Abstract
A depolarization-activated K(+) channel capable of carrying the large K(+) currents that flow from shrinking cells during movements of Samanea saman leaflets has been described in the plasmalemma of Samanea motor cell protoplasts (N Moran et al [1988] Plant Physiol 88:643-648). We now characterize this channel in greater detail. It is selective for K(+) over other monovalent ions, with the following order of relative permeability: K(+) > Rb(+) > Na(+) approximately Cs(+) approximately Li(+). It is blocked by Cs(+) and by Ba(2+) in a voltage dependent manner, exhibiting a ;long-pore' behavior, similarly to various types of K(+) channels in animal systems. Cadmium, known for its blockage of Ca(2+) channels in animal systems, and Gd(3+), closely related to La(3+), which also blocks Ca(2+) channels in animal cells, both block K(+) currents in Samanea in a voltage-independent manner, and without interfering with the kinetics of the currents. The suggested mechanism of block is either (a) by a direct interaction with the K(+) channel, but external to its lumen, or, alternatively, (b) by blocking putative Ca(2+) channels, and preventing the influx of Ca(2+), on which the activation of the K(+) channels may be dependent.