sau 23 Flashcards
Describe enzyme-coupled receptors briefly
Like GPCRs, enzyme-coupled receptors are transmembrane proteins that display their ligand-binding domains on the outer surface of the plasma membrane. Instead of associating with a G protein, the cytoplasmic domain of the receptor either acts as an enzyme itself or forma complex with another protein that acts as an enzyme. Most of these signal proteins function as local mediators and can act at very low concentrations (about 10-9 to 10-11 M). Responses to them are typically slow (on the order of hours), and their effects may require many intracellular transduction steps that usually lead to a change in gene expression.
Enzyme-coupled receptors, however, can also mediate direct, rapid reconfigurations of the cytoskeleton, changing the cell’s shape and the way that it moves. The extracellular signals that induce such changes are often not diffusible signal proteins, but proteins attached to the surfaces over which a cell is crawling.
The largest class of enzyme-coupled receptors consists of receptors with a cytoplasmic domain that functions as a tyrosine kinase, which phosphorylates particular tyrosines on specific intracellular signaling proteins. These receptors are called receptor tyrosine kinases (RTKs).
Abnormal cell growth, proliferation, differentiation, survival, and migration are fundamental features of a cancer cell, and abnormalities in signaling via RTKs and other enzyme-coupled receptors have a major role in the development of most cancers.
Explain why activated RTKs recruit a complex of intracellular signaling proteins
An enzyme-coupled receptor has to switch on the enzyme activity of its intracellular domain (or of an associated enzyme) when an external signal molecule binds to its extracellular domain. Unlike GPCRs, enzyme-coupled receptor proteins usually have only one transmembrane segment, which spans the lipid bulayer as a single α helix. Because a single α helix is poorly suited to transmit a conformational change across the bilayer, enzyme-coupled receptors have a different strategy for transducing the extracellular signal. In many cases, the binding of an extracellular signal molecule causes two receptor molecules to come together in the plasma membrane, forming a dimer. This pairing brings the two intracellular taiils of the receptors together and activates their kinase domains, such that each receptor tail phoshorylates the other. In the case of RTKs, the phosporylations occur on specific tyrosines.
This tyrosine phosphorylation then triggers the assembly of a transient but elaborate intracellular signaling complex on the cytoslic tail of the receptors. The newly phosphorylated tyrosines serve as docking sites for many of the intracellular signaling proteins (10-20 molecules). Some of these proteins become phosphorlated and activated on binding to the receptors, and they then propagate the signal: others function solely as scaffolds, which couple the receptors to other signaling proteins, thereby helping to build the active signaling complex. All of these docked intracellular signaling proteins possess a specialized interaction domain, which recognizes specific phosphorylated tyrosines on the receptor tails. Other interaction domains allow intracellular signaling proteins to recognize phosphorylated lipids that are produced on the cytosolic side of the plasma membrane in response to certain signals.
As long as they remain together, the signaling protein complexes assembled on the cytosolic tails of the RTKs can transmit a signal along several routes simultaneously to many destinations in the cell, thus activating and coordinating the numerous biochemical changes that are required to trigger a complex response such as cell proliferation or differentiation. To help terminate the response, the tyrosine phosphorylations are reversed by tyrosine phosphatases, which remove the phosphates that were added to the tyrosines of both the RTKs and other intracellular signaling proteins in response to the extracellular signal. In some cases, activated RTKs (as well as some GPCRs) are inactivated in a more brutal way: they are dragged into the interior of the cell by endocytosis and then destroyed by digestion in lysosomes.
Different RTKs recruit different collections of intracellular signaling proteins, producing different effects: however, certain components are used by most RTKs. These include, for example, a phospholipase C that functions in the same way as the phoshoæipase C activated by GCPRs to trigger the inositol phospholipid signaling pathway. Another intracellular signaling protein that is activated by almost all RTKs is a small GTP-binding protein called Ras.
Show how Activation of an RTK stimulates the assembly of an intracellular signaling complex.
Explain how most RTKs activate the monomeic GTPase Ras
The signaling compex Ras - is a small GTP-binding protein that is bound by a lipid tail to the cytosolic face of the plasma membrane. Virtually all RTKs activate Ras, including platelet-derived growth factor (PDGF) receptors, which mediate cell proliferation in would healing, and nerve growth factor (NGF) receptors, which play an important part in the development of certain vertebrate neurons.
The Ras protein is a member of a large family of small GTPbinding proteins, called monomeric GTPases. Ras resembles the α subunit of a G protein and functions as a molecular switch in much the same way. It cycles between two distinct conformational states - active when GTP is bound and inactive when GDP is bound. Interaction with an activating proteins called Ras-GEF encourages Ras to exchange its GDP for GTP, thus switched Ras to its activated state; after a delay, Ras is switched off by a GAP valled Ras-GAP, which promotes the hydrolysis of its bound GTP to GDP.
In its active state, Ras initiates a phosphorylation cascade in which a series of serine/threonine kinases phosphorylate and activate one another in sequence, like an intracellular game of dominoes. This relay system, which carries the signal from the plasma memrane to the nucleus, includes a three-kinase module called the MAP-kinase signaling module, in honor of the final enzyme in the chain, the mitogen-activated protein kinase, or MAP kinase. In this pathway, MAP kinase is phosphorylated and activated by an enzyme called, MAP kinase kinase. This protein is itself switched on by a MAP kinase kinase kinase (which is activated by Ras). At the end of the MAP-kinase cascade, MAP kinase phosphorylates various effector proteins, including certain transcription regulators, altering their ability to control gene transcription. The resulting change in the pattern of gene expression may stimulate cell proliferation, promote cell survival, or induce cell differentiation: the precise outcome will depend on which other genes are active in the cell and what other signals the cell receives.
Show how Ras activates a MAP-kinase signaling module.
Explain how RTKs activate PI 3-kinase to produce lipid docking sites in the plasma membrane
Many of the extracellular signal proteins that stimulate cells to survive and grow, including signal proteins belonging to the insulin-like growth factor (IGF) family, act through RTKs. One crucially important signaling pathway that these RTKs activate to promote cell growth and survival involves the enzyme phosphoinositide 3-kinas (PI 3-kinase), which phosphorylates inositol phospholipids in the plasma membrane. These phosphorylated lipids serve as docking sites for specific intracellular signaling proteins, which relocate one another. One of the most important of these relocated signaling proteins is the serine/threonine kinase Akt.
Akt, also called protein kinase B (PKB), promotes the growth and survival of many cell types, often by inactivating the signaling proteins it phosphorylates. For example, Akt phosphorylates and inactivates a cytoslic protein called Bad. In its active state, Bad encourages the cell to kill itself by indirectly activating a cell-suicide program called apoptosis. Phosphorylation by Akt thus promotes cell survival by inactivating a protein that otherwise promotes cell death.
In addition to promoting cell survival, the PI-3-kinase-Akt signaling pathway stimulates cells to grow in size. It does so by indirectly acticvating a large serine/threonine kinase called Tor. Tor stimulates cells to grwo both by enhancing protein synthesis and by inhiiting protein degradation. The anticancer drug rapamycin works by inactivating Tor, indicating the importance of this signaling pathway in regulating cell growth and survival - and the consequences of its disregulation in cancer.
Show how Activated Akt promotes cell survival.
Show how Akt stimulates cells to grow in size by activating the serine/threonine kinase Tor.
Show how Both GPCRs and
RTKs activate multiple intracellular
signaling pathways.
Explain how some receptors activate a fast track to the nucleus
Not all receptors trigger complex signaling cascades that use multiple components to carry a message to the nucleus. Some tak a more direct route to control gene expression. One such receptor is the protein Notch.
In this simple signaling pathway, the receptor itself acts as a transcription regulator. When activating by the binding of a Delta, a transmembrane signal protein on the surface of a neighboring cell, the Notch receptor is cleaved. This cleavage releases the cytosolic tail of the receptor, which is then free to move to the nucleus, where it helps to activate the appropriate set of Notch-responsive genes.
Explain how some extracellular signal molecules cross the plasma membrane and bind to intracellular receptors
Another direct route to the nucleus is taken by extracellular signal molecules that rely on intracellular receptor proteins. These molecules include the steroid hormones - cortisol, estradiol and testosterone - and the thyroid hormones such as thyroxine. All of these hydrophobic molecules pass through the plasma membrane of the target cell and bind to receptor proteins located in either the cytosol or the nucleus. Regardless of their initial location, these intracellular receptor proteins are referred to as nuclear receptors because, when activated by hormone binding, they enter the nucleus, where they regulate the trancription of genes. In unstimulated cells, nuclear receptors are typically present in an inactive form. When a hormone binds, the receptor undergoes a large conformational change that activates the protein, allowing it to promote or inhibit the transcription of specific target genes. Each hormone binds to a different nuclear receptor, and each receptor acts as at a different set of regulatory sites in DNA. Moreover, a given hormone usually regulates different sets of genes in different cell types, thereby evoking different physiological responses in different target cells.
Show how The steroid hormone
cortisol acts by activating a transcription
regulator.
Explain how protein kinase networks integrate information to control complex cell behaviors
A cell receivevs messages from many sources, and it must integrate this information to generate an appropriate response: to live or to die, to divide, to differentiate, to change shape, to move, to send out chemical messages of its own, and so on. This integration is made possible by connections and interactions that occur between different signaling pathways. Such cross-talk allows the cell to bring together multiple streams of information and react to a rich combination of signals.
The most extensive links among the pathways are mediated by the protein kinases present in each. These kinases often phosphorylate, and hence regulate, components in other signaling pathways, in addition to components in their own pathway.
Many intracellular signaling proteins have several potential phoshprylation sites, each of whih can be phosphorylated by a different protein kinase. These proteins can thus act as integrating devices. Infromation received from different intracellular signaling pathways can converge on such proteins, which then convert a multicomponent input to a single outgoing signal. These integrating proteins, in turn, can deliver a signal to many downstream targets. In this way, the intracellular signaling system may act like a network of nerve cells in the brain - or like a collection of microprocessors in a computer - interpreting complex information and generating complex responses.
Show how Intracellular signaling
proteins serve to integrate incoming
signals.
