Ga,b,g

PDB file 1GP2

This file depicts a rat heterotrimeric inhibitory G Protein, with bound GDP. This structure was solved by M. A. Wall et al., in 1996.

Recommended display options:

Display as cartoon, and color chain, to distinguish the subunits a (chain a), b (chain b), g (chain g). To see the GDP, select hetero-ligand, display as spacefill, and color CPK.

The WD-repeat in the b subunit forms a sevenfold b propeller.
Select
protein-sheet and specify a different color, to emphasize the b structure.
Locate where the ends of the b protein overlap, with both ends of the protein segment contributing to one of the propeller blades.
Question: Why might this overlap of N and C-terminal ends of the protein have evolved? 

Examine the domain of subunit a that makes contact with subunit b at the face of the b-propeller.
Select
200-215a and color magenta.
This is the "switch II" domain of subunit a, that changes position when GTP replaces GDP, resulting in dissociation of Ga from Gbg.

With your teammate at an adjacent computer, compare this display of a-GDP complexed to bg, and the one of a-GTP (GTP analog). To facilitate this comparison, to make the b & g subunits on this display disappear, use the menu to select chain b then select hide selected; then select chain g and hide selected. Rotate so that both images have the same orientation in relation to the guanine ring and first two phosphates of GDP/GTP.
Compare the position of switch II (should be colored magenta) with GDP and the GTP analog.


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