LIBERTY UNIVERSITY MSCI 660
INTRACELLULAR MEMBRANE
TRAFFIC AND CELLULAR SORTING
QUIZ QUESTIONS WITH COMPLETE
SOLUTIONS
Course
MSCI 660
1) What is the primary function of the endoplasmic reticulum (ER) in protein sorting?
Answer: The ER synthesizes and folds proteins and directs them to their proper destinations via
the secretory pathway.
Rationale: Proteins are co-translationally translocated into the ER, where chaperones ensure
proper folding and signal sequences determine subsequent targeting.
2) How do signal sequences direct proteins to the ER?
Answer: N-terminal hydrophobic sequences on nascent polypeptides are recognized by the
signal recognition particle (SRP), which pauses translation and directs the ribosome to the ER
membrane.
Rationale: This ensures that secretory and membrane proteins enter the ER co-translationally for
proper folding and modification.
3) What is the role of COPII-coated vesicles?
Answer: COPII vesicles mediate transport of proteins from the ER to the Golgi apparatus.
Rationale: COPII proteins select cargo and drive vesicle budding, ensuring proper trafficking
from the ER to the cis-Golgi.
4) How do COPI-coated vesicles function in membrane traffic?
Answer: COPI vesicles mediate retrograde transport from the Golgi back to the ER.
Rationale: This retrieves escaped ER-resident proteins and recycles Golgi enzymes, maintaining
compartment identity.
5) What is the role of clathrin-coated vesicles?
, Answer: Clathrin vesicles mediate transport from the Golgi to endosomes and from the plasma
membrane during endocytosis.
Rationale: Clathrin coats provide structural scaffolding for vesicle formation, while adaptins
select cargo proteins.
6) How do SNARE proteins ensure vesicle targeting specificity?
Answer: v-SNAREs on vesicles pair with t-SNAREs on target membranes to drive specific
membrane fusion.
Rationale: The SNARE complex provides molecular recognition, preventing vesicles from
fusing with incorrect compartments.
7) What is the role of the Golgi apparatus in protein sorting?
Answer: The Golgi modifies, sorts, and packages proteins for delivery to lysosomes, the plasma
membrane, or secretion.
Rationale: Proteins undergo glycosylation, phosphorylation, and other modifications that
determine their final destination.
8) How are lysosomal enzymes targeted from the Golgi?
Answer: Lysosomal enzymes are tagged with mannose-6-phosphate (M6P) in the Golgi and
recognized by M6P receptors for transport to endosomes/lysosomes.
Rationale: This tagging system ensures that hydrolases reach lysosomes rather than being
secreted extracellularly.
9) What is the difference between constitutive and regulated secretion?
Answer: Constitutive secretion delivers proteins continuously to the plasma membrane, whereas
regulated secretion stores proteins in vesicles until a signal triggers release.
Rationale: Secretory cells like endocrine or neuronal cells use regulated pathways to release
hormones or neurotransmitters in response to stimuli.
10) How do endosomes contribute to membrane traffic?
Answer: Endosomes sort internalized cargo, recycling some to the plasma membrane and
sending others to lysosomes for degradation.
INTRACELLULAR MEMBRANE
TRAFFIC AND CELLULAR SORTING
QUIZ QUESTIONS WITH COMPLETE
SOLUTIONS
Course
MSCI 660
1) What is the primary function of the endoplasmic reticulum (ER) in protein sorting?
Answer: The ER synthesizes and folds proteins and directs them to their proper destinations via
the secretory pathway.
Rationale: Proteins are co-translationally translocated into the ER, where chaperones ensure
proper folding and signal sequences determine subsequent targeting.
2) How do signal sequences direct proteins to the ER?
Answer: N-terminal hydrophobic sequences on nascent polypeptides are recognized by the
signal recognition particle (SRP), which pauses translation and directs the ribosome to the ER
membrane.
Rationale: This ensures that secretory and membrane proteins enter the ER co-translationally for
proper folding and modification.
3) What is the role of COPII-coated vesicles?
Answer: COPII vesicles mediate transport of proteins from the ER to the Golgi apparatus.
Rationale: COPII proteins select cargo and drive vesicle budding, ensuring proper trafficking
from the ER to the cis-Golgi.
4) How do COPI-coated vesicles function in membrane traffic?
Answer: COPI vesicles mediate retrograde transport from the Golgi back to the ER.
Rationale: This retrieves escaped ER-resident proteins and recycles Golgi enzymes, maintaining
compartment identity.
5) What is the role of clathrin-coated vesicles?
, Answer: Clathrin vesicles mediate transport from the Golgi to endosomes and from the plasma
membrane during endocytosis.
Rationale: Clathrin coats provide structural scaffolding for vesicle formation, while adaptins
select cargo proteins.
6) How do SNARE proteins ensure vesicle targeting specificity?
Answer: v-SNAREs on vesicles pair with t-SNAREs on target membranes to drive specific
membrane fusion.
Rationale: The SNARE complex provides molecular recognition, preventing vesicles from
fusing with incorrect compartments.
7) What is the role of the Golgi apparatus in protein sorting?
Answer: The Golgi modifies, sorts, and packages proteins for delivery to lysosomes, the plasma
membrane, or secretion.
Rationale: Proteins undergo glycosylation, phosphorylation, and other modifications that
determine their final destination.
8) How are lysosomal enzymes targeted from the Golgi?
Answer: Lysosomal enzymes are tagged with mannose-6-phosphate (M6P) in the Golgi and
recognized by M6P receptors for transport to endosomes/lysosomes.
Rationale: This tagging system ensures that hydrolases reach lysosomes rather than being
secreted extracellularly.
9) What is the difference between constitutive and regulated secretion?
Answer: Constitutive secretion delivers proteins continuously to the plasma membrane, whereas
regulated secretion stores proteins in vesicles until a signal triggers release.
Rationale: Secretory cells like endocrine or neuronal cells use regulated pathways to release
hormones or neurotransmitters in response to stimuli.
10) How do endosomes contribute to membrane traffic?
Answer: Endosomes sort internalized cargo, recycling some to the plasma membrane and
sending others to lysosomes for degradation.
