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Chapter 16: Problem 9

The Ras protein functions as a molecular switch that is set to its "on" state by other proteins that cause it to release its bound GDP and bind GTP. A GTPase-activating protein helps reset the switch to the "off" state by inducing Ras to hydrolyze its bound GTP to GDP much more rapidly than it would without this encouragement. Thus, Ras works like a light switch that one person turns on and another turns off. You are studying a mutant cell that lacks the GTPase-activating protein. What abnormalities would you expect to find in the way in which Ras activity responds to extracellular signals?

Short Answer

Expert verified
Ras activity would remain 'on' too long, causing excessive signaling responses.

Step by step solution

01

Understand Normal Ras Function

Ras functions as a switch that is turned on by binding GTP, which is encouraged by other proteins, and turned off by hydrolyzing GTP to GDP, a process accelerated by GTPase-activating proteins (GAPs). Normally, this allows Ras to respond rapidly to signals and then turn off to prevent excessive signaling.
02

Identify the Mutant Cell's Missing Component

In the mutant cell, the GTPase-activating protein (GAP) is absent. GAP is responsible for speeding up the hydrolysis of GTP to GDP, thereby turning Ras 'off'. Without GAP, Ras will have difficulty switching to the off state.
03

Predict the Effect on Ras Activity

Without GAP, Ras will remain in the 'on' state for much longer because the hydrolysis of GTP to GDP will be significantly slowed down. This prolongation in the 'on' state means that Ras will continue to signal even when extracellular signals have ceased.
04

Determine the Impact on Cellular Signaling

The prolonged 'on' state of Ras can lead to excessive signal transduction pathways being activated continuously, resulting in potential abnormalities like uncontrolled cell growth or failure to respond appropriately to other signals, often associated with cancerous growth.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Molecular Switch
The Ras protein acts like a molecular switch in the cell, oscillating between an "on" state and an "off" state. This switching mechanism is crucial for the regulation of various cellular processes. When in the "on" state, Ras is bound to GTP, a molecule that acts as an energy source within the cell. When Ras is bound to GDP, it is in the "off" state.

Other proteins facilitate the switch by helping Ras release GDP so that GTP can bind. This switch is incredibly important because it allows the cell to respond to signals from the outside environment. Just like a light switch, it ensures that signals are on only when needed and off when not.
GTPase-activating Protein
GTPase-activating proteins, or GAPs, are pivotal in controlling the Ras molecular switch. These proteins work by accelerating the hydrolysis of GTP to GDP. By doing this, GAPs help turn the Ras protein "off" much faster than it would occur naturally.

Think of GAPs as the person who turns off the light switch. Without them, Ras remains in the "on" state longer, potentially leading to overactive cellular responses. This careful control is essential, as prolonged Ras activity can lead to issues such as excessive cell division, which is often associated with tumor development.
Extracellular Signals
Cells constantly receive information from their environment through extracellular signals. These signals are usually chemical messengers, like hormones or growth factors, that trigger cellular responses.

When an extracellular signal reaches a cell, it interacts with proteins on the cell surface, starting a cascade of events inside the cell that involves Ras. Ras responds to these signals by switching "on," allowing the cell to adapt to changing conditions. However, if Ras stays "on" even when the signals have stopped, as would happen in a mutant cell lacking GAP, the cell might not respond accurately, potentially leading to issues like abnormal growth.
Signal Transduction Pathways
Signal transduction pathways are the series of steps by which a cell responds to extracellular signals. These pathways convert a simple signal into a complex response, allowing the cell to act accordingly.

Within these pathways, Ras serves as a critical control point. When in the "on" state, Ras can activate other proteins, leading to a variety of cellular responses, such as cell growth and division. Proper regulation is necessary to ensure signals are transmitted appropriately.
  • Without GAP, Ras may stay "on," activating these pathways indefinitely.
  • This can result in uncontrolled signaling, similar to a stuck throttle in a car.
Proper function of these pathways is essential for maintaining cellular health and preventing diseases such as cancer.

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Most popular questions from this chapter

A. Compare and contrast signaling by neurons, which secrete neurotransmitters at synapses, with signaling carried out by endocrine cells, which secrete hormones into the blood. B. Discuss the relative advantages of the two mechanisms.

Animal cells and plant cells have some very different intracellular signaling mechanisms but also share some common mechanisms. Why do you think this is so?

Two intracellular molecules, \(X\) and \(Y\), are both normally synthesized at a constant rate of 1000 molecules per second per cell. Molecule \(X\) is broken down slowly: each molecule of \(X\) survives on average for 100 seconds. Molecule \(Y\) is broken down 10 times faster: each molecule of Y survives on average for 10 seconds. A. Calculate how many molecules of \(X\) and \(Y\) the cell contains at any time. B. If the rates of synthesis of both \(X\) and \(Y\) are suddenly increased tenfold to 10,000 molecules per second per cellwithout any change in their degradation rates- how many molecules of \(X\) and \(Y\) will there be after one second? C. Which molecule would be preferred for rapid signaling?

What are the similarities and differences between the reactions that lead to the activation of G proteins and the reactions that lead to the activation of Ras?

Which of the following statements are correct? Explain your answers. A. The extracellular signal molecule acetylcholine has different effects on different cell types in an animal and often binds to different cell-surface receptor molecules on different cell types. B. After acetylcholine is secreted from cells, it is long-lived, because it has to reach target cells all over the body. C. Both the GTP-bound \(\alpha\) subunits and nucleotide-free \(\beta \gamma\) complexes \(-\) but not GDP-bound, fully assembled G proteins-can activate other molecules downstream of GPCRs. D. \(\mathrm{IP}_{3}\) is produced directly by cleavage of an inositol phospholipid without incorporation of an additional phosphate group. E. Calmodulin regulates the intracellular \(\mathrm{Ca}^{2+}\) concentration. F. Different signals originating from the plasma membrane can be integrated by cross-talk between different signaling pathways inside the cell. G. Tyrosine phosphorylation serves to build binding sites for other proteins to bind to RTKs.
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