Voltageactivated Cationselective Channel Permeable To Nh4 On The Symbiosome Membrane Of The Model Legume Lotus Japonicus

D.M. Roberts1, S.D. Tyerman2

department of Biochemistry, Cellular and Molecular Biology, Univ. Tennessee, Knoxville, TN, USA

Department of Biology, Flinders University of South Australia, SA, Australia

The symbiosome membrane is responsible for the metabolite exchange that represents the crux of nitrogen-fixing symbioses: the efflux of fixed nitrogen (NH4+) and the uptake of dicarboxylates as an energy source for the symbiont (Udvardi, Day 1997). In the present study we have examined the transport activities of the symbiosome membrane of Lotus japonicus by patch clamp recording. Excised L. japonicus symbiosome membrane patches show a rectified, inward (towards the cytosolic compartment) current that is activated by negative (with respect to the cytosol) voltage potentials. A predominant selectivity for monovalent cations was observed, although evidence for a low calcium permeability was also obtained. The channel is permeable to both K+ and NH4+, but NH4+ showed a slightly higher conductance (apparent Km=17 mM). The channel shows no intrinsic rectification, but depends upon the presence of divalent cations. In the absence of divalent cations (either Mg2+ or Ca2+) the channel is largely open and symmetrical, but the addition of divalent cations results in inhibition of the current with the direction of rectification dependent upon which side of the membrane possesses a higher divalent cation activity. However, based on measurements with intact symbiosomes, the direction of current is proposed to be towards the cytosol and Mg2+ on the cytosolic side is proposed to mediate the rectification and voltage-dependence of the current. Overall, the data suggest that L. japonicus possesses an ammonium-permeable cation channel that is similar to the cation channel previously described on the soybean symbiosome (Tyerman et al. 1995; Whitehead et al. 1998) and has the properties necessary to transport fixed NH4+ from the symbiosome space to the plant cytosol. In addition, coordinate regulation of this channel and the electrogenic H+-ATPase may also play a role in symbiosome homeostasis by regulation of the transmembrane potential and pH of the symbiosome space.

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