Flashcards in 9.1.16 Lecture Deck (52):
What are lysosomes?
Acidic organelles found in every eukaryotic cell that contain hydrolytic enzymes and mediate intracellular digestion.
Typically, lysosomal hydrolytic enzymes degraded macromolecules in a ___ fashion.
What keeps the pH low in lysosomes?
If a lysosome were to burst, what would happen?
The released enzymes would alarm the cell but would not be lethal, since the optimal pH for enzymatic action is less than 7, which is the pH of the cell.
Acid hydrolases work optimally at a pH of ___.
What are the 7 main acid hydrolases?
___ evolve into lysosomes as they move to the cell interior and acquire ___.
Endosomes; hydrolytic enzymes
___ are the end of the endocytic pathway.
Describe the endocytic pathway.
Plasma membrane > early endosome > late endosome > endolysosome > lysosome (note that cargo can enter the early endosome from the TGN as well)
As the endocytic pathway proceeds, the pH ___.
Where do the enzymes needed for degradation in the lysosomes come from? Where do the substrates targeted for degradation in the lysosome come from?
Fed to lysosomes from the TGN; Early endocytic pathways
What happens to hydrolytic enzymes in the cis-golgi?
Fully folded but inactive glycoproteins from the ER get phsophorylated on mannose residues, destining them for the lysosome.
What happens to hydrolytic enzymes in the trans-golgi?
Phospho-mannose marked proteins get sorted for vesicular transport to endosomes and then to lysosomes.
Where do the substrates for lysosomal degradation come from?
Many sources, both extracellular and cytoplasmic
What are the three general ways substrates are transported to the lysosome?
1. Bacteria from the extracellular space get phagocytosed (form phagosome)
2. Substrates from outside the cell are brought in via receptor mediated endocytosis (form endosome)
3. Dying/damaged organelles from inside the cell are enclosed by a double cell membrane from the ER via autophagy (form autophagosome)
True or false - lysosomes are defined by their appearance.
False - lysosomes are defined by a common set of properties.
What are the common set of properties that define a lysosome?
1. Luminal pH of ~4.5
2. Presence of active forms of lysosomal hydrolases
3. Presence of lysosomal membrane proteins (LAMPs)
What do LAMPs do?
Form an inner lining to protect the lysosomes from enzymes
Lysosomal enzymes are phosphorylated on ___ residues in the ___.
Virtually all proteins from the ER have ___ glycans.
How are lysosomal enzymes phosphorylated on their mannose residues?
Signal patch on the lysosomal hydrolase (in its inactive zymogen form) and UDP-GlcNac bind to the enzyme GlcNac phosphotransferase. This allows for transfer of GlcNac-P to the 6-OH on mannose (release of UMP). Another enzyme removes GlcNac, leaving mannose-6-phosphate.
The transport of enzymes to the lysosomes involves ___ receptors.
Proteins marked with M-6-P move to the ___, where they bind to receptors.
M6P receptors in the TGN are ___ proteins with cytoplasmic tails that interact with ___. Clathrin mediated budding from here makes transport vesicles that bind to ___. Here, the cargo is released and ___ removed by acid phosphatases. What happens to the empty receptors?
Transmembrance; clathrin adaptors; endosomes; 6-P; they are incorporated into retromer vesicles that bud from the endosomes and return to TGN.
The one-way flow of proteins to the lysosome is caused by what?
Slowly decreasing pH levels in various organelles; note that this ultimately comes from the H+ ATPases.
Once in the lysosome, digestive enzymes are activated by ___.
Lysosomal proteases are synthesized as ___. These proteases acquire limited proteolytic activity at a ___ pH. Once activated, the protease can do what?
Inactivate precursors; proenzymes; low; process additional inactive precursors
Lysosomal enzymes that are accidentally secreted are recovered by ___. How does this work?
Receptor-mediated endocytosis; Plasma-membrane M6P receptors bind the escaped lysosomal enzyme. This is then returned to the lysosome by clathrin and receptor-mediated endocytosis.
Lysosomes degrade plasma membrane proteins via ___.
Describe the process of lysosomal degradation of plasma membrane proteins via multivesicular bodies.
The proteins receive a mark on the cytoplasmic tails (often ubiquitination conjugation). This mark places the proteins into a complex of proteins called ESCRT which stimulate an inward pinching off of a membrane vesicle.
What is internal membrane fission?
Fission of a vesicle into a region that is the topological equivalent of the outside of the cell.
Integral membrane proteins can be sorted into vesicles on the basis of what 3 things?
2. Intrinsic signals
How can cell signaling be terminated?
By degrading receptors in lysosomes
Many endocytosed proteins will avoid lysosomal degradation in what two ways?
1. Endocytic vesicles can be recycled back to the plasma membrane.
2. Endosomes bud and travel to the other side of a polarized cell via transcytosis
Give an example of transcytosis.
Babies get protective maternal antibodies in this way. Maternal IgA binds to a baby's Fc receptors. This transcytoses across intestinal epithelia and populate the internal fluids of the baby.
Proteins are marked for ___ sorting from the ___.
Transmembrane proteins can sort at the TGN into or out of ___ before being selectively incorporated into vesicles.
Lipid raft microdomains
Raft vesicles often sort to ___ cell surfaces while non-rafts often sort to ___ cell surfaces.
Apical (faces inward to lumen); Basolateral (faces outward away from lumen)
Cells can encapsulate cytoplasmic constituents and organelles in ___ vacuoles for eventual degradation.
Autophagosome formation requires a ___ and ___.
Donor organelle (usually the ER); nucleating signals
Cells can also bring in dead material via ___, in which the dead cell is surrounded by a lipid bilayer at completion.
Some macromolecules brought to the lysosome are hard to digest. Why?
Extracellular matrix proteins, cell surface proteins, and membrane lipids internalize via endocytosis and phagocytosis. These include membrane glycosphingolipids, glycoproteins, and proteoglycans in both the carbohydrate-rich glycocalyx on the surface of cells and in the surrounding connective tissue matrix.
___ maintain and remodel connective tissue. This involves synthesis of matrix constituents and their degradation within lysosomes. This process must be maintained in a ___.
Fibroblasts; steady state
N-linked glycoproteins found in biological membranes and the extracellular matrix contain 1+ branched, complex oligosaccharide chains. Their degradation requires cooperation of several ___.
What are lysosomal storage diseases?
Genetic conditions due to lysosomal enzyme deficiencies with the corresponding accumulation of substrates within lysosomes.
What is the incidence of lysosomal storage disease?
What is the inheritance pattern of lysosomal storage diseases?
Autosomal recessive (except Hunter syndrome and Fabry disease, which are X-linked recessive)
The onset of clinical symptoms of lysosomal storage diseases usually occurs ___ in life.
Early; note that variants exist with onsets in adulthood.
True or false - lysosomal storage diseases affect all tissues equally.
False - this depends in part on the tissue distribution of the relevant substrate
How are lysosomal storage disease classified?
Based on the identity of the stored substrate
What are several examples of lysosomal storage diseases?
Mucolipidosis II (I-Cell Disease)
Metachromatic Leukodystrophy (MLD)