Vesicle transport Flashcards
Vesicle
A membrane bilayer enclosed compartment with a hydrophilic interior. It is used to transport soluble proteins into, out of, and within the cell. Vesicles facilitate the cross of membranes via fusion. They also transport hydrophobic proteins to membranes via fusion. These hydrophobic proteins are stuck in the membrane of the vesicle
Cargo
The contents of vesicles
Vesicles vs micelles
Micelles are relevant to detergents and have a completely hydrophobic interior- they are not bilayer enclosed. On the other hand, vesicles are bilayer enclosed. They have a hydrophilic center and a hydrophobic region
Vesicle transport- budding and fusion
If cargo is being transported from the ER to another organelle, the vesicle would bud off of the ER membrane. The interior of the vesicle would contain hydrophilic cargo while hydrophobic cargo would be embedded in the vesicle membrane. The vesicle would travel through the cytoplasm and fuse with a target membrane, like the Golgi apparatus membrane. Once the vesicle fuses, it releases hydrophilic and hydrophobic proteins into the organelle
Exocytosis
Important for secreting protein cargo out of the cell. The vesicle membrane fuses with the cell membrane, and then the cargo can be released out of the cell. Also important for delivering proteins and lipids to the membrane to promote cellular expansion. Each time endocytosis occurs, lipids are lost from the membrane. Therefore, exocytosis must also occur to balance endocytosis and maintain the cell membrane
Biosynthetic-secretory pathway
Vesicles that bud from the ER and travel to a membrane. The vesicle buds off and fuses with the Golgi membrane. The contents go through the Golgi and another vesicle buds off the Golgi and travels to its final destination. Their final destination is usually endosomes or lysosomes, as well as the cell membrane for exocytosis. Vesicles generally deliver digestive enzymes to the endosome. If the vesicle fuses with the cell membrane, it’s contents will be secreted out of the cell
Endocytic pathway
Brings cargo from outside the cell at the membrane surface, usually bringing them over to the endosomes and lysosomes, fusing with these organelles. The vesicle takes in contents from outside the cell and pinches off the cell membrane. It fuses with the endosome, delivering the endocytosed material
Retrieval pathways
Bring membrane lipids and proteins back to their compartment of origin. Contents are returned to the Golgi apparatus or to the ER. This helps to “recycle” some components that were lost during the biosynthetic-secretory pathway
Endosomes and lysosomes function
They have enzymes that can break down endocytosed material
How do vesicles form?
They form from specialized, coated regions of the cell membrane, and they bud off as coated vesicles
Coated vesicles
The “coat” is a distinctive cage of structural proteins that surrounds the vesicle itself. The coat is extremely important to the formation of vesicles. Once the vesicle has formed and budded off a membrane, the coat is shed- the coat must be shed for the vesicle to fuse with other membranes
Functions of vesicle coats (2)
- The coat physically molds the forming and vesicle and it allows the vesicle to form
- Recruits or concentrates membrane proteins in a specialized patch. Some of the proteins are cargo receptors, which help to recruit soluble cargo proteins to the vesicle
3 types of coated vesicles
- Clathrin coated
- COP1 coated
- COP2 coated
How are the types of coated vesicles distinguished?
They are distinguished by the proteins that make up the coat. There are several types of each coated vesicle, and each one is specialized for different transport steps in different places of the cell. Some are specialized in forming vesicles at the ER, others are specialized in forming vesicles at the cell membrane. They incorporate different coat protein subunits that modify their properties
Clathrin coated vesicles
Major protein is clathrin. They mediate transport between the Golgi, lysosomes, endosomes, and cell membrane. The vesicle generally originates at the cell membrane and travels to one of the other organelles. This is important for the endocytic pathway
COP1 coated vesicles
Mediate transport from the Golgi cisternae (Golgi subunits) to the ER. This is important in the retrieval pathway. It helps to mediate transport between the different subunits of the Golgi. It can also mediate transport between the Golgi and the cell membrane
COP2 coated vesicles
Mediate transport from the ER to the Golgi cisternae. This is important for the biosynthetic-secretory pathway
Where do clathrin coated vesicles form?
They usually form at the cell membrane and usually fuse with the endosome or the Golgi compartments
Where do COP1 coated vesicles form? (2)
- Form in the Golgi and deliver things to the ER in the retrieval pathway
- Form in the Golgi and deliver things to the cell membrane in the second phase of the biosynthetic-secretory pathway
Where do COP2 coated vesicles form?
- Form in the ER and fuse with the Golgi in the first phase of the biosynthetic-secretory pathway
Clathrin structure
Contains subunits that are comprised of 3 large and 3 small polypeptide chains (heavy and light chains). It forms a triskelion (a 3 legged structure). The N terminals of the triskelions extend into the cage, forming contacts with adaptor proteins. The clathrin triskelions are the major structural proteins of the coat.
Clathrin coated pits
Clathrin assembles into a basketlike, convex framework of hexagons and pentagons to form coated pits on the cytosolic surface of membranes. The framework allows for the physical molding of that vesicle. A clathrin coated pit is when the vesicle is being molded and the clathrin is starting to assemble
Adaptor proteins
These proteins form a separate, second layer of the coat between the clathrin cage and the vesicle membrane. They physically bind the clathrin coat to the membrane and trap cargo receptors. Adaptor proteins recruit cargo receptors and therefore cargo to the forming vesicle. The receptors and cargo compartmentalize into membrane regions that form the vesicle
Cargo receptors
Transmembrane receptors that capture soluble cargo molecules in the vesicle. Once cargo receptors are recruited, they recruit and bind to soluble cargo proteins