title: Molecular and enzymatic characterizations of novel bifunctional 3beta-hydroxysteroid dehydrogenases/C-4 decarboxylases from Arabidopsis thaliana.
abstract: We have isolated two cDNAs from Arabidopsis thaliana encoding bifunctional 3beta-hydroxysteroid dehydrogenase/C-4 decarboxylases (3betaHSD/D) involved in sterol synthesis, termed At3betaHSD/D1 and At3betaHSD/D2. Transformation of the yeast ergosterol auxotroph erg26 mutant, which lacks 3betaHSD/D activity, with the At3betaHSD/D1 isoform or with At3betaHSD/D2 isoform containing a C-terminal At3betaHSD/D1 endoplasmic reticulum-retrieval sequence restored growth and ergosterol synthesis in erg26. An in vitro enzymatic assay revealed high 3betaHSD/D activity for both isoenzymes in the corresponding microsomal extracts. The two At3betaHSD/D isoenzymes showed similar substrate specificities that required free 3beta-hydroxyl and C-4-carboxyl groups but were quite tolerant in terms of variations of the sterol nucleus and side chain structures. Data obtained with 4alpha-carboxy-cholest-7-en-3beta-ol and its 3alpha-deuterated analog revealed that 3alpha-hydrogen-carbon bond cleavage is not the rate-limiting step of the reaction. In planta reduction on the expression of the 3betaHSD/D gene as a consequence of VIGS-mediated gene silencing in Nicotiana benthamiana led to a substantial accumulation of 3beta-hydroxy-4beta,14-dimethyl-5alpha-ergosta-9beta,19-cyclo-24(24(1))-en-4alpha-carboxylic acid, consistent with a decrease in 3betaHSD/D activity. These two novel oxidative decarboxylases constitute the first molecularly and functionally characterized HSDs from a short chain dehydrogenase/reductase family in plants.
authors.affiliation: Institut de Biologie Moléculaire des Plantes, CNRS UPR2357, 28 Rue Goethe, 67083 Strasbourg Cedex, France. enzymo@bota-ulp.u-strasbg.fr
authors.forename: Alain
authors.initials: A
authors.lastname: Rahier
authors.forename: Sylvain
authors.initials: S
authors.lastname: Darnet
authors.forename: Florence
authors.initials: F
authors.lastname: Bouvier
authors.forename: Bilal
authors.initials: B
authors.lastname: Camara
authors.forename: Martin
authors.initials: M
authors.lastname: Bard
mesh_terms: D015067:17-Hydroxysteroid Dehydrogenases / Q000737:chemistry*; D017360:Arabidopsis / Q000201:enzymology*; D002244:Carbon / Q000737:chemistry; D002262:Carboxy-Lyases / Q000737:chemistry*; D002384:Catalysis; D003001:Cloning, Molecular; D018076:DNA, Complementary / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism; D020868:Gene Silencing; D008956:Models, Chemical; D008969:Molecular Sequence Data; D010641:Phenotype; D010802:Phylogeny; D013261:Sterols / Q000737:chemistry; D014026:Nicotiana / Q000201:enzymology
chemical_list: D018076:DNA, Complementary; D013261:Sterols; D002244:Carbon; D015067:17-Hydroxysteroid Dehydrogenases; C044256:3 (or 17)-beta-hydroxysteroid dehydrogenase; C119754:4alpha-carboxysterol-C3 dehydrogenase; D002262:Carboxy-Lyases