Rough%20Endoplasmic%20Reticulum

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Rough Endoplasmic Reticulum
Lecture 19 BSCI 420/421 Oct 16, 2002 If you
want the rainbow, you gotta put up with the
rain -Dolly Parton
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Fluorescent Micrographs of ER
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TEMs of RER
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F37 Free And Membrane- Bound R-somes
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The signal hypothesis That an ER signal sequence
in the nacent Polypeptide chain directs
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• Evidence for the signal hypothesis
• Without signal sequences, proteins are not
  imported into RER.
• (Protease protection assay)
• 2. When signal sequences are added to a gene
  coding for a normally cytosolic protein, the
  polypeptide it makes is incorporated into RER.
• 3. A different sized (smaller) polypeptide is
  made in RER than when the same mRNA is translated
  on free polysomes in an
• in vitro system (reticulocyte lysate wo RER).

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The ER Signal is a sequence of about 10
hydrophobic AAs That becomes anchored in the
membrane by hydrophylic AAs On either side.
E.g. Ser-leu-leu-leu-val-gly-Ile-leu-phe-trp-ala-
thr near the NH2 terminal The signal sequence
initially binds to a RNP particle called
the Signal Recognition Particle (SRP).
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How the SRP directs ribosomes to the RER
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Evidence for an aqueous pore thru which the
polypeptide Passes into the ER
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A soluble protein released into the ER lumen to
be secreted
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A single-pass membrane protein
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Hydrophobicity plot of rhodopsin, a 7-pass
transmembrane protein