The activity of the Arabidopsis bifunctional lysine-ketoglutarate reductase/saccharopine dehydrogenase enzyme of lysine catabolism is regulated by functional interaction between its two enzyme domains

Lysine-ketoglutarate reductase/saccharopine dehydrogenase (LKR/SDH) is a bifunctional enzyme catalyzing the first two steps of lysine catabolism in animals and plants. To elucidate the biochemical signification of the linkage between the two enzymes of LKR/SDH, namely lysine ketoglutarate and saccharopine dehydrogenase, we employed various truncated and mutated Arabidopsis LKR/SDH polypeptides expressed in yeast. Activity analyses of the different recombinant polypeptides under conditions of varying NaCl levels implied that LKR, but not SDH activity, is regulated by functional interaction between the LKR and SDH domains, which is mediated by the structural conformation of the linker region connecting them. Because LKR activity of plant LKR/SDH enzymes is also regulated by casein kinase 2 phosphorylation, we searched for such potential regulatory phosphorylation sites using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and site-directed mutagenesis. This analysis identified Ser-458 as a candidate for this function. We also tested a hypothesis suggesting that an EF-hand-like sequence at the C-terminal part of the LKR domain functions in a calcium-dependent assembly of LKR/SDH into a homodimer. We found that this region is essential for LKR activity but that it does not control a calcium-dependent assembly of LKR/SDH. The relevance of our results to the in vivo function of LKR/SDH in lysine catabolism in plants is discussed. In addition, because the linker region between LKR and SDH exists only in plants but not in animal LKR/SDH enzymes, our results suggest that the regulatory properties of LKR/SDH and, hence, the regulation of lysine catabolism are different between plants and animals.