Electrostatic surface representation of the SLC9C1 monomer and cartoon representation of the half helices TM5a-b and TM12a-b (yellow) and ion-binding site (dotted black circle and purple Na + ion): the crossover half helices are unique to the NhaA-fold and the half-helical dipoles that are formed are denoted (left). Middle, in SLC9C1 Arg431 (green-stick) neutralizes the negatively charged half-helical dipole, and Asp209 (green-stick) neutralizes the positively charged half-helical dipoles. Asn237 and Asp238 in TM6 form the canonical ND motif 15 , 37 , characteristic of electroneutral NHEs. Right, ion-binding site residues for sea urchin SLC9C1 (green sticks) and the corresponding position of ion-binding site residues from horse NHE9 (Protein Data Bank (PDB) 6Z3Z ; light blue). In SLC9C1 a salt-bridge (dashed line) is formed between Arg399 in TM11 and Glu233 in TM6.
