Figure 3. HmBRI packing in hexagonal crystals.

(A) Packing in the membrane plane. There are two trimers in the unit cell (shown in yellow and green) that are oriented in opposite directions. (B) Packing of the membrane-like layers. (C) Crystal contacts between the layers. The contacts rely on the ionic interactions between the SO₄ ²⁻ ions and the R172 and K176 side-chains, and on the hydrogen bond between the N166 side-chain of one protomer and the G71 backbone oxygen of another. A second putative SO₄ ²⁻ ion is observed close to the first one that interacts with K176 and R179 side-chains. Potential interactions are marked by dashed lines. F_o-F_c difference electron densities before the insertion of the ions into the model are shown at the level of 3 σ. The symbols ', ” and ’” denote different crystallographic symmetry-related HmBRI molecules. The black oval denotes the crystallographic symmetry rotation axis (C₂).