A new scheme of symbiosis: ligand- and voltage-gated anion channels in plants and animals

Abstract

Anion channels in the plasma membrane of both plant and animal cells participate in a number of important cellular functions such as volume regulation, trans-epithelial transport, stabilization of the membrane potential and excitability. Only very recently attention has turned to the presence of anion channels in higher plant cells. A dominant theme among recent discoveries is the role of Ca 2+ in activating or modulating channel current involved in signal transduction. The major anion channel of stom atal guard cell protoplasts is a 32-40 pS channel which is highly selective for anions, in particular NO 3 - , Cl - and malate. These channels are characterized by a steep voltage dependence. Anion release is elicited upon depolarization and restricted to a narrow voltage span of — 100 mV to the reversal potential of anions. During prolonged activation the current slowly inactivates. A rise in cytoplasmic calcium in the presence of nucleotides evokes activation of the anion channels. Following activation they catalyse anion currents 10-20 times higher than in the inactivated state thereby shifting the resting potential of the guard cell from a K + -conducting to an anionconducting state. Patch-clamp studies have also revealed that growth hormones directly affect voltage-dependent activity of the anion channel in a dose-dependent manner. Auxin binding resulted in a shift of the activation potential towards the resting potential. Auxin-dependent gating of the anion channel is side- and hormone-specific. Its action is also channel-specific as K + channels coexisting in the same membrane patch were insensitive to this ligand. It remains to be established w hether the anion channel possesses an auxin binding site or consists of a hetero-complex with a member of the emerging family of auxin binding proteins. Known inhibitors of anion channels from anim al epithelial cells reversibly blocked the anion channel from the extracellular side. At the single-channel level, channel block is caused by a reduction of the long open transitions into flickering bursts. This plant anion channel as characterized by its steep voltagedependence and interaction with intracellular and extracellular ligands shares interesting similarities with members of gene families of anim al anion channels.