Flashcards in 09_Synthesis of Membrane Proteins Deck (22)
What are trans-membrane segments?
hydrophobic segments in integral proteins that interfere with their transfer into the RER lumen
What assists the proper orientation of transmembrane sequences?
What is the charge on the majority of extramembrane domans facing the cytosol?
What allows the TM segment to remain in the bilayer?
the hydrophobic nature of the TM segment
The TM segment is inserted (laterally/horizontally) into the phospholipid bilayer.
Where does protein glycosylation occur?
Where is the core sugar (later to be added to protein) assembled? What lipid is it attached to?
built on both sides of the ER membrane
the sugar is attached to dolichol-pyrophosphate
What amino acid sequence does N-linked glycosylation target?
Which enzyme transfers the oligosaccharyl group to the Asn sidechain of a newly synthesized protein?
How many terminal glucose residues are on a gycosylated protein?
What three types of sugars are in the protein-linked oligosaccharide?
What is overall the role of chaperones in the ER?
ensure proteins are properly folded
Misfolded proteins are destroyed by ________ in the (cytoplasm/ER lumen).
How does Calnexin recognize misfolded proteins?
Calnexin uses glucose as a signal to recognize misfolded glycoproteins.
What are BiP and membrane sensor proteins?
chaperones that recognize misfolded proteins
How are misfolded glycoproteins tagged?
by a terminal glucose
Which enzymes remove glucose residues in quality control of glycoproteins?
Glucosidases (I and II)
After removal of 2 of the 3 glucose residues on a glycoprotein, where does the protein bind?
Which enzyme removes the terminal glucose residue after binding of the glycoprotein to Calnexin?
If, after binding to Calnexin ad removal of the terminal glucose, the protein is still misfolded, what occurs?
Glucosyl transferase (GT or UGGT) adds a terminal glucose again, and the protein goes through a second round of quality control.
How is a misfolded protein marked for degradation?
A polyubiquitin tail is added, marking the protein for degradation by the proteosome.