BSCI330 Study Guide Exam 3 Chapter 12 Intracellular Compartments and Protein Sorting Eukaryotic cells o 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 microvilli 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 development o Chloroplasts in plant cells Patches of specialized membrane in the proplastid inner membrane invaginate and pinch off to form thylakoid vesicles which develop into the mature thylakoid o 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 cell 1 1 BSCI330 Study Guide Exam 3 Topologically equivalent compartments o Molecules that travel within each set of topologically equivalent compartments do o B c of nuclear pore complexes the nucleus is topologically equivalent to cytosol not have to cross a membrane to do so b c the two membranes are continuous o o Ex Cytosol to nucleoplasm ER lumen to extracellular space Movement of proteins between cellular compartments 3 fundamental mechanisms o 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 conformation o 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 2 2 BSCI330 Study Guide Exam 3 Unidirectional Transported protein must unfold to snake through the translocator Ex 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 another Bud from donor compartment and fuse with target compartment Can only move proteins between topologically equivalent compartments because the transported proteins do not cross a membrane Bidirectional or unidirectional Ex Transfer of proteins from ER to Golgi o 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 export Ran exists in 2 states o GTP ase activity switches signal transduction on and off o 1 RAN GEF catalyzes the binding of GTP to RAN in nucleus o 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 proteins 3 3 BSCI330 Study Guide Exam 3 o Driven by RAN GTP gradient RAN GTP diffuses back through complex into cytosol with empty receptor protein o 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 GDP exchange in the nucleus sets up a RAN GTP concentration gradient 2 RAN GTP binds to nuclear export receptors causing them to bind their cargo 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 GAP o o 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 calcineurin o 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 4 4 BSCI330 Study Guide Exam 3 Post translational translocation o Mitochondrial protein transport Mitochondrial proteins are first synthesized as precursor proteins in the cytosol and then translocated into mitochondria Mitochondria need to import proteins from the cytosol because the mitochondrial genome does not code for all of the proteins that they need so they
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