BSCI330 Study Guide Exam 3Chapter 12: Intracellular Compartments and Protein Sorting- Eukaryotic cellso Membrane-enclosed organelles The cytosol, ER, Golgi, nucleus, mitochondria, endosome, lysosome, and peroxisome are all distinct compartments isolated from the rest of the cell by at least one selectivity permeable membrane Internal membranes can have a larger surface area than the plasma membrane- ER membranes of liver and pancreatic cells are 12-15x the plasma membrane surface area- Complex organelle structure developmento Chloroplasts in plant cells Patches of specialized membrane in the proplastid inner membrane invaginate and pinch off to form thylakoid vesicles, which develop into themature thylakoido Nucleus with 2 lipid bilayers Extensions of the ER DNA attached to an invagination of the plasma membrane, which rearranges to form an envelope around the DNA Envelope is pinched off from plasma membrane, producing a nuclear compartment surrounded by a double membrane Nuclear pore complexes (gated passageways) penetrate nuclear envelope o Ingestion of a prokaryote to create mitochondria Mitochondria may have originated when a bacterium was engulfed by a larger pre-eukaryotic cellmicrovilli11BSCI330 Study Guide Exam 3- Topologically equivalent compartmentso Molecules that travel within each set of topologically equivalent compartments donot have to cross a membrane to do soo B/c of nuclear pore complexes, the nucleus is topologically equivalent to cytosol b/c the two membranes are continuous oo Ex: Cytosol to nucleoplasm ER lumen to extracellular space- Movement of proteins between cellular compartments: 3 fundamental mechanismso 1. Gated transport- protein traffic between cytosol and nucleus occurs through nuclear pore complexes Bidirectional Nuclear pore complexes- extend across 2 bilayers and function as selective gates that actively transport specific macromolecules and macromolecular assemblies through a large aqueous pore and allow free passive diffusion of smaller molecules (5000 daltons or less) through aqueous channels- Each complex has a molecular mass=125 million Daltons and is composed of >50 different proteins called nucleoporins- The nuclear envelope contains 3000-4000 pore complexes- Transports ~100 histone protein molecules/min from cytosol into nucleus to synthesize DNA (106 histone molecules every 3 min)- Transports ~6 newly assembled ribosomal subunits/min from nucleus into cytosol if the cell is rapidly growing- Nuclear proteins can be transported through a pore complex in a fully folded conformationo 2. Transmembrane transport- membrane-bound protein translocators directly transport specific proteins across a membrane from the cytosol into a space that is topologically distinct (topologically non-equivalent b/c it has to cross a bilayer)22BSCI330 Study Guide Exam 3 UnidirectionalTransported protein must unfold to snake through the translocatorEx: Transport proteins from cytosol to ER lumen or from cytosol to mitochondria o 3. Vesicular transport- membrane-enclosed transport intermediates ferry proteins from one compartment to anotherBud from donor compartment and fuse with target compartmentCan only move proteins between topologically equivalent compartments because the transported proteins do not cross a membraneBidirectional or unidirectionalEx: Transfer of proteins from ER to Golgio A signal sequence (signal peptide) “targets” a protein produced in the cytosol to a specific membrane compartment Removal of the signal sequence off the protein after use by enzymes “signal peptidases” trapping the protein in its target compartment-Nuclear Transport o Nuclear import receptor proteins needed for nuclear import to recognize nuclear localization signals Importing nuclear proteins concentrates specific proteins in the nucleus-a process which must consume energy The energy is provided by the hydrolysis of GTP by GTPase Ran- GTPase Ran is found in cytosol and nucleus and is required for nuclear import and exporto GTP-ase activity switches signal transduction on and off- Ran exists in 2 stateso 1. RAN-GEF catalyzes the binding of GTP to RAN in nucleuso 2. RAN-GAP hydrolyzes GTP attached to RAN in cytosol- A gradient of RAN-GTP is created across the nuclear pore (more RAN-GTP inside the nucleus than outside)o RAN-GTP binds to nuclear import receptors after they diffuse into the nucleus, causing the release cargo proteins33BSCI330 Study Guide Exam 3o Driven by RAN-GTP gradient, RAN-GTP diffuses back through complex into cytosol with empty receptor proteino Nuclear export receptor proteins needed to recognize nuclear export signals on the proteins for export Both receptor proteins bind to the signal sequence and to nucleoporins to guide cargo through the pore complex 1. Hydrolysis of RAN-GTP in the cytoplasm by RAN-GAP and GTP/GDPexchange in the nucleus sets up a RAN-GTP concentration gradient 2. RAN-GTP binds to nuclear export receptors, causing them to bind theircargo inside the nucleus (rather than release it like with import) 3. Driven by RAN-GTP gradient, the export receptor, RAN-GTP and cargo diffuse through the nuclear pore into the cytosol - The cargo is released when RAN is hydrolyzed in the cytosol to RAN-GDP by RAN-GAPoo Nuclear transport of transcription factors (gene regulatory proteins)Nuclear import sequences on transcription factors can be exposed by conformational changes that follow the phosphorylation or dephosphorylation of adjacent amino acids by specific kinases or phosphatases which are activated in response to cellular signals-Ex: NF-AT protein- regulates the activation of T-lymphocytes (white blood cells) by foreign antigens during the human immune response-Dephosphorylation of NF-AT causes a conformational change, exposing a nuclear import sequence on the protein’s surface-Detection of antigens (that pose a threat to the body) in the blood by T-cell receptor proteins causes a rise in cytosolic calcium ion concentration, which activates a protein, calcineurino Calcineurin dephosphorylates NF-AT and allows it to enter the nucleus, where it triggers gene expression appropriate to the T-cell’s role in the immune response 44BSCI330 Study Guide Exam 3--Post-translational translocation o Mitochondrial protein transportMitochondrial proteins are first synthesized as precursor proteins in the cytosol and then translocated into
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