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 reconstituti on of ion channels from the endoplasmic reticulum of a higher plant. T his was obtained at high purity from touch-sensitive tendrils of Bryon ia dioica, In this preparation, a calcium-selective strongly rectifyin g channel is prevailing whose single-channel properties have been char acterized, The single-channel conductance is 29 pS in 50 mM CaCl2. The Ca2+:K+ selectivity was determined to be similar to 6.6. The channel is voltage-gated and, more importantly, the gating voltage is strongly shifted towards more negative voltages when a transmembrane Ca2+ grad ient is applied, Thus, at physiological voltages across the endoplasmi c reticulum membrane, the channel's open probability will be governed largely by the chemical potential gradient of Ca2+, generated by the C a2+-ATPase in that same membrane, The calcium release channel describe d 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 f irst known member of a hitherto unrecognized class of calcium channels .