Voltage-dependent Ca2+ influx into right-side-out plasma membrane vesicles isolated from wheat roots: characterization of a putative Ca2+ channel.

We report on the identification of a voltage-dependent Ca2+ transport system that mediates Ca2+ influx across the plasma membrane (PM) of wheat (Triticum aestivum) root cells. The experimental approach involved the imposition of transmembrane electrical potentials (via K+ diffusion potentials) in populations of purified, right-side-out PM vesicles isolated from wheat roots. Using 45Ca2+ to quantify Ca2+ influx into the PM vesicles, the voltage-dependent characteristics of Ca2+ transport were found to be similar to those exhibited by L-type voltage-gated Ca2+ channels in animal cells. The putative PM Ca2+ channel opened upon depolarization of the membrane potential, and Ca2+ flux increased to a maximum upon further depolarization and then decreased back to zero upon further successive depolarizations. This channel was found to be selective for Ca2+ over Mg2+, Sr2+, K+, and Na+; was blocked by very low concentrations of La3+; was unaffected by high concentrations of the K+ channel blocker tetraethylammonium; and exhibited Michaelis-Menten-type transport kinetics. Based on these transport properties, we argue that this transport system is a PM Ca2+ channel. We suggest that the use of radiotracer flux analysis of voltage-clamped PM vesicles derived from plant roots is a straightforward approach for the characterization of certain voltage-gated ion channels functioning in cellular membranes of higher plant cells.