Combining structure and function in the nicotinic superfamily: the single-channel activation mechanism for the prokaryotic model channel ELIC

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The purpose of our work is to understand how ion channels function as molecules. The key to this is to know the 3-D structure of the channel (by X-ray crystallography) and to find out how this structure moves when the channel is activated, mainly by recording its electrical activity. It is rarely possible to get both sorts of information for the same molecule. New data mean that this is now possible for the group of channels we work on.Ion channels are proteins coded in our genome and are essential components of many cells in our bodies. For instance, they allow cells to communicate with each other at cell-to-cell junctions called synapses. This is essential not only in the brain but also to in the rest of our bodies where it allows the appropriate commands to reach muscles in our limbs and to regulate our blood pressure and heart rate. Channels also allow each neurone to process the information it receives from other neurones. Channels are important for normal human physiology and for disease. Mutations in the genes that code for channels can damage channel function and produce inherited human disease, such as cystic fibrosis. In addition to that, many drugs used for common diseases or in anaesthesia act by binding to channels. In particular, the group of channels that we study, the nicotinic superfamily, are targeted by sleeping pills, drugs for epilepsy, nicotine in tobacco and some insecticides. There are two main sources of information about channels: the first is X-ray crystallography, which provides us with information about the shape of the protein and the second is electrophysiology, which measures the electrical signal the channel produces. In the most advanced form, which is our special expertise, this technique detects the current that passes through a single protein molecule in real time, even though it is very small (more than a billion times smaller than the current in a kettle). This technique is very useful, because it allows us to measure the speed with which molecular events in the function of the channel occur. Hence we can understand channel function precisely as a chemical reaction, quantifying each step, from the binding of the neurotransmitter to the opening of the channel. By doing this in channels in the nicotinic group, we have found how tightly neurotransmitters and drugs bind to the protein when it is active or inactive and why some drugs act more strongly than others. Ideally we should study the structure and the function of the SAME channel. This is not easy because channels are difficult to crystallize, and so far we have good structures only for three channels in this group (GLIC, ELIC and GluCl). Of these, GLIC produces electrical signals that are too small for good electrophysiology. As for GluCl, we don't know how good its signal is (the structure has literally just been published). A potential problem with GluCl is that it does not open like all other channels in the group do, in response to a neurotransmitter-like compound, but it requires TWO different substances, binding to different places, so we don't know how good a model it will be.Until now ELIC was thought not to be able to open. Other scientists have now discovered the right substances that activate ELIC, and it turns out to open well and to give an excellent, big signal. We want to apply the single-molecule recording that is our special skill to ELIC, so that we can understand how it functions as a molecule. Once we have that, we can push our understanding much further, because we can refer to precise structural information (available for ELIC) in interpreting the effect of drugs and the effect of mutations in the channel. This is basic research but it is what is needed if we want to explain what bits of the molecule change and how they move when the channel is activated, where exactly drugs bind to the protein and how we should modify the structure of drugs in order to make them more effective.