ProteinTrafficking Flashcards
Why is compartmentalization important in cells?
Creates microenvironments for organelle function.
• Ensures proteins are directed to precise locations.
What are the two major protein targeting pathways?
Cytoplasmic Pathway:
• Proteins translated in cytoplasm.
• Become cytoplasmic proteins or move to the nucleus (e.g., transcription factors).
- Endoplasmic Reticulum (ER) Pathway:
• Proteins are transported via vesicles to the Golgi, lysosomes, peroxisomes, or plasma membrane.
What is the role of signal sequences in protein targeting?
Signal sequences act as “tags” to direct proteins to specific locations.
• Example: ER signal sequence binds to the signal recognition particle (SRP), which guides the protein to the ER membrane.
How can signal sequences alter protein destination?
Swapping signal sequences can redirect a protein to a different cellular location.
What are the destinations of proteins entering the ER?
Remain in the ER lumen and are transported further.
- Become embedded in the membrane as transmembrane proteins.
What is the mechanism of vesicle fusion with the plasma membrane?
Vesicles contain v-SNAREs, and the plasma membrane has t-SNAREs.
• v-SNAREs and t-SNAREs form a complex, bringing the vesicle close to the membrane for fusion.
• Calcium facilitates the SNARE interaction, allowing vesicles to release their contents.
What protein modifications occur in the ER and Golgi?
ER Modifications:
• Protein folding via chaperones and disulfide bond formation.
- Golgi Modifications:
• Glycosylation by glycosyltransferases.
• Example: Glycosyltransferases define blood group antigens (O, A, B).
What is the role of glycosylation in red blood cells?
Determines blood groups:
• O: Universal red blood cell donor.
• AB: Universal plasma donor.
What is the lysosomal postcode system?
Mannose-6-phosphate (M6P) is added to proteins in the Golgi.
• M6P receptor ensures proteins are delivered to lysosomes
What is I-cell disease?
A lysosomal postcode disorder caused by defective M6P addition, leading to improper protein targeting.
How are proteins imported into mitochondria?
Signal sequence forms an amphipathic helix (hydrophobic and positively charged sides).
- Protein remains partially unfolded by chaperones.
- Enters via TOM (outer membrane) and TIM (inner membrane) complexes.
- Signal sequence is cleaved, and the protein matures.
How does nuclear-cytoplasmic trafficking occur?
Import: Nuclear localization signal binds importin → interacts with nuclear pore fibrils.
• Export: Exportin binds cargo → uses GTP hydrolysis via RAN GTPase to facilitate exit.
What is the role of gated, transmembrane, and vesicular transport in protein trafficking?
Gated Transport: Proteins move through nuclear pores.
- Transmembrane Transport: Proteins cross membranes using translocators (e.g., TOM/TIM).
- Vesicular Transport: Proteins move via vesicles (e.g., between ER and Golgi).
