Cryo-EM structure of the mechanically activated ion channel OSCA1.2

Abstract

Mechanically activated (MA) ion channels underlie touch, hearing, shear-stress sensing, and response to turgor pressure 1,2 . Previously reported as putative ion channels sensitive to osmolality 3-5 , members of the OSCA/TMEM63 family have been identified as a conserved class of eukaryotic MA ion channels that can be opened directly by membrane tension 6 . The OSCA/TMEM63 family are not homologous to other known MA ion channels, and the structural underpinnings of their function remain unexplored. Here, we report cryo-electron microscopy (cryo-EM) structures of OSCA1.2 from Arabidopsis thaliana in lipidic nanodiscs and lauryl maltose neopentyl glycol (LMNG) detergent at overall resolutions of 3.1 and 3.5 A, respectively. The structures reveal that OSCA1.2 is a trapezoid-shaped homodimer with each subunit containing 11 transmembrane (TM) helices and a folded intracellular domain (ICD) that mediates dimerization. The TM organization of OSCA1.2 has unexpectedly close resemblance to the TMEM16 family of 10 TM helix-containing calcium-dependent ion channels and scramblases 7-9 . We locate the ion permeation pathway within each subunit by demonstrating that a conserved acidic residue is a determinant of channel conductance. Molecular dynamics (MD) simulations provide insights regarding the local membrane environment and lipid interactions, suggesting how OSCA1.2 may be gated by membrane tension. Our work lays the foundation for physiological and biophysical studies on a conserved family of proteins with a newly assigned function as MA ion channels. Moreover, given the role of OSCA proteins as mechanosensors regulating the osmotic stress response in plants 3,6 , our structure could inform strategies to improve plant drought and salt tolerance.