Cell Biology: Nuclear Transport and Endomembrane System

Study Notes

The nuclear envelope is composed of two lipid bilayers continuous with the endoplasmic reticulum.
The nuclear envelope contains thousands of nuclear pores which function as portals into and out of the nucleus.
Nuclear pores are complex structures with over 50 different proteins.
These proteins determine what can pass through the nuclear pore.
Messenger RNA (mRNA) and ribosomes are synthesized in the nucleus and exported to the cytoplasm.
Proteins needed in the nucleus are imported from the cytosol.
Proteins destined for the nucleus have a nuclear localization signal (NLS) that allows them to enter.
Proteins leaving the nucleus have a nuclear export signal (NES) for exit.

The endomembrane system is composed of the rough endoplasmic reticulum (RER), smooth endoplasmic reticulum (SER), and Golgi apparatus.
It's the primary system for protein and lipid synthesis.
Ions, ATP, amino acids, and other small molecules diffuse freely throughout the cell.
The movement of most proteins and other large molecules is tightly regulated.

Proteins targeted to the endomembrane system have a "zip code" called an ER signal sequence.
This signal sequence binds to a signal recognition particle (SRP).
SRP binds to a receptor in the ER membrane.
SRP is a complex of proteins and RNA.
In the RER lumen, proteins are folded and glycosylated.

Proteins produced in the RER are transported within the endomembrane system in transport vesicles.
Transport vesicles bud off from the ER and deliver their contents to other organelles or to the cell surface.

The Golgi apparatus is a "postal sorting facility" that modifies, sorts, and packages proteins.
The Golgi apparatus is dynamic and changes in composition over time.
New cisternae form at the cis face and old cisternae break off from the trans face.
Proteins move through the Golgi, entering at the cis face and exiting at the trans face.
Enzymes in the Golgi attach specific carbohydrate chains to proteins.
Glycosylation in the Golgi apparatus further sorts proteins into vesicles.

Each protein has a molecular tag that directs it to a specific transport vesicle.
Each type of transport vesicle also has a tag that ensures it is transported to the correct destination.
Some proteins are sent to the cell surface in vesicles that fuse with the plasma membrane, releasing their contents into the extracellular space in a process called exocytosis.

Materials are delivered to the lysosomes by three processes: phagocytosis, autophagy, and receptor-mediated endocytosis.
Endocytosis is a process by which the cell membrane pinches off a vesicle to bring outside material into the cell.
Once in the lysosome, macromolecules are hydrolyzed.

Receptor-mediated endocytosis uses receptors to bind to macromolecules outside the cell.
The plasma membrane pinches in to form a vesicle that delivers its contents to the lysosome.