Bacterial mechanosensitive channels protect cells from hypoosmotic shocks by opening and releasing ions and solutes in response to an increase in membrane tension. In the case of the homoheptameric mechanosensitive channel of small conductance (MscS), opening of the ion-conducting pore involves tilting and rotating of its transmembrane helices while the cytoplasmic domain, which contains lateral fenestrations for ion entry, remains unchanged. In addition to MscS, Escherichia coli expresses five MscS-like channels. Here, we structurally characterized the mechanosensitive channel of mini conductance (MscM), an MscS-like channel that features eight additional transmembrane helices and a large periplasmic domain. A cryo-EM structure of MscM in the closed conformation reveals that the periplasmic domain assembles into a ring, while the transmembrane domain is curved. The cryo-EM map of MscM in the open conformation does not resolve the periplasmic ring but shows a flattened and expanded transmembrane domain with a transmembrane pore of ∼20 Å in diameter. The cytoplasmic domain also changes, opening the cytoplasmic fenestrations that are closed in the closed conformation. The conformation of the cytoplasmic domain is coupled to the transmembrane domain through a cytoplasmic extension of transmembrane helix 7. Despite the greater open pore diameter of MscM compared to MscS (∼20 Å vs. ∼14 Å), the conductance of MscM of ∼0.3 nS (vs. ∼1 nS for MscS) is limited by its smaller cytoplasmic fenestrations. We propose that in MscM, the transmembrane domain serves only as the tension sensor, while the cytoplasmic fenestrations have evolved to gate the channel.
Hiotis et al. (Sun,) studied this question.