Cell Biology: Week 9

What does the ER's extensive network of membrane sheets and tubules provide?

Approximately 20-30 times more surface area than the plasma membrane in a mammalian cell.

How is the nuclear envelope thought to have originated?

From plasma membrane infoldings surrounding the DNA.

What is the endomembrane system?

The ER, Golgi apparatus, endosomes, and lysosomes, which exchange material through vesicular transport, together with the nuclear envelope and plasma membrane.

What allows a protein to be recognized by a cytosolic nuclear import receptor?

An exposed nuclear localization signal (NLS).

How can regulatory binding partners retain proteins in the cytosol?

By masking their NLS or anchoring them.

What can permit nuclear import when a protein is retained in the cytosol?

Cellular signals can release the restraint and permit nuclear import.

What is the function of protein B in the protein A example?

Protein B helps retain protein A in the cytosol, preventing hormone-independent nuclear accumulation.

What is one possible mechanism by which protein B retains protein A in the cytosol?

NLS masking: protein B covers protein A's localization signal, and hormone signaling exposes it.

What is another possible mechanism by which protein B retains protein A?

Protein B may anchor protein A in the cytoplasm.

What are Golgi cisternae?

Flattened membrane sacs that make up the Golgi apparatus.

How many flattened membrane sacs make up a typical Golgi stack?

3-20 cisternae.

Where do proteins and lipids enter the Golgi apparatus?

At the ER-facing cis Golgi network.

Where do proteins and lipids exit the Golgi apparatus?

At the trans Golgi network.

How does cargo move through the Golgi stack?

Through transport vesicles and cisternal maturation, in which cisternae progress toward the trans face.

Where does glycosylation begin?

In the ER, where a core oligosaccharide is attached.

What happens to oligosaccharides in the Golgi apparatus?

Enzymes in successive Golgi compartments modify their sugar composition.

What is the order of glycosylation processing?

ER → cis Golgi → central Golgi → trans Golgi.

What happens if an early step in glycosylation is blocked?

It can also prevent later sugar additions.

What is low-density lipoprotein (LDL)?

A particle that carries cholesterol through the bloodstream.

What do LDL particles bind to on target cells?

LDL receptors.

What normally causes LDL to separate from its receptor?

Endosomal acidification.

What happens to the LDL receptor after LDL separates from it?

The receptor recycles.

Where does LDL proceed after separating from its receptor?

Toward lysosomes.

What can happen if LDL cannot dissociate from its receptor?

Normal sorting is disrupted, reducing efficient delivery of LDL cholesterol for release and cellular use.

What does centrifugation separate in an organelle protein-import experiment?

Free proteins from proteins that co-sediment with organelles.

Why is co-sedimentation alone insufficient to prove protein import?

It cannot distinguish proteins inside an organelle from proteins attached to its surface.

What protects imported soluble proteins from external protease?

An intact organelle membrane.

What happens to surface-exposed proteins when external protease is added?

They are digested.

What does detergent do in a protein-import experiment?

It disrupts the organelle membrane, allowing protease to digest previously protected proteins in the organelle lumen.