5.3.4 G proteins
A large family of proteins, called G proteins, are regulated by binding of GTP. All known forms of life on Earth use G proteins to regulate protein synthesis and in eukaryotes there are ten families of G proteins which, between them, regulate many functions, from signal transduction to transport processes, cytoskeletal rearrangements and protein synthesis.
Generally, when they are bound to GTP, G proteins are in their active conformations and can bind to and stimulate effector proteins. Hydrolysis of the GTP is catalysed by the G protein itself; G proteins are therefore also known as a GTPases. On hydrolysis of GTP, the γ-phosphate is released and the GDP remains bound to the protein. In this GDP-bound form, the protein is inactive and cannot bind to effector proteins. Thus there are similarities in the regulation of G proteins with the regulation of proteins by direct phosphorylation.
In the same way that the phosphorylation state of some proteins is determined by the balance of kinase and phosphatase activities, the switch between GTP-bound and GDP-bound conformations of a G protein depends on the activity of other proteins, as well as the G protein's own GTPase activity. Proteins known as GTPase activating proteins (GAPs) enhance the GTPase activity of the G protein whilst guanine nucleotide exchange factors (GEFs) accelerate the dissociation of GDP from the G protein, allowing it to be replaced by GTP. Note that, in some cases, these regulatory functions may reside in intrinsic domains of the G protein itself. We will concentrate here on two of the main families of G protein: small G proteins, of which Ras is an example; and trimeric G proteins.
Small G proteins consist of a single domain of about 200 residues and require extrinsic GAPs to stimulate the hydrolysis of bound GTP. There are a number of different families of small G proteins. Ras is the prototypical small G protein and is involved in transduction of growth factor signals. Other small G proteins are involved in a variety of cellular processes, including intracellular transport, vesicle formation and targeting, cytoskeletal changes and cell polarisation, regulation of cell growth, and assembly of the mitotic spindle.
For now, we will focus on the structural aspects of this protein. Ras (Mr = 21 000) is equivalent to the core GTP-binding domain common to all G proteins. It contains a six-stranded β sheet sandwiched between five α helices. Figure 35 shows the structure of Ras in both its GTP- and GDP-bound forms. The nucleotides bind in a shallow groove formed by loops at the surface of the protein and this binding requires Mg2+ ions.
Trimeric G proteins (Figure 36) have three subunits (α, β and γ).They transduce signals from a family of receptor proteins known as 7-helix transmembrane receptors, of which there are more than 1000. Many species express numerous different α, β and γ subunits and there is considerable diversity in the G proteins assembled from them. Gα subunits have Mr values in the range 39 000–45 000 and consist of two domains: a GTP binding domain similar to the single domain of small G proteins such as Ras, and a domain that further enhances binding of the GTP molecule. Both the Gα and the Gγ subunits are anchored to the intracellular surface of the plasma membrane. Gα is usually myristoylated whilst Gγ is prenylated (Figure 25). The Gβ and Gγ subunits bind to each other very tightly but bind reversibly to Gα. The association of Gα with Gβγ and the activity of these subunits depend on whether GTP or GDP is bound to Gα.
Other G proteins include certain proteins involved in the initiation and elongation stages of translation of mRNA. As well as binding GTP, these proteins bind amino-acyl tRNAs and deliver them to the ribosome. Hydrolysis of the GTP occurs when the appropriate amino acid is added to the peptide.