3D structure of Tolerant Nona Bokra (orange cartoon) embedded in a membrane bilayer (membrane head-group P and N atoms are shown in blue and tan beads). The position of Val395 and Asp332 (shown in sticks) are just at the opposite end of the channel.
HKT1;5 is a ion transporter which removes excess sodium from the shoots back to the roots
HKT1;5 has functional variability among salt sensitive and tolerant varieties and multiple sequence alignment of sequences of HKT1;5 from Oryza species and P. coarctata showed 4 major amino acid substitutions (140 P/A/T/I, 184 H/R, D332H, V395L). A molecular dynamics simulation study showed that valine is unable to generate strong hydrophobic network with its surroundings in comparison to leucine due to reduced side chain length. The resultant alteration in pore rigidity increases the likelihood of Na+ transport back from xylem sap (shoot) to parenchyma (root) and further to soil. The model also proposes that the presence of aspartate at the 332 position possibly leads to frequent polar interactions with the extracellular loop polar residues which may shift the loop away from the opening of the constriction at the pore and therefore permit easy efflux of the Na+. These two substitutions of the HKT1;5 transporter probably help tolerant varieties maintain better Na+/K+ ratio for survival under salt stress.