MscM uses a novel gating mechanism for bacterial mechanosensitive channels

The mechanosensitive channel of small conductance (MscS) is the founding member of the family of MscS-like channels, which share a structurally conserved core but feature additional structural elements that define their specific channel characteristics. Here, we characterize the structure and function of the Escherichia coli mechanosensitive channel of mini conductance (MscM), which features eight additional transmembrane (TM) helices and a large periplasmic domain. Our cryo-EM structures reveal that channel gating involves conformational changes in all domains of MscM. In particular, a cytoplasmic extension of TM7 couples the conformation of the TM domain to that of the cytoplasmic domain, resulting in gating of its lateral fenestrations, where ions enter the channel. Thus, different from all other MscS-like channels studied to date, channel gating in MscM is mediated by its cytoplasmic domain and not the TM domain, which senses changes in membrane tension and operates the cytoplasmic gates.