logo_sm.gif (1977 bytes)Ras Changes Conformation upon "Switching"

The two forms of  ras (i.e. rasGTP and   rasGDP) have different conformations. If one looks at the locations of the alpha carbons of each residue in both structures you find that, although there are small changes in conformation throughout the ras molecule, two small sections show major changes in structure as the molecule switches from the GDP complex to the GTP complex (or vice versa). These section are called the "switch regions". Each switch region comprises about eight amino acids. Thus, only about ten percent of the protein (22 out of about 170 residues total) undergoes major structural changes upon switching from "off" to "on".

Use the CHIME model shown below to explore this conformational switch:

Highlight switch regions (toggle on/off)

residues 30-38 and residues 57-74

Switch Conformations (toggle between GTP & GDP complexes)

Space-filled Model


Cartoon Model (default)


Stick Model (all atoms)


Carefully examine the spacefilled model to see how the surface of the protein changes. It is the differences in this surface that cause effector molecules to bind to the rasGTP complex  but not the rasGDP complex.

Examine the cartoon model to see how the secondary structure changes. Notice how residues 64-74 (at one end of the "orange group" of amino acids) form an alpha helix in the rasGDP complex. This helix "loosens up" upon the conformational change to the GTP complex. (It is still recognizable as a helix, but does not have the proper geometry to be an alpha helix.)

At the other end of the "orange group" of amino acids one residue (alanine 58) is "added" to a strand of beta structure when GDP is replaced with GTP. At the same time two residues from the "violet group" (35 and 36) are also converted to beta structure. Taken together these changes cause a lengthening of a section of beta sheet when the ras protein switches from the GDP complex to the GTP complex.

Copyright 2000, 2007 F.R. Gorga Last update: