The lysine-dependent stimulation of lysine catabolism in tobacco seed requires calcium and protein phosphorylation.
The accumulation of free lysine in tobacco seed triggers the stimulation of lysine-ketoglutarate reductase, an enzyme that acts in lysine catabolism. The mechanism of lysine-ketoglutarate reductase stimulation was studied in two different systems: (1) developing seeds of wild-type plants in which the low basal lysine-ketoglutarate reductase activity can be stimulated by the exogenous addition of lysine; and (2) developing seeds of transgenic tobacco plants expressing a bacterial dihydrodipicolinate synthase in which lysine-ketoglutarate reductase activity is stimulated by endogenous lysine overproduction. In both systems, the stimulation of lysine-ketoglutarate reductase activity was significantly reduced when treated with the Ca2+ chelator EGTA. Moreover, the inhibitory effect of EGTA was overcome by the addition of Ca2+ but not Mg2+, suggesting that the lysine-dependent activation of lysine-ketoglutarate reductase requires Ca2+. This was further confirmed by a significant stimulation of lysine-ketoglutarate reductase activity following the treatment of wild-type seeds with ionomycin (an ionophore that increases Ca2+ flow into the cytoplasm). In addition, treatment of wild-type seeds with the protein phosphatase inhibitor okadaic acid triggered a significant induction in lysine-ketoglutarate reductase activity, whereas treatment of the transgenic seeds with the protein kinase inhibitor K-252a caused a significant reduction in its activity. Thus, we conclude that the stimulation of lysine-ketoglutarate reductase activity by lysine in tobacco seed operates through an intracellular signaling cascade mediated by Ca2+ and protein phosphorylation.
Karchi
H
H
Department of Plant Genetics, Weizmann Institute of Science, Rehovot, Israel.
Miron
D
D
Ben-Yaacov
S
S
Galili
G
G
D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002227:Carbazoles / Q000494:pharmacology; D002614:Chelating Agents / Q000494:pharmacology; D004533:Egtazic Acid / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D026121:Indole Alkaloids; D007700:Kinetics; D008239:Lysine / Q000378:metabolism* / Q000494:pharmacology*; D008274:Magnesium / Q000494:pharmacology; D010766:Phosphorylation; D010940:Plant Proteins / Q000378:metabolism*; D010947:Plants, Toxic; D011493:Protein Kinase C / Q000037:antagonists & inhibitors; D012443:Saccharopine Dehydrogenases / Q000378:metabolism*; D012639:Seeds / Q000378:metabolism; D014026:Nicotiana / Q000378:metabolism*
D002227:Carbazoles; D002614:Chelating Agents; D004791:Enzyme Inhibitors; D026121:Indole Alkaloids; D010940:Plant Proteins; D004533:Egtazic Acid; C049985:staurosporine aglycone; D012443:Saccharopine Dehydrogenases; D011493:Protein Kinase C; D008274:Magnesium; D008239:Lysine; D002118:Calcium