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PLANT PHYSIOLOGY , Vol 103, Issue 2 385-390, Copyright © 1993 by American Society of Plant Biologists
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METABOLISM AND ENZYMOLOGY |
Controlled Proteolysis Activates the Plasma Membrane Ca2+ Pump of Higher Plants (A Comparison with the Effect of Calmodulin in Plasma Membrane from Radish Seedlings)
F. Rasi-Caldogno, A. Carnelli and M. I. De Michelis
Centro di Studio del Consiglio Nazionale delle Ricerche per la Biologia Cellulare e Molecolare delle Piante, Dipartimento di Biologia, Universita di Milano, via G. Celoria 26, 20133 Milano, Italy (F.R.-C., A.C.)
The effects of calmodulin and of controlled trypsin treatments on the
activity of the Ca2+ pump were investigated in plasma membrane purified
from radish (Raphanus sativus L.) seedlings. Treatment of the plasma
membrane with ethylenediaminetetra-acetate (EDTA), which removed about
two-thirds of the plasma membrane-associated calmodulin, markedly increased
the stimulation of the Ca2+ pump by calmodulin. In EDTA-treated plasma
membrane, stimulation by calmodulin of the Ca2+ pump activity was maximal
at low free Ca2+ (2-5 [mu]M) and decreased with the increase of free Ca2+
concentration. The Ca2+ pump activity was stimulated also by a controlled
treatment of the plasma membrane with trypsin: the effect of trypsin
treatment depended on the concentration of both trypsin and plasma membrane
proteins and on the duration of incubation. Stimulation of the Ca2+ pump
activity by trypsin treatment of the plasma membrane was similar to that
induced by calmodulin both in extent and in dependence on the free Ca2+
concentration in the assay medium. Moreover, the Ca2+ pump of
trypsin-treated plasma membrane was insensitive to further stimulation by
calmodulin, suggesting that limited proteolysis preferentially cleaves a
regulatory domain of the enzyme that is involved in its activation by
calmodulin.
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