Chapter 11 and 12 - Nucleus and Control of Gene Expresion

Phosphorylation and dephosphorylation the nuclear

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Unformatted text preview: r envelope acts as the barrier between the nucleus and the cytoplasm many proteins synthesized in the cytoplasm are required in the nucleus for replication and transcription mRNAs, tRNAs and ribosomal subunits manufactured in the nucleus must enter the cytoplasm A single HeLa cell (comes from crucial cancer cell, studied because they reproduce fast) must import about 560000 ribosomal proteins and export 14000 ribosomal subunits every minute Particulate material (like the ribosomal subunit) and gold particles (which were injected to follow) have been imaged by EM passing single file through the nuclear pores, in their normal 3D shape Nuclear Pores o o But pores are not just open channels Nuclear pores contain the nuclear pore complex (NPC) that appears to fill the pore like a stopper projects into the cytoplasm and nucleoplasm o o o o o o Huge, supramolecular complex Eightfold repetition of most structures (octagonal symmetry) 8 cytoplasmic filaments It is composed of about 30 proteins called nucleoporins Subset of nucleoporins with a large number of phenylalanine-glycine repeats (FG) It is thought to line the channel of the NPC with filamentous FG domain extending into the channel FG domains form hydrophobic sieve blocking large molecules NPC Transport Through Nuclear Pores o o o o Protein synthesized in the cytoplasm are targeted for the nucleus by the nuclear localization signal (NLS) classical NLS consists of one or two short stretches of positively charged amino acids Family of proteins bring macromolecules across the nuclear envelope act as mobile transport receptors Importins move macromolecules from cytoplasm to nucleus Exportins move macromolecules from nucleus to cytoplasm 6 Import Through Nuclear Pores o o o o o o o o o Import beings as the NLS containing protein binds to a soluble NLS receptor in the cytoplasm importin alpha or beta Importing and NLS-protein (receptor cargo complex) travels to the outer membrane and docks with the cytoplasmic filaments extending outward from the nuclear pore complex The receptor cargo complex moves through the nuclear pore through interactions with FG domains now the protein is in the nuclear compartment and must interact with Ran-GTP Ran-GTP works depending on concentration gradient across the nuclear envelope concentration of active Ran-GTP is higher in the nucleus than in the cytoplasm Ran-GTP is converted to inactive form, Ran-GDP, in cytoplasm to maintain the concentration gradient Ran-GTP binds to receptor-cargo complex and causes dissaembly Imported cargo is released into the nucleplasm and importin Beta subunit is shuttled back to cytoplasm bound to Ran-GTP In the cytoplasm, Ran-GTP is hydrolyzed to Ran-GDP and importin is released Importin alpha is transported back to the cytoplasm by an exportin Export Through Nuclear Pore o o o Proteins exported from nucleus contain a nuclear export signal (NES) NES is recognized by transport receptors that carry proteins through NPC to cytoplasm Most of the export is RNAs (mRNAs, rRNAs and tRNAs) RNAs move through the NPC as ribonucleoproteins (RNPs) Chromosomes and Chromatin o o o o o o o o o o Chromosomes appear the beginning of mitosis and disappear at the end of cell division so where are the chromosomes in non-mitotic cells? The human cell contains 46 chromosomes Each chromosome contain...
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This document was uploaded on 04/10/2014.

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