Concept 11.4 Response: Cell signaling leads to regulation of transcription or cytoplasmic activities

Question 1

We now take a closer look at the cell’s subsequent response to an extracellular signal—what some researchers call the

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“output response.”

Question 2

The response at the end of the pathway may occur in the

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nucleus of the cell or in the cytoplasm

Question 3

Many signaling pathways ultimately regulate protein synthesis, usually by turning specific

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genes on or off in the nucleus.

Question 4

Like an activated steroid receptor (see Figure 11.9), the final activated molecule in a signaling pathway may function as a

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transcription factor.

Question 5

The response to this growth factor signal is transcription, the synthesis of one or more specific mRNAs, which will be translated in the

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cytoplasm into specific proteins.

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In other cases, the transcription factor might regulate a gene by

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turning it off

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Sometimes a signaling pathway may regulate the activity of proteins rather than causing their

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synthesis by activating gene expression

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This directly affects proteins that function outside the

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nucleus

Question 9

The final step in the signaling pathway that begins with epinephrine binding activates the

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enzyme that catalyzes the breakdown of glycogen.

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participate in a variety of pathways, leading to both nuclear and cytoplasmic responses, including cell division

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Signal receptors, relay molecules, and second messengers

Question 11

Malfunctioning of growth factor pathways like the one in Figure 11.15 can contribute to abnormal cell division and the development of

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cancer

Question 12

Whether the response occurs in the nucleus or in the cytoplasm, it is not simply turned ______________ Rather, the extent and specificity of the response are regulated in multiple ways.

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“on” or “off.”

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Here we’ll consider four aspects of this regulation. First, as mentioned earlier, signaling pathways generally

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amplify the cell’s response to a single signaling event.

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The degree of amplification depends on the

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function of the specific molecules in the pathway

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Second, the many steps in a multistep pathway provide control points at which the cell’s response can be further

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regulated, contributing to the specificity of the response and allowing coordination with other signaling pathways.

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Third, the overall efficiency of the response is enhanced by the presence of proteins known as

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scaffolding proteins.

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Finally, a crucial point in regulating the response is the

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termination of the signal.

Question 18

Elaborate enzyme cascades amplify the cell’s response to a

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signal.

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At each catalytic step in the cascade, the number of activated products can be

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much greater than in the preceding step

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For example, in the epinephrine-triggered pathway in Figure 11.16, each adenylyl cyclase molecule catalyzes the formation of 100 or so cAMP molecules, each molecule of protein kinase A phosphorylates about

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10 molecules of the next kinase in the pathway, and so on.

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The amplification effect stems from the fact that these proteins persist in their

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active form long enough to process multiple molecules of substrate before they become inactive again.

Question 22

As a result of the signal’s amplification, a small number of epinephrine molecules binding to receptors on the surface of a liver cell or muscle cell can lead to the

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release of hundreds of millions of glucose molecules from glycogen.

Question 23

Consider two different cells in your body—a liver cell and a heart muscle cell, for example. Both are in contact with your

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bloodstream and are therefore constantly exposed to many different hormone molecules, as well as to local regulators secreted by nearby cells.

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Yet the liver cell responds to some signals but ignores others, and the same is true for the

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heart cell.

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epinephrine stimulates the liver cell to break down glycogen, but the main response of the heart cell to epinephrine is

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contraction, leading to a more rapid heartbeat.

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The explanation for the specificity exhibited in cellular responses to signals is the same as the basic explanation for virtually all differences between cells: .

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Because different kinds of cells turn on different sets of genes, different kinds of cells have different collections of proteins

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The response of a cell to a signal depends on its particular collection of

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signal receptor proteins, relay proteins, and proteins needed to carry out the response.

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In many cases, the efficiency of signal transduction is apparently increased by the presence of

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scaffolding proteins, large relay proteins to which several other relay proteins are simultaneously attached

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Researchers have found scaffolding proteins in brain cells that permanently hold together

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networks of signaling pathway proteins at synapses.

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This hardwiring enhances the

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speed and accuracy of signal transfer between cells because the rate of protein-protein interaction is not limited by diffusion.

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The importance of the relay proteins that serve as points of branching or intersection in signaling pathways is highlighted by the problems arising when these proteins are

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defective or missing.

Question 32

in an inherited disorder called __________________________ , the absence of a single relay protein leads to such diverse effects as abnormal bleeding, eczema, and a predisposition to infections and leukemia.

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Wiskott-Aldrich syndrome (WAS)

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These symptoms are thought to arise primarily from the

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absence of the protein in cells of the immune system.

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By studying normal cells, scientists found that the WAS protein is located just beneath the

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immune cell surface.

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The protein interacts both with microfilaments of the cytoskeleton and with several different components of signaling pathways that relay information from the

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cell surface, including pathways regulating immune cell proliferation.

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This multifunctional relay protein is thus both a branch point and an important intersection point in a complex signal transduction network that controls

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Immune cell behavior

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When the WAS protein is absent, the cytoskeleton is not properly organized and signaling pathways are disrupted, leading to the

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WAS symptoms.

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For a cell of a multicellular organism to remain capable of responding to incoming signals, each molecular change in its signaling pathways must

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last only a short time.

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if a signaling pathway component becomes locked into one state, whether active or inactive, consequences for the organism

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can be serious.

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The ability of a cell to receive new signals depends on reversibility of the changes produced by

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prior signals.

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The binding of signaling molecules to receptors is

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reversible.

Question 42

As the external concentration of signaling molecules falls, fewer receptors are bound at any given moment, and the unbound receptors

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revert to their inactive form.

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The cellular response occurs only when the concentration of receptors with bound signaling molecules is above a certain

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threshold.

Question 44

When the number of active receptors falls below that threshold, the cellular response ceases. Then, by a variety of means, the relay molecules return to their inactive forms: .

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The GTPase activity intrinsic to a G protein hydrolyzes its bound GTP; the enzyme phosphodiesterase converts cAMP to AMP; protein phosphatases inactivate phosphorylated kinases and other proteins; and so forth. As a result, the cell is soon ready to respond to a fresh signal