Mechanosensitive (MS) channels that provide protection against hypoosmotic shock are found in the membranes of organisms from the three domains of life: bacteria, archaea, and eucarya. Two families of ubiquitous MS channels are recognized, and these have been designated the MscL and MscS families. A high-resolution X-ray crystallographic structure is available for a member of the MscL family, and extensive molecular genetic, biophysical, and biochemical studies conducted in many laboratories have allowed postulation of a gating mechanism allowing the interconversion of a tightly closed state and an open state that controls transmembrane ion and metabolite fluxes. In contrast to the MscL channel proteins, which are of uniform topology, the much larger MscS family includes protein members with topologies that are predicted to vary from 3 to 11 alpha-helical transmembrane segments (TMSs) per polypeptide chain. Sequence analyses reveal that the three C-terminal TMSs of MscS channel proteins are conserved among family members and that the third of these three TMSs exhibits a 20-residue motif that is shared by the channel-forming TMS (TMS 1) of the MscL proteins. We propose that this C-terminal TMS in MscS family homologues serves as the channel-forming helix in a homooligomeric structure. The presence of a conserved residue pattern for the putative channel-forming TMSs in the MscL and MscS family proteins suggests a common structural organization, gating mechanism, and evolutionary origin.