FIGURE 3.

Altered helix topologies of monomer and dimer subunits. A, left, the subunits of the ^NcNTD dimer structure are superimposed with the ^EaNTD monomer crystal structure (A72R; PDB: 4FBS) (10). Right, the subunits of the ^EaNTD dimer crystal structure are superimposed with the ^EaNTD monomer crystal structure. Both superimpositions illustrate the structural rearrangement of the five-helix bundle that occurs during the monomer to dimer transition. The semitransparent cyan and red spheres represent disordered loops in the ^NcNTD subunit A and ^EaNTD A72R structures, respectively. B, the two subunits of the ^NcNTD dimer are shown as a ribbon representation and are superimposed as in Fig. 1D. Side chains for residues Asp¹⁷ and Asp⁵³ are shown as sticks, and the handshake interaction (black dashed line) between these two residues in subunit A (cyan) is shown. The semitransparent cyan spheres represent the disordered loop containing residues 83–86 of subunit A. C, H1, H2, and H3 of both ^NcNTD dimer subunits are shown in a ribbon representation, and residues Asp¹⁷ and Asp⁵³ of both subunits are shown as sticks. The subunits are oriented to show the difference in the angle at which H2 crosses H3 due to the presence or absence of the Asp¹⁷-Asp⁵³ handshake interaction (black dashed line).