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Published on December 22, 2006; 10.1104/pp.106.085555

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Received June 21, 2006
Accepted December 7, 2006

Two Arabidopsis Genes (IPMS1 and IPMS2) Encode Isopropylmalate Synthase, the Branchpoint Step in the Biosynthesis of Leucine

Jan-Willem de Kraker , Katrin Luck , Susanne Textor , James G. Tokuhisa , and Jonathan Gershenzon *

Max-Planck Institute for Chemical Ecology, Beutenberg Campus, Hans-Knöll-Straße 8, D-07745 Jena, Germany

* Corresponding author; email: gershenzon{at}ice.mpg.de.

Heterologous expression of the Arabidopsis thaliana IPMS1 (At1g18500) and IPMS2 (At1g74040) cDNAs in Escherichia coli yields isopropylmalate synthases (IPMS; EC 2.3.3.13). These enzymes catalyze the first dedicated step in Leu biosynthesis, an aldol-type condensation of acetyl-CoA and 2-oxoisovalerate to isopropylmalate. Most biochemical properties of IPMS1 and IPMS2 are similar: broad pH optimum around pH 8.5, Mg2+ as cofactor, feedback inhibition by Leu, Km for 2-oxoisovalerate of approximately 300 µM, and a Vmax of about 2x103 µmol min-1 gram-1. However, IPMS1 and IPMS2 differ in their Km for acetyl-CoA (45 µM and 16 µM, respectively) and apparent quaternary structure (dimer and tetramer, respectively). A knockout insertion mutant for IPMS1 showed an increase in Val content, but no changes in Leu content; two insertion mutants for IPMS2 did not show any changes in soluble amino acid content. Apparently, in planta each gene can adequately compensate for the absence of the other, consistent with available micro-array and RT-PCR data which show that both genes are expressed in all organs at all developmental stages. Both encoded proteins accept 2-oxo acid substrates in vitro ranging in length from glyoxylate to 2-oxohexanoate, and catalyze at a low rate the condensation of acetyl-CoA and 4-methylthio-2-oxobutyrate, i.e. a reaction involved in glucosinolate chain elongation normally catalyzed by methylthioalkylmalate synthases (MAM). The evolutionary relationship between IPMS and MAM enzymes is discussed in view of their amino acid sequence identity (60%) and overlap in substrate specificity.




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