A CA2-DEPENDENT AND VOLTAGE-DEPENDENT CATION CHANNEL IN THE NUCLEAR-ENVELOPE OF RED BEET()

The patch-clamp technique was applied to study ion conductances in var ious configurations of the nuclear envelope of non-enzyme-treated red beet (Beta vulgaris L.) nuclei. With excised patches a non-selective c ation channel was observed, that was activated by micromolar concentra tions of Ca2+ on the nucleoplasmic side of the envelope. The channel a ctivity was also voltage-dependent and the voltage threshold of channe l activation changed with the nucleoplasmic Ca2+ concentration. The mo st prominent conductance level was 110 +/- 22 pS with 150 mM KCl in th e bath and pipette. The channel was permeable to small cations: permea bilities relative to K+ were P-K congruent to P-Na = 1, P-Cs = 0.3, bu t P-Cl = 0.09. Calcium ions also permeated the channel with P-Ca = 0.4 3, estimated from reversal potential, or 0.14, estimated from conducta nce ratio. Zn2+ (1 mM) when applied to the cytoplasmic side of the env elope blocked the channel activity completely, while amiloride (2 mM) reduced; the channel current by 86% from the nucleoplasmic side. The p roperties of the whole-nucleus current (voltage-, time- and Ca2+-depen dence) paralleled those observed with excised patches. The channel may provide a Ca2+-regulated pathway for passive diffusion of cations acr oss the nuclear envelope and thus may play an important role in Ca2+-d ependent nuclear processes ranging from gene transcription to apoptosi s. (C) 1998 Elsevier Science B.V. All rights reserved.