IN-VITRO DEPHOSPHORYLATION INHIBITS THE ACTIVITY OF SOYBEAN LYSINE-KETOGLUTARATE REDUCTASE IN A LYSINE-REGULATED MANNER

In plant seeds, the essential amino acid lysine autoregulates its own level by modulating the activity of its catabolic enzyme lysine-ketogl utarate reductase via an intracellular signaling cascade, mediated by Ca2+ and protein phosphorylation/dephosphorylation. In the present rep ort, it has been further tested whether the activity of soybean lysine -ketoglutarate reductase, as well of saccharopine dehydrogenase, the s econd in the pathway of lysine catabolism, are modulated by direct pho sphorylation of the bifunctional polypeptide containing both of these linked activities. Incubation of purified lysine-ketoglutarate reducta se/saccharopine dehydrogenase with casein kinase II resulted in a sign ificant phosphorylation of the bifunctional enzyme. Moreover, in vitro dephosphorylation of the bifunctional polypeptide with alkaline phosp hatase significantly inhibited the activity of lysine-ketoglutarate re ductase, but not of its linked enzyme saccharopine dehydrogenase. The inhibitory effect of alkaline phosphatase on lysine-ketoglutarate redu ctase activity was dramatically stimulated by binding of lysine to the enzyme. Our results suggest that in plant seeds, active lysine-ketogl utarate reductase is a phospho-protein, and that its activity is modul ated by opposing actions of protein kinases and phosphatases. Moreover , this modulation is subject to a compound regulation by lysine.