Gadolinium-sensitive, voltage-dependent calcium release channels in the endoplasmic reticulum of a higher plant mechanoreceptor organ.

The lipid bilayer technique was adapted to the functional reconstitution of ion channels from the endoplasmic reticulum of a higher plant. This was obtained at high purity from touch-sensitive tendrils of Bryonia dioica. In this preparation, a calcium-selective strongly rectifying channel is prevailing whose single-channel properties have been characterized. The single-channel conductance is 29 pS in 50 mM CaCl2. The Ca2+: K+ selectivity was determined to be approximately 6.6. The channel is voltage-gated and, more importantly, the gating voltage is strongly shifted towards more negative voltages when a transmembrane Ca2+ gradient is applied. Thus, at physiological voltages across the endoplasmic reticulum membrane, the channel's open probability will be governed largely by the chemical potential gradient of Ca2+, generated by the Ca(2+)-ATPase in that same membrane. The calcium release channel described here is effectively blocked by Gd3+ which also completely suppresses a tendril's reaction to touch, suggesting that this channel could be a key element of calcium signaling in higher plant mechanotransduction. Its molecular characteristics and inhibitor data show it to be the first known member of a hitherto unrecognized class of calcium channels.