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Lecture 17

BIOL 3113 Lecture Notes - Lecture 17: Oligosaccharyltransferase, Membrane Transport Protein, Dolichol


Department
BIOL
Course Code
BIOL 3113
Professor
Barbara S
Lecture
17

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Compartments of Eucaryotic Cells
Vesicular Transport - Secretory Pathways
Secretion (Exocytosis)
Proteins and other macromolecules produced in the ER are often exported from the cell
through the process of secretion - exocytosis
Transport from the ER to the Golgi and from the Golgi to the plasma membrane is
mediated by coated vesicles
Proteins synthesized on the RER are usually further modified in the lumen of the ER and
in the Golgi before they become functional and are secreted
Modification of Proteins
In the ER:
*formation of disulfide bonds
*glycosylation glycoproteins
In the Golgi:
*oligosaccharide processing
Glycosylation of Polypeptides in the ER
Most proteins synthesized on the ER are glycosylated on the lumenal side
N-linked Glycosylation:
*14-sugar oligosaccharide added to asparagine side chains
as they enter the lumen of the ER
*enzyme - oligosaccharyl transferase
*oligosaccharide donor - dolichol phosphate (a membrane lipid)
ER-Golgi Transport
Some proteins made in the ER stay in the ER - ER resident proteins
ER retention signal - four-amino acid sequence
recognized by a membrane-bound receptor protein in the ER and Golgi membranes
ER resident proteins that escape to the Golgi are returned to the ER
Most proteins made in the ER are destined for other compartments and are transported
from the ER to the Golgi
Mediated by vesicles budding from ribosome-free areas of the ER

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Two pathways:
*Forward pathway
no transport signal required (default pathway)
mediated by COP-coated vesicles
proteins must be properly folded before they can enter the vesicles
retention in the ER mediated by chaperone proteins
*Return pathway
requires transport signal - ER retention signal
returns ER-resident proteins to the ER
mediated by COP-coated vesicles
retention in the ER mediated by chaperone proteins
Cystic fibrosis -
Mutation production of slightly misfolded plasma membrane transport protein -
chloride channel (functional if reaches plasma membrane) recognized by chaperones
as misfolded retained in the ER and degraded
Golgi Apparatus
Compartment in which ER proteins are processed, sorted, and packaged for shipment to
other compartments
Structure:
Series of flattened and tubular vesicles with associated small vesicles
*Two surfaces - "faces":
cis face (forming or entering face) - entry point for proteins from the ER
trans face (exit face) - exit site for proteins leaving Golgi
*Golgi stacks (4-6 cisternae):
cis -Golgi network
cis cisternae
medial cisternae
trans cisternae
trans Golgi network
*Golgi vesicles - provide transport between Golgi cisternae
Oligosaccharide processing in the Golgi
Primary function of the Golgi - glycosylation of proteins and modification of
oligosaccharide chains
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