Review The Concepts

1. Signaling by soluble extracellular molecules can be classified into three types: endocrine, paracrine, and au-tocrine. Describe how these three methods of cellular signaling differ. Growth hormone is secreted from the pituitary, which is located at the base of the brain, and acts through growth hormone receptors located on the liver. Is this an example of endocrine, paracrine, or autocrine signaling? Why?

2. A ligand binds two different receptors with a Kd value of 10~7 M for receptor 1 and a Kd value of 10~9 M for receptor 2. For which receptor does the ligand show the greater affinity? Calculate the fraction of receptors that have a bound ligand ([RL]/RT) for ligand with receptor 1 and for ligand with receptor 2, if the concentration of free ligand is 10~8 M.

3. A study of the properties of cell-surface receptors can be greatly enhanced by isolation or cloning of the cell-surface receptor. Describe how a cell-surface receptor can be isolated by affinity chromatography. How can you clone a cell surface receptor using a functional-expression assay?

4. Signal-transducing trimeric G proteins consist of three subunits designated a, p, 7. The Ga subunit is a GTPase switch protein that cycles between active and inactive states depending upon whether it is bound to GTP or to GDP. Review the steps for ligand-induced activation of effector proteins mediated by the trimeric-G-protein complex. Suppose that you have isolated a mutant Ga subunit that has an increased GTPase activity. What effect would this mutation have on the G protein and the effector protein?

5. Membrane proteins are often found clustered. Describe how protein clustering can be mediated by adapter proteins or by specialized lipid rafts termed caveolae. What advantage might there be to having a cluster of membrane proteins involved in a signaling pathway rather than spread out in the membrane?

6. Epinephrine binds to both p-adrenergic and a-adrener-gic receptors. Describe the opposite actions on the effector protein, adenylyl cyclase, elicited by the binding of epineph-rine to these two types of receptors. Describe the effect of adding an agonist or antagonist to a p-adrenergic receptor on the activity of adenylyl cyclase.

7. In liver and muscle cells, epinephrine stimulates the release of glucose from glycogen by inhibiting glycogen synthesis and stimulating glycogen breakdown. Outline the molecular events that occur after epinephrine binds to its receptor and the resultant increase in the concentration of in-tracellular cAMP. How are the cAMP levels returned to normal? Describe the events that occur after cAMP levels decline.

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8. Continuous exposure of a Gs protein-coupled receptor to its ligand leads to a phenomenon known as desensitiza-tion. Describe several molecular mechanisms for receptor de-sensitization. How can a receptor be reset to its original sensitized state? What effect would a mutant receptor lacking serine or threonine phosphorylation sites have on a cell?

9. A number of different molecules act as second messengers. Activation of rhodopsin by light induces the closing of gated cation channels with cyclic GMP as a second messenger. Describe the effect of light on rhodopsin. On what effector protein does the trimeric G protein act? What type of trimeric G protein is involved in this event?

10. Visual adaptation and receptor desensitization involve similar phosphorylation mechanisms. Describe how the p-adrenergic receptor kinase (BARK) and rhodopsin kinase play important roles in these processes. What role does de-phosphorylation play in these reactions?

11. Inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) are second messenger molecules derived from the cleavage of the phosphoinositide PIP2 (phosphatidylinositol 4,5-bisphosphate) by activated phospholipase C. Describe the role of IP3 in the release of Ca2+ from the endoplasmic reticulum. How do cells replenish the endoplasmic reticulum stores of Ca2+? What is the principal function of DAG?

12. In 1992, the journal .Science named nitric oxide the Molecule of the Year. Describe how this important second messenger is synthesized. How does nitric oxide cause relaxation of smooth muscle cells?

13. Ligand binding to G protein-coupled receptors can result in activation of gene transcription. Describe how the second messengers PIP2 and cAMP can activate transcription of genes.

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