D. Miron et al., IN-VITRO DEPHOSPHORYLATION INHIBITS THE ACTIVITY OF SOYBEAN LYSINE-KETOGLUTARATE REDUCTASE IN A LYSINE-REGULATED MANNER, Plant journal, 12(6), 1997, pp. 1453-1458
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.