Nematocida displodere Mechanosensitive Ion Channel of Small Conductance 2 assembles into a unique 6-channel super-structure in vitro

Mechanosensitive ion channels play an essential role in reacting to environmental signals and sustaining cell integrity by facilitating ion flux across membranes. For obligate intracellular pathogens like microsporidia, adapting to changes in host environment is crucial for survival and propagation. Despite representing a eukaryote of extreme genome reduction, microsporidia have expanded the gene family of mechanosensitive ion channels of small conductance (mscS) through repeated gene duplication and horizontal gene transfer. All microsporidian genomes that are characterized to-date contain mscS genes of both eukaryotic and bacterial origin, and have at least 5 different mscS copies. Here, we investigated the cryo-electron microscopy structure of the bacterially derived mechanosensitive ion channel of small conductance 2 (MscS2) from Nematocida displodere, an intracellular pathogen of Caenorhabditis elegans. MscS2 is the most compact MscS known, and assembles into a unique superstructure in vitro with six heptameric MscS2 channels oligomerizing through their transmembrane domains. Individual MscS2 channels are oriented in a heterogeneous manner to one another, resembling an asymmetric, flexible six-way cross joint. Finally, we show that, despite the extreme compaction, microsporidian MscS2 still forms a heptameric membrane channel, conserving the most important structural features of bacterial MscS.