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

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Abstract
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, smt1 and smt2; in addition, smt2 cDNAs were shown to encode a 24‐methylene lophenol C241 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 Ntsmt1‐1 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 C241 methyltransferase. Meanwhile, two cDNAs were isolated from Oriza sativa and shown to belong to smt1 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.

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