Lecture 16 & 17 questions

What is the default location for a protein to be translated if no signal is present?

The cytosol.

Where are cytosolic proteins translated?

On free ribosomes in the cytosol.

What determines whether a protein enters the ER during translation?

The presence of an ER signal sequence on the protein.

How is the structure of the ER related to its function?

Its extensive membrane network allows for efficient protein and lipid synthesis and modification.

How does the structure of the Golgi relate to its function?

The stacked cisternae allow sequential processing, tagging, and sorting of proteins.

What is the role of vesicles in protein trafficking?

Vesicles transport proteins between the ER, Golgi, and other destinations via budding and fusion processes.

How does budding of vesicles occur?

It is receptor-mediated, involving coat proteins and cargo recognition, similar to receptor-mediated endocytosis.

Which proteins are shipped through the ER-Golgi pathway?

Membrane proteins, secreted proteins, and proteins destined for lysosomes or other organelles.

Which proteins are NOT shipped through the ER-Golgi pathway?

Cytosolic proteins, nuclear proteins, mitochondrial proteins, and peroxisomal proteins.

Can you tell if a protein uses the ER-Golgi pathway by knowing its final destination?

Yes, proteins destined for secretion, membranes, or lysosomes will have an ER signal and use the pathway.

How does the Golgi "know" where to ship a particular protein?

Proteins are tagged with molecular "zip codes" that are recognized by receptors in the Golgi.

Where are protein tags (zip codes) added?

In the ER and the Golgi apparatus.

What happens if a protein does not receive the correct tag?

It may be misrouted, accumulate in the wrong location, or be degraded.

What happens if the Golgi is missing a receptor for a protein’s tag?

The tagged protein cannot be recognized and may be misdelivered or degraded.

How does a protein enter the nucleus despite the double membrane?

Through nuclear pore complexes using a nuclear localization signal (NLS) and import receptor.

How do nuclear-encoded mitochondrial proteins reach the mitochondria?

They contain a mitochondrial targeting signal recognized by receptors on the mitochondrial surface.

What are the three major categories of proteins we've discussed?

Enzymes, transporters/pumps, and receptors.

What happens when a receptor binds its signal (ligand)?

It often undergoes a conformational change and catalyzes a chemical or physical response.

What kind of chemical reaction do some receptors catalyze?

GTP hydrolysis is a common example.

What determines a receptor’s signal specificity?

The shape and R-group interactions at the receptor's active site.

What can prevent a receptor from functioning properly?

Mutations, misfolding, or changes in temperature or pH.

What happens when you increase the amount of a signal molecule?

More receptors are activated, increasing the rate of signaling—until saturation.

What does it mean when receptors are saturated?

All binding sites are occupied, so adding more signal has no effect.

What is an agonist?

A molecule that mimics the natural ligand and activates the receptor.

What is an antagonist?

A molecule that binds the receptor but does not activate it; it blocks the natural signal.

How are receptor proteins transported?

Through the endomembrane system if they are membrane-bound.

How are protein signal molecules secreted for cell-cell communication?

Via the endomembrane system (ER → Golgi → vesicles → exocytosis).

Where can receptors be found in the cell?

On the cell surface, in internal membranes (e.g., Golgi), or free-floating in the cytoplasm (e.g., nuclear import receptors).