Figure 3: The crucial role of the N-terminal domain in the gating of MscL shown by a parametric FE simulation.

(a) Mesh representation of a subunit of WT EcMscL obtained from an EcMscL homology model based on MtMscL (PDB: 2OAR; left panel—red mesh)¹⁷ and the FE model of a subunit of EcMscL without the C-terminal domain (right panel—solid red rods). The α-helices are modelled as elastic rods and the loops are modelled as nonlinear springs. (b) The FE structure of EcMscL is embedded into the lipid bilayer with the mesh distribution shown. (c) Superposition of FE EcMscL open structure with a previously obtained restrained MD simulation of EcMscL²⁰. (d) Effective (Von Mises) stress distribution in the open state (top view). The membrane tensional stress is made dimensionless using the Young’s modulus of EcMscL, E, that is, stress/E. The nondimensional stress is 0.6. (e,f) Channel pore in an expanded state, with and without the N terminus (top view). The nondimensional exerted stress on the membrane is 0.3 in both models, and thus they do not represent fully open structures^20,35. The light grey dashed circles in e,f represent the position of the effective pore with respect to the plane of the membrane. This diameter is, however, not the actual pore size, since it does not show the side chains on each TM1. The effective pore diameter of the WT model is ∼24 Å, and the model that lacks the N terminus is ∼18 Å. (g) Side view of a WT subunit showing that the angle between the N-terminal domain and the TM1 helix increases as the channel begins to gate. Moreover, they both tilt upwards towards the membrane midplane as the membrane is stretched. (h) TM1 has less out-of-plane tilting in the absence of the N terminus (θ*=33°) compared with the WT channel (θ=45°). Overall, these results suggest that the N-terminal helices have a significant role in transferring the force from the lipid to the pore-lining TM1 helix, guiding both its tilting and expansion.