A new counting method developed by researchers at the Salk Institute and the Max Planck Institute for Biophysical Chemistry, Göttingen/Germany, showed that glutamate receptors are, unlike other ligand-gated ion channels that are assumed to be an assembly of five subunits, tetramers (Science, vol. 280, 5 June 1998).
Glutamate receptor channels are important mediators of cell-to cell communication in the brain. They are formed by proteins localized at the surface of neurons. Each protein is thought to be able to bind a neurotransmitter molecule. After binding of the neurotransmitter, that was released by the neighboring nerve cell, the receptor induces a current flow into the receiving nerve by opening a pore within the receptor. The core receptor is thought to be formed by assembly of five proteins, as is assumed for all neurotransmitter activated receptors. This view is now challenged by a counting method developed by Christian Rosenmund, Yael Stern-Bach and Charles Stevens.
The key observation upon which this counting method is based is, that the flow of current through the receptor channel is dependent on how many of the receptors binding sites have glutamate molecules bound.
Measuring current flow through glutamate receptors is usually hampered by receptor desensitisation, a rapid shut-down of the receptors once they are activated by the neurotransmitter, says Christian Rosenmund, a former postdoctoral fellow of Prof. Charles Stevens, Howard Hughes Medical Institute, Laboratory for Molecular Neurobiology, and since three years research fellow at the Max Planck Institute for Biophysical Chemistry in Göttingen. The molecular biologist Yael Stern-Bach, formerly postdoc in the neighboring lab of Prof Steven Heinemann and now assistent professor at the Hadassah Dental School, Hebrew University, Jerusalem, created mutated glutamate receptors that did not desensitize. These mutant receptors were than used to understand how neurotransmitter binding leads to the opening of the channel pore.
Using a pharmacological trick, the minute currents flowing through single channels could be observed during slow, stepwise occupation of the binding sites by the neurotransmitter glutamate. Three sizes of current were observed; a small current flow when two glutamate were bound, a medium sized current when three molecules were bound and was fully open with four molecules bound. The easiest interpretation of that result is that the receptor has a tetrameric structure, similar to most voltage dependent channels.
The finding that the current flow depends on the number of subunits activated by the neurotransmitter could change the theory how ligand gated receptors become activated by its ligand in general. Before it was thought that the receptor opens its pore (to various degrees) only when all binding sites are occupied. The new finding suggests that the current flow rather depends on the concerted action of all participating subunits forming the receptor. Recently other groups are finding similar behavior on other ligand-gated and voltage gated channels. This international group continues its collaboration to understand in more detail how these receptor function, particularly of interest is how the specific conformations induced by the binding of the neurotransmitter cause the specific current sizes.
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