Chapter 13: Protein Sorting

Set of flashcards covering key concepts of protein sorting and transport within eukaryotic cells, focusing on the secretory pathway, organelle targeting, and protein import mechanisms.

What is the main function of the secretory pathway?

To sort and transport proteins to their correct destinations: plasma membrane, endoplasmic reticulum, Golgi complex, lysosomes, and extracellular matrix.

What is the role of the ER signal sequence in protein targeting?

The ER signal sequence directs newly synthesized proteins to the endoplasmic reticulum for co-translational import.

What do Tom20 and Tom22 receptors do?

They act as import receptors in the outer mitochondrial membrane, facilitating the entry of mitochondrial proteins.

What is the significance of Günter Blobel's work?

He was awarded the Nobel Prize in 1999 for his discovery of the signal sequence for protein targeting.

What type of proteins do free ribosomes primarily synthesize?

Free ribosomes primarily synthesize soluble proteins that function in the nucleus, chloroplasts, mitochondria, and peroxisomes.

How does the translocation channel function for protein transfer across membranes?

The translocation channel facilitates the transfer of unfolded proteins into or across the membrane bilayer.

What are the energy sources employed during the import of proteins into organelles?

GTP hydrolysis, ATP hydrolysis (in mitochondria and peroxisomes), and cycling of Ran GTPase for nuclear transport.

What is the role of signal recognition particle (SRP) during protein synthesis?

SRP binds to the ER signal sequence and the ribosomal subunit to pause translation until the ribosome is directed to the translocon.

What do the targeting sequences for mitochondria generally consist of?

An amphipathic helix, 20-50 residues in length, with basic amino acids on one side and hydrophobic residues on the other.

Describe the mechanism of quality control for protein folding in the ER.

Improperly folded proteins are recognized by chaperones and can be retained in the ER or targeted for degradation via the ubiquitin/proteasome pathway.

What is the basic structure of the nuclear pore complex (NPC)?

The NPC consists of a complex of nucleoporins arranged in an 8-fold symmetry, facilitating the transport of proteins and RNAs between the nucleus and cytoplasm.

What prevents improperly folded proteins from progressing through the secretory pathway?

Unfolded or misfolded proteins induce the unfolded protein response (UPR) and may be shipped back to the cytosol for degradation.

What distinguishes constitutive secretion from regulated secretion in the secretory pathway?

Constitutive secretion occurs continuously and involves proteins that are immediately fused to the plasma membrane, while regulated secretion involves stored vesicles that release proteins in response to specific signals.