FIGURE 4.

Loops important for nucleotide binding in Ras adopt noncanonical conformations in the Cdc73 C-domain. Structural superposition of the Cdc73 C-domain (green) with Rab33 bound to a GTP analog (Protein Data Bank entry 1Z06). Within the Rab33 structure, the P-loop (magenta), and Switch I and II (red) are indicated, and the position of the GTP analog is shown in sticks. A, loops corresponding to P-loop and Switch I are altered in Cdc73. Residues making important hydrogen bonding interactions with nucleotide and/or Mg ion in Rab33 are indicated as are equivalent residues in Cdc73. B, the Switch II loop region is shifted within Cdc73. Switch II from Rab33 (red) and its interaction with the terminal phosphate is indicated, whereas the position of the equivalent loop in the C-domain is shown in green.