Glutamate receptors and potassium channels both allow the passage of charged ions between the inside and outside of cells, but are mechanistically quite different. So the discovery in a bacterium of what looks like a ‘hybrid’ between them, as described by G.-Q. Chen and colleagues elsewhere in this issue (Nature 402, 817–821; 1999), is quite an event.
Glutamate receptors are permeable to several different ions and are opened, or ‘gated’, by their ligand, glutamate. By contrast, as their name suggests, potassium channels show exquisite selectivity for potassium ions. These channels can be gated either by intracellular ligands, or by voltage (a mechanism that is coincidentally investigated in two other papers in this issue, on pages 809 and 813).
Chen et al. have identified a glutamate receptor, GluR0, from the cyanobacterium Synechocystis. This is the first prokaryotic glutamate receptor to be described, and a surprise in itself. But remarkably the authors also reveal that it is endowed with a potassium-selective pore.
The extracellular regions of GluR0 show sequence similarity to the ligand-binding domains of glutamate receptors (red in the figure here) and the pore region looks like an inverted potassium-channel pore (blue). The exciting implication is that potassium channels and glutamate receptors may have originated from a common ancestor, and so may have common architectural and functional features.
Last year, the first potassium-channel structure was solved by D. A. Doyle et al. (Science 280, 69–77; 1998). The channel, known as KcsA, is found in the bacterium Streptomyces lividans, and is gated by an intracellular ligand. The structural information allowed predictions to be made about the structure of other potassium-channel pores. Until now, however, there has been no such information available for glutamate receptors. A further aspect of the work by Chen et al. is that they show that the KcsA structure can be used to make predictions about glutamate-receptor pores as well.