Endocytosis

Question 1

Define phagocytosis and pinocytosis.

Answer 1

• phagocytosis: cellular eating, MO, dead cells, phagosomes, >250nm, macrophages, neutrophils, dendritic cells, microglia…
• pinocytosis: cellular drinking, fluids and solutes, aprox. 100nm, all eukaryotic cells, ingestion of the PM fragments, may involve clathrin-coated vesicular transport

Question 2

What is the role of LDL receptor in clathrin coated endocytosis? Define the LDL receptor briefly.

Answer 2

LDL receptor contains a core of about 1500 cholesterol molecules esterified to long chain fatty acids that is surrounded by a lipid monolayer composed of about 800
phospholipid and 500 unesterified cholesterol molecules .
A single molecule of a 500,000 dalton protein organizes the particle and mediates the specific binding of LDL to cell surface receptor proteins.

LDL receptor proteins binding to a coated pit in the plasma membrane of a normal cell. The site in the cytoplasmic domain enables the LDL receptor to bind to adaptins in the clathrin coated pits.

Question 3

Describe a general route of endocyted cargo.

Answer 3

The endosomal compartments is a set of heterogeneous, membrane enclosed tubes extending from the periphery of the cell to the perinuclear region, where it is often close to the Golgi apparatus, and near the nucleus.

The process of internalization is rather quick, where concentrated pits with cargo enter the cell in early endosomes within minutes. After 5-15 min, the endosomes mature and turn into late endosomes and lysosomes which have more and more acidic environment. Early and late endosomes differ in their protein compositions; they are associated with different Rab proteins. Processes are accompanied by fussion of such compartments (maturation).

Many molecules, however, are recycled instead from the early endosomes back to the plasma membrane via transport vesicles. Only molecules that are not retrieved from endosomes in this way are delivered to lysosomes for degradation.

Question 4

What happens to receptors that are endoctyted? What happens if a ligand does not dissociate from a receptor? Name examples.

Answer 4

• Recycling: Retrieved receptors are returned to the same plasma membrane domain from which they came from. (LDL receptor)
• Transcytosis: Retrieved receptors are returned to a different domain. (Fc receptor)
• Degradation: Receptors that are not specifically retrieved from endosomes follow the pathway from the endosomal compartment to lysosomes, where they are degraded, guided by formation of oligomeric aggregates in the endosomal membrane. (EGF receptor)

-If the ligand that is endocytosed with its receptor stays bound to the receptor in the acidic environment of the endosome, it follows the same pathway as the receptor; otherwise it is delivered to lysosomes.

Question 5

How is the cytosolic part of EGF receptor degraded even though it isnot facing the lumenal compartment?

Answer 5

The invagination is essential to achieve complete digestion of endocytosed membrane proteins. Because the outer membrane of the multivesicular body becomes continuous with the lysosomal membrane, lysosomal hydrolases
could not digest the cytosolic domains of transmembrane proteins such as the EGF receptor shown here, if it were not for the invagination. Modification with the small protein ubiquitin facilitates the uptake of the receptors into endocytic vesicles.