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
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
what are protein tags?
amino acid sequence, ahydrophobic domain
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