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TRANSPORT FROM THE ER THROUGH THE GOLGI APPARATUS Golgi complex is site of carbohydrate synthesis. It is also sorting and dispatching center for products of the ER. Proteins leave the ER in COPII-coated transport vesicles o Proteins are put into COPII coated vesicles o These vesicles bud from “ER exit sites” o This is a selective process o Proteins display exit signals or transport signals on their cytosolic surface that the COPII coat can recognize o These coat components act as cargo receptors and are recycled back to the ER after they have delivered their cargo to the Golgi o Membrane proteins in the ER are packaged into budding transport vesicles through interactions of exit signals on their cytosolic tails with the COPII coat o Soluble cargo proteins in the ER lumen have exit signals that attach them to transmembrane cargo receptors which bind through exit signals in their cytoplasmic tails to components of the COPII coat o Proteins without any signals can leak out too Only proteins that are properly folded and assembled can leave the ER o Only good proteins can leave the ER, bad ones get stuck to chaperone proteins like BiP or calnexin o The chaperones cover up exit signals or anchor the protein o Failed proteins eventually transferred into cytosol to be degraded by proteasomes o Process of continual degradation of ER proteins PROVIDES EARLY WARNING SYSTEM TO ALERT IMMUNE SYSTEM WHEN A VIRUS INFECTS CELLS o ER imports peptide fragments of viral proteins and sticks them onto MHC proteins in the ER lumen and sticks those on the cell surface o T lymphocytes recognize the peptides and kill the infected cells o This is bad sometimes, mutations result in production of misfolded protein THAT IS ACTUALLY IMPORTANT FOR CHLORINE TRANSPORT Vesicular tubular clusters mediate transport from the ER to the golgi apparatus
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o After vesicles budded from ER exit sites shed their coats, they fuse with each other o This is called homotypic fusion o Homotypic fusion requires a set of matching snares o Both membranes contribute v-snares and t-snares o First NSF separates the v-snares and t-snares o Then membrane fusion occurs, leads to formation of 1 continuous compartment called a VESICULAR TUBULAR CLUSTER o Endosomes can also do this o Rab proteins help regulate the size o These clusters are short-lived because they move quickly o The clusters form and begin to bud off transport vesicles of their own o Unlike the COPII vesicles that bud from the ER, these are COPI coated o These carry back escaped proteins and cargo receptors The retrieval pathway to the ER uses sorting signals o Retrieval pathway for returning escaped proteins back to the ER depends on ER RETRIEVAL SIGNALS o Resident ER membrane proteins have signals that bind to COPI coats so they are put into COPI coated vesicles for retrograde delivery to the ER o Best signal is 2 lysines followed by any 2 other AA at the C-terminal end of the ER membrane protein o THIS IS CALLED A KKXX SEQUENCE!!!!!! o
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This note was uploaded on 12/30/2010 for the course BIO 320 taught by Professor Staff during the Spring '08 term at University of Texas at Austin.

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