1.A.23.4.3
Mechanosensative cation-selective channel with a conductance of 100 pS, YnaI (344aas; 4 - 6 TMSs).  Protects against hypoosmotic shock (Edwards et al. 2012).  The structure has been solved by cryo-electron microscopy to a resolution of 13 Å (Böttcher et al. 2015). While the cytosolic vestibule is structurally similar to that in MscS, additional density is seen in the transmembrane region, consistent with the presence of two additional TMSs predicted for YnaI. The location of this density suggests that the extra TMSs are tilted, which could induce local membrane curvature extending the tension-sensing paddles seen in MscS. Off-center lipid-accessible cavities are seen that resemble gaps between the sensor paddles in MscS. The conservation of the tapered shape and the cavities in YnaI suggest a mechanism similar to that of MscS (Böttcher et al. 2015). The voltage dependence of inactivation occurs independently of the positive charges of R46, R54, and R74 (Nomura et al. 2016). A 3.8 Å structure by cryoEM revealed a heptamer structural fold similar to previously studied MscS channels. The ion-selective filter is formed by seven hydrophobic methionines (Met158) in the transmembrane pore (Yu et al. 2017). Details of the gating transition for MscS have been predicted (Zhu et al. 2018). YnaI has a gating mechanism based on flexible pore helices (Flegler et al. 2020), and thus, MscS-like channels of different sizes have a common core architecture but show different gating mechanisms and fine-tuned conductive properties. Attempted Cryo-EM structural determination of detergent-free YnaI Using SMA2000 revealed limitations of this method (Catalano et al. 2021).