Lysosome Transport Flashcards
What is the pH of lysosomes?
~5
Lysosomes contain a variety of hydrolytic enzymes that are active at acidic pH.
How is the acidic pH of a lysosome maintained?
by a proton pump in the membrane which uses the energy of ATP hydrolysis to move H+ into the lysosome against a concentration gradient.
What is the major function of lysosomes?
site of intracellular degradation of macromolecules
Vesicles destined for lysosomes from the Golgi contain what coat protein and adaptor?
Clathrin and GGA adaptor
Many proteins targeted to the lysosome possess what modification?
mannose 6-phosphate modification
Most soluble hydrolases are targeted to the lysosome by this modification
Where is the mannose 6-phosphate tag added to proteins bound for lysosomes?
Cis Golgi
What does the mannose 6-phosphate attach to on proteins bound for lysosomes?
the N-linked oligosaccarhride
The addition of mannose 6-phosphate is dependent on which enzymes?
N-acetylglucosamine phosphotransferase and an uncovering enzyme (for what?)
How are lysosome proteins tagged with mannose 6-phosphate transferred from the golgi to the lysosome?
Lysosomal proteins with a mannose 6-phosphate residue bind a mannose 6-phosphate receptor in the trans Golgi network. These complexes become incorporated into specialized vesicles that are transported to the late endosome and then to the lysosome.
What is I-cell disease?
This disease is due to a deficiency in multiple lysosomal hydrolases.
It is often due to a defect in N-acetylglucosamine phosphotransferase, which prevents the addition of mannose 6-phosphate tag. In the absence of the tag, many (but not all) of the hydrolases are secreted instead of being targeted to the lysosomes
What is Tay Sachs disease caused by?
due to the absence of the lysosomal hydrolase, β-hexosaminadase A, which results in the accumulation of GM2 ganglioside.
What is autophagy?
mechanism of targeting intracellular organelles (old or damaged) and cytosolic proteins to the lysosome for degradation
Glucose degradation also occurs this way
Extracellular material brought into the cell through engulfing in called what?
Phagocytosis. Bacteria/ foreign particle is enveloped in a phagosome
Phagocytosis occurs primarily in what kinds of cells?
White blood cells (macrophages, neutrophils, etc). These mediate phagocytosis
What does phagocytosis require to work?
Requires binding to specific receptors on cell. Binging to receptors triggers formation of a pseudopod that engulfs the particle
Also requires coating of the bacterium or particle with an antibody (FC receptors on the membrane can recognize antibodies on the foreign object) or complement
Following internalization, the phagosome (the vesicle enveloping the particle) fuses with lysosomes
Psuedopod formation requires what?
actin filaments just under the membrane. Remember, actin is always involved when deformation of the cell membrane occurs
Mechanism is not sure
How do bacteria like Mycobacterium, Salmonella, Chlamydia disrupt lysosome function?
These pathogens, when phagocytosed by cells prevent phagosome-lysosome fusion. This allows for growth of the organism in the phagosome in a protective niche. Eventually it is released from the cell to infect another cell
What is pinocytosis?
The nonspecific uptake of extracellular fluid into a cell. Material dissolved in the fluid is taken up at a rate proportional to its concentration. Non-saturable.
No binding at the membrane is necessary
Is receptor mediated endocytosis saturable?
Yes
What is receptor mediated endocytosis?
allows for the selective uptake of extracellular proteins and small particles. Ultimate fate of such proteins or particles may be degradation, utilization or storage in the cell, or transport across the cell. The process is saturable.
Pinocytosis and receptor mediated endocytosis usually occur where?
at coated pits - pits in the membrane with a protein layer made up of clathrin on the cytoplasmic side of the membrane.
What are the steps of receptor mediated endocytosis?
i. Cell surface receptors bind extracellular ligands
ii. The cytosolic tail of the receptor then binds adaptins (AP2), which recruit clathrin to the cytoplasmic face of the plasma membrane
iii. Clathrin drives membrane budding and pinching off of the vesicle from the plasma membrane (requires the dynamin GTPase)
iv. Following budding from the plasma membrane, the clathrin coat dissociates from the endosome and the cell surface receptor buds off to return to the cell surface
v. Certain ligands, such as LDL, dissociate from their receptor in the acidic pH of the early endosome (pH 6.0-6.2)
vi. In the case of LDL, the receptor then recycles back to the cell surface, and LDL is sorted first to late endosomes then to lysosomes where it is degraded. Free cholesterol is then released into the cytoplasm
vii. Certain cell surface receptors, such as the EGF receptor, do not dissociate from their bound ligand in the acidic pH of the early endosome. Rather, the receptor and ligand are delivered to lysosome where they are both degraded. This forces new receptors to be synthesized and thus uptake can be mediated in time
What is Familial hypercholesterolemia?
affected individuals can not take up LDL (containing cholesterol) from the blood due to mutation in LDL receptor tail so binding of AP2 and recruitment of clathrin cannot occur
- Affected individuals have high levels of serum cholesterol and develop atherosclerosis leading to plaque in the arteries (thus, prone to myocardia)
- Some cases of inherited familial hypercholesterolemia are due to a mutation in the LDL receptor which can not bind adaptins, and therefore can not be internalized via clathrin
- Heritable mutations are also found in specific adaptins for the LDL receptor. Mutations in these adaptins result in autosomal recessive hypercholesterolemia
What is the strategy for treatment of individuals with
lysosomal storage disorder?
“Enzyme-replacement therapy”
1) Deliver recombinant enzyme with mannose 6-phosphate
tag to the cells of individuals with the disease (there are receptors for this on the cell surface).
2) Surface mannose 6-phosphate receptors bind and mediate
the uptake of the enzyme by receptor-mediated endocytosis (with clathrin and adoption 2).
3) The enzyme is delivered to the lysosome where it degrades its substrate.
Blood-brain barrier may disrupt this therapy
