Extracellular cation binding pocket is essential for ion conduction of OsHKT2;2

Authors: RIEDELSBERGER, JANIN - University Of Talca; VERGARA-JAQUE, ARIELA - University Of Talca; Pineros, Miguel; DREYER, INGO - University Of Talca; GONZALEZ, WENDY - University Of Talca

Submitted to: BMC Plant Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/27/2019
Publication Date: 7/15/2019
Citation: Riedelsberger, J., Vergara-Jaque, A., Pineros, M., Dreyer, I., Gonzalez, W. 2019. Extracellular cation binding pocket is essential for ion conduction of OsHKT2;2. Biomed Central (BMC) Plant Biology. 19(1);316.

Interpretive Summary: High salt accumulation in agricultural soils underlies an important abiotic stress that limits crop productivity. Salt toxicity experienced in these environmental is due to the fact that at high enough levels Na+ ions can mirror the functions of K+ ions, outcompeting it and causing K+ deficiency symptoms in plants. Plants have developed refined mechanisms to cope with salt stress, allowing them to maintain proper balance between sodium (Na+) and potassium (K+) internal levels in order to achieve normal plant growth. HKTs belong to a family of membrane transport proteins that mediate Na+ and K+ transport in plants, and thereby play a crucial role in regulating Na+ usage and detoxification. However, the fundamental biochemical, functional, and structural characteristics of these membrane transport proteins is largely unknown. This study investigates the structural – functional relations of the rice OsHKT2;2 transporter, providing information of the molecular nature of an extracellular region in the protein which is involved in the recognition and binding of the transported ion. Given the functional and natural diversity identified on HKTs, and therefore their idealness as targets to improve salinity tolerance, understanding the structural and functional relation of this family of membrane transporters provides an initial platform for future efforts aimed at regulating Na+ usage in crops.

Technical Abstract: HKT channels mediate sodium uniport or sodium and potassium symport transport in plants. Monocotyledons express a much higher number of HKT proteins than dicotyledons, and it is only within this clade of HKT channels that cation symport mechanisms are found. The prevailing ion composition in the extracellular medium affects the transport abilities of various HKT channels by changing their selectivity or ion transport rates. How this mutual effect is achieved at the molecular level is still unknown. Here, we built a homology model of the monocotyledonous OsHKT2;2, which shows sodium and potassium symport activity, and performed molecular dynamics simulations in the presence of sodium and potassium ions. By analysing ion-protein interactions, we identified a cation binding pocket on the extracellular protein surface, which is formed by residues P71, D75, D501 and K504. Proline and the two aspartate residues coordinate cations, while K504 forms salt bridges with D75 and D501 and may be involved in the forwarding of cations towards the pore entrance. Functional validation via electrophysiological experiments confirmed the biological relevance of the predicted ion binding pocket and identified K504 as a central key residue. Mutation of the cation coordinating residues affected the functionality of HKT only slightly. Additional in silico mutants and simulations of K504 supported experimental results.