Plants Do It Differently. A New Basis for Potassium/Sodium Selectivity in the Pore of an Ion Channel

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Abstract
Understanding of the molecular architecture necessary for selective K+ permeation through the pore of ion channels is based primarily on analysis of the crystal structure of the bacterial K+ channel KcsA, and structure:function studies of cloned animal K+ channels. Little is known about the conduction properties of a large family of plant proteins with structural similarities to cloned animal cyclic nucleotide-gated channels (CNGCs). Animal CNGCs are nonselective cation channels that do not discriminate between Na+ and K+ permeation. These channels all have the same triplet of amino acids in the channel pore ion selectivity filter, and this sequence is different from that of the selectivity filter found in K+-selective channels. Plant CNGCs have unique pore selectivity filters; unlike those found in any other family of channels. At present, the significance of the unique pore selectivity filters of plant CNGCs, with regard to discrimination between Na+ and K+ permeation is unresolved. Here, we present an electrophysiological analysis of several members of this protein family; identifying the first cloned plant channel (AtCNGC1) that conducts Na+. Another member of this ion channel family (AtCNGC2) is shown to have a selectivity filter that provides a heretofore unknown molecular basis for discrimination between K+ and Na+ permeation. Specific amino acids within the AtCNGC2 pore selectivity filter (Asn-416, Asp-417) are demonstrated to facilitate K+ over Na+ conductance. The selectivity filter of AtCNGC2 represents an alternative mechanism to the well-known GYG amino acid triplet of K+ channels that has been identified as the critical basis for K+ over Na+ permeation through the pore of ion channels.