BIOLOGY 2B03 Chapter Notes - Chapter 5: Endoplasmic Reticulum, Calnexin, Glycosylation
17 Jun 2018
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BIOLOGY 2B03 - Module 5 Lecture I
Visualizing the Rough ER
ER is seen in cultured epithelial cells
(Contrast image)
● Showing all the membranes
● More details
(Immunofluorescence image)
● Using a antibody to a single protein; specific to the ER
● WE can specifically see just hose membrane of the ER
Dynamic Movements of the RER
● DNA in blue
● Peroxisomes in green
● The ER in red
○ The membranes of ER are constantly changing
shape and structure
○ Undergoing fission and fusion
○ Migrating to new locations in the cell
Rough ER and Smooth ER Functions
Membranes of each are continuous, but different functions happen
within them
● They look different in using electron microscopy
● Allow researchers to see individual ribosome complexes
Rough ER: the rough appearance attributable to the ribosomes that dot its surface
● Is the site of co-translational transport, protein modification, and the formation of vesicles
Smooth ER: Clear of ribosomes
● Is the site of fatty acid and phospholipid synthesis
● Carbohydrate metabolism occurs
● Calcium is sequestered to regulate calcium concentration in the cytosol
Post-Translational Modification (PTMs) in the ER
There are series of PTMs that occur within the ER
● Such as glycosylation or the addition of carbohydrate groups, formation of disulfide bonds,
protein folding, and proteolytic cleavage
ER lumen targeted proteins’ modification
● Can occur along the entire length of the protein
Modification of protein embedded in the ER membrane
● Will only occur on the luminal portion of the protein and not on the transmembrane domain
or on the cytosolic portion of the protein
Protein Glycosylation: N-linked
Protein Glycosylation: addition of a polysaccharide or sugar group to a protein
● Common on a proteins that are secreted from the cell and proteins embedded in the cell
membrane
● Common on protein that was secreted from the cell and proteins embedded in the cell
membrane
● IMPORTANT: for proteins mediate cell interactions with the extracellular matrix and for
receptor-ligand recognition
N-linked Glycosylation: most common form of glycosylation
find more resources at oneclass.com
find more resources at oneclass.com
● Addition of a polysaccharide to the NH2 of the R-group of asparagine
● The modified portion of the protein will remain on the luminal side or exoplasmic side of the
membrane through transport
○ Appear on the exterior surface of a protein embedded in the Er membrane
● Further modification can occur to the polysaccharide within ER/different regions of Golgi
Body
Protein Folding in the ER
Endoplasmic Reticulum is like home to a
collection of enzymes that facilitate protein
folding
● Some protein foldings are assisted by
recognizing amino acids carrying
added N-linked polysaccharides
● Lectins: recognize modified proteins
and assist in protein folding in a
similar manner to molecular
chaperones
○ Calnexin and calreticulin are
part of lectins protein family
○ Calnexin: is found throughout
the ER membrane
BiP is an ER-resident HSP70 Chaperone
BiP: an ER-resident protein and is a member of the HSP70 family of proteins
● Roles depend upon the ability of BP to recognize and bind to unfolded proteins
● BiP and its co-chaperones are crucial for efficient ER protein folding
○ Hsp40 & NEF (a nucleotide exchange factor)
● Plays role in the transfer of proteins from the ER through the translocon
○ By binding to proteins as soon as the appear on the luminal side of the membrane
during cotranslational transport
● Initiates the unfolded protein response in the ER
Disulphide Bond Formation in the ER
Disulphide Bonds: are covalent linkages between the sulfhydryl groups (-SH groups) of two cysteine
residues
● Could be essential to the formation of the tertiary or quaternary structure of the protein
● May occur within a single protein; intramolecular bond
● Or between two different bond; intermolecular bond
● In eukaryotic cells → the oxidative reaction forms the disulphide bond
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Er is seen in cultured epithelial cells (contrast image) Using a antibody to a single protein; specific to the er. We can specifically see just hose membrane of the er. The membranes of er are constantly changing shape and structure. Migrating to new locations in the cell. Membranes of each are continuous, but different functions happen within them. They look different in using electron microscopy. Allow researchers to see individual ribosome complexes. Rough er: the rough appearance attributable to the ribosomes that dot its surface. Is the site of co-translational transport, protein modification, and the formation of vesicles. Is the site of fatty acid and phospholipid synthesis. Calcium is sequestered to regulate calcium concentration in the cytosol. There are series of ptms that occur within the er. Such as glycosylation or the addition of carbohydrate groups, formation of disulfide bonds, protein folding, and proteolytic cleavage. Can occur along the entire length of the protein.
