Mutagenesis of the bovSERPINA3-3 demonstrates the requirement of aspartate-371 for intermolecular interaction and formation of dimers

The family of serpins is known to fold into a metastable state that is required for the proteinase inhibition mechanism. One of the consequences of this conformational flexibility is the tendency of some mutated serpins to form polymers, which occur through the insertion of the reactive center loop of one serpin molecule into the A-sheet of another. This "A-sheet polymerization" has remained an attractive explanation for the molecular mechanism of serpinopathies. Polymerization of serpins can also take place in vitro under certain conditions (e.g., pH or temperature). Surprisingly, on sodium dodecyl sulfate/polyacrylamide gel electrophoresis, bovSERPINA3-3 extracted from skeletal muscle or expressed in Escherichia coli was mainly observed as a homodimer. Here, in this report, by site-directed mutagenesis of recombinant bovSERPINA3-3, with substitution D371A, we demonstrate the importance of D371 for the intermolecular linkage observed in denaturing and reducing conditions. This residue influences the electrophoretic and conformational properties of bovSERPINA3-3. By structural modeling of mature bovSERPINA3-3, we propose a new "non-A-sheet swap" model of serpin homodimer in which D371 is involved at the molecular interface.

Three bovine alpha2-fucosyltransferase genes encode enzymes that preferentially transfer fucose on Galbeta1-3GalNAc acceptor substrates

To investigate the synthesis of alpha2-fucosylated epitopes in the bovine species, we have characterized cDNAs from various tissues. We found three distinct alpha2-fucosyltransferase genes, named bovine fut1, fut2, and sec1 which are homologous to human FUT1, FUT2, and Sec1 genes, respectively. Their open reading frames (ORF) encode polypeptides of 360 (bovine H), 344 (bovine Se), and 368 (bovine Sec1) amino acids, respectively. These enzymes transfer fucose in alpha1,2 linkage to ganglioside GM(1)and galacto- N -biose, but not to the phenyl-beta-D-galactoside, type 1 or type 2 acceptors, suggesting that their substrate specificity is different and more restricted than the other cloned mammalian alpha2-fucosyltransferases. Southern blot analyses detected four related alpha2-fucosyltransferase sequences in the bovine genome while only three have been described in other species. The supernumerary entity seems to be related to the alpha2-fucosyltransferase activity which can also use type 1 and phenyl-beta-D-galactoside substrate acceptors. It was exclusively found in bovine intestinal tract. Our results show that, at least in one mammalian species, four alpha2-fucosyltransferases are present, three adding a fucose on alpha1,2 linkage on type 3/4 acceptor (Galbeta1-3GalNAc) and another able to transfer also fucose on phenyl-beta-D-galactoside and type 1 (Galbeta1-3GlcNAc) acceptors. The phylogenetic tree of the enzymes homologous to those encoded by the bovine fut1, fut2, and sec1 genes revealed two main families, one containing all the H-like proteins and the second containing all the Se-like and Sec1-like proteins. The Sec1-like family had a higher evolutionary rate than the Se-like family.

Cloning and characterization of the Arabidopsis thaliana SQS1 gene encoding squalene synthase--involvement of the C-terminal region of the enzyme in the channeling of squalene through the sterol pathway

Squalene synthase (SQS) catalyzes the first committed step of the sterol biosynthetic pathway. A full-length Arabidopsis thaliana SQS cDNA has been isolated by combining library screening and PCR-based approaches. Arabidopsis SQS is encoded by a small gene family of two genes (SQS1 and SQS2) which are organized in a tandem array. SQS1 and SQS2 have an identical organization with regard to intron positions and exon sizes and encode SQS isoforms showing a high level of sequence conservation (79% identity and 88% similarity). The isolated cDNA has been assigned to the SQS1 gene product, SQS1. RNA blot analysis has shown that the 1.6-kb SQS1 mRNA is detected in all plant tissues analyzed (inflorescenses, leaves, stems and roots) although the transcript is especially abundant in roots. Arabidopsis SQS1 isoform is unable to complement the SQS-defective Saccharomyces cerevisiae strain 5302, although SQS activity was detected in the microsomal fraction of the transformed yeast strain. However, a chimeric SQS resulting from the replacement of the 66 C-terminal residues of the Arabidopsis enzyme by the 111 C-terminal residues of the Schizosaccharomyces pombe enzyme was able to confer ergosterol prototrophy to strain 5302. Labeling studies using [3H]farnesyl-P2 and microsomal fractions obtained from yeast strains expressing either Arabidopsis SQS1 or chimeric Arabidopsis/S. pombe SQS derivatives indicated that the C-terminal region of the enzyme is involved in the channeling of squalene through the yeast sterol pathway.

Cloning and characterization of the Arabidopsis thaliana SQS1 gene encoding squalene synthase--involvement of the C-terminal region of the enzyme in the channeling of squalene through the sterol pathway

Squalene synthase (SQS) catalyzes the first committed step of the sterol biosynthetic pathway. A full-length Arabidopsis thaliana SQS cDNA has been isolated by combining library screening and PCR-based approaches. Arabidopsis SQS is encoded by a small gene family of two genes (SQS1 and SQS2) which are organized in a tandem array. SQS1 and SQS2 have an identical organization with regard to intron positions and exon sizes and encode SQS isoforms showing a high level of sequence conservation (79% identity and 88% similarity). The isolated cDNA has been assigned to the SQS1 gene product, SQS1. RNA blot analysis has shown that the 1.6-kb SQS1 mRNA is detected in all plant tissues analyzed (inflorescenses, leaves, stems and roots) although the transcript is especially abundant in roots. Arabidopsis SQS1 isoform is unable to complement the SQS-defective Saccharomyces cerevisiae strain 5302, although SQS activity was detected in the microsomal fraction of the transformed yeast strain. However, a chimeric SQS resulting from the replacement of the 66 C-terminal residues of the Arabidopsis enzyme by the 111 C-terminal residues of the Schizosaccharomyces pombe enzyme was able to confer ergosterol prototrophy to strain 5302. Labeling studies using [3H]farnesyl-P2 and microsomal fractions obtained from yeast strains expressing either Arabidopsis SQS1 or chimeric Arabidopsis/S. pombe SQS derivatives indicated that the C-terminal region of the enzyme is involved in the channeling of squalene through the yeast sterol pathway.

Polyprenol formation in the yeast Saccharomyces cerevisiae: effect of farnesyl diphosphate synthase overexpression

Biosynthesis of polyprenols was followed in the erg mutants of Saccharomyces cerevisiae impaired in various steps of the mevalonate pathway. The end products of the enzymatic reaction carried out in vitro, in the wild type yeast and all mutants tested, were identified as dehydrodolichols (alpha-unsaturated polyprenols) whereas in vivo, yeast synthesize dolichols (alpha-saturated polyprenols) (Biochimie, 1996.78:111-112.) The strain defective in the farnesyl diphosphate (FPP) synthase, (coded by the erg20-2 gene) required the presence of exogenous FPP for synthesis of dehydrodolichols to occur in vitro. Overexpression of the ERG20 gene restored synthesis of polyprenols in vitro indicating that FPP is the allylic "starter" for cis-prenyltransferase in yeast. Overexpression of the ERG20 gene in the erg 9 mutant, defective in squalene synthase activity, not only restored synthesis of dehydrodolichols in vitro, but also increased the synthesis of dolichols in vivo, almost 10-fold in comparison with wild type yeast. On the other hand overexpression of the mutated FPP synthase, coded by the gene erg20-2 in the same genetic background, resulted in a 100-fold increase of the amount of dehydrodolichols. Interestingly, in addition to the family of typical for yeast C60-C80 compounds, dehydrodolichols of chain length up to C135 were synthesized both in vitro and in vivo.

Arabidopsis thaliana contains two differentially expressed farnesyl-diphosphate synthase genes

The enzyme farnesyl-diphosphate synthase (FPS; EC 2.5.1.1/EC 2.5.1.10) catalyzes the synthesis of farnesyl diphosphate (FPP) from isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). This reaction is considered to be a rate-limiting step in isoprenoid biosynthesis. Southern blot analysis indicates that Arabidopsis thaliana contains at least 2 genes (FPS1 and FPS2) encoding FPS. The FPS1 and FPS2 genes have been cloned and characterized. The two genes have a very similar organization with regard to intron positions and exon sizes and share a high level of sequence similarity, not only in the coding region but also in the intronic sequences. Northern blot analysis showed that FPS1 and FPS2 have a different pattern of expression. FPS1 mRNA accumulates preferentially in roots and inflorescences, whereas FPS2 mRNA is predominantly expressed in inflorescences. The cDNA corresponding to the FPS1 gene was isolated by functional complementation of a mutant yeast strain defective in FPS activity (Delourme, D., Lacroute, F., and Karst, F. (1994) Plant Mol. Biol. 26, 1867-1873). By using a reverse transcription-polymerase chain reaction strategy we have cloned the cDNA corresponding to the FPS2 gene. Analysis of the FPS2 cDNA sequence revealed an open reading frame encoding a protein of 342 amino acid residues with a predicted molecular mass of 39,825 Da. FPS1 and FPS2 isoforms share an overall amino acid identity of 90.6%. Arabidopsis FPS2 was able to rescue the lethal phenotype of an ERG20-disrupted yeast strain. We demonstrate that FPS2 catalyzes the two successive condensations of IPP with both DMAPP and geranyl diphosphate leading to FPP. The significance of the occurrence of different FPS isoforms in plants is discussed in the context of the complex organization of the plant isoprenoid pathway.

Cloning of an Arabidopsis thaliana cDNA coding for farnesyl diphosphate synthase by functional complementation in yeast

A cDNA encoding farnesyl diphosphate synthase, an enzyme that synthesizes C15 isoprenoid diphosphate from isopentenyl diphosphate and dimethylallyl diphosphate, was cloned from an Arabidopsis thaliana cDNA library by complementation of a mutant of Saccharomyces cerevisiae deficient in this enzyme. The A. thaliana cDNA was also able to complement the lethal phenotype of the erg20 deletion yeast mutant. As deduced from the full-length 1.22 kb cDNA nucleotide sequence, the polypeptide contains 343 amino acids and has a relative molecular mass of 39,689. The predicted amino acid sequence presents about 50% identity with the yeast, rat and human FPP synthases. Southern blot analyses indicate that A. thaliana probably contains a single gene for farnesyl diphosphate synthase.

General resistance to sterol biosynthesis inhibitors in Saccharomyces cerevisiae

Screening for resistance to fenpropimorph was undertaken in order to isolate yeast mutants affected in the regulation of the ergosterol pathway. Among the mutants isolated, one bearing the recessive fen1-1 mutation was characterized by a 1.5-fold increase in the ergosterol level and a general resistance to sterol biosynthesis inhibitors. The fen1-1 mutation was linked to MAT locus on chromosome III. The measurement of enzyme activities involved in the ergosterol pathway revealed that isopentenyl diphosphate (IPP) isomerase activity was specifically increased 1.5-fold as compared to the wild type strain. However, overexpression of IPP isomerase in the wild type strain was not by itself sufficient to lead to sterol increase or resistance to sterol biosynthesis inhibitors, showing that IPP isomerase is not a limiting step in the pathway. The fen1-1 mutation permits viability in aerobiosis of yeast disrupted for sterol-14 reductase in absence of exogenous ergosterol supplementation, whereas the corresponding strain bearing the wild type FEN1 allele grows only in anaerobiosis. This result shows that ignosterol is able to efficiently replace ergosterol as bulk membrane component and that the fen1-1 mutation eliminates the specific ergosterol requirement in yeast.