Arabidopsis thaliana NADP-malic enzyme isoforms: high degree of identity but clearly distinct properties

• GERRARD WHEELER, M. C., ARIAS, C. L., TRONCONI, M. A., MAURINO, V. G., ANDREO, C. S. AND DRINCOVICH, M. F

PLANT MOLECULAR BIOLOGY

Springer Netherlands

Año: 2008 vol. 67 p. 231 - 242

0167-4412

 The Arabidopsis thaliana genome contains fourNADP-malic enzymes genes (NADP-ME1-4). NADP-ME4is localized to plastids whereas the other isoforms arecytosolic. NADP-ME2 and 4 are constitutively expressed,while NADP-ME1 is restricted to secondary roots andNADP-ME3 to trichomes and pollen. Although the fourisoforms share remarkably high degree of identity(75–90%), recombinant NADP-ME1 through 4 show distinctkinetic properties, both in the forward (malateoxidative decarboxylation) and reverse (pyruvate reductivecarboxylation) reactions. The four isoforms behave differentlyin terms of reversibility, with NADP-ME2presenting the highest reverse catalytic efficiency. Whenanalyzing the activity of each isoform in the presence ofmetabolic effectors, NADP-ME2 was the most highlyregulated isoform, especially in its activation by certaineffectors. Several metabolites modulate both the forwardand reverse reactions, exhibiting dual effects in some cases.Therefore, pyruvate reductive carboxylation may be relevantin vivo, especially in some cellular compartments andconditions. In order to identify residues or segments of theNADP-ME primary structure that could be involved in thedifferences among the isoforms, NADP-ME2 mutants anddeletions were analysed. The results obtained show thatArg115 is involved in fumarate activation, while theamino-terminal part is critical for aspartate and CoA activation,as well as for the reverse reaction. As a whole, thesestudies show that minimal changes in the primary structureare responsible for the different kinetic behaviour of eachAtNADP-ME isoform. In this way, the co-expression ofsome isoforms in the same cellular compartment would notimply redundancy but represents specificity of function.