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Plant Physiol, January 2003, Vol. 131, pp. 215-227
Alanine Aminotransferase Homologs Catalyze the
Glutamate:Glyoxylate Aminotransferase Reaction in Peroxisomes of
Arabidopsis1
Aaron H.
Liepman2 and
Laura J.
Olsen*
Department of Molecular, Cellular, and Developmental Biology,
University of Michigan, Ann Arbor, Michigan 48109-1048
Plant peroxisomal glyoxylate aminotransferases play central
roles within the photorespiratory pathway. Genes encoding glyoxylate aminotransferases have been isolated from several animals and microbes,
but only recently have plant homologs been identified. Three
Arabidopsis homologs of alanine (Ala):glyoxylate aminotransferase 2 (AGT2) contain a putative type 1 peroxisomal targeting signal (PTS1),
but the metabolic significance of these AGT2 homologs is unknown. GGT1
and GGT2 are Ala aminotransferase (AlaAT) homologs from Arabidopsis
that represent another type of glyoxylate aminotransferase. These
proteins are class I aminotransferases, each containing a putative
PTS1. GGT1 and GGT2 are members of a small family of AlaATs in
Arabidopsis. When expressed as recombinant proteins in
Escherichia coli, GGT1 and GGT2 displayed biochemical
characteristics very similar to one another, and to the Arabidopsis
protein purified from leaves. Four aminotransferase activities were
specifically associated with GGT1 and GGT2, using the substrate pairs
glutamate (Glu):glyoxylate, Ala:glyoxylate, Glu:pyruvate, and
Ala:2-oxoglutarate. GGT1 and GGT2 may
have partially redundant functions; transcripts of both genes were
detected in many of the same tissues. Although Glu:glyoxylate
aminotransferase (GGT) activity has been observed in several locations
in different plants and algae, including the cytoplasm and
mitochondria, our subcellular fractionation data indicate that GGT
activity was exclusively peroxisomal in Arabidopsis. Thus, glyoxylate
aminotransferase reactions in plant peroxisomes appear to be catalyzed
by at least two distinct types of aminotransferases: an AGT1 homolog
with serine:glyoxylate aminotransferase activity (A.H. Liepman,
L.J. Olsen [2001] Plant J 25: 487-498), and a pair of
closely related, potentially redundant AlaAT homologs with GGT activity.
1
This work was supported in part by the
University of Michigan Cellular Biotechnology Training Program
(fellowship no. NIH-GM08353 to A.H.L.) and by the Rackham School of
Graduate Studies (fellowship to A.H.L.).
2
Present address: Department of Energy Plant
Research Lab, Michigan State University, East Lansing, MI
48824-1312.
*
Corresponding author; e-mail ljo{at}umich.edu; fax
734-647-0884.
© 2003 American Society of Plant Biologists
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