title: Arabidopsis thaliana contains a single gene encoding squalene synthase.
abstract: Squalene synthase (SQS) catalyzes the condensation of two molecules of farnesyl diphosphate (FPP) to produce squalene (SQ), the first committed precursor for sterol, brassinosteroid, and triterpene biosynthesis. Arabidopsis thaliana contains two SQS-annotated genomic sequences, At4g34640 (SQS1) and At4g34650 (SQS2), organized in a tandem array. Here we report that the SQS1 gene is widely expressed in all tissues throughout plant development, whereas SQS2 is primarily expressed in the vascular tissue of leaf and cotyledon petioles, and the hypocotyl of seedlings. Neither the complete A. thaliana SQS2 protein nor the chimeric SQS resulting from the replacement of the 69 C-terminal residues of SQS2 by the 111 C-terminal residues of the Schizosaccharomyces pombe SQS were able to confer ergosterol prototrophy to a Saccharomyces cerevisiae erg9 mutant strain lacking SQS activity. A soluble form of SQS2 expressed in Escherichia coli and purified was unable to synthesize SQ from FPP in the presence of NADPH and either Mg2+ or Mn2+. These results demonstrated that SQS2 has no SQS activity, so that SQS1 is the only functional SQS in A. thaliana. Mutational studies revealed that the lack of SQS activity of SQS2 cannot be exclusively attributed to the presence of an unusual Ser replacing the highly conserved Phe at position 287. Expression of green fluorescent protein (GFP)-tagged versions of SQS1 in onion epidermal cells demonstrated that SQS1 is targeted to the endoplasmic reticulum (ER) membrane and that this location is exclusively dependent on the presence of the SQS1 C-terminal hydrophobic trans-membrane domain.
authors.affiliation: Departament de Bioquímica i Biologia Molecular, Facultat de Farmàcia, Universitat de Barcelona, Avda. Diagonal 643, Barcelona, Spain.
authors.forename: Antoni
authors.initials: A
authors.lastname: Busquets
authors.forename: Verónica
authors.initials: V
authors.lastname: Keim
authors.forename: Marta
authors.initials: M
authors.lastname: Closa
authors.forename: Ana
authors.initials: A
authors.lastname: del Arco
authors.forename: Albert
authors.initials: A
authors.lastname: Boronat
authors.forename: Montserrat
authors.initials: M
authors.lastname: Arró
authors.forename: Albert
authors.initials: A
authors.lastname: Ferrer
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000201:enzymology / Q000235:genetics*; D029681:Arabidopsis Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism; D013186:Farnesyl-Diphosphate Farnesyltransferase / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D020869:Gene Expression Profiling; D005816:Genetic Complementation Test; D049452:Green Fluorescent Proteins / Q000032:analysis; D008969:Molecular Sequence Data; D019697:Onions / Q000235:genetics / Q000648:ultrastructure; D030821:Plants, Genetically Modified / Q000378:metabolism; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D012441:Saccharomyces cerevisiae / Q000235:genetics; D016415:Sequence Alignment
chemical_list: D029681:Arabidopsis Proteins; D049452:Green Fluorescent Proteins; D013186:Farnesyl-Diphosphate Farnesyltransferase