Ion channels

Voltage-gated ion channels

Voltage-gated channels (VGICs) open in response to potentials across the membrane, and control functions such as nerve signals and heart beats. All VGICs are comprised of four subunits where each subunit consists of six TM helices, where the first four (S1-4) constitute the voltage-sensing domain (VSD), with the S4 helix and its four Arginines acting as the actual voltage sensor. However, despite a recent high-resolution structure of a Kv1.2/2.1 chimera, the nature of the S4 gating motion has been hotly debated. Based on the observation of 310 helix growth in our previous work, we started to believe the activation occurs by a gradual change in secondary structure as the segment moves, and have done a number of studies where we have calculated how the structural change occurs. We are doing a number of studies in collaboration with the Elinder lab in Linköping, where we have been able to use voltage-clamp electrophysiology to derive large sets of new constraints in several different states, and then used these to derive new models not only of open/closed structures of voltage sensors, but a sequence of intermediate states that for the first time has made it possible to track a complete voltage-sensor cycle.

Ligand-gated ion channels

Synaptic transmission in the nervous system is controlled by ligand-gated ion channels (LGIC) that combine ligand-binding and transmembrane units. Among the most important classes are the Cys-loop receptors that include acetylcholine (AChR), serotonin (5-HT3), glycine (GlyR) and GABA. However, due to the low sequence conservation and lack of high-resolution structures of the trans-membrane domain (TMD), surprisingly little is known about potential allosteric binding sites despite their extreme pharmacological importance. The receptors also differ in the sense that AChR & and 5HT3 are cation-selective, while GlyR and GABAR are anion-selective, i.e. they cause opposite effects! Until recently there were no high-resolution eukaroytic structures at all, but the crystal structure of the open form of the Gloeobacter violaceus pentameric ligand-gated ion channel (GLIC) has been solved at 2.9Å. These largely confirm our previous models for a twisting open/close motion, and the pH-regulation of GLIC has even made it possible for us to model the closing in microsecond-scale simulations. Here too we have successfully used combinations of voltage clamping in combination with molecular simulations, and have suggested a new dual-site model for anesthetic action where there is one inhibiting binding site (inside subunits) and a second potentiating one between subunits. In particular, we have been able to correlate experimental results for single-point-mutants with predictions from free energy calculations, and are working on designing new classes of anesthetics.