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Intracellular Transport.docx

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BIOL 130
Heidi Engelhardt

Intracellular Compartments and Protein Sorting  intracellular compartments revisited  3 ways to import proteins into organelles 1) nuclear import, nuclear pore complex, nuclear localization sequence 2) import across membranes  mitochondria / chloroplasts  endoplasmic reticulum – ER signal sequence, signal recognition particle (SRP) o soluble versus transmembrane proteins 3) vesicular transport – coated vesicles, inward vs outward traffic  secretory pathways o processing in ER o protein folding, unfolded protein response o processing in Golgi apparatus o exocytosis o constitutive vs regulated secretion  endocytic pathways o pinocytosis, phagocytosis, receptor-mediated endocytosis o lysosomal function Evolution of Internal Membranes of Eukaryotes  precursors of eukaryotes believed to be organisms (like bacteria) with no internal membranes o plasma membrane carried out all membrane-related functions  endomembrane system thought to have evolved as invagination of plasma membrane  mitochondria and chloroplasts thought to have evolved as endosymbionts Organization  cytoskeleton important in: o cell shape o cell motility o movement / position of organelles o movement of materials within cell  movement of chromosomes during mitosis Protein Sorting  protein sorting = transfer of proteins into compartments where they are needed  synthesis of virtually of proteins starts in cytosol, on free ribosomes  all protein transport requires energy  Signal Sequence o stretch of amino acids, 15-60 AAs long, that directs proteins to particular organelles  signal sequences for nucleus, mito/chloro, peroxisomes or ER  usually removed after sorting o if sequence is deleted or transferred to another protein  protein goes to wrong ‘address  Transport through Nuclear Pores o Nucleus has double membrane (envelope) o Nucleur pores  very high traffic through nuclear pores (500 molecules through each of the 3000-4000 pores per second) but flow is SELECTIVE  small molecules (even small proteins) freely pass through nuclear pores  passage of larger proteins is active (energy-requiring) process  nuclear localization signal - amino acid sequence that ‘tags’ a protein for import into the nucleus by nuclear transport  nuclear export signal tags a protein for export  proteins pass through nuclear pores without unfolding o Moves out:  mature, properly processed mRNA  ribosomal RNA (manufactured in nucleolus) o Moves in:  histones, ribosomal proteins, proteins required for transcription, DNA replication  dNTPs, rNTPs  Transport across Membranes o Chloroplasts and Mitochondria  mitochondria and chloroplasts have double membrane  chloroplasts have third membrane - thylakoids  although they have their own genomes and ribosomes, most of their proteins encoded by nuclear genome, so must be imported  proteins destined for mitochondria / chloroplasts made by free ribosomes in the cytosol  signal sequence at N terminus  proteins must be moved across both outer and inner membranes at special sites where layers are in contact  proteins must unfold to be imported, then refold and signal sequence removed  subsequent transport within organelle requires another signal sequence (exposed after first one removed) o Endoplasmic Reticulum  ER is most extensive of endomembrane system  12% of cell volume  serves as entry point for not only proteins bound for ER itself, but rest of endomembrane system (Golgi, lysosomes, endosomes), cell surface, secretory proteins  once in ER (in membrane or lumen), proteins will never re-enter cytosol  Two types of proteins transferred to ER:  water soluble proteins translocated completely across into ER lumen o destined for secretion, or lumen of an organelle  prospective transmembrane proteins translocated only partially across o destined for plasma membrane, ER membrane or membrane of another organelle  1. ER signal sequence and an SRP direct ribosome to ER  Solub
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