極低温電子顕微鏡で解析された水とイオンチャネルの構造と機能

抄録

A biological cell, such as a nerve cell regulates a cell signalling mainly by ion channels. Water movement through membrane should therefore be strictly separated from the movement of ions. This means water channels must be highly specific for water to prevent any ions. The studies on function of a water channel, aquaporin-1 for the last decade put us some puzzling questions. For answering the puzzling questions, structure of aquaporin-1 was analysed at a resolution of 3.8 Å by one of the third generation cryo-EM and electron crystallography. We also analysed a structure of another water channel expressed in brain. The muscle-derived electric organ of the Torpedo electric ray is highly enriched in acetylcholine (ACh) receptor-containing membranes which have been a major source of material for structural study of the receptor. The membranes are readily converted into tubular crystals, having helical symmetry. By imaging of the tubular crystals in thin films of amorphous ice, structure of ACh receptor was analysed at a resolution of 4 Å.The ACh receptor has a cation-selective pore, delineated by a ring of five subunits. In each subunit, four membrane-spanning segments, M1-M4 were predicted and the second membrane-spanning segment, M2, shapes the lumen of the pore and forms the gate of the channel. The gate is a constricting hydrophobic girdle at the middle of the lipid bilayer, formed by weak interactions between neighbouring inner helices. When ACh enters the ligand-binding domain, it triggers rotations of the inner parts of the domains of α-subunits. These rotations are communicated through the inner (M2) helices and open the pore by breaking the girdle apart. We analysed structure of voltage sensitive Na⁺-channel as well as IP₃ receptor by single particle method. By utilizing our cryo-EM we have now good chance to understand functions of channels from structural point of view.