FIGURE 2.

Structural changes produced by the S205L mutation. A, magnification of the intersubunit connections in the S129R mutant structure. Here, His¹⁷⁷ engages with Glu¹⁷³ and Asp¹⁷⁵ to form a triad that may stabilize the tetrameric structure. B, magnification of the intersubunit interactions in the S129R/S205L mutant. In this structure, His¹⁷⁷ has changed its rotamer conformation and interacts with Asp¹⁹⁷. Also, a novel interaction is observed between Asp¹⁹⁷ and Arg²⁰². His¹⁷⁷, Asp¹⁹⁷, and Arg²⁰² form a novel triad of interactions at the cytoplasmic domain interface. C, alignment of subunit interface residues of the S129R mutant (gray) and the S129R/S205L mutant (gold). When His¹⁷⁷ is engaged with Glu¹⁷³ and Asp¹⁷⁵, these residues are located within the intersubunit space. By contrast, when His¹⁷⁷ switches its rotamer conformation and interacts with Asp¹⁹⁷ and Arg²⁰², Glu¹⁷³ and Asp¹⁷⁵ are released from the intersubunit space and line the cytoplasmic pore.