Abstract
YnaI is a member of the family of bacterial MscS (mechanosensitive channel of small conductance)-like channels. Channel gating upon hypoosmotic stress and the role of lipids in this process have been extensively studied for MscS, but are less well understood for YnaI, which features two additional transmembrane helices. Here, we combined cryogenic electron microscopy, molecular dynamics simulations and patch-clamp electrophysiology to advance our understanding of YnaI. The two additional helices move the lipid-filled hydrophobic pockets in YnaI further away from the lipid bilayer and change the function of the pocket lipids from being a critical gating element in MscS to being more of a structural element in YnaI. Unlike MscS, YnaI shows pronounced gating hysteresis and remains open to a substantially lower membrane tension than is needed to initially open the channel. Thus, at near-lytic membrane tension, both MscL and YnaI will open, but while MscL has a large pore and must close quickly to minimize loss of essential metabolites, YnaI only conducts ions and can thus remain open for longer to continue to facilitate pressure equilibration across the membrane.