1. Name three features common to the activation of cytokine receptors and receptor tyrosine kinases. Name one difference with respect to the enzyme activity of these receptors.

2. Erythropoietin (Epo) is a hormone that is produced naturally in the body in response to low O₂ levels in the blood. The intracellular events that occur in response to Epo binding to its cell-surface receptor are well characterized. What molecule translocates from the cytosol to the nucleus after (a) JAK2 activates STAT5 and (b) GRB2 binds to the Epo receptor? Why did some endurance athletes use Epo to improve their performance (“blood doping”) until it was banned by most sports?

3. Explain how expression of a dominant-negative mutant of JAK blocks the erythropoietin (Epo)-cytokine signaling pathway.

4. Even though GRB2 lacks intrinsic enzyme activity, it is an essential component of the epidermal growth factor (EGF) signaling pathway that activates MAP kinase. What is the function of GRB2? What roles do the SH2 and SH3 domains play in the function of GRB2? Many other signaling proteins possess SH2 domains. What determines the specificity of SH2 interactions with other molecules?

5. Once an activated signaling pathway has elicited the proper changes in target-gene expression, the pathway must be inactivated. Otherwise, pathological consequences may result, as exemplified by persistent growth factor–initiated signaling in many cancers. Many signaling pathways possess intrinsic negative feedback by which a downstream event in a pathway turns off an upstream event. Describe the negative feedback that down-regulates signals induced by (a) erythropoietin and (b) TGF-β.

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6. A mutation in the Ras protein renders Ras constitutively active (Ras^D). What is constitutive activation? How does constitutively active Ras promote cancer? What type of mutation might render the following proteins constitutively active: (a) Smad3, (b) MAP kinase, and (c) NF-κB?

7. The enzyme Ste11 participates in several distinct MAP kinase signaling pathways in the budding yeast S. cerevisiae. What is the substrate for Ste11 in the mating factor signaling pathway? When a yeast cell is stimulated by mating factor, what prevents the induction of osmolytes required for survival in high-osmotic-strength media, given that Ste11 also participates in the MAP kinase pathway initiated by high osmolarity?

8. Describe the events required for full activation of protein kinase B. Name two effects of insulin mediated by PKB in muscle cells.

9. Describe the function of the PTEN phosphatase in the PI-3 kinase signaling pathway. Why does a loss-of-function mutation in PTEN promote cancer? Predict the effect of constitutively active PTEN on cell growth and survival.

10. Binding of TGF-β to its receptors can elicit a variety of responses in different cell types. For example, TGF-β induces plasminogen activator inhibitor 1 in epithelial cells and specific immunoglobulins in B cells. In both cell types, Smad3 is activated. Given the conservation of the signaling pathway, what accounts for the diversity of the response to TGF-β in various cell types?

11. How is the signal generated by binding of TGF-β to cell-surface receptors transmitted to the nucleus, where changes in target-gene expression occur? What activity in the nucleus ensures that the concentration of active Smads closely reflects the level of activated TGF-β receptors on the cell surface?

12. The extracellular signaling protein Hedgehog can remain anchored to cell membranes. What modifications to Hedgehog enable it to be membrane bound? Why is this property useful?

13. Explain why loss-of-function hedgehog and smoothened mutations yield the same phenotype in flies, but a loss-of-function patched mutation yields the opposite phenotype.

14. Most mammalian cells have a single immobile cilium called the primary cilium, in which intraflagellar transport (IFT) motor proteins (discussed in greater detail in Chapter 18) move elements of the Hedgehog (Hh) signaling pathway along microtubules. What parts of the Hh signaling pathway would mutations in the IFT motor proteins Kif3A, Kif7, and dynein disrupt?

15. Why is the signaling pathway that activates NF-κB considered to be relatively irreversible compared with cytokine or RTK signaling pathways? Nonetheless, NF-κB signaling must be down-regulated eventually. How is the NF-κB signaling pathway turned off?

16. Describe two roles for polyubiquitinylation in the NF-κB signaling pathway.

17. What feature of Delta ensures that only neighboring cells are signaled?

18. What biochemical reaction is catalyzed by γ-secretase? Why was it proposed that a chemical inhibitor of this activity might be a useful drug for treating Alzheimer’s disease? What possible side effects of such a drug would complicate this use?