PROTEASOME - a protein digesting
"organelle?"
- an enzyme complex for non-lysosomal protein degradation
| Protein degradation is essential to the cell... | ||
| to supply amino acids for fresh protein synthesis | ||
| to remove excess enzymes | ||
| to remove transcription factors (gene action) that are no longer needed. | ||
| There are two major intracellular organelles to digest damaged or unneeded proteins | ||
|
1) lysosomes : for extracellular proteins from endocytosis, receptor mediated endocytosis & phagocytosis |
||
|
and 2)
proteasomes: for endogenous proteins (proteins synthesized within cell) transcription factors, cell cycle cyclins, virus coded proteins, improperly folded proteins due to translation errors (made by faulty genes) and proteins damaged by cytosol molecules. [cystic fibrosis is due to the accelerated degradation of Cl transporter] |
||
|
|
Proteasomes degrade
proteins to short peptides, followed by --> hydrolysis of these peptides via cytoplasmic exopeptidases |
Structure* |
| back 150 back 255 | ||
| Proteasomes... were discovered by Alfred Goldberg (Harvard Med) & Martin Rechsteiner (Utah) in 1980's are large multi-enzyme complexes that digest proteins average human cell* holds about 20,000 to 30,000 proteasomes. they can lead to disease via overzealous degradation (CF) or neglecting to digest proteins |
| Proteins digested by
proteasomes
include... 90% of all abnormal, misfolded proteins all short-lived (2-3 hr ½ life) regulatory proteins (ala --> transcription factors) also digests many longer-lived proteins of cells: in all 80% to 90% of intracellular are degraded by proteasomes. |
| Protein Digestion... begins when cells add small polypeptide (ubiquitin) to a protein to be digested Ubiquitin is a globular protein [76 aa] with a C-terminus that sticks into aqueous space of cytosol conserved in all cell types; is virtually identical in sequence in bacteria, yeast, or mammals 3 enzymes of cells [ E1, E2, E3 ] add ubiquitin to proteins to be digested... E1 (Ub-activating enzyme) modify Ub so that its C-terminal GLY reacts w LYS on protein E2 (Ub-conjugating enzyme) attach Ub to the protein targeted for digestion E3 (Ub-ligases) play a role in recognizing the substrate protein & linking Ub to it.  addition of
uniquitin targets a protein's
entry into a Proteasome complex |
STRUCTURE of
PROTEASOME
complex...
the active proteasome has a MW of 2,400kD
[
electron micrograph ref-1
]
each complex
consists of a base and top & bottom lids, with binding sites
for
ubquitinized proteins & 6 ATPases to unfold proteins for entry
it's a
barrel
shaped structure* made of 4 separate parts
1. a
lid (cap) of 9 polypeptides, with binding
specificity for ubiquitinzed proteins
2. a regulatory cap on top (& sometimes bottom)
made of lid & regulator proteins
called PA700
(19s) is gatekeeper allowing only uniquitinized
proteins in
made of 14 different proteins with 6 of them being ATPases
3. a base of 4 stacked rings (like a stack
bagels ) of 7 proteins each...
two central
β rings with threonine protease catalytic activity sites
- 2 sites with chymotrypsin activity which digests
hydrophobic aa
- 2 sites with trypsin activity that digests
basic aa
- 2 sites with
caspase cleaving acidic aa
2 outer
α rings - with no known catalytic activity
4. a small
base cap (a 11s piece called PA28)
Sometimes blocking proteasome digestion of
cellular proteins can help protect a cell.
blocking protein
degradation using Proteasome Inhibitors cause
ubiquitin-tagged
misfolded & damaged
proteins to accumulate within cells which triggers a
HEAT-SHOCK
response thus
protecting the cell from toxic agents and high temps.
Drug companies are looking for
proteasome
inhibitors to bolster a cell's ability to
1. withstand some injury protecting
cells against ischemia or maintain organs after transplant
2. modulate amount and life span of
cyclins & transcription factors, etc...
3. to
treat
cancers with drugs as Velcade that inhibity ubiquitin-mediated proteolysis
Some links to Proteasome
Biology -
Proteasomes by Kimball
Groll et al, Yeast Proteasome in Nature 1997
Proteasome Research Products - Affiniti,
Inc.
References: 1. Human proteasomes reacted with a monoclonal antibody with specificity for a subunit protein. F. Kopp et al, J. Mol. Biol., 1995, 248:264-272.
Back
