Lecture 15 - Endosomes, lysosomes and proteosomes

Question 1

Ubiquitin-proteosome pathway

Answer 1

the major pathway of selective protein degradation in eukaryotic cells
- uses ubiquitin as a marker that targets cytosolic and nuclear proteins for rapid proteolysis

Question 2

Polyubiquitination

Answer 2

a post-translational modification that directs proteins to the interior of a proteasome for degradation

Question 3

What are the 3 enzymes that handle ubiquitination?

Answer 3

E1, E2, and E3 enzymes
- E1 detects free ubiquitin & activates it
- E2 = ubiquitin carrier protein
- E3 (ubiquitin ligase) picks the protein to be degraded
- first ubiquitin is added to the target protein, more ubiquitins join to form a polyubiquitin chain on the target protein

Question 4

Proteasome

Answer 4

a compartmentalized protease with sequestered active sites

Question 5

Endocytosis

Answer 5

material to be ingested is progressively enclosed by a small portion of the plasma membrane, which first invaginates and then pinches off to form an endocytic vesicle containing the ingested substance or particle

Question 6

What happens to endocytic vesicles? / process of endocytosis

Answer 6

Most endocytic vesicles fuse with a common receiving compartment, the early endosome, where internalized cargo is sorted: some molecules are returned to the plasma membrane, either directly or via a recycling endosome, and other remain as the early endosome changes into a late endosome by a process termed endosome maturation

Question 7

What coats most membrane invagination and pinocytic vesicles?

Answer 7

clathrin

Question 8

Receptor-mediated endocytosis

Answer 8

used by cells to import selected extracellular macromolecules
- lipids are transported in lipid-protein particles known as low-density lipoproteins

Question 9

What are the main sorting stations in the endocytic pathway?

Answer 9

early endosomes

• specific proteins are retrieved from early endosomes and returned to the plasma membrane

Question 10

What are lysosomes/

Answer 10

membrane-enclosed organelles filled with soluble acid hydrolases that digest molecules at acidic pH

Question 11

What maintains the acidic environment of lysosomes?

Answer 11

a vacuolar ATPase in the membrane uses energy from ATP hydrolysis to pump H+ into the lysosome

Question 12

What are endolysosomes?

Answer 12

a late endosome that contains molecules and newly synthesized lysosomal hydrolases that fuses with preexisting lysosomes

endolysosomes fuse with each other

Question 13

Intralumenal vesicles

Answer 13

formed by patches of maturing endosome membranes invaginating
- they carry endocytosed membrane proteins that need to be degraded

Question 14

What mediates the formation of intralumenal vesicles in multivesicular bodies?

Answer 14

ESCRT protein complexes
- complex forms to push the membrane in

Question 15

Routes to lysosome

Answer 15

• golgi delivers most digestive enzymes
• late endosome to lysosome
• autophagosome to lysosome
• phagosome to lysosome

Question 16

Phagocytosis

Answer 16

a special form of endocytosis in which a cell uses large endocytic vesicles called phagosomes to ingest large particles such as microorganisms and dead cells
- a cargo-triggered process: requires the activation of cell-surface receptors that transmit signals to the cell interior

Question 17

Autophagy

Answer 17

organelles are engulfed into a membrane structure called the autophagosome that subsequently fuses with the lysosome
- when cells experience stress or starvation, autophagy is used to recycle existing proteins and macromolecules

Question 18

How do autophagosomes assemble?

Answer 18

ATG8-marked vesicles undergo homotypic fusion with each other