Protein Sorting Flashcards
What is the nature of nuclear localization signals (NLS)?
Short amino acid sequences that direct proteins to the nucleus, typically rich in basic residues (e.g., lysine, arginine).
What is the function of nuclear pores?
Large protein complexes that allow selective transport of molecules between the nucleus and cytoplasm.
What soluble factors are involved in nuclear import?
Importins (karyopherins), Ran-GTP, and NTF2 facilitate nuclear import by recognizing NLS and regulating transport through the pore.
How does Ran-GTP regulate nuclear import?
Ran-GTP binds importin in the nucleus, causing cargo release, while Ran-GDP in the cytoplasm promotes importin-cargo binding.
What are mitochondrial targeting signals (MTS)?
Amphipathic alpha-helical sequences at the N-terminus of proteins destined for mitochondria.
How are proteins sorted to different mitochondrial compartments?
Additional sorting signals (e.g., TIM/TOM complexes) direct proteins to the matrix, inner membrane, outer membrane, or intermembrane space.
What is the signal sequence for ER targeting?
A hydrophobic N-terminal sequence recognized by the signal recognition particle (SRP).
How does protein translocation into the ER occur?
Via the Sec61 translocon, which allows polypeptide chains to cross or integrate into the ER membrane.
What post-translational modifications occur in the ER?
N-glycosylation, disulfide bond formation, and GPI-anchor addition.
What is ER quality control?
Chaperones (e.g., BiP) and folding enzymes ensure proper protein folding; misfolded proteins are degraded via ERAD (ER-associated degradation).
What drives vesicle formation in the secretory pathway?
Coat proteins (COPII for ER→Golgi, COPI for retrograde) and GTPases (e.g., Sar1, ARF) deform membranes into vesicles.
How are transport vesicles targeted to the correct membrane?
Rab GTPases and t-SNAREs on target membranes bind v-SNAREs on vesicles to ensure specific docking/fusion.
What is receptor-mediated endocytosis?
Clathrin-coated pits internalize specific ligands (e.g., LDL) bound to receptors via adaptor proteins (e.g., AP2).
How does ubiquitin regulate receptor downregulation?
Ubiquitin tags receptors for endocytosis and lysosomal degradation, reducing cell surface levels (e.g., EGFR signaling attenuation).
What is the role of the TOM complex in mitochondrial protein import?
Translocase of the Outer Membrane (TOM) imports all nucleus-encoded mitochondrial proteins across the outer membrane.
What is the role of the TIM complex in mitochondrial protein import?
Translocase of the Inner Membrane (TIM) sorts proteins to the matrix or inner membrane using membrane potential.
What is the function of the signal recognition particle (SRP) in ER targeting?
SRP binds the ER signal sequence, pauses translation, and delivers the ribosome to the SRP receptor on the ER membrane.
What is the unfolded protein response (UPR)?
A stress pathway activated by misfolded ER proteins, increasing chaperone production or triggering apoptosis if overloaded.
What are the main types of vesicle coats in the secretory pathway?
COPII (anterograde ER→Golgi), COPI (retrograde Golgi→ER), and clathrin (Golgi→lysosomes/endosomes/plasma membrane).
How do SNARE proteins mediate vesicle fusion?
v-SNAREs on vesicles and t-SNAREs on target membranes form a tight complex that pulls membranes together for fusion.
What is the difference between constitutive and regulated secretion?
Constitutive secretion is continuous; regulated secretion (e.g., hormones) stores cargo in vesicles until a signal triggers release.
What is phagocytosis?
Uptake of large particles (e.g., bacteria) via actin-driven membrane protrusions, distinct from clathrin-mediated endocytosis.
What is pinocytosis?
Non-specific uptake of extracellular fluid and small molecules via small vesicles (bulk endocytosis).
How does dynamin function in endocytosis?
A GTPase that pinches off clathrin-coated vesicles from the plasma membrane by constricting the vesicle neck.
What is the role of ESCRT complexes in ubiquitin-dependent sorting?
ESCRT (Endosomal Sorting Complex Required for Transport) recognizes ubiquitinated cargo and directs it to intralumenal vesicles for degradation.
What are the main compartments of the mitochondrial import machinery?
TOM (outer membrane), TIM23 (matrix/inner membrane), TIM22 (inner membrane), and SAM/TOB (outer membrane assembly).
What is the role of Hsp70 in mitochondrial protein import?
Matrix Hsp70 binds incoming polypeptides, using ATP hydrolysis to pull them through the TIM23 translocon.
What is the difference between co-translational and post-translational ER import?
Co-translational: ribosome docks to ER during translation. Post-translational: fully synthesized proteins are imported (common in yeast).
What is the function of calnexin/calreticulin in ER quality control?
They bind monoglucosylated N-glycans on folding proteins, retaining them in the ER until properly folded.
What is the role of PDI in the ER?
Protein Disulfide Isomerase (PDI) catalyzes disulfide bond formation and rearrangement during protein folding.
What is the function of the Golgi apparatus in the secretory pathway?
Modifies proteins (e.g., glycosylation), sorts cargo to lysosomes, plasma membrane, or secretory vesicles.
What is retrograde transport?
Vesicle movement from the Golgi back to the ER (e.g., for ER resident protein retrieval or toxin entry).
What are early endosomes?
Sorting stations for endocytosed material: recycle receptors to the membrane or degrade cargo via late endosomes/lysosomes.
What is macropinocytosis?
Actin-dependent uptake of large fluid volumes, often triggered by growth factors (e.g., in immune cells).
How does the mannose-6-phosphate (M6P) pathway target proteins to lysosomes?
M6P tags are added in the Golgi, recognized by M6P receptors, and directed to endosomes/lysosomes.
