Functional characterization of a potassium-selective prokaryotic glutamatereceptor

Ion channels are molecular pores that facilitate the passage of ions across cell membranes and participate in a range of biological processes, from ex citatory signal transmission in the mammalian nervous system to the modulat ion of swimming behaviour in the protozoan Paramecium(1). Two particularly important families of ion channels are ionotropic glutamate receptors (GluR s)(2) and potassium channels(3,4). GluRs are permeable to Na+, K+ and Ca2are gated by glutamate, and have previously been found only in eukaryotes(2 ), In contrast, potassium channels are selective for K+, are gated by a ran ge of stimuli, and are found in both prokaryotes and eukaryotes(3,4). Here we report the discovery and functional characterization of GluR0 from Synec hocystis PCC 6803, which is the first GluR found in a prokaryote. GluR0 bin ds glutamate, forms potassium-selective channels and is related in amino-ac id sequence to both eukaryotic GluRs and potassium channels. On the basis o f amino-acid sequence and functional relationships between GluR0 and eukary otic GluRs, we propose that a prokaryotic GluR was the precursor to eukaryo tic GluRs. GluR0 provides evidence for the missing link between potassium c hannels and GluRs, and we suggest that their ion channels have a similar ar chitecture, that GluRs are tetramers and that the gating mechanisms of GluR s and potassium channels have some essential features in common.