Effect of the ferredoxin electron donor on sunflower (Helianthus annuus) desaturases

Lipidomic Analysis of Arabidopsis T-DNA Insertion Lines Leads to Identification and Characterization of C-Terminal Alterations in FATTY ACID DESATURASE 6

Mass-spectrometry-based screening of lipid extracts of wounded and unwounded leaves from a collection of 364 Arabidopsis thaliana T-DNA insertion lines produced lipid profiles that were scored on the number and significance of their differences from the leaf lipid profiles of wild-type plants. The analysis identified Salk_109175C, which displayed alterations in leaf chloroplast glycerolipid composition, including a decreased ratio between two monogalactosyldiacylglycerol (MGDG) molecular species, MGDG(18:3/16:3) and MGDG(18:3/18:3). Salk_109175C has a confirmed insertion in the At5g64790 locus; the insertion did not co-segregate with the recessive lipid phenotype in the F2 generation of a wild-type (Columbia-0) × Salk_109175C cross. The altered lipid compositional phenotype mapped to the At4g30950 locus, which encodes the plastidial ω-6 desaturase FATTY ACID DESATURASE 6 (FAD6). Sequencing revealed a splice-site mutation, leading to the in-frame deletion of 13 amino acids near the C-terminal end of the 448 amino acid protein. Heterologous expression in yeast showed that this deletion eliminates desaturase activity and reduces protein stability. Sequence comparison across species revealed that several amino acids within the deletion are conserved in plants and cyanobacteria. Individual point mutations in four conserved residues resulted in 77-97% reductions in desaturase activity, while a construct with all four alanine substitutions lacked activity. The data suggest that the deleted region of FAD6, which is on the C-terminal side of the four putative transmembrane segments and the histidine boxes putatively involved in catalysis, is critical for FAD6 function.

Ectopically expressed sweet pepper ferredoxin PFLP enhances disease resistance to Pectobacterium carotovorum subsp. carotovorum affected by harpin and protease-mediated hypersensitive response in Arabidopsis

Plant ferredoxin-like protein (PFLP) is a photosynthesis-type ferredoxin (Fd) found in sweet pepper. It contains an iron-sulphur cluster that receives and delivers electrons between enzymes involved in many fundamental metabolic processes. It has been demonstrated that transgenic plants overexpressing PFLP show a high resistance to many bacterial pathogens, although the mechanism remains unclear. In this investigation, the PFLP gene was transferred into Arabidopsis and its defective derivatives, such as npr1 (nonexpresser of pathogenesis-related gene 1) and eds1 (enhanced disease susceptibility 1) mutants and NAHG-transgenic plants. These transgenic plants were then infected with the soft-rot bacterial pathogen Pectobacterium carotovorum subsp. carotovorum (Erwinia carotovora ssp. carotovora, ECC) to investigate the mechanism behind PFLP-mediated resistance. The results revealed that, instead of showing soft-rot symptoms, ECC activated hypersensitive response (HR)-associated events, such as the accumulation of hydrogen peroxide (H2 O2 ), electrical conductivity leakage and expression of the HR marker genes (ATHSR2 and ATHSR3) in PFLP-transgenic Arabidopsis. This PFLP-mediated resistance could be abolished by inhibitors, such as diphenylene iodonium (DPI), 1-l-trans-epoxysuccinyl-leucylamido-(4-guanidino)-butane (E64) and benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (z-VAD-fmk), but not by myriocin and fumonisin. The PFLP-transgenic plants were resistant to ECC, but not to its harpin mutant strain ECCAC5082. In the npr1 mutant and NAHG-transgenic Arabidopsis, but not in the eds1 mutant, overexpression of the PFLP gene increased resistance to ECC. Based on these results, we suggest that transgenic Arabidopsis contains high levels of ectopic PFLP; this may lead to the recognition of the harpin and to the activation of the HR and other resistance mechanisms, and is dependent on the protease-mediated pathway.

Overexpression of membrane proteins from higher eukaryotes in yeasts

Heterologous expression and characterisation of the membrane proteins of higher eukaryotes is of paramount interest in fundamental and applied research. Due to the rather simple and well-established methods for their genetic modification and cultivation, yeast cells are attractive host systems for recombinant protein production. This review provides an overview on the remarkable progress, and discusses pitfalls, in applying various yeast host strains for high-level expression of eukaryotic membrane proteins. In contrast to the cell lines of higher eukaryotes, yeasts permit efficient library screening methods. Modified yeasts are used as high-throughput screening tools for heterologous membrane protein functions or as benchmark for analysing drug-target relationships, e.g., by using yeasts as sensors. Furthermore, yeasts are powerful hosts for revealing interactions stabilising and/or activating membrane proteins. We also discuss the stress responses of yeasts upon heterologous expression of membrane proteins. Through co-expression of chaperones and/or optimising yeast cultivation and expression strategies, yield-optimised hosts have been created for membrane protein crystallography or efficient whole-cell production of fine chemicals.

The sunflower plastidial omega3-fatty acid desaturase (HaFAD7) contains the signalling determinants required for targeting to, and retention in, the endoplasmic reticulum membrane in yeast but requires co-expressed ferredoxin for activity

Although plant plastidial omega3-desaturases are closely related to microsomal desaturases, heterologous expression in yeast of the Helianthus annuus FAD7 omega3-desaturase showed low activity in contrast to similar expression of microsomal FAD3 omega3-desaturases. However, the removal of the plastidial transit peptide and the incorporation of a KKNL motif to the C-terminus of HaFAD7 increased the activity by 10-fold compared to the native protein. N-terminal fusion of transmembrane-domains from either the yeast microsomal ELO3, (a type III signal anchor domain), or FAE1, an endoplasmic reticulum membrane anchoring domain, resulted in moderate increases in enzyme activity (5- and 7-fold, respectively), suggesting that the first, most hydrophobic transmembrane domain of HaFAD7 is sufficient to direct targeting to, and insertion into, the endoplasmic reticulum membrane. Furthermore, fusing a hemagglutinin (HA) epitope tag upstream of an endogenous C-terminal KEK motif resulted in a significant loss of activity compared to the un-tagged construct, indicating that the endogenous KEK C-terminal di-lysine motif is capable of directing in yeast the ER-retention of this normally plastidial-located protein. Western blotting analysis of constructs with internal HA epitope revealed that in whole cell extracts, with the exception of the one bound to C-terminal, it did not display a reduced level of protein accumulation. Whilst ferredoxin was shown to be required for HaFAD7 activity in yeast, it appears not necessary for protein stability and accumulation of this plastidial desaturase in the endoplasmic reticulum.