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Chapter 15: Problem 22

What would happen to proteins bound for the nucleus if there were insufficient energy to transport them?

Short Answer

Expert verified
Proteins will not enter the nucleus and will remain bound to importins in the cytoplasm due to insufficient energy for transport.

Step by step solution

01

Understanding Nuclear Import

To understand the exercise, we need to know how proteins are imported into the nucleus. Proteins meant for the nucleus have specific sequences known as nuclear localization signals (NLS). These signals are recognized by nuclear transport receptors called importins that transport proteins into the nucleus.
02

Role of Energy in the Process

The import of proteins into the nucleus is an energy-dependent process. This is driven by the small GTPase, Ran, which requires the hydrolysis of GTP to GDP to provide the energy needed to disassemble the protein-importin complex so that the protein can be released inside the nucleus.
03

Consequences of Insufficient Energy

If there is insufficient energy (lack of GTP), the Ran protein cannot hydrolyze GTP. As a result, the importin-protein complex cannot disassemble, preventing the release of the protein into the nucleus. Consequently, proteins destined for the nucleus would remain bound to importin and possibly accumulate in the cytoplasm.
04

Overall Biological Impact

Without proper transport into the nucleus, essential proteins such as transcription factors and other regulatory molecules cannot reach their functional site. This could inhibit critical operations like gene expression regulation and cellular response to stimuli, affecting overall cellular function.

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Key Concepts

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

Nuclear Localization Signal
Proteins targeted for the nucleus carry essential "tags" known as nuclear localization signals (NLS). These signals are short amino acid sequences that act like postal addresses, ensuring proteins reach the correct cellular compartment. When a protein possesses an NLS, it essentially waves a guiding flag for the cellular machinery to recognize.

Without the proper localization signals, proteins may drift aimlessly in the cytoplasm without reaching their destination. Thus, NLS are crucial for ensuring proteins that regulate nuclear processes, such as replication and transcription, get to where they need to be in the nucleus.
Importins
Importins are specialized receptors that act as the means of transport for proteins carrying nuclear localization signals. Consider importins as the "taxi service" for cellular proteins. Once a protein with an NLS binds to an importin, it forms a complex that is competent to enter the nuclear pore complex of the nuclear envelope.

This transport system ensures efficient and targeted delivery of proteins to the nucleus, avoiding potential dysfunction from misplaced proteins. Importins play a significant role in maintaining cellular specificity, helping only the correct proteins gain access into the nucleus.
Energy-Dependent Process
Transporting proteins into the nucleus is not a passive process; it requires energy. This energy is necessary to facilitate the movement of the importin-protein complex through the nuclear pore complex.

The cellular energy currency, GTP, is hydrolyzed in this process, ensuring that the importin-protein complex can disassemble once inside the nucleus. Without sufficient GTP, the process stalls, preventing proteins from being released into the nucleus and halting further cellular functions reliant on these proteins.
GTPase Ran
Ran is a small GTPase central to the energy-dependent transport of proteins into the nucleus. Operating as a molecular switch, Ran binds and hydrolyzes GTP to GDP, providing the necessary energy to modify the importin-protein complex.

With high concentrations of RanGTP found inside the nucleus, importins dissociate from their cargo after arriving at their destination. If energy levels are low and GTP is scarce, this mechanism falters. Proteins remain bound to importins, and their successful entry into the nucleus is impeded, disrupting necessary nuclear operations.

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Most popular questions from this chapter

A particular type of Drosophila mutant becomes paralyzed when the temperature is raised. The mutation affects the structure of dynamin, causing it to be inactivated at the higher temperature. Indeed, the function of dynamin was discovered by analyzing the defect in these mutant fruit flies. The complete paralysis at the elevated temperature suggests that synaptic transmission between nerve and muscle cells (discussed in Chapter 12 ) is blocked. Suggest why signal transmission at a synapse might require dynamin

Iron (Fe) is an essential trace metal that is needed by all cells. It is required, for example, for synthesis of the heme groups and iron-sulfur centers that are part of the active site of many proteins involved in electron-transfer reactions; it is also required in hemoglobin, the main protein in red blood cells. Iron is taken up by cells by receptor-mediated endocytosis. The iron-uptake system has two components: a soluble protein called transferrin, which circulates in the bloodstream; and a transferrin receptor-a transmembrane protein that, like the LDL receptor in Figure \(15-33,\) is continually endocytosed and recycled to the plasma membrane. Fe ions bind to transferrin at neutral pH but not at acidic pH. Transferrin binds to the transferrin receptor at neutral \(\mathrm{pH}\) only when it has an Fe ion bound, but it binds to the receptor at acidic pH even in the absence of bound iron. From these properties, describe how iron is taken up, and discuss the advantages of this elaborate scheme.

What would happen to proteins bound for the nucleus if there were insufficient energy to transport them?

Consider a protein that contains an ER signal sequence at its N-terminus and a nuclear localization sequence in its middle. What do you think the fate of this protein would be? Explain your answer.

Edit each of the following statements, if required, to make them true: "Because nuclear localization sequences are not cleaved off by proteases following protein import into the nucleus, they can be reused to import nuclear proteins after mitosis, when cytosolic and nuclear proteins have become intermixed. This is in contrast to ER signal sequences, which are cleaved off by a signal peptidase once they reach the lumen of the ER. ER signal sequences cannot therefore be reused to import ER proteins after mitosis, when cytosolic and ER proteins have become intermixed; these ER proteins must therefore be degraded and resynthesized."
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