Separation of membrane vesicles from maize roots having different calcium transport activities

Two calcium transport activities have been identified in membrane vesicles isolated from a corn (Zea mays L. cv. Dekalb XL 85) root microsomal fraction. Using a 5/10% discontinuous dextran-sorbitol gradient, it was possible to separate membranes with different characteristics and presenting different calcium transport mechanisms. In particular, a fraction isolated at the 0/5% interface possessed both a Ca2+/H+ antiport, which was uncoupled by ionophores, and a Ca2+ transport activity which was stimulated by calmodulin. A second fraction collected at the 5/10% interface was insensitive to ionophores and possessed only the Ca2− transport activity stimulated by calmodulin. A possible localisation on different membrane systems of these two mechanisms is discussed.