dihydroxybenzoic acid. Anthranilic acid and 4-hydroxyphenyllactic acid (as precursor for RA) were not used as substrates. p-Coumaroylesters and -amides are substrates hydroxylated by CYP98 hydroxylases. The only CYP98 sequence from Anthoceros agrestis is CYP98A147. The best substrates for the NADPH-dependent hydroxylation were p-coumaroylanthranilic and p-coumaroyl-3-hydroxyanthranilic acids while p-coumaroylshikimic and p-coumaroyl-4-hydroxyphenyllactic acids were poor substrates. The biosynthetic pathway towards rosmarinic acid thus still remains open and other enzyme classes as well as an earlier introduction of the 3-hydroxyl group to afford the caffeic acid substitution pattern must be taken into consideration.

Crowding within synaptic junctions influences the degradation of nucleotides by CD39 and CD73 ectonucleotidases

Synapsed cells can communicate using exocytosed nucleotides like adenosine triphosphate (ATP). Ectonucleotidases localized to synaptic junctions degrade nucleotides into metabolites like adenosine monophosphate (AMP) or adenosine. Oftentimes nucleotide degradation occurs in a sequential manner, of which ATP degradation by CD39 and CD73 is a representative example. Here, CD39 first converts ATP and adenosine diphosphate (ADP) into AMP, after which AMP is dephosphorylated into adenosine by CD73. Hence, the concerted activity of CD39 and CD73 can help shape cellular responses to extracellular ATP. In a previous study, we demonstrated that coupled CD39 and CD73 activity within synapse-like junctions is strongly controlled by the enzymes' co-localization, their surface charge densities, and the electrostatic potential of the surrounding cell membranes. In this study, we demonstrate that crowders within synaptic junctions, which can include globular proteins like cytokines and membrane-bound proteins, impact coupled CD39 and CD73 ectonucleotidase activity and, in turn, the availability of intrasynapse ATP. Specifically, we developed a spatially explicit, reaction-diffusion model for the coupled conversion of ATP → AMP and AMP → adenosine in a model synaptic junction with crowders that is solved via the finite element method. Our modeling results suggest that the association rate for ATP to CD39 is strongly influenced by the density of intrasynaptic protein crowders, as increasing crowder density generally suppressed ATP association kinetics. Much of this suppression can be rationalized based on a loss of configurational entropy. The surface charges of crowders can further influence the association rate, with the surprising result that favorable crowder-nucleotide electrostatic interactions can yield CD39 association rates that are faster than crowder-free configurations. However, attractive crowder-nucleotide interactions decrease the rate and efficiency of adenosine production, which in turn increases the availability of ATP and AMP within the synapse relative to crowder-free configurations. These findings highlight how CD39 and CD73 ectonucleotidase activity, electrostatics, and crowding within synapses influence the availability of nucleotides for intercellular communication.

Characterization of evolutionarily distinct rice BAHD-Acyltransferases provides insight into their plausible role in rice susceptibility to Rhizoctonia solani

Plants produce diverse secondary metabolites in response to different environmental cues including pathogens. The modification of secondary metabolites, including acylation, modulates their biological activity, stability, transport, and localization. A plant-specific BAHD-acyltransferase (BAHD-AT) gene family members catalyze the acylation of secondary metabolites. Here we characterized the rice (Oryza sativa L.) BAHD-ATs at the genome-wide level and endeavor to define their plausible role in the tolerance against Rhizoctonia solani AG1-IA. We identified a total of 85 rice OsBAHD-AT genes and classified them into five canonical clades based on their phylogenetic relationship with characterized BAHD-ATs from other plant species. The time-course RNA sequencing (RNA-seq) analysis of OsBAHD-AT genes and qualitative real-time polymerase chain reaction (qRT-PCR) validation showed higher expression in sheath blight susceptible rice genotype. Furthermore, the DNA methylation analysis revealed higher hypomethylation of OsBAHD-AT genes that corresponds to their higher expression in susceptible rice genotype, indicating epigenetic regulation of OsBAHD-AT genes in response to R. solani AG1-IA inoculation. The results shown here indicate that BAHD-ATs may have a negative role in rice tolerance against R. solani AG1-IA possibly mediated through the brassinosteroid (BR) signaling pathway. Altogether, the present analysis suggests the putative functions of several OsBAHD-AT genes, which will provide a blueprint for their functional characterization and to understand the rice-R. solani AG1-IA interaction.

Molecular and structural characterization of agmatine coumaroyltransferase in Triticeae, the key regulator of hydroxycinnamic acid amide accumulation

Hydroxycinnamic acid amides (HCAAs) are involved in stress-induced defense in many plant species. Barley accumulates high concentrations of HCAAs irrespective of exogenous stressors, while other major cereals such as wheat and rice accumulate relatively low levels of HCAAs in intact tissues. The primary HCAA species in barley are biosynthesized by agmatine p-coumaroyltransferase (ACT), an N-acyltransferase of the BAHD superfamily. However, the molecular basis underlying barley's uniquely high HCAA accumulation has not been elucidated, and information regarding the structural details of BAHD N-acyltransferases is limited. Hence, we aimed to investigate the ACTs of family Poaceae. We isolated ACT (-like) genes, including those previously undescribed, and investigated their enzymatic and genetic features. All the identified enzymes belonged to clade IVa of the BAHD superfamily. The barley and wheat ACTs were further categorized, based on catalytic properties and primary structures, into ACT1 and ACT2 groups, the encoding loci of which are neighbors on the same chromosome. While all ACTs exhibited similar K_m values for CoA-thioesters (acyl-group donors), members of the ACT1 group showed a distinctly higher affinity for agmatine (acyl-acceptor). Among the ACTs tested, an ACT isozyme in barley (HvACT1-1) showed the highest catalytic efficiency and transcript level, indicating that ACT regulates high-level HCAA accumulation in barley. For further enzymatic characterization of the ACTs, we crystalized wheat ACT2 (TaACT2) and determined its structure at 2.3 Å resolution. Structural alignment of TaACT2 and HvACT1-1 showed that the architectures of the substrate binding pockets were well conserved. However, the structure of a loop located at the entrance to acyl-acceptor binding site may be more flexible in TaACT2, which could be responsible for the lower affinity of TaACT2 to agmatine. Mutations of HvACT1-1 at Glu372 and Asp374 within one of the clade-IV specific motifs facing the deduced acyl-acceptor binding pocket caused significant catalytic deterioration toward agmatine both in K_m and k_cat, suggesting their key roles in acyl acceptor binding by the clade-IV enzymes. This study elucidated the molecular basis of how plants accumulate defensive specialized metabolites and provided insights into developing efficient and eco-friendly agricultural methods.

Genome-wide analysis of the lignin toolbox for morus and the roles of lignin related genes in response to zinc stress

Mulberry (Morus, Moraceae) is an important economic plant with nutritional, medicinal, and ecological values. Lignin in mulberry can affect the quality of forage and the saccharification efficiency of mulberry twigs. The availability of the Morus notabilis genome makes it possible to perform a systematic analysis of the genes encoding the 11 protein families specific to the lignin branch of the phenylpropanoid pathway, providing the core genes for the lignin toolbox in mulberry. We performed genome-wide screening, which was combined with de novo transcriptome data for Morus notabilis and Morus alba variety Fengchi, to identify putative members of the lignin gene families followed by phylogenetic and expression profile analyses. We focused on bona fide clade genes and their response to zinc stress were further distinguished based on expression profiles using RNA-seq and RT-qPCR. We finally identified 31 bona fide genes in Morus notabilis and 25 bona fide genes in Fengchi. The putative function of these bona fide genes was proposed, and a lignin toolbox that comprised 19 genes in mulberry was provided, which will be convenient for researchers to explore and modify the monolignol biosynthesis pathway in mulberry. We also observed changes in the expression of some of these lignin biosynthetic genes in response to stress caused by excess zinc in Fengchi and proposed that the enhanced lignin biosynthesis in lignified organs and inhibition of lignin biosynthesis in leaf is an important response to zinc stress in mulberry.

Function of the HYDROXYCINNAMOYL-CoA:SHIKIMATE HYDROXYCINNAMOYL TRANSFERASE is evolutionarily conserved in embryophytes

The plant phenylpropanoid pathway generates a major class of specialized metabolites and precursors of essential extracellular polymers that initially appeared upon plant terrestrialization. Despite its evolutionary significance, little is known about the complexity and function of this major metabolic pathway in extant bryophytes, which represent the non-vascular stage of embryophyte evolution. Here, we report that the HYDROXYCINNAMOYL-CoA:SHIKIMATE HYDROXYCINNAMOYL TRANSFERASE (HCT) gene, which plays a critical function in the phenylpropanoid pathway during seed plant development, is functionally conserved in Physcomitrium patens (Physcomitrella), in the moss lineage of bryophytes. Phylogenetic analysis indicates that bona fide HCT function emerged in the progenitor of embryophytes. In vitro enzyme assays, moss phenolic pathway reconstitution in yeast and in planta gene inactivation coupled to targeted metabolic profiling, collectively indicate that P. patens HCT (PpHCT), similar to tracheophyte HCT orthologs, uses shikimate as a native acyl acceptor to produce a p-coumaroyl-5-O-shikimate intermediate. Phenotypic and metabolic analyses of loss-of-function mutants show that PpHCT is necessary for the production of caffeate derivatives, including previously reported caffeoyl-threonate esters, and for the formation of an intact cuticle. Deep conservation of HCT function in embryophytes is further suggested by the ability of HCT genes from P. patens and the liverwort Marchantia polymorpha to complement an Arabidopsis thaliana CRISPR/Cas9 hct mutant, and by the presence of phenolic esters of shikimate in representative species of the three bryophyte lineages.

Anthocyanin 5,3'-aromatic acyltransferase from Gentiana triflora, a structural insight into biosynthesis of a blue anthocyanin

The acylation of anthocyanins contributes to their structural diversity. Aromatic acylation is responsible for the blue color of anthocyanins and certain flowers. Aromatic acyltransferase from Gentiana triflora Pall. (Gentianaceae) (Gt5,3'AT) catalyzes the acylation of glucosyl moieties at the 5 and 3' positions of anthocyanins. Anthocyanin acyltransferase transfers an acyl group to a single position, such that Gt5,3'AT possesses a unique enzymatic activity. Structural investigation of this aromatic acyl group transfer is fundamental to understand the molecular mechanism of the acylation of double positions. In this study, structural analyses of Gt5,3'AT were conducted to identify the underlying mechanism. The crystal structure indicated that Gt5,3'AT shares structural similarities with other BAHD family enzymes, consisting of N and C terminal lobes. Structural comparison revealed that acyl group preference (aromatic or aliphatic) for the enzymes was determined by four amino acid positions, which are well conserved in aromatic and aliphatic CoA-binding acyltransferases. Although a complex structure with anthocyanins was not obtained, the binding of delphinidin 3,5,3'-triglucoside to Gt5,3'AT was investigated by evaluating the molecular dynamics. The simulation indicated that acyl transfer by Gt5,3'AT preferentially occurs at the 5-position rather than at the 3'-position, with interacting amino acids that are mainly located in the C-terminal lobe. Subsequent assays of chimeric enzymes (exchange of the N-terminal lobe and the C-terminal lobe between Gt5,3'AT and lisianthus anthocyanin 5AT) demonstrated that acyl transfer selectivity may be caused by the C-terminal lobe.

In vitro inhibition of shikimate hydroxycinnamoyltransferase by acibenzolar acid, the first metabolite of the plant defence inducer acibenzolar-S-methyl

Acibenzolar acid, the first metabolite formed in planta from the defence inducer acibenzolar-S-methyl (ASM), has been shown to be an inhibitor of the enzyme shikimate hydroxycinnamoyltransferase (HST), extracted from grapevine or tobacco cell suspension cultures. Using a purified recombinant Arabidopsis thaliana HST, the inhibition was found to be competitive, acibenzolar acid binding reversibly to the shikimate binding site of the HST:p-coumaroyl-CoA complex, with a Ki value of 250 μM. The other hydroxycinnamoyltransferases tested in the course of this study, using either hydroxypalmitic acid, putrescine, tyramine, or quinic acid as acyl acceptors were not, or only slightly, inhibited by acibenzolar acid. To understand the specificity of the interaction of acibenzolar acid with HST, we analyzed the structure-activity relationship of a series of benzoic or acibenzolar acid analogues, tested either as AtHST substrates or as inhibitors. This analysis confirmed previously published data on the substrate flexibility of HST and demonstrated that both the carboxyl group and the thiadiazole moiety of acibenzolar acid are playing an important role in the interaction with the shikimate binding site. Acibenzolar acid, which cannot form an ester bond with p-coumaric acid, was however a less potent inhibitor than protocatechuic or 3-hydroxybenzoic acids, which are used as acyl acceptors by HST. Our results show that the interaction of acibenzolar acid with HST, which is probably directly linked to the substrate promiscuity of HST, is unlikely to play a direct role in the defence-inducing properties of ASM in plants.

Parameterization of a drug molecule with a halogen σ-hole particle using ffTK: Implementation, testing, and comparison

Halogen atoms are widely used in drug molecules to improve their binding affinity for the receptor proteins. Many of the examples involve "halogen bonding" between the molecule and the binding site, which is a directional interaction between a halogen atom and a nucleophilic atom. Such an interaction is induced by an electron cloud shift of the halogen atom toward its covalently bonded neighbor to form the σ-bond, leaving a small electrostatic positive region opposite to the bond called the "σ-hole." To mimic the effect of the σ-hole in the CHARMM non-polarizable force field, recently CGenFF added a positively charged massless particle to halogen atoms, positioned at the opposite side of the carbon-halogen bond. This particle is referred to as a lone pair (LP) particle because it uses the lone pair implementation in the CHARMM force field. Here, we have added support for LP particles to ffTK, an automated force field parameterization toolkit widely distributed as a plugin to the molecular visualization software VMD. We demonstrate the updated optimization process using an example halogenated drug molecule, AT130, which is a capsid assembly modulator targeting the hepatitis B virus. Our results indicate that parameterization with the LP particle significantly improves the accuracy of the electrostatic response of the molecule, especially around the halogen atom. Although the inclusion of the LP particle does not produce a prominent effect on the interactions between the molecule and its target protein, the protein-ligand binding performance is greatly improved by optimization of the parameters.

Characterization and functional analysis of the Hydroxycinnamoyl-CoA: shikimate hydroxycinnamoyl transferase (HCT) gene family in poplar

Hydroxycinnamoyl-CoA: shikimate hydroxycinnamoyl transferase (HCT) divides the mass flux to H, G and S units in monolignol biosynthesis and affects lignin content. Ten HCT homologs were identified in the Populus trichocarpa (Torr. & Gray) genome. Both genome duplication and tandem duplication resulted in the expansion of HCT orthologs in Populus. Comprehensive analysis including motif analysis, phylogenetic analysis, expression profiles and co-expression analysis revealed the divergence and putative function of these candidate PoptrHCTs. PoptrHCT1 and 2 were identified as likely involved in lignin biosynthesis. PoptrHCT9 and 10- are likely to be involved in plant development and the response to cold stress. Similar functional divergence was also identified in Populus tomentosa Carr. Enzymatic assay of PtoHCT1 showed that PtoHCT1 was able to synthesize caffeoyl shikimate using caffeoyl-CoA and shikimic acid as substrates.

Cell-Free Expression of a Plant Membrane Protein BrPT2 From Boesenbergia Rotunda

Prenylation of aromatic natural products by membrane-bound prenyltransferases (PTs) is an important biosynthesis step of many bioactive compounds. At present, only a few plant flavonoid-related PT genes have been functionally characterized, mainly due to the difficulties of expressing these membrane proteins. Rapid and effective methods to produce functional plant membrane proteins are thus indispensable. Here, we evaluated expression systems through cell-based and cell-free approaches to express Boesenbergia rotunda BrPT2 encoding a membrane-bound prenyltransferase. We attempted to express BrPT2 in Escherichia coli and tobacco plants but failed to detect this protein using the Western-blot technique, whereas an intact single band of 43 kDa was detected when BrPT2 was expressed using a cell-free protein synthesis system (PURE). Under in vitro enzymatic condition, the synthesized BrPT2 successfully catalyzed pinostrobin chalcone to pinostrobin. Molecular docking analysis showed that pinostrobin chalcone interacts with BrPT2 at two cavities: (1) the main binding site at the central cavity and (2) the allosteric binding site located away from the central cavity. Our findings suggest that cell-free protein synthesis could be an alternative for rapid production of valuable difficult-to-express membrane proteins.

Crystal structure of barley agmatine coumaroyltransferase, an N-acyltransferase from the BAHD superfamily

The enzymes of the BAHD superfamily, a large group of acyl-CoA-dependent acyltransferases in plants, are involved in the biosynthesis of diverse secondary metabolites. While the structures of several O-acyltransferases from the BAHD superfamily, such as hydroxycinnamoyl-CoA shikimate hydroxycinnamoyl transferase, have been elucidated, no structural information on N-acyltransferases is available. Hordeum vulgare agmatine coumaroyltransferase (HvACT) is an N-acyltransferase from the BAHD superfamily and is one of the most important enzymes in the secondary metabolism of barley. Here, an apo-form structure of HvACT is reported as the first structure of an N-acyltransferase from the BAHD superfamily. HvACT crystals diffracted to 1.8 Å resolution and belonged to the monoclinic space group P2₁, with unit-cell parameters a = 57.6, b = 59.5, c = 73.6 Å, α = 90, β = 91.3 , γ = 90°. Like other known BAHD superfamily structures, HvACT contains two domains that adopt a two-layer αβ-sandwich architecture and a solvent-exposed channel that penetrates the enzyme core.

Structural and Biochemical Insights Into Two BAHD Acyltransferases (AtSHT and AtSDT) Involved in Phenolamide Biosynthesis

Phenolamides represent one of the largest classes of plant-specialized secondary metabolites and function in diverse physiological processes, including defense responses and development. The biosynthesis of phenolamides requires the BAHD-family acyltransferases, which transfer acyl-groups from different acyl-donors specifically to amines, the acyl-group acceptors. However, the mechanisms of substrate specificity and multisite-acylation of the BAHD-family acyltransferases remain poorly understood. In this study, we provide a structural and biochemical analysis of AtSHT and AtSDT, two representative BAHD-family members that catalyze the multisite acylation of spermidine but show different product profiles. By determining the structures of AtSHT and AtSDT and using structure-based mutagenesis, we identified the residues important for substrate recognition in AtSHT and AtSDT and hypothesized that the acyl acceptor spermidine might adopt a free-rotating conformation in AtSHT, which can undergo mono-, di-, or tri-acylation; while the spermidine molecule in AtSDT might adopt a linear conformation, which only allows mono- or di-acylation to take place. In addition, through sequence similarity network (SSN) and structural modeling analysis, we successfully predicted and verified the functions of two uncharacterized Arabidopsis BAHD acyltransferases, OAO95042.1 and NP_190301.2, which use putrescine as the main acyl-acceptor. Our work provides not only an excellent starting point for understanding multisite acylation in BAHD-family enzymes, but also a feasible methodology for predicting possible acyl acceptor specificity of uncharacterized BAHD-family acyltransferases.