Structure of activated Gs

The experiment also focuses on the structure of the activated G protein.
The G protein consists of three subunits: GαS, Gβs and Gγs. Upon activation of the G protein by binding with the receptor, nucleotide exchange occurs. This is when the GDP bound to Gs is released from the Gαs subunit upon complex formation and GTP binds to the now nucleotide free aplha subunit.

This nucleotide exchange causes the dissociation of the Gβγ and Gα subunit from the receptor.

The Gαs subunit further consists of two domains (shown in figure 1):

• GαsRas domain - Ras-like GTPase domain
• GαsAH domain - Alpha helical domain

Figure 1: Structure of Gαs domains and how they interact with the receptor (PDB
ID: 3SN6). (Gβs and Gγs domains hidden in this view. GTP shown as yellow spheres)

Conformational changes

Upon activation of Gs, there are many conformational changes to the protein. These conformational changes were observed by comparing the structure of Gαs in the β₂AR complex with the GTPγS-bound Gαs. GTPγS is an analogue to GTP.

These conformational changes include:

• A 127 degree rotation about the junction between the domains (figure 2a)
• The Gαs α5 helix is displaced 6 Å towards the β₂AR and rotated as the C-terminal end projects into the transmembrane core of the  β₂AR (figure 2b)
• β6-α5 loop is rotated inwards in active form

Figure 2: Structural changes observed in Gαs in the β₂AR complex compared with the GTPγS-bound Gαs. (Ramussen et al, 2011)

a. This image shows the 127 degree rotation of GαsAH in the active GTPγS-bound form (grey) compared to the inactive form (orange). b. This shows the displacement of the α5 helix in the active state (grey) towards the β₂AR (green). (Rasmussen et al, 2011)

Figure 3: Pymol image showing how the β1-α1 and β6-α5 loop 
interact with the phosphates and purine ring, respectively, in the
active GTPγS-bound form (PDB ID: 1AZT).

Upon addition of the GTPγS, the β1-α1 loop interacts with the phosphates in this structure, and the β6-α5 loop interacts with the purine ring (see figure 3). A comparison of these arrangements in the active and non-active form can be seen in figure 4, which shows the relative positions of the loops in the various structures.

Although there has been successful comparisons of the structure of β₂AR-Gs with the active Gαs-GTPγS form, the structure for the GDP-bound Gs heterotrimer has not yet been determined. This means that a direct comparison cannot yet be made between the Gαs-Gβγ interface before and after formation of the β₂AR-Gs complex. 

Figure 4: Comparison of β1-α1 and β6-α5 structures in active form (Green, as above) and inactive form (blue)