Two families of sterol methyltransferases are involved in the first and the second methylation steps of plant sterol biosynthesis.
Two methyl transfers are involved in the biosynthesis of 24-methyl and 24-ethyl sterols, which play major roles in plant growth and development. The first methyl transfer applies to cycloartenol, the second to 24-methylene lophenol. About ten cDNA clones encoding S-adenosyl-L-methionine (AdoMet) sterol methyltransferases (SMTs) have been isolated so far from various plants. According to their deduced amino acid sequences, they were classified in two families, smtl and smt2; in addition, smt2 cDNAs were shown to encode a 24-methylene lophenol C24 methyltransferase [Bouvier-Navé, P., Husselstein, T., Desprez, T. & Benveniste, P. (1997) Eur. J. Biochem. 246, 518-529]. We now report the comparison of two cDNAs isolated from Nicotiana tabacum, Ntsmt1-1 which belongs to the first SMT cDNA family and Ntsmt2-1 which belongs to the second. Both cDNAs were expressed in the yeast null mutant erg6, deficient in SMT. Whereas erg6 is devoid of 24-alkyl sterols, erg6 Ntsmt1-1 contained a majority of 24-methylene sterols and erg6 Ntsmt2-1, a majority of 24-ethylidene sterols, indicating distinct functions for the expression products of these cDNAs. In the presence of AdoMet, delipidated microsomes from erg6 Ntsm1-1 efficiently converted cycloartenol into 24-methylene cycloartanol, but did not produce any 24-ethylidene lophenol upon incubation with 24-methylene lophenol. This demonstrates that cDNA Ntsmt1-1 (and most probably the other plant SMT cDNAs of the first family) encode(s) a cycloartenol C24 methyltransferase. In contrast, delipidated microsomes of erg6 Ntsmt2-1 were shown to methylate preferentially 24-methylene lophenol, as expected from an SMT encoded by an smt2 cDNA. In summary, among various cDNAs isolated from N. tabacum, one (Ntsmt1-1) belongs to the first family of plant SMT cDNAs according to its deduced amino acid sequence and was shown to encode a cycloartenol C24 methyltransferase, whereas another (Ntsmt2-1) belongs to the second family and was shown to encode a 24-methylene lophenol C24 methyltransferase. Meanwhile, two cDNAs were isolated from Oriza sativa and shown to belong to smtl and to smt2 families, respectively. These data disclose the coexistence, in a given plant species, of two distinct SMTs, each catalyzing one step of methylation in the sterol biosynthesis pathway.
Bouvier-Navé
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P
Institut de Biologie Moléculaire des Plantes, Département de Biologie Cellulaire et Moléculaire, Strasbourg, France. pierre@medoc.u-strasbg.fr
Husselstein
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T
Benveniste
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P
D000595:Amino Acid Sequence; D002775:Cholestadienols / Q000378:metabolism; D005816:Genetic Complementation Test; D008745:Methylation; D008780:Methyltransferases / Q000145:classification / Q000378:metabolism*; D008954:Models, Biological; D008969:Molecular Sequence Data; D012275:Oryza; D010802:Phylogeny; D010840:Phytosterols / Q000096:biosynthesis* / Q000378:metabolism; D010947:Plants, Toxic; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity; D014026:Nicotiana; D014315:Triterpenes
C114722:24-methylenelophenol; D002775:Cholestadienols; D010840:Phytosterols; D014315:Triterpenes; D008780:Methyltransferases; C022670:delta 24-sterol methyltransferase; C100089:cycloartenol