Figure 7.

Canonical interactions between the SA and Switch I regions within the Ras, Rho, and Ran families. Perspective is as shown in Figure 4C. Representations and coloring are as described in Figure 4. (A) Human Ras-GTP (pdb code: 1QRA). Note that at position 23 the canonical Phe or Tyr is replaced by Leu, which is highly conserved in the Ras subfamily. (B) The Ras family Rap2a GTPase from human in complex with GTP (pdb code: 2RAP). In the Rap2 subfamily, the canonical residue occurs at position 23. (C) The Rho family Rac1 GTPase from human (pdb code: 1MH1). (D) Human Ran in complex with importin-β and a GTP analog (pdb code: 1IBR). Note that H30 of Ran is noncanonical and that the canonical hydrogen bond between the previous residue and the SA region involves a N154 side-chain atom rather than a main-chain atom. These deviant features appear to be important to Ran's C-terminal switching mechanism (see Fig. 8).