The Heterotrimeric G Protein Complex

A Molecular Nanomachine

Hundreds of cell-surface receptors for hormones and other ligands use 'G proteins' to transduce intracellular signalling pathways. Dramatic recent advances by several laboratories have provided exciting details of the structure of the heterotrimeric G proteins. With mating receptor proteins that act like molecular levers, a trio of 'switch domains' that can adopt 'open' or 'closed' conformations, and a beta subunit that resembles a propeller, these proteins may be thought of as molecular 'nanomachines'.

The typical heterotrimeric G protein consists of an alpha subunit of 45-47 kD

a beta subunit of 35 kD

and a gamma subunit of 7-9 kD

The Alpha Subunit

The alpha subunit of heterotrimeric G proteins consists of two domains, a GTPase domain and an alpha-helical domain. The GTPase domain is similar in structure to p21ras and other members of the GTPase superfamily of proteins. It consists of five helices surrounding a six-stranded beta sheet with five strands running parallel and one strand antiparallel to the others. The second of the five helices is a 3(10) helix, rather than an alpha helix.

The alpha-helical domain is unique to the heterotrimeric G proteins and has a long central helix surrounded by five shorter helices .

The alpha helical domain is joined to the GTPase domain by two extended strands, linker 1 (res 54-58 ) and linker 2 (res 173-179 ). Between these two linking segments lies a deep cleft within which the nucleotide (GTP or GDP) is tightly bound . In this image, the nucleotide is GDP and the structure is the inactive complex of GDP and the heterotrimeric G protein.

The alpha subunit of the heterotrimer undergoes a conformation change when GDP is exchanged for GTP. This causes the alpha subunit to dissociate from the beta-gamma complex. Both the alpha-GTP complex and the beta-gamma complex can then act as signals, binding to downstream targets and activating or inhibiting their functions. The structural changes induced by nucleotide exchange are localized to the surfaces of the alpha subunit that contact the beta subunit, including the amino-terminal alpha helix of the alpha subunit . and three adjacent regions on one face of the protein. Since the protein's signalling activity and GTPase activity are switched on and off by this conformation change, these regions are designated Switch I, Switch II and Switch III. Switch I consists of residues 173-183 .
Switch II is comprised of residues 195-215 . and switch III includes residues 227-238 .

The structure of the beta subunit of the G protein heterotrimer is interesting for at least two reasons. First, it is a "beta propeller" protein, with seven distinct beta sheet domains arranged like blades on a propeller. The N-terminus of the beta subunit contains a 25-residue alpha helix . and a loop (residues 26-45) that connects the helix with the propeller blades . .

The propeller blades each comprise a four-stranded antiparallel beta sheet. Using the buttons below, you can separately examine each of the seven propeller blades. Before you begin, click here to reposition the molecule: )
At any time during the viewing of the propeller blades, you can zoom and rock the molecule for a closer look: )

Blade 1 ( )
Blade 2 ( )
Blade 3 ( )
Blade 4 ( )
Blade 5 ( )
Blade 6 ( )
Blade 7 ( )

What is the purpose of the 7-bladed propeller design of the beta subunit? One hypothesis suggests that this design provides up to seven different faces on the protein to interact with target ligands.

The WD motif

The second novel feature of the beta subunit is that it contains multiple copies of the so-called 'WD40' or 'WD' repeat sequence motif, which has been found in a variety of regulatory proteins and enzymes. The general form of the WD motif is:

[x_6-94-(GH-x_23-41-WD]

that is, a variable N-terminal domain followed by a core of relatively constant length and bounded by GH (Gly-His) on one end and WD (Trp-Asp) on the other. Proteins that contain this motif generally contain four to eight repeats of it. There are seven WD motifs in the G protein beta subunit. Interestingly, each of the WD repeats in the beta subunit does not form one beta-blade structure. Rather, each WD motif overlaps two of the beta-blades in the structure.

Click here to see the third WD motif in the beta subunit .

Here are the earmark residues of this WD motif:

In the middle of the domain:

Gly 182

and

His 183

and at the end of the motif:

Trp 211

and

Asp 212

Now take one more look at the third WD motif of the beta subunit. Gly-182 is yellow,
His-183 is green,
Trp-211 is cyan
and
Asp-212 is magenta:

Membrane Interactions

One of the most interesting questions about the heterotrimeric G proteins concerns how they are arranged with respect to the membrane. Both the alpha and gamma subunits are lipid- anchored. The alpha subunit may be myristoylated or palmitoylated on its N-terminus, whereas the gamma subunit may be farnesylated or geranylgeranylated on its C-terminus. Interestingly, these two subunit termini are juxtaposed in the structure shown here. The alpha subunit N-terminus is shown in yellow and the gamma subunit C-terminus is shown in green.

With lipid anchors attaching the heterotrimer to the membrane at these points, we may imagine that the membrane might lie either along the left side of this structure or along the bottom.

You may wish to manipulate this image yourself: