The effects of calmodulin and of controlled trypsin treatments on the activity of the Caz+ pump were investigated in plasma membrane purified from radish (Raphanus sativus 1.) seedlings. Treatment of the plasma membrane with ethylenediaminetetraacetate (EDTA), which removed about two-thirds of the plasma membrane-associated calmodulin, markedly increased the stimulation of the Caz+ pump by calmodulin. In EDTA-treated plasma membrane, stimulation by calmodulin of the Caz+ pump activity was maximal at low free Caz+ (2-5 p~) and decreased with the increase of free CaZ+ concentration. l h e CaZ+ 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 CaZ+ 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 CaZ+ concentration in the assay medium. Moreover, the Caz+ 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.In plant cells, the extrusion of Ca2+ from the cytoplasm to the apoplast is catalyzed by a Mg-ATP-dependent Ca2+ pump. During the last few years, the transport and hydrolytic activities of the plant PM Ca2+ pump have been characterized in some detail both in native PM vesicles and in proteoliposomes reconstituted with the solubilized and partially purified enzyme (reviewed in Briskin, 1990; Evans et al., 1991;De Michelis et al., 1992a). The most striking characteristics of the plant PM Ca2+ pump are its ability to use ITP or GTP besides ATP as a substrate (Williams et al., 1990; Camelli et al., 1992) and its high sensitivity to inhibition by fluorescein derivatives such as erythrosin B (Rasi-Caldogno et al., 1987, This work was supported by a grant of the Italian Ministry for University and Scientific and Technologic Research (40% quote) and by the Italian Ministry of Agriculture in the framework of the project 'Resistenze genetiche delle piante agrarie agli stress biotici ed abiotici."