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J. Biol. Chem., Vol. 275, Issue 21, 15962-15968, May 26, 2000
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From the Plants synthesize S-methylmethionine
(SMM) from S-adenosylmethionine (AdoMet), and methionine
(Met) by a unique reaction and, like other organisms, use SMM as a
methyl donor for Met synthesis from homocysteine (Hcy). These reactions
comprise the SMM cycle. Two Arabidopsis cDNAs
specifying enzymes that mediate the SMM The nucleotide sequence(s) reported in this paper has been submitted to the GenBankTM/EMBL Data Bank with accession number(s) AF219222 (AtHMT-1) and AF219223 (AtHMT-2).
Characterization and Functional Expression of cDNAs
Encoding Methionine-sensitive and -insensitive Homocysteine
S-Methyltransferases from Arabidopsis*
,,,
Horticultural Sciences Department,
University of Florida, Gainesville, Florida 32611, the
§ Department of Horticulture, Purdue University,
West Lafayette, Indiana 47907, and the ¶ Biochemistry
Department, Michigan State University,
East Lansing, Michigan 48824
Met reaction (SMM:Hcy
S-methyltransferase, HMT) were identified by homology and
authenticated by complementing an Escherichia coli yagD
mutant and by detecting HMT activity in complemented cells. Gel blot
analyses indicate that these enzymes, AtHMT-1 and -2, are encoded by
single copy genes. The deduced polypeptides are similar in size (36 kDa), share a zinc-binding motif, lack obvious targeting sequences, and
are 55% identical to each other. The recombinant enzymes exist as
monomers. AtHMT-1 and -2 both utilize L-SMM or
(S,S)-AdoMet as a methyl donor in vitro and
have higher affinities for SMM. Both enzymes also use either methyl donor in vivo because both restore the ability to utilize
AdoMet or SMM to a yeast HMT mutant. However, AtHMT-1 is strongly
inhibited by Met, whereas AtHMT-2 is not, a difference that could be
crucial to the control of flux through the HMT reaction and the SMM
cycle. Plant HMT is known to transfer the pro-R methyl
group of SMM. This enabled us to use recombinant AtHMT-1 to establish
that the other enzyme of the SMM cycle, AdoMet:Met
S-methyltransferase, introduces the pro-S
methyl group. These opposing stereoselectivities suggest a way to
measure in vivo flux through the SMM cycle.
*
This work was supported in part by National Science
Foundation Grants IBN-9816075 (to A. D. H) and IBN-9904263 (to
D. A. G.), by Department of Energy Grant DE-FG02-99ER20344
(to D. R.), by an endowment from the C. V. Griffin, Sr.
Foundation, and by the Florida Agricultural Experiment Station. Journal
series no. R-07506.The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in
accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
To whom correspondence should be addressed: Horticultural
Sciences Dept., University of Florida, P. O. Box 110690, Gainesville, FL 32611. Tel.: 352-392-1928; Fax: 352-392-6479; E-mail:
adha@gnv. ifas.ufl.edu.
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