Two families of sterol methyltransferases are involved in the first and the second methylation steps of plant sterol biosynthesis

Eur J Biochem. 1998 Aug 15;256(1):88-96. doi: 10.1046/j.1432-1327.1998.2560088.x.


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.

Publication types

  • Comparative Study

MeSH terms

  • Amino Acid Sequence
  • Cholestadienols / metabolism
  • Genetic Complementation Test
  • Methylation
  • Methyltransferases / classification
  • Methyltransferases / metabolism*
  • Models, Biological
  • Molecular Sequence Data
  • Oryza
  • Phylogeny
  • Phytosterols / biosynthesis*
  • Phytosterols / metabolism
  • Plants, Toxic
  • Sequence Homology, Amino Acid
  • Substrate Specificity
  • Tobacco
  • Triterpenes


  • 24-methylenelophenol
  • Cholestadienols
  • Phytosterols
  • Triterpenes
  • Methyltransferases
  • delta 24-sterol methyltransferase
  • cycloartenol

Associated data

  • GENBANK/AF042332
  • GENBANK/AF042333
  • GENBANK/AF053766
  • GENBANK/U81312