Fig. 1.

Structure and classification of HKT proteins. (A) Basic structure of the HKT proteins. HKTs have eight transmembrane domains (M1_A–M2_D) and four pore‐loop domains (P_A–P_D). Serine (blue) and glycine (green) residues responsible for Na⁺/K⁺ selection are indicated by arrows in the first pore‐loop region, which is known as the selectivity filter position. N and C indicate N‐ and C‐terminus of HKT proteins. (B) HKT1 proteins are divided into two classes, classes I and II. Most members of class I possess a serine residue in the selectivity filter position, whereas most class II transporters contain a glycine residue at the same position.