State-stabilizing Interactions in Bacterial Mechanosensitive Channel Gating and Adaptation

Abstract

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-A 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.

FIGURE 1.

Modeled gating cycle of MscS. The splayed peripheral TM1 (gold) and TM2 (green) helices of the crystal structure are aligned with the TM3a segments (cyan) in simulations, and the missing N terminus (red) was modeled de novo (44). The reconstructed resting state (lower left) with the closed gate (Leu¹⁰⁵ and Leu¹⁰⁹; yellow van der Waals spheres) equilibrated in the explicit lipid bilayer, and TM3s bent at Gly¹²¹. Opening results in kink-free TM3s dilating the gate by 8 Å (lower right). Inactivation is associated with re-forming of the crystallographic kink at Gly¹¹³ and uncoupling of the TM1-TM2 pairs from the gate-bearing TM3s.