VOLTAGE-DEPENDENT ANION CHANNEL OF ARABIDOPSIS HYPOCOTYLS - NUCLEOTIDE REGULATION AND PHARMACOLOGICAL PROPERTIES

Plasma membrane anion channels are thought to play important roles in osmoregulation and signal transduction in higher plant cells. Knowledg e of their pharmacology and regulation is of importance to unravel the ir physiological functions. In this study, we explore the pharmacologi cal properties and the nucleotide regulation of the voltage-dependent anion channel of Arabidopsis hypocotyls. The pharmacological profile o f this channel is characterized by a low sensitivity to most anion cha nnel blockers. It is inhibited by niflumic acid with an IC50 of 80 mu M, but poorly sensitive to IAA-94 and NPPB and insensitive to 9-AC and DIDS. Nucleotides alter the amplitude, the kinetics and the voltage-d ependence of the channel. The main effect of nucleotides is a shift of the voltage-dependent gate of the channel toward depolarized potentia ls leading to a strong reduction of the current amplitude. This regula tion does not require ATP hydrolysis as nonhydrolyzable ATP analogues- -AMPPNP and ATP gamma S--also regulate the anion current. This suggest s that a nucleotide binding site is involved in the regulation. The st udy of the properties of this putative nucleotide binding site reveals that (i) ATP regulates the channel with an EC50 of 0.7 mM, (ii) adeny l nucleotides modulate the channel with the following order of effecti veness: ATP > ADP much greater than AMP, and (iii) thiophosphate nucle otide analogues are the most potent agonists with EC50 in the range of 80 mu M. The hypothesis that this regulation may couple the electrica l properties of the membrane with the metabolic status of the cell is discussed.