State-stabilizing Interactions in Bacterial Mechanosensitive Channel Gating and Adaptation

We outline several principles that we believe define the gating of two bacterial mechanosensitive channels, MscL and MscS. Serving as turgor regulators in bacteria and other walled cells, these molecules are tangible models for studying conformational transitions in membrane proteins driven directly by membrane tension. MscL, a compact pentamer, reversibly opens a gigantic 30-Å pore at near-lytic tensions. MscS, a heptameric complex, exhibits transient activation of a smaller pore at moderate tensions, thereby entering a tension-insensitive inactivated state. By comparing the structures and predicted transitions in these channels, we concluded that opening is commonly achieved through tilting and outward motion of the pore-lining helices, which is kinetically limited by hydration of the pore. The intricate adaptive behavior in MscS appears to depend on specific interhelical associations and the flexibility of the pore-lining helices. We discuss physical factors that may direct the transitions and stabilize main functional states in these channels.Osmotic forces are strong, which necessitated development of osmoregulation along with the first semipermeable membrane delineating the early cell. A simple estimation shows that a 1-μm cell behaving as an ideal osmometer would sustain a downshock no stronger than 20 mm, after which membrane tension would exceed the lytic limit of 10–12 dynes/cm. Thus, a cell without a reinforcing envelope or protective valves is very vulnerable. Free-living and enteric microorganisms cycling through the soil and experiencing drastic environmental changes developed robust mechanisms to maintain volume and integrity (1). The mechanosensitive channels MscS and MscL (mechanosensitive channels of small and large conductance, respectively) have been identified as primary osmolyte release valves limiting the turgor pressure under acute osmotic shock (2–4).Without mscS and mscL genes, Escherichia coli survives a 300 mosm osmotic downshock (2), its resistance attributed to the peptidoglycan layer partially restraining swelling. However, expression of either MscS or MscL allows cells to withstand a 700–800 mosm downshock through release of small osmolytes (2). Purification and reconstitution proved that MscL and MscS respond directly to tension in the lipid bilayer (5–7). Both channels reside in the inner (cytoplasmic) membrane (8), with MscL localized at the cell poles, bearing high curvature (9).As primary components of the turgor regulation system, E. coli MscS and MscL became convenient models for studies of tension-driven conformational transitions in membrane proteins (10). The crystal structures of closed-state Mycobacterium tuberculosis MscL (11) and E. coli MscS in two distinct conformations (12, 13) provided invaluable initial points to explore their gating mechanisms, in which computational methods play increasingly important roles.