9.3Intracellular Signal Transduction Pathways Mediated by Receptors
Actual intracellular signal transduction is performed through a combination of the aforementioned mechanisms. The intracellular signal transduction pathways, mediated by four main receptors - enzyme-linked receptors, G protein-coupled receptors, channel receptors and transcription factor receptors, which respond to extracellular signaling molecules - are shown in Figure. 9-5 for discussion on the signal transduction pathways through which a stimulus applied to the cell surface activates nuclear genes.
Many of the enzyme-linked receptors on the plasma membrane have the kinase domain inside the cell. These receptors form dimers when bound with a signaling molecule, and the pair receptors perform mutual tyrosine phosphorylation. In this way, the binding of a signaling molecule to a receptor transduces information to the interior of the cell.
Figure 9-5(1) shows the signal transduction of a growth factor called epidermal growth factor (EGF). Binding with EGF phosphorylates the tyrosine of the receptor protein, and a protein with the SH2 region that recognizes phosphorylated tyrosine binds to it to form an activated complex. This complex activates Ras - a small G protein - by transforming it to the GTP-bound form. The activated Ras regulates nuclear gene expression through a MAPK chain reaction.
Some enzyme-linked receptors have, in addition to tyrosine kinase, a domain that phosphorylates serine or threonine in a protein. Conversely, some proteins have a domain for phosphatase, an enzyme that removes a phosphate group. These proteins also play an important role in signal transduction.
Fig. 9-5. Signal transduction pathways mediated by four main receptors
(1) Enzyme-linked receptors: The signal transduction pathway of a growth factor receptor is shown. When bound with EGF, the two receptors form a dimer and phosphorylates each other. A protein that recognizes and binds to this phosphorylated site activates Ras, a low-molecular-weight G protein, thereby initiating the kinase cascade reaction.
(2) G protein-coupled receptor: When a signaling molecule binds to the G protein trimer, the GDP-GTP exchange reaction occurs in the Gα of the G protein trimer, and the G protein trimer released from the receptor activates target enzymes such as adenylate cyclase and phospholipase C. As a result, the level of cAMP and Ca2+ increase, thereby transduceing signals.
(3) Channel receptor: Binding with a signaling molecule regulates the opening and closing of a channel, thereby transduceing signals.
(4) Transcription factor receptor: since receptors for fat-soluble signaling molecules are located in the cytoplasm and the nucleus, the signaling molecules cross the plasma membrane into these areas, where they bind to the receptors and transduce signals.
Intracellular Signal Transduction by Protein Degradation
As a means of signal transduction, a mechanism is used in which a target protein is cleaved through the activation of a protein cleavage enzyme. The cleaved protein may be activated and used for signal transduction, or may be fully degraded.
As examples of cleavage through activation, membrane-bound proteins may be cleaved and move from the plasma membrane to the nucleus, thereby activating genes, or signals may be transduceed through a chain reaction involving protease, as in the cell death discussed later (Fig. 9-11).
There is another mechanism in which particular proteins are selectively degraded using ATP as energy. Ubiquitin, a small protein, is bound to a protein as a marker for degradation. This ubiquitinated protein is promptly degraded in a proteasome - a complex consisting of many proteins. Since ubiquitination acts on particular proteins, a complex mechanism regulates this process. The action of the plant hormone auxin, which causes the bending of plant stems, is such an example.
G Protein-coupled Receptors
This is a family of receptors that are encoded by more genes in humans than other proteins are (see the Column in 9.4). Approximately on thousand types of G protein-coupled receptors are encoded by the human genome, and are involved in the reception of various kinds of information, from stimuli such as light and odor to the actions of blood pressure regulation hormones. The proteins are given this name because, as shown in Figure. 9-5(2), they penetrate the plasma membrane seven times and are bound with a trimeric G protein in the cytoplasm. The trimeric G protein bound with the receptor consists of three subunits - α, β and γ. When the receptor is activated and the Gα subunit is transformed from the GDP-bound form to the GTP-bound form, Gα is released from the receptor and binds to adenylate cyclase, thereby activating this enzyme. Subsequently, the GTP loses the phosphate group by cleavage and becomes GDP, causing the inactivation of the G protein. cAMP, which is generated by adenylate cyclase, activates a kinase called cAMP-dependent kinase and regulates gene expression.
In addition, Gα released by the receptor, together with a Gβ-Gγ complex, generates inositol trisphosphate by activating an enzyme called phospholipase C. This inositol trisphosphate increases the Ca2+ concentration in the cell by opening the Ca2+ channel on an endoplasmic reticulum. As a result, Ca2+-dependent kinase is activated, and gene expression is regulated.
Channel receptors on the plasma membrane and the endoplasmic reticulum membrane are opened when they are bound with signaling molecules. The resultant changes in intracellular ion concentration transduce signals (Fig. 9-5(3), see Chapter 5).
Transcription Factor Receptors
Fat-soluble steroid hormones such as adrenocortical hormones and male and female hormones (and many other fat-soluble signaling molecules) cross the plasma membrane into cells. Transcription factor receptors that specifically bind to these signaling molecules exist in cells and nuclei (Fig. 9-5(4)). These receptors are also called nuclear receptors, and are a type of transcription factor equipped with a special zinc finger structure for binding to DNA.
In the case of steroid hormones, for example, the receptor binds to a regulatory protein in the cytoplasm. However, if a steroid hormone binds to the receptor, the latter changes its structure and dissociates itself from the regulatory protein. The free receptor then moves to the nucleus and regulates gene expression there.