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Lecture 13

MCD BIO 165A Lecture 13: lecture 13 lysosomes & peroxisomes

Molecular, Cell, and Developmental Biology
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lecture 13
lysosomes & peroxisomes
- Image of lysosome
In dead cells: Immunostain for protein in lysosome; in live cells: tag with GFP
This image: (uses neither of those 2) using dye (lysotracker) selective for
lysosomes; dye is everywhere else in the cell, but only fluorescing in lysosome
dye takes advantage of lysosome property that makes it only fluoresce there
Dynamic organelles responsible for receiving and degrading macromolecules from the
secretory, endocytic, autophagic and phagocytic membrane trafficking pathways
- Garbage disposal, specialized to destroy things that are toxic, can accumulate
Destroys protein in endomembrane system
- recycling amino acids, nucleotides, small molecules cell will be using again
Discovery: fractionate cell -> which component degrades glucose 6 phosphate?
- Discovered Enzyme glucose 6 phosphatase in a fraction of cell associated with
- EM looking at that fraction of cell -> they’re lysosomes
Heterogeneous appearance including acid hydrolases
Wide size range: 25nm 1um in diameter
- Because flux of degraded material differs in size
Present in all EK cells with broad number of functions
Lysosomal enzymes are a bunch of enzymes degrading stuff, e.g. phospholipase, acid
phosphatase, collagenase
LDL is a protein lipid aggregate from diet; needs to be cleared from blood vessels

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- Receptors on endothelial cells take them up thru endocytosis -> secretary pathway
thru endosomes; sorted so that receptor recycled back to cell surface, LDL shuttled to
lysosome to be destroyed -> provides breakdown products e.g. free cholesterol cell
can use (in this case, pathway important for clearing up LDL & prevent blood vessel
- After internalization: Cargo sorted in endosomes, those destined for degradation
sorted to lysosomes
- LDL taken up thru endocytosis -> secretary pathway into early endosome -> receptors
recycled back to surface, LDL transferred to lysosome & degraded
Like most membranous organelles (except chloroplasts & mitochondria), membrane
originally bud off of trans Golgi (where sorting takes place) => right proteins sorted into
lysosomes, lysosome matures

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In ER, Lysosomal proteins tagged with phosphorylated mannose residues
- Lysosomal protein has sequence that allows it to be recognized by
phosphotransferase that transfers phosphate to mannose
- N-glycosylation in RER
- Tagged lysosomal enzymes recognized & captured by mannose 6 phosphate receptors
(MPRs) in Golgi
- Gathered in clathrin-coated vesicle, destined for lysosome
- Clathrin dissociates from vesicle; MPRs dissociate from lysosome (because of low pH)
& recycled
If MPR doesn’t dissociate: either lysosomal enzyme goes back to Golgi & comes
back out, or receptors ends up in lysosome & degraded
Why are lysosomal enzymes only active in lysosome but not before that?
- Lysosomal enzymes evolved to be functional at low pH (4-5), which occurs in
- To make lysosome acidic: proton pumps pump H+ into lysosome using ATP (against
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