Position of β hairpins in the NhaA dimer. (A) 2mFo-DFc electron density averaged over the four molecules of the asymmetric unit (contoured at 1.5 σ). The map was calculated directly after molecular replacement using a search model where the β hairpins and loops had been omitted. (B) Cartoon representation of the NhaA dimer viewed from the periplasmic-facing side of the membrane, with the two protomers shown in light brown and green, respectively. (C) The 14–amino acid β hairpins from each promoter, shown in light brown and green, form a four-stranded antiparallel β sheet, as viewed from the cytoplasmic side of the membrane. The residues facing the membrane are hydrophobic except for Lys57. (D) Comparison of the positions of the β hairpins in each of the determined NhaA structures: crystal structure of the dimer (this work; light brown and green), monomeric crystal structure (Hunte et al., 2005; pale green), and dimer from cryo-EM (Appel et al., 2009; yellow).