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Plants Do It Differently. A New Basis for Potassium/Sodium Selectivity in the Pore of an Ion Channel1
Richard W. Mercier,
Qiang Leng3, and
Gerald A. Berkowitz*
Agricultural Biotechnology Laboratory, Department of Plant Science, 1390
Storrs Road U4163, University of Connecticut, Storrs, Connecticut
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 familyof plant
proteins with structural similarities to cloned animalcyclic nucleotide-gated
channels (CNGCs). Animal CNGCs arenonselective cation channels that do not
discriminate betweenNa+ and K+ permeation. These
channels all have the same tripletof amino acids in the channel pore ion
selectivity filter,and this sequence is different from that of the
selectivityfilter found in K+-selective channels. Plant CNGCs have
uniquepore selectivity filters; unlike those found in any other familyof
channels. At present, the significance of the unique poreselectivity filters
of plant CNGCs, with regard to discriminationbetween Na+ and
K+ permeation is unresolved. Here, we presentan
electrophysiological analysis of several members of thisprotein family;
identifying the first cloned plant channel(AtCNGC1) that conducts
Na+. Another member of this ion channelfamily (AtCNGC2) is shown
to have a selectivity filter thatprovides a heretofore unknown molecular
basis for discriminationbetween K+ and Na+ permeation.
Specific amino acids withinthe AtCNGC2 pore selectivity filter (Asn-416,
Asp-417) aredemonstrated to facilitate K+ over Na+
conductance. The selectivityfilter of AtCNGC2 represents an alternative
mechanism to thewell-known GYG amino acid triplet of K+ channels
that has beenidentified as the critical basis for K+ over
Na+ permeationthrough the pore of ion channels.