Figure 5.

Three ecMscS channels embedded in a membrane. ecMscS is composed of seven identical subunits (each shown in a different color). ecMscS (shown in shades of green) has a transmembrane domain (faintly visible in the membrane in case of the ecMscS closest to the viewer), forming a channel that opens and closes in the presence or absence of significant osmotic pressure across the cell membrane. ecMscS also has a large extra-membrane domain pointing into the cell interior, the cytoplasm. This domain of ecMscS is called the cytoplasmic domain and is prominently visible in the figure for all three ecMscS proteins. The membrane is a bilayer of lipids; lipids are composed of head groups (shown as orange and blue spheres) and tails (shown as white lines). Ions (positive ions shown as blue spheres, negative ions as red spheres) diffuse through the bulk solvent, the cytoplasm, as seen here below the membrane. Ions enter the ecMscS cytoplasmic domains through the side openings into the domain interior and, in case of osmotic stress having induced an opening of the ecMscS trans-membrane channel, pass through the channel towards the space outside of the cell, the periplasm. The intricate geometry of the cytoplasmic domain, a roughly spherical interior connected to the cytosol through seven narrow openings, plays a determining role in the manner in which ions leave an osmotically challenged cell. The labels shown on the figure correspond to: 1. Periplasm; 2. cytoplasm; 3. membrane; 4. cytoplasmic domain of ecMscS; 5. side openings; 6. ions.