The term G protein refers to proteins that bind the nucleotide guanine as guanosine triphosphate (GTP) and guanosine diphosphate (GDP). There are two types of G proteins: heterotrimeric, or large, G proteins and small G proteins. Heterotrimeric G proteins are membrane-associated and, along with G protein-coupled receptors (GPCRs), function primarily in cell signalling and signal transduction. Small GTP-binding proteins function in diverse cellular processes including signal transduction, cytoskeletal reorganization, and vesicle trafficking. The small G protein superfamily includes the Ras family (signal transduction), the Rho/Rac family (cytoskeleton), the Rab and Sar1/Arf families (vescicle trafficking), and the Ran family (nuclear import/export) (Takai et al., 2001).
A molecular switch
G protein activity is dependent on whether it is binding GTP or GDP. This useful property has led to the appropriation of G proteins by many cellular processes to be used as "molecular switches". G proteins are generally thought to be "active" when binding GTP and "inactive" when binding GDP. The transition from the GTP-bound state to the GDP-bound state depends on the hydrolysis of GTP. This GTPase activity is either completely intrinsic to the G protein or is enhanced by another class of proteins, "GTPase activating proteins" (GAPs). The GDP to GTP transition requires the dissociation of GDP, so that GTP may again bind at the active site. Proteins that mediate this GDP dissociation are known as guanine nucleotide exchange factors (GEFs). Figure 1 schematizes the switch mechanism for the Rho protein. Figure 1. Image taken from Luo, 2000.
Heterotrimeric G proteins are unique in that they exist as a complex (Gαβγ) in the GDP-bound state but dissociate (into Gα and Gβγ) upon the release of GDP/binding of GTP.
Heterotrimeric G protein families
Heterotrimeric G proteins have been divided into four families on the basis of sequence similarity: Gs, Gi, Gq, and G12/13. These four families have been shown to have different, but often overlapping, effects on the cell (see figure 3, Neves, 2002). Figure 3. Image taken from Neves, 2002
The original GPCR cell signaling pathway described included Gs proteins. Gαs, among other things, activates adenylate cyclase. Gi pathways are characterized by the ability of Gαi to inhibit adenylate cyclase, and of Gβγ to activate its own downstream effectors, which include phosphatidylinositol 3-kinase (PI3K). The Gq pathway is activated by calcium-mobilizing hormones and acts through inositol trisphosphate (IP3), diacylglycerol (DAG), and protein kinase C (PKC). The G12/13 family is the most recently identified and the least well studied. It is not known as to what extent Gα12 and Gα13 act through distinct effectors (Neves, 2002).
G Protein-Coupled Receptors
Heterotrimeric G proteins associate with 7-transmembrane domain receptors called G protein-coupled receptors (GPCRs) at the cell membrane. There are as many as 865 GPCR-encoding genes in humans (Milligan, 2006). The association of the receptor with all three G protein subunits, Gα, Gβ, and Gγ, requires that GDP is bound to Gα. When the receptor protein is activated with the appropriate ligand, the ligand/receptor complex acts as a GEF, allowing the GDP to dissociate and GTP to bind. The G proteins then dissociate from the receptor and from each other, with only the β- and γ-subunits remaining bound to one another. Gβγ and Gα-GTP may then activate downstream effectors. Figure 2 is a schematic of this dissociation. Gα-GTP is shown activating adenylate cyclase, which produces cyclic adenosine monophosphate (cAMP). cAMP is an important second messenger.Figure 2. Image modified from Firestein, 2001.
For more information on G protein-coupled receptors, see the GPCR wikipedia entry
The Rho/Rac family of small GTPases
The Rho family of small G proteins, which includes Rho, Rac, and CDC42, are important effectors that regulate actin dynamics. These proteins are of particular importance at the growth cone, where they mediate growth and collapse in response to chemoattractants and repellents. Axon guidance receptors are directly or indirectly coupled to Rho GEFs and GAPs, which regulate Rho activity. Figure 4 describes the relationship between Rho, Rac, CDC42, Rho GEF/GAPs, and actin (Huber, 2003).Figure 4. Image taken from Huber, 2003
References
1. Takai Y, Sasaki T, Matozaki T. Small GTP-Binding Proteins. Physiol Rev. 81, 153-208 (2001).
2. Luo L. Rho GTPases in neuronal morphogenesis Nat Rev Neurosci. 1, 173-180 (2000).
3. Milligan G, Kostenis E. Heterotrimeric G-proteins: a short history. Br J Pharmacol. 147 Suppl 1:S46-55 (2006)
4. Firestein, S. How the olfactory system makes sense of scents. Nature 413, 211-218 (2001)
5. Neves S, Ram P, Iyengar R. G protein pathways. Science 296, 1636-1639 (2002)
6. Huber A, Kolodkin A, Ginty D, Cloutier JF. Signaling at the growth cone: ligand-receptor complexes and the control of axon growth and guidance. Ann Rv Neurosci 26, 509-63 (2003)
