Osmotically Activated Anion Current of Phycomyces blakesleeanus-Filamentous Fungi Counterpart to Vertebrate Volume Regulated Anion Current

Abstract

Studies of ion currents in filamentous fungi are a prerequisite for forming a complete understanding of their physiology. Cytoplasmic droplets (CDs), obtained from sporangiophores of Phycomyces blakesleeanus, are a model system that enables the characterization of ion currents in the native membrane, including the currents mediated by the channels not yet molecularly identified. Osmotically activated anionic current with outward rectification (ORIC) is a dominant current in the membrane of cytoplasmic droplets under the conditions of hypoosmotic stimulation. We have previously reported remarkable functional similarities of ORIC with the vertebrate volume regulated anionic current (VRAC), such as dose-dependent activation by osmotic difference, ion selectivity sequence, and time and voltage dependent profile of the current. Using the patch clamp method on the CD membrane, we further resolve VRAC-like ORIC characteristics in this paper. We examine the inhibition by extracellular ATP and carbenoxolone, the permeation of glutamate in presence of chloride, selectivity for nitrates, and activation by GTP, and we show its single channel behavior in excised membrane. We propose that ORIC is a functional counterpart of vertebrate VRAC in filamentous fungi, possibly with a similar essential role in anion efflux during cell volume regulation.