Intracellular Transport and Protein Sorting Mechanisms

What is the key concept regarding protein targeting after synthesis?

Proteins need to be correctly targeted to their compartment or membrane of function.

What are the major organelles involved in intracellular transport?

Nucleus, Endoplasmic Reticulum (ER), Golgi apparatus, Lysosome, Plasma Membrane, Mitochondria, Peroxisome.

What are the two types of Endoplasmic Reticulum (ER)?

Rough ER and Smooth ER.

What is the function of Rough ER?

Rough ER is studded with ribosomes and is abundant in cells with secretory functions, making proteins that are secreted, inserted into membranes, or localized within organelles.

What is the role of the Golgi apparatus?

The Golgi apparatus modifies proteins and sorts them to other locations, producing secretory vesicles.

What is a sorting signal in the context of protein transport?

A sorting signal is an amino acid sequence that specifies where a protein is sent.

What are the three mechanisms of protein import into organelles?

1. Transport through nuclear pores, 2. Transport across membranes via protein translocators, 3. Transport by vesicles.

How does protein import through nuclear pores work?

Proteins contain a nuclear localization signal (NLS) that binds to importin, which associates with the nuclear pore for GTP-mediated import.

What is required for mitochondrial import of proteins?

Proteins must be unfolded for translocation across the double membrane.

What is the process of mitochondrial import?

Proteins contain a mitochondrial localization signal (MLS), bind to a receptor protein in the membrane, and are translocated in an unfolded state before being refolded.

What is the significance of the ER in protein synthesis?

The ER serves as an entry point to the cell surface, lysosomes, endosomes, and Golgi, and proteins enter the ER while being synthesized.

What is the role of the signal recognition particle (SRP) in ER import?

SRP recognizes the ER signal on the protein, pauses translation, and guides the ribosome to the SRP receptor on the ER membrane.

What happens to soluble proteins during ER import?

The signal sequence is recognized by SRP, translation pauses, and the protein is sent through the translocation channel co-translationally.

How are membrane proteins imported into the ER?

Membrane proteins have additional internal stop-transfer sequences that embed within the membrane and become membrane-spanning regions.

What is the difference between soluble and membrane-bound proteins in terms of ER import?

Soluble proteins are fully translocated into the ER lumen, while membrane proteins have stop-transfer sequences that anchor them in the membrane.

What are the characteristics of nuclear pores?

Nuclear pores regulate the entry and exit of macromolecules and are composed of over 100 different proteins.

What is the nuclear export process?

Proteins contain a nuclear export signal (NES) that binds to CRM-1, associates with the nuclear pore, and undergoes GTP-mediated export.

What is the importance of sorting signals in protein transport?

Sorting signals are crucial for determining the correct localization of proteins within the cell.

What are the two types of proteins synthesized in the ER?

Soluble proteins and membrane-bound proteins.

What is the function of chaperone proteins in mitochondrial import?

Chaperone proteins assist in the refolding of proteins after they have been translocated into the mitochondrial matrix.

What happens to the signal sequence during ER import?

The signal sequence is cleaved off after the protein is imported into the ER.

What is the role of the SRP receptor in the ER?

The SRP receptor on the ER membrane binds the SRP-protein complex and facilitates the translocation of the protein into the ER.

What is a common feature of proteins that span the membrane multiple times?

They have multiple start and stop transfer sequences that alternate in the polypeptide.

How do proteins achieve their final conformation after import?

Proteins are often refolded with the help of chaperone proteins after being imported into organelles.

What is the significance of the N-terminal signal for ER import?

The N-terminal signal marks the protein for import into the ER.

What is the basic mechanism of transport by vesicles?

Proteins are sorted from the ER to specific locations within transport vesicles.

What is the function of the nuclear localization signal (NLS)?

The NLS is a basic sequence that directs proteins to the nucleus.

What is the function of the nuclear export signal (NES)?

The NES is a leucine-rich sequence that directs proteins out of the nucleus.