15–6 Even though plants and animals independently evolved multicellularity, they use virtually all the same signaling proteins and second messengers for cell–cell communication.
Discuss the following problems.
15–7 Suppose that the circulating concentration of hormone is 10–10 M and the Kd for binding to its receptor is 10–8
M. What fraction of the receptors will have hormone bound? If a meaningful physiological response occurs when 50% of the receptors have bound a hormone molecule, how much will the concentration of hormone have to rise to elicit a response? The fraction of receptors (R) bound to hormone (H) to form a receptor–hormone complex (R–H) is [R–H]/ ([R] + [R–H]) = [R–H]/[R]TOT = [H]/([H] + Kd).
15–8 Cells communicate in ways that resemble human communication. Decide which of the following forms of human communication are analogous to autocrine, paracrine, endocrine, and synaptic signaling by cells.
A. A telephone conversation b.
Talking to people at a cocktail party
C.
D.
Talking to yourself 15–9 Why do signaling responses that involve changes in proteins already present in the cell occur in milliseconds to seconds, whereas responses that require changes in gene expression require minutes to hours?
15–10 How is it that different cells can respond in different ways to exactly the same signaling molecule even when they have identical receptors?
15–11 Why do you suppose that phosphorylation/ dephosphorylation, as opposed to allosteric binding of small molecules, for example, has evolved to play such a prominent role in switching proteins on and off in signaling pathways?
15–12 Consider a signaling pathway that proceeds through three protein kinases that are sequentially activated by phosphorylation. In one case, the kinases are held in a signaling complex by a scaffolding protein; in the other, the kinases are freely diffusible (Figure Q15–1). Discuss the properties of these two types of organization in terms of signal amplification, speed, and potential for cross-talk between signaling pathways.
Figure Q15–1 A kinase cascade organized by a scaffolding protein or composed of freely diffusing components (Problem 15–12).
15–13 Describe three ways in which a gradual increase in an extracellular signal can be sharpened by the target cell to produce an abrupt or nearly all-or-none response.
15–14 Activation (“maturation”) of frog oocytes is signaled through a MAP kinase signaling module. An increase in the hormone progesterone triggers the module by stimulating the translation of Mos mRNA, which is the frog’s MAP kinase kinase kinase (Figure Q15–2). Maturation is easy to score visually by the presence of a white spot in the middle of the brown surface of the oocyte (see Figure Q15–2). To determine the dose–response curve for pro-gesterone-induced activation of MAP kinase, you place 16 oocytes in each of six plastic dishes and add various concentrations of progesterone. After an overnight incubation, you crush the oocytes, prepare an extract, and determine the state of MAP kinase phosphorylation (hence, activation) by SDS polyacrylamide-gel electrophoresis (Figure Q15–3A). This analysis shows a graded response of MAP kinase to increasing concentrations of progesterone.
progesterone Figure Q15–2 Progesterone-induced MAP kinase activation, leading to oocyte Mos maturation (Problem 15–14). (Courtesy of Helfrid Hochegger.) (A) POOLED OOCYTES –+ of frog oocytes (Problem 15–14).
pooled oocytes.
(B) Phosphorylation of