Cryo-EM structure of OSCA1.2 from <i>Oryza sativa</i> elucidates the mechanical basis of potential membrane hyperosmolality gating

TLDR

Plants must sense osmotic stress for survival, and the recently discovered OSCA1 ion channel—whose transmembrane domain resembles the TMEM16 family of calcium‑dependent channels—serves this function. We determined the cryo‑EM structure and function of rice OSCA1.2 to model its role in hyperosmolality sensing and gating. The extended helical arms sense membrane tension and, via computational dynamics, appear to couple to the transmembrane domain to open the transport pathway. The cryo‑EM structure shows a dimeric OSCA1.2 with 11 transmembrane helices and a cytosolic RNA‑recognition‑like domain bearing helical arms that, as confirmed by HDXMS, couple to the membrane to translate turgor‑induced tension into channel opening, providing a structural basis for hyperosmolality sensing in rice and expanding knowledge of the anoctamin superfamily.

Abstract

Sensing and responding to environmental water deficiency and osmotic stresses are essential for the growth, development, and survival of plants. Recently, an osmolality-sensing ion channel called OSCA1 was discovered that functions in sensing hyperosmolality in Arabidopsis . Here, we report the cryo-electron microscopy (cryo-EM) structure and function of an OSCA1 homolog from rice ( Oryza sativa ; OsOSCA1.2), leading to a model of how it could mediate hyperosmolality sensing and transport pathway gating. The structure reveals a dimer; the molecular architecture of each subunit consists of 11 transmembrane (TM) helices and a cytosolic soluble domain that has homology to RNA recognition proteins. The TM domain is structurally related to the TMEM16 family of calcium-dependent ion channels and lipid scramblases. The cytosolic soluble domain possesses a distinct structural feature in the form of extended intracellular helical arms that are parallel to the plasma membrane. These helical arms are well positioned to potentially sense lateral tension on the inner leaflet of the lipid bilayer caused by changes in turgor pressure. Computational dynamic analysis suggests how this domain couples to the TM portion of the molecule to open a transport pathway. Hydrogen/deuterium exchange mass spectrometry (HDXMS) experimentally confirms the conformational dynamics of these coupled domains. These studies provide a framework to understand the structural basis of proposed hyperosmolality sensing in a staple crop plant, extend our knowledge of the anoctamin superfamily important for plants and fungi, and provide a structural mechanism for potentially translating membrane stress to transport regulation.