Chapter 13 (Cell Biology)
No description yet. This curated deck is ready for daily study.
Card preview (24)
What is the secretory pathway?
A series of steps that transport newly made proteins, lipids, and carbohydrates from the ER through the Golgi apparatus to secretory vesicles and then to the plasma membrane for secretion.
What is the purpose of the secretory pathway?
It ensures biomolecules reach the correct destinations, undergo proper modifications, and are secreted or inserted into membranes as needed.
What types of molecules move through the secretory pathway?
Proteins, lipids, and carbohydrates.
What are signal sequences in the secretory pathway?
Short amino acid ‘address tags’ that direct proteins to their appropriate organelles or destinations.
What is the general direction of the secretory pathway?
ER → Golgi → secretory vesicles → plasma membrane → secretion.
How do molecules move between compartments?
Using transport vesicles that bud off one compartment and fuse with the next.
What ensures correct protein movement between compartments?
Quality control systems that monitor folding and prevent misfolded proteins from advancing.
What is the main function of the rough ER in the secretory pathway?
Synthesis and initial processing of soluble and membrane-bound proteins.
What are the major modifications occurring in the ER?
Folding, disulfide bond formation, multimer assembly, and N-linked glycosylation.
What enzyme catalyzes disulfide bond formation in the ER?
Disulfide isomerase (and QSOX).
Why are disulfide bonds not formed in the cytosol?
The cytosol is a reducing environment that prevents disulfide bond formation.
What is the purpose of disulfide bonds in secreted proteins?
They stabilize the protein structure and protect against pH and temperature changes.
What is glycosylation?
The enzymatic addition of sugar chains to proteins, aiding in stability and cell recognition.
Where does N-linked glycosylation begin?
In the ER, where a 14-sugar oligosaccharide is added to the lipid carrier dolichol.
To which amino acid is the glycan attached in N-linked glycosylation?
To an asparagine residue within the sequence N-X-S/T.
What happens to the glycan after addition in the ER?
It is trimmed to a high-mannose form.
What does high-mannose glycosylation indicate?
The protein is still within or recently left the ER.
What are the key functions of glycosylation?
Protein protection, signaling, recognition, and transport targeting.
What sequence keeps soluble ER proteins from leaving?
The KDEL sequence (Lys-Asp-Glu-Leu).
What receptor recognizes KDEL-tagged proteins?
The KDEL receptor in the ER and Golgi membranes.
How are ER membrane proteins retained?
By a KKXX signal in their cytosolic tail, interacting with COPI coat proteins.
Which vesicles mediate Golgi → ER retrograde transport?
COPI-coated vesicles.
What happens to misfolded proteins in the ER?
They are retained by chaperones like BiP or degraded via ER-associated degradation (ERAD).
What does BiP do?
Binds misfolded proteins to prevent export from the ER.