title: Controlled Proteolysis Activates the Plasma Membrane Ca2+ Pump of Higher Plants (A Comparison with the Effect of Calmodulin in Plasma Membrane from Radish Seedlings).
abstract: 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.
authors.affiliation: 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.).
authors.forename: F.
authors.initials: F
authors.lastname: Rasi-Caldogno
authors.forename: A.
authors.initials: A
authors.lastname: Carnelli
authors.forename: M. I.
authors.initials: MI
authors.lastname: De Michelis