A Calcium-Selective Channel from Root-Tip Endomembranes of Garden Cress

A Ca(2+) channel from root-tip endomembranes of garden cress (Lepidium sativum L.) (LCC1) was characterized using the planar lipid-bilayer technique. Investigation of single-channel recordings revealed that LCC1 is voltage gated and strongly rectifying. In symmetrical 50 mm CaCl(2) solutions, the single-channel conductance was 24 picosiemens. LCC1 showed a moderate selectivity for Ca(2+) over K(+) (9.4:1) and was permeable for a range of divalent cations (Ca(2+), Ba(2+), and Sr(2+)). In contrast to Bryonia dioica Ca(2+) channel 1, a Ca(2+)-selective channel from the endoplasmic reticulum of touch-sensitive tendrils, LCC1 showed no bursting channel activity and had a low open probability and mean open time (2.83 ms at 50 mV). Inhibitor studies demonstrated that LCC1 is blocked by micromolar concentrations of erythrosin B (inhibitor concentration for 50% inhibition [IC(50)] = 1.8 μm) and the trivalent cations La(3+) (IC(50) = 5 μm) and Gd(3+) (IC(50) = 10 μm), whereas verapamil showed no blocking effect. LCC1 may play an important role in the regulation of the cytoplasmic free Ca(2+) concentration in root-tip and/or root-cap cells. The question of whether this ion channel is part of the gravitropic signal transduction pathway deserves further investigation.