THE APPARENT TURNOVER OF 1-AMINOCYCLOPROPANE-1-CARBOXYLATE SYNTHASE IN TOMATO CELLS IS REGULATED BY PROTEIN-PHOSPHORYLATION AND DEPHOSPHORYLATION

In suspension-cultured cells of tomato (Lycopersicon esculentum Mill.) , the activity of 1-aminocyclopropane-1-carboxylate synthase (ACC-S) r apidly increases in response to fungal elicitors. The effect of inhibi tors of protein kinases and protein phosphatases on the regulation of ACC-S was studied. K-252a, an inhibitor of protein kinases, prevented induction of the enzyme by elicitors and promoted its apparent turnove r in elicitor-stimulated cells, causing a 50% loss of activity within 4 to 8 min in both the presence and absence of cycloheximide. Calyculi n A, an inhibitor of protein phosphatases, caused a rapid increase of ACC-S in the absence of elicitors and an immediate acceleration of the rate of ACC-S increase in elicitor-stimulated cells. In the presence of cycloheximide there was no such increase, indicating that the effec t depended on protein synthesis. Cordycepin, an inhibitor of mRNA synt hesis, did not prevent the elicitor-induced increase in ACC-S activity but strongly reduced the K-252a-induced decay and the calyculin A-ind uced increase of its activity. In vitro, ACC-S activity was not affect ed by K-252a and calyculin A or by treatments with protein phosphatase s. These results suggest that protein phosphorylation/dephosphorylatio n is involved in the regulation of ACC-S, not by regulating the cataly tic activity itself but by controlling the rate of turnover of the enz yme.