Intracellular Transport Flashcards
(30 cards)
What are the three main functions of cellular membranes?
Barrier – Separates organelle contents from the cytoplasm.
Selective permeability – Controls what enters and exits.
Surface area increase – Supports biochemical reactions.
Why do specialized cells have different organelle distributions?
Cells adjust organelle shape & abundance based on function.
Example → Plasma cells have expanded rough ER to produce antibodies.
What are the two types of ER?
Rough ER – Covered in ribosomes, involved in protein synthesis.
Smooth ER – Lacks ribosomes, involved in lipid metabolism & detoxification.
What percentage of cellular membranes does the ER make up?
About 50% of total membrane volume
What is co-translational protein transport?
Most proteins enter the ER before translation is complete.
Proteins for the nucleus, peroxisomes & mitochondria enter post-translationally.
What is the Signal Hypothesis?
Short stretch of amino acids (~15–25) targets proteins to the ER.
Signal sequence lacks exact homology but has hydrophobic structure.
Signal peptide is cleaved by signal peptidase in the ER lumen.
What does the Signal Recognition Particle (SRP) do?
Binds emerging signal sequence on translating ribosome.
Halts translation temporarily.
Guides the ribosome to the ER membrane.
How do proteins pass into the ER lumen?
Through the Sec61 translocon, an aqueous pore.
Some translocation is co-translational; others occur post-translationally.
What is a Single-Pass Transmembrane Protein?
Contains a ‘stop-transfer’ sequence that halts passage into the lumen
How do Multi-Pass Proteins like GPCRs integrate into membranes?
Combination of start and stop transfer sequences controls how segments embed
What percentage of proteins in the ER undergo glycosylation?
The majority of ER-synthesized proteins are glycosylated.
What amino acids participate in N-linked glycosylation?
Asparagine (N-linked glycan) in the ER
What amino acids participate in O-linked glycosylation?
Serine, threonine, and hydroxylysine (O-linked glycan) in both ER and Golgi
What is the function of Protein Disulfide Isomerase (PDI)?
Catalyzes disulfide bond formation (S-S) between cysteine residues
How does BiP assist protein folding?
Helps translocation through Sec61.
Recognizes misfolded proteins, preventing aggregation.
Blocks incorrect proteins from entering the Golgi.
How does the ER track folding errors?
Oligosaccharides mark protein folding status.
Calnexin & Calreticulin retain incomplete proteins in the ER.
What enzyme acts as a ‘timer’ for protein quality control?
Mannosidase slowly trims a core mannose, distinguishing new vs. misfolded proteins
What happens to misfolded proteins?
Lectins recognize altered oligosaccharides.
Poly-ubiquitin tags mark them for degradation via proteasomes.
What triggers the Unfolded Protein Response (UPR)?
Accumulation of misfolded proteins in the ER
What are the three parallel UPR pathways?
1️ IRE1 – Cleaves mRNA to produce chaperone proteins.
2️PERK – Slows translation to reduce protein overload.
3️ ATF6 – Activates genes encoding folding regulators.
What is the function of the Golgi Apparatus?
Carbohydrate synthesis.
Sorting & dispatching ER products (like an Amazon depot).
What are COPII-coated vesicles?
ER proteins packaged into COPII vesicles for transport to the Golgi.
Can be selective (via COPII interactions) or passive.
How do ER proteins get retrieved from the Golgi?
KKXX sorting signals → Direct ER membrane proteins back.
KDEL signals → Recover cytosolic ER proteins like BiP.
pH changes regulate retrieval efficiency between compartments.
What happens to N-linked glycans in the Golgi?
Trimmed and modified, forming complex oligosaccharides.
High-mannose glycans are trimmed but no new sugars are added.
