4. Protein tyrosine kinases induce phosphorylation cascades

26.3 G proteins may activate or inhibit target proteins

Key terms defined in this section

G proteins are guanine nucleotide-binding proteins. Trimeric G proteins are associated with the plasma membrane. When bound by GDP the trimer remains intact and is inert. When the GDP is replaced by GTP, the a subunit is released from the bg dimer. Either the a monomer or the bg dimer then activates or represses a target protein. Monomeric G proteins are cytosolic and work on the same principle that the form bound to GDP is inactive, but the form bound to GTP is active.
Receptor is a transmembrane protein, located in the plasma membrane, that binds a ligand in a domain on the extracellular side, and as a result has a change in activity of the cytoplasmic domain. (The same term is sometimes used also for the steroid receptors, which are transcription factors that are activated by binding ligands that are steroids or other small molecules.)
Second messengers are small molecules that are generated when a signal transduction pathway is activated. The classic second messenger is cyclic AMP, which is generated when adenylate cyclase is activated by a G protein (when the G protein itself was activated by a transmembrane receptor).
Serpentine receptor has 7 transmembrane segments. Typically it activates a trimeric G protein.

G proteins transduce signals from a variety of receptors to a variety of targets. The components of the general pathway can be described as:

• The receptor is a resident membrane protein that is activated by an extracellular signal.
  
• A G protein is converted into active form when an interaction with the activated receptor causes its bound GDP to be replaced with GTP.
  
• An effector is the target protein that is activated (or Xless often Xinhibited) by the G protein; sometimes it is another membrane-associated protein.
  
• Second messengers are small molecules that are released as the result of activation of (certain types) of effectors.

Another terminology that is sometimes used to describe the relationship of the components of the transduction pathway is to say that the receptor is upstream of the G protein, while the effector is downstream.

Figure 26.10 Classes of G proteins are distinguished by their effectors and are activated by a variety of transmembrane receptors.

The effectors linked to different types of G proteins are summarized in Figure 26.10. The important point is that there is a large variety of G proteins, activated by a wide variety of receptors. The activation of an individual G protein may cause it to stimulate or to inhibit a particular effector; and some G proteins act upon multiple effectors (causing the activation in turn of multiple pathways). Two of the classic G proteins are G_s, which stimulates adenylate cyclase (increasing the level of cAMP), and G_t, which stimulates cGMP phosphodiesterase (decreasing the level of cGMP). The cyclic nucleotides are a major class of second messengers; another important group consists of small lipid molecules, such as inositol phosphate or DAG (diacylglycerol; for review see Divecha and Irvine, 1995).

Although the receptors that couple to G proteins respond to a wide variety of ligands, they have a common type of structure and mode of binding the ligand. They are serpentine receptors, with 7 transmembrane regions, and function as monomers. The greatest conservation of sequence is found in hydrophobic transmembrane regions, which in fact are used to classify the serpentine receptors into individual families (for review see Strader, 1994).

The binding sites for small hydrophobic ligands lie in the transmembrane domains, so that the ligand becomes bound in the plane of the membrane. The smallest ligands, such as biogenic amines, may be bound by a single transmembrane segment. Larger ligands, such as extended peptides, may have more extensive binding sites in which extracellular domains provide additional points of contact. Large peptide hormones may be bound mainly by the extracellular domains.

When the ligand binds to its site, it triggers a conformational change in the receptor that causes it to interact with a G protein. A well characterized (although not typical) case is that of rhodopsin, which contains a retinal chromophore covalently linked to an amino acid in a transmembrane domain. Exposure to light converts the retinal from the 11-cis to the all trans conformation, which triggers a conformational change in rhodopsin that causes its cytoplasmic domain to associate with the G_t protein (transducin).

Figure 26.11 Activation of G_s causes the a subunit to activate adenylate cyclase.

G proteins are trimers whose function depends on the ability to dissociate into an α monomer and a βγ dimer. The dissociation is triggered by the activation of an associated receptor. In its inactive state, the α subunit of the G protein is bound to GDP. Figure 26.11 shows that the activated receptor causes the GDP to be replaced by GTP. This causes the G protein to dissociate into a free α-GTP subunit and a free βγ dimer. (The basic interactions of G proteins are reviewed in the supplement G proteins).

The interaction between receptor and G protein is catalytic. After a G protein has dissociated from an activated receptor, the receptor binds another (inactive) trimer, and the cycle starts again. So one ligand-receptor complex can activate many G protein molecules in a short period, amplifying the original signal.

The most common pathway for the next stage in the pathway calls for the activated α subunit to interact with the effector. In the case of G_s, the α_s subunit activates adenylate cyclase; in the case of G_t, the α_t subunit activates cGMP phosphodiesterase. In other cases, however, it is the βγ dimer that interacts with the effector protein. In some cases, both the α subunit and the βγ dimer interact with effectors (for review see Neer and Clapham, 1988; Clapham and Neer, 1993; Neer, 1995).

Consistent with the idea that it is more often the α subunits that interact with effectors, there are more varieties of α subunits (16 known in mammals) than of β (5) or γ subunits (11). However, irrespective of whether the α or βγ subunits carry the signal, the common feature in all of these reactions is that a G protein acts upon an effector enzyme that in turn changes the concentration of some small molecule(s) in the cell.

In either the intact or dissociated state, G proteins are associated with the cytoplasmic face of the plasma membrane. But the individual subunits are quite hydrophilic, and none of them appears to have a transmembrane domain. The βγ dimer has an intrinsic affinity for the membrane because the γ subunit is prenylated. The α_i and α_o types of subunit are myristoylated, which explains their ability to remain associated with the membrane after release from the βγ dimer. The α_s subunit is palmitoylated.

Because several receptors can activate the same G proteins, and since (at least in some cases) a given G protein has more than one effector, we must ask how specificity is controlled. The most common model is to suppose that receptors, G proteins, and effectors all are free to diffuse in the plane of the membrane. In this case, the concentrations of the components of the pathway, and their relative affinities for one another, are the important parameters that regulate its activity. We might imagine that an activated α-GTP subunit scurries along the cytoplasmic face of the membrane from receptor to effector. But it is also possible that the membrane constrains the locations of the proteins, possibly in a way that restricts interactions to local areas. Such compartmentation could allow localized responses to occur (for review see Spring, 1997).

Reviews

Clapham, D. E. and Neer, E. J. (1993). New roles of G protein (&#szlig;g-dimers in transmembrane signaling. Nature 365, 403-406.

Divecha, N. and Irvine, R. F. (1995). Phospholipid signaling. Cell 80, 269-278.

Neer, E. J. (1995). Heterotrimeric G proteins: organizers of transmembrane signals. Cell 80, 249-257.

Neer, E. J. and Clapham, D. E. (1988). Roles of G protein subunits in transmembrane signaling. Nature 333, 129-134.

Spring, S. R. (1997). G protein mechanisms: insights from structural analysis. Ann. Rev. Biochem 66, 639-678.

Strader, D. (1994). Structure and function of G protein-coupled receptors. Ann. Rev. Biochem 63, 101-132.