CH 15 - Chapter 15 Intracellular compartments and transport...

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11/11/09 1 Chapter 15 Intracellular compartments and transport What is the fate of each protein after synthesis?
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11/11/09 2 Most proteins function in defined locations within a eukaryotic cell Compartmentalization What are these compartments and how do newly synthesized proteins get there? Cellular compartmentalization is achieved by many different membrane- enclosed organelles Selectively-permeable membranes
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11/11/09 3 Primary functions of the membrane-enclosed compartments of a eukaryotic cell Compartment Cytosol Nucleus ER Golgi Lysosomes Endosomes Mitochondria Chloroplasts Peroxisomes Main Function Metabolic pathways, protein synthesis Contains main genome, DNA/RNA synthesis Synthesis of most lipids, protein distribution Protein and lipid modification for distribution Intracellular degradation Sorting of endocytosed materials ATP synthesis - oxidative phosphorylation Photosynthesis Oxidation of toxic compounds See text and/or glossary for review Protein sorting : Energy-dependent mechanisms for protein entry into an organelle from the cytosol
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11/11/09 4 Signal sequences are necessary and sufficient for protein sorting For some signal sequences, structural features are more important than specific amino acid sequence Signal sequences direct proteins to the correct organelle
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11/11/09 5 Transport through nuclear pores The nucleus has two membranes The outer membrane is continuous with the ER Nuclear pores are “gates” through both membranes of the nucleus that allow movement of molecules in both directions: 15_08_nuclear_pore.jpg A nuclear pore is a complex of ~100 proteins Small molecules can freely diffuse through the nuclear pore but large molecules like proteins cannot pass without an appropriate sorting signal - nuclear localization signal (short stretch of lysines/arginines, positive charges) and helper proteins ( nuclear transport receptors )
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11/11/09 6 15_09_pore_transport.jpg Protein transport through nuclear pores is active GTP hydrolysis Proteins remain in their fully folded conformation throughout the transport process
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11/11/09 7 Transport across mitochondrial and chloroplast membranes Proteins must be unfolded during transport Chaperonins help re-fold the protein after transport N-terminal signal sequence (structure/charge) 15_11_ER.jpg ER is the most extensive membrane system in the cell and is also the entry point for proteins destined for several other organelles and the cell exterior Transport across ER membrane
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11/11/09 8 Two kinds of proteins are transferred from cytosol to ER 1. Soluble proteins Transferred completely to the ER lumen Destined for secretion out of the cell or to the lumen of another organelle (not mitochondrion or plastid) 2. Transmembrane proteins
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