Molecular mechanisms of gating and selectivity in the MscS and MRS2 superfamilies of channels - Project Summary Ion channels are central to electrical excitability, hormone secretion, immune response, and neuronal function. Dysfunction of ion channels underlies an array of pathological conditions such as asthma, hypertension, cancer, diabetes, heart failure and chronic pain. Our long-term goals are to gain a comprehensive understanding of the physicochemical principles underlying ion channel function and to establish new foundations for rational therapeutics. Toward this end, we strive to understand the innerworkings of ion channels using a combination of biochemical, biophysical, and electrophysiological approaches. This proposal focuses on two superfamilies of ion channels, the mechanosensitive channel of small conductance (MscS) superfamily and the MRS2/CorA superfamily of magnesium channels. The MscS superfamily of mechanosensitive channels, found in many organisms including bacteria, fungi, algae, and plants, is crucial for management of osmotic pressure. While literature has well documented that McsS channels can be directly activated by increased lateral membrane tension (‘force-from-lipids’), the biophysical and structural basis of mechanical gating (opening and closing of the channel pore in response to force stimulation) remains elusive, mainly because we cannot apply mechanical force in structural approaches. Our recent innovative structural and functional studies of several MscS homologs have uncovered a potentially unifying gating mechanism depicted as ‘flattening and expansion’ of the transmembrane domain. We are now able to combine electrophysiology, structural biology and computational biology to further address one of the central questions in mechanobiology: how do mechanosensitive channels gate? Magnesium ion, the most abundant divalent cation in living organisms, plays pivotal physiological roles, and abnormal cellular magnesium levels are associated with metabolic disorders including obesity, diabetes and cardiovascular disease. Accumulating literature has established that MRS2, which belongs to the superfamily of CorA magnesium channels that are ubiquitously found in bacteria, fungi, plants and animals, is the molecular conduit that imports cytosolic magnesium into the mitochondrial matrix to regulate mitochondrial metabolism. Intriguingly, we find that human MRS2, unlike its prokaryotic counterpart operating as a magnesium-gated magnesium channel, is instead a calcium-regulated, nonselective cation channel. These unprecedented findings suggest that our understanding of magnesium channel selectivity is still rudimentary. Moreover, the open, conductive conformations are unknown for many of the MRS2/CorA channels. Therefore, mechanistic understanding of the two most fundamental properties of an ion channel, selectivity and gating, remains to be elucidated in the MRS2/CorA superfamily of channels and will be addressed in this proposal.
