Chapter 9: Nucleus Flashcards
Is all protein trafficking through nuclear pores unidirectional?
No, protein trafficking through nuclear pores is not unidirectional. It can occur bidirectionally. Proteins can be imported into the nucleus or exported from the nucleus, depending on their specific signals and the cellular needs.
Importin bound to NLS-containing cargo is transported into the nucleus where the complex binds Ran-GTP and gets dissociated. True or false?
True.
Importin bound to cargo containing a nuclear localization signal (NLS) is transported into the nucleus. Once inside the nucleus, the importin-cargo complex binds to Ran-GTP (a small GTPase protein) and undergoes a conformational change, leading to the dissociation of the complex. This allows the cargo to be released into the nucleus while importin and Ran-GTP are recycled back to the cytoplasm.
How would you design an experiment to demonstrate that the NLS (nuclear localization signal) of the yeast protein Swi5 is sufficient for the transport of this protein into the nucleus?
1) Alter the sequence of Swi5 so that the NLS is deleted and demonstrate that the altered Swi5 protein cannot be detected in the nucleus.
2) Create a fusion between the NLS of Swi5 and a gene that is not normally found in the nucleus and demonstrate that this new fusion protein is now detected in the nucleus.
Answer: Either experiment A or B would demonstrate that the NLS of Swi5 is sufficient for nuclear transport.
The transport of proteins across the nuclear envelope has some distinct mechanisms compared to the transport of proteins across the mitochondrial membrane, although both are mediated by the presence of a signal sequence. What are the other differences?
The nuclear localization signal (NLS) is not cleaved but remains an integral part of the protein
Both U snRNAs and microRNAs are transcribed by polymerase II and then further modified including cleavage of the RNAs. What are RNAs?
Transcribed in the nucleus, exported to the cytoplasm and then cleaved and modified in the cytoplasm.
RNAs are molecules transcribed from DNA that have diverse functions within cells, including carrying genetic information, regulating gene expression, and participating in various cellular processes.
Explain the property of the nuclear pore and how it was discovered.
Particles 9nm or smaller in diameter can passively diffuse through the nuclear pore complex, while larger particles cannot. The maximum size of particles able to freely diffuse across the nuclear envelope was determined by injecting gold particles of precise sizes on one side of the nuclear envelope and assaying their ability to move to the other side.
Leptomycin is a compound that inhibits the growth of Human Immunodeficieny Virus in human cells, yet it cannot be used to treat HIV patients because it is toxic to all of a patient’s cells. Explain how leptomycin functions in human cells.
Leptomycin binds to the exportin protein Crm1, thereby inhibiting its ability to recognize nuclear export signals. Because it interferes with nuclear export, it blocks the nuclear/cytoplasmic cycling of HIV Rev protein and thereby stops HIV growth. But because it stops virtually all nuclear export, it is toxic to all cells, regardless of whether they are infected with HIV.
The protein IκB functions in two independent and complimentary roles to control the import and export of the transcription factor NFκB. Explain these two roles.
Role 1 is to control import of NFκB into the nucleus. In the cytosol, IκB binds to NFκB, thereby preventing NFκB from exposing its nuclear localization signal and subsequent binding of importin α. When IκB is phosphorylated, it is degraded, allowing NFκB to bind importin α and move into the nucleus.
Role 2 is to control export of NFκB from the nucleus-newly synthesized IκB is transported into the nucleus, where it binds to both NFκB and exportin proteins, thereby promoting export of NFκB into the cytosol
The nucleus is completely filled with chromatin. True or false?
FALSE.
There are interchromosomal domains where is RNA being transported to the nuclear envelope for export.
Within the nucleus, there are interchromosomal domains, which are regions that do not contain densely packed chromatin. These interchromosomal domains provide space for various nuclear activities, including the transport of RNA molecules to the nuclear envelope for export.
RNA molecules that are synthesized in the nucleus, such as mRNA molecules, undergo processing and maturation steps before being transported out of the nucleus into the cytoplasm for translation. This export process involves the transport of RNA molecules from interchromosomal domains to the nuclear envelope, where they can exit the nucleus through nuclear pore complexes.
The interchromosomal domains allow for the movement of RNA molecules and provide a less condensed space for RNA processing, assembly of export complexes, and subsequent export from the nucleus. These regions facilitate the efficient export of RNA to ensure proper gene expression and cellular function.
Active genes are found adjacent to interchromosomal domains. True or false?
True,
highly transcribed genes are found on the outside edges of chromosome territories which are adjacent to the interchromosomal domains.
What are nuclear lamin proteins making up?
Answer: make up a meshwork that lie directly beneath and support the inner membrane of the nucleus.
Nuclear lamin proteins make up the nuclear lamina, which is a structural network located at the inner surface of the nuclear envelope in eukaryotic cells. The nuclear lamina provides mechanical support, stability, and organization to the nucleus.
Your textbook describes a simple experiment illustrating that export of transfer RNA (tRNA) from the nucleus to the cytoplasm is under the control of the small GTP binding protein Ran. Summarize the results of this experiment and explain how Ran is functioning in this process.
In the study, Ran-GAP was injected into the nucleus of a cell. Ran GAP is usually in the cytoplasm and hydrolyzes Ran-GTP to Ran-GDP. With Ran-GAP in the nucleus, all the Ran-GTP was converted to Ran-GDP and could not function as part of the exportin complex so tRNA remained in the nucleus.
What advantage does possession of a nucleus offer a cell? because of all the different things that get transported in and out of a nucleus (ions, RNAs, a variety of proteins, etc.). True or false?
The cell can have a larger genome because the nucleus protects the DNA from breakage during interphase.
What is the nucleolus?
Answer: It is where rRNAs are synthesized and where ribosomal subunits are assembled.
The nucleolus is where rRNAs are made and ribosomal subunits assembled. The nucleolus is also a storage center for nuclear proteins that aren’t active at the time.
The nuclear localization signal (NLS) is recognized by and binds to which protein in the process of nuclear protein transport?
(importin b) Karyopherin
The NLS tags are recognized by and bind to importins and in some cases the nuclear pore complexes itself.
In addition to importins, specifically importin β (also known as karyopherin β), playing a crucial role in recognizing and binding to the nuclear localization signal (NLS) of cargo proteins, the nuclear pore complexes (NPCs) themselves also have the ability to directly interact with NLS tags.
