Arabidopsis thaliana encodes a bacterial-type heterodimeric isopropylmalate isomerase involved in both Leu biosynthesis and the Met chain elongation pathway of glucosinolate formation.
The last steps of the Leu biosynthetic pathway and the Met chain elongation cycle for glucosinolate formation share identical reaction types suggesting a close evolutionary relationship of these pathways. Both pathways involve the condensation of acetyl-CoA and a 2-oxo acid, isomerization of the resulting 2-malate derivative to form a 3-malate derivative, the oxidation-decarboxylation of the 3-malate derivative to give an elongated 2-oxo acid, and transamination to generate the corresponding amino acid. We have now analyzed the genes encoding the isomerization reaction, the second step of this sequence, in Arabidopsis thaliana. One gene encodes the large subunit and three encode small subunits of this enzyme, referred to as isopropylmalate isomerase (IPMI) with respect to the Leu pathway. Metabolic profiling of large subunit mutants revealed accumulation of intermediates of both Leu biosynthesis and Met chain elongation, and an altered composition of aliphatic glucosinolates demonstrating the function of this gene in both pathways. In contrast, the small subunits appear to be specialized to either Leu biosynthesis or Met chain elongation. Green fluorescent protein tagging experiments confirms the import of one of the IPMI small subunits into the chloroplast, the localization of the Met chain elongation pathway in these organelles. These results suggest the presence of different heterodimeric IPMIs in Arabidopsis chloroplasts with distinct substrate specificities for Leu or glucosinolate metabolism determined by the nature of the different small subunit.
Knill
Tanja
T
Institut Molekulare Botanik, Universität Ulm, Albert-Einstein-Allee 11, 89069 Ulm, Germany.
Reichelt
Michael
M
Paetz
Christian
C
Gershenzon
Jonathan
J
Binder
Stefan
S
D000596:Amino Acids / Q000378:metabolism; D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D001419:Bacteria / Q000201:enzymology; D053898:Biosynthetic Pathways; D015152:Blotting, Northern; D002736:Chloroplasts / Q000378:metabolism; D015971:Gene Expression Regulation, Enzymologic; D018506:Gene Expression Regulation, Plant; D005961:Glucosinolates / Q000378:metabolism*; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D007535:Isomerases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D007930:Leucine / Q000096:biosynthesis*; D008293:Malates / Q000737:chemistry / Q000378:metabolism; D008715:Methionine / Q000378:metabolism*; D008856:Microscopy, Fluorescence; D015394:Molecular Structure; D009154:Mutation; D055503:Protein Multimerization; D021122:Protein Subunits / Q000235:genetics / Q000378:metabolism; D021381:Protein Transport; D011523:Protoplasts / Q000166:cytology / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014026:Nicotiana / Q000166:cytology / Q000235:genetics / Q000378:metabolism
C502920:2-isopropylmalic acid; D000596:Amino Acids; D029681:Arabidopsis Proteins; D005961:Glucosinolates; D008293:Malates; D021122:Protein Subunits; D011993:Recombinant Fusion Proteins; D049452:Green Fluorescent Proteins; D008715:Methionine; C032241:isopropylmalate isomerase; D007535:Isomerases; D007930:Leucine