Voltage-Dependent Cation Channels Permeable to NH₄⁺, K⁺, and Ca²⁺ in the Symbiosome Membrane of the Model Legume Lotus japonicus

Abstract

The symbiosome of nitrogen fixing root nodules mediates metabolite exchange between endosymbiotic rhizobia bacteria and the legume host. In the present study, the ion currents of the symbiosome membrane of the model legume Lotus japonicus were analyzed by patch-clamp recording. Both excised and symbiosome-attached patches exhibited a large inward (toward the cytosolic side of the membrane) current that is activated in a time-dependent manner by negative (on the cytosolic side) potentials. Based on reversal potential determinations and recordings with the impermeant cation N-methyl-glucamine, this current shows a high permeability for monovalent cations with no apparent permeability for anions. The current also showed a finite Ca²⁺ permeability. However, the currents were predominantly carried by univalent cations with a slightly greater selectivity for NH₄⁺ over K⁺. Increased Ca²⁺ concentration inhibited the current with a K₀.₅ for inhibition of 0.317 mM. The current showed strong rectification that is mediated by divalent cations (either Mg²⁺ or Ca²⁺). The influence of divalent cations is symmetrical in nature, because rectification can be exerted in either direction depending upon which side of the membrane has the highest concentration of divalent cations. However, based on observations with symbiosome-attached patches, the direction of the current in vivo is proposed to be toward the cytosol with cytosolic Mg²⁺ acting as the putative gating regulator. The findings suggest that L. japonicus possesses a voltage-dependent cation efflux channel that is capable of exporting fixed NH₄⁺, and may also play an additional role in Ca²⁺ transport.