Roles of the ER and Golgi Complex in Protein Trafficking Flashcards Preview

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Flashcards in Roles of the ER and Golgi Complex in Protein Trafficking Deck (16):

Describe the N-linked glycosylation steps of proteins in the ER.

(Figure 12-7 in the textbook)

1. glycosylation begins as dolichol phosphate, an oligosaccharide carrier, is inserted into the ER membrane

2. GlcNAc and mannose groups are then added to the phosphate group

3. the growing core oligosaccharide is translocated to the ER lumen by a flippase

4. once inside the lumen, more mannose and glucose are added

5. the completed core oligosaccharide is transferred from dolichol to the asparagine residue of the recipient protein

6. the core oligosaccharide attached to the protein is trimmed and modified


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What is cotranslational glycosylation?

the oligosaccharide is added to the recipient protein as the polypeptide is being synthesized, this promotes proper protein folding


Give a quick overview of golgi trafficking.

sorting of proteins begins in the ER compartments of the Golgi

There are mechanisms to retrieve or retain compartment-specific proteins

the final sorting of material that wil leave the Golgi complex occurs in the TGN

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what are protein tags?

amino acid sequence, ahydrophobic domain

oligosaccharide chain

some other feature


what are lipid tags?

membrane lipids may be tagged to help vesicles reach their destinations

one or more phosphate groups attached to positions 3, 4, and 5 of a membrane phosphatidyl inositol


what does the RXR tag do?

How does this work with assembly of the protein?

RXR (arg-X-arg) tag promotes protein retention in the ER

proper assembly masks the RXR sequence, allowing the assembled complex to leave the ER


what are the two retrieval tags found in proteins?

how do they work?

where did evidence of these tags come from?

KDEL and HDEL are short C-terminal sequences

when a protein with this tag binds to a receptor , the receptor-ligand complex is packaged into a transport vesicle for return to the ER

evidence of the tags came from experiements involving chimeric proteins (fusion proteins)



how are golgi complex proteins sorted according to the lengths of their membrane-spanning domains?

some proteins resident to the Golgi complex also contain retention or retrieval tags

large complexes that are excluded from transport vesicles may play a role in maintaining the protein composition of the Golgi complex

a third mechanism involves hydrophobic regions of golgi proteins


how are proteins determined to enter the cisternae they need to enter?

the length of the hydrophobic domains may determine into which cisterna of the Golgi complex each membrane-bound protein is incorporated as it moves through the organelle.


how is the length of hydrophobic domains correlated with location in the golgi complex?

the thickness of the membranes in the golgi increases from the CGN to the TGN

Proteins move from compartment to compartment until the membrane thickness exceeds the length of the transmembrane domains

this blocks further migration


How are the lysosomal proteins distinguished from other proteins?

Mannose residues on the carbohydrate side chain of lysosomal enzymes are phosphorylated with form an oligosaccharide mannose-6-phosphate


How is the phosphorylation of mannose residues catalyzed?

two Golgi specific enzymes 

the first located in the early Golgi compartments that is a phosphotransferase that adds GlcNAc-1-phosphate to carbon 6 of mannose 

the second located in the mid-Golgi compartments removes the GlcNAc leaving behinde the mannose-6-phosphate


What are MPR's?

mannose-6-phosphate receptors (MPR) that bind to the mannose-6-phosphate residues of lysosomal proteins


what are the vesicles called that come from the TGN and what do they develop from?

late endosomes develop from early endosomes


How does an early endosome mature into a late endosome?

What does a late endosome mature into?

the pH of the lumen decreases to about 5.5 causing the boud lysosomal enzymes to dissociate from the MPRs

a late endosome matures into a lysosome