Cryo-EM Structure of OSCA1.2 from Oryza sativa: Mechanical basis of hyperosmolality-gating in Plants

Abstract

1Cryo-EM structure of OSCA1.2 from Oryza sativa: Mechanical basis of 2 hyperosmolality-gating in plants 3 4 5Abstract 46Sensing and responding to environmental water deficiency and osmotic stresses is 47 essential for the growth, development and survival of plants. Recently, sensing ion channel called OSCA1 was discovered that functions in sensing 49 hyperosmolality in Arabidopsis. Here, we report the cryo-EM structure and function of 50 an ion channel from rice (Oryza sativa; OsOSCA1.2), showing how it mediates 51 hyperosmolality sensing and ion permeability. The structure reveals a dimer; the 52 molecular architecture of each subunit consists of eleven transmembrane helices and 53 a cytosolic soluble domain that has homology to RNA recognition proteins. The 54transmembrane domain is structurally related to the TMEM16 family of calcium 55 dependent ion channels and scramblases. The cytosolic soluble domain possesses a 56 distinct structural feature in the form of extended intracellular helical arms that is 57parallel to the plasma membrane. These helical arms are well positioned to sense 58 lateral tension on the inner leaflet of the lipid bilayer caused by changes in turgor 59pressure. Computational dynamic analysis suggests how this domain couples to the 60 transmembrane portion of the molecule to open the channel. Hydrogen-deuterium 61 exchange mass spectrometry (HDXMS) experimentally confirms the conformational 62 dynamics of these coupled domains. The structure provides a framework to 63understand the structural basis of hyperosmolality sensing in an important crop plant, 64 extends our knowledge of the anoctamin superfamily important for plants and fungi, 65and provides a structural mechanism for translating membrane stress to ion transport 66 regulation. 67 68Introduction 69Hyperosmolarity and osmotic stress are among the first physiological responses to 70 changes in salinity and drought. Hyperosmolality triggers increases in cytosolic free 71Ca 2+ concentration and thereby initiates an osmotic stress-induced signal transduction 72 cascade in plants (1-3). Salinity and drought stress trigger diverse protective 73 mechanisms in plants enabling enhanced drought tolerance and reduction of water 74 loss in leaves. 75 76Ion channels have long been hypothesized as sensors of osmotic stress. A candidate 77 membrane protein named OSCA was isolated in a genetic screen for mutants that 78impair the rapid osmotic stress-induced Ca 2+ elevation in plants (1). OSCA1 encodes 79 a multi-spanning membrane protein that functions in osmotic/mechanical stress-80induced activation of ion currents. However, the underlying mechanisms and whether 81OSCA1 itself encodes an ion conducting pore specific for Ca 2+ requires further 82analysis. OSCA1 is a member of a larger gene family in Arabidopsis with 15 members 83(4), and with many homologs encoded in other plants and fungal genomes. 84Furthermore, evolutionary analyses have revealed that OSCA is distantly related to 85 the anoctamin superfamily, that includes the TMEM16 family...

Expansion of the Transporter-Opsin-G protein-coupled receptor superfamily with five new protein families

“…Additional evidence supporting the structure of the superfamily has been recently reported [60,61]. These findings were useful for solving the structure of the CSC member OSCA1.2 from Oryza sativa [62] and Arabidopsis thaliana [63], which is distantly related to the fungal homolog TMEM16 (TC: 1.A.17.1.18), an ANO family member with reported 3D structure [64]. Furthermore, after the original submission of this manuscript for publication, a 3D structure of a KDEL member (PDB: 6I6B; TC: 9.…”

Biophysical Principles of Ion-Channel-Mediated Mechanosensory Transduction

“…Based on the gating mechanism of MscL, it has been proposed that for force-fromlipids to effectively gate a mechanosensitive channel, there should be a part of the channel that acts as a sensor by intimately interacting with the lipid bilayer (e.g., MscL N terminus), while being directly connected to a pore-forming unit of the protein (e.g., TM2 in MscL) (Bavi et al, 2016a;Iscla et al, 2008;Kung, 2005;Teng et al, 2015;Martinac et al, 2018). Recent structural evidence from Piezo1 channels and functional data from TREK-1 and OSCA1.2 channels support this idea (Honoré et al, 2006;Saotome et al, 2018;Maity et al, 2019). However, proposing such a mechanism by no means excludes the possibility of long-range allosteric interactions within the protein structure, which may also regulate channel function.…”

“…Since it seems that His1423 residue in NOMPC interacts directly with the lipid bilayer, this specific lipid-protein interaction is also likely to be critical for activation by mechanical stimuli. This proposal seems plausible given the role that horizontal membrane-coupling helices, such as the S4-S5 linker of NOMPC, play in other mechanosensitive channels (Bavi et al, 2016a;Cox et al, 2016b;Maity et al, 2019). How membrane tension is modulated by cytoskeletal proteins is thus an exceedingly relevant question.…”

Mammalian Mechanoelectrical Transduction: Structure and Function of Force-Gated Ion Channels

“…AtOSCA1.1, AtOSCA3.1 and AtOSCA1.2 are dimers with a two-fold symmetry axis perpendicular to the membrane, with each subunit comprising 11 TMs and a pore domain (Jojoa-Cruz et al, 2018;Liu et al, 2018;Maity et al, 2019;Zhang et al, 2018)( Figure 1C). Both subunits are separated by a large central cavity filled with lipids that might help stabilize the complex.…”