The Proteasome

Protein degradation is as essential to the cell as protein synthesis. For example,

to supply amino acids for fresh protein synthesis
to remove excess enzymes
to remove transcription factors that are no longer needed.

There are two major intracellular devices in which damaged or unneeded proteins are broken down:

lysosomes and
proteasomes

Lysosomes deal primarily with

extracellular proteins, e.g., plasma proteins, that are taken into the cell, e.g., by endocytosis
cell-surface membrane proteins that are used in receptor-mediated endocytosis.
the proteins (and other macromolecules) engulfed by autophagosomes.

Link to a discussion of lysosomes.

Proteasomes deal primarily with endogenous proteins; that is, proteins that were synthesized within the cell such as:

transcription factors
cyclins (which must be destroyed to prepare for the next step in the cell cycle)
proteins encoded by viruses and other intracellular parasites
proteins that are folded incorrectly because
- of translation errors
- they are encoded by faulty genes
  Most cases of cystic fibrosis are caused by the accelerated degradation of a mutant version of a chloride transporter. [Link]
- they have been damaged by other molecules in the cytosol.

Structure of the Proteasome

The Core Particle (CP)

The core particle is made of 2 copies of each of 14 different proteins.
These are assembled in groups of 7 forming a ring.
The 4 rings are stacked on each other (like 4 donuts)

The Regulatory Particle (RP)

There are two identical RPs, one at each end of the core particle.
Each is made of 14 different proteins (none of them the same as those in the CP).
6 of these are ATPases.
Some of the subunits have sites that recognize the small protein ubiquitin.

Ubiquitin

a small protein (76 amino acids)
conserved throughout all the kingdoms of life; that is, virtually identical in sequence whether in bacteria, yeast, or mammals.
used by all these creatures to target proteins for destruction.

The Process

Proteins destined for destruction

are conjugated to a molecule of ubiquitin which binds to the terminal amino group of a lysine residue.
Additional molecules of ubiquitin bind to the first forming a chain.
The complex binds to ubiquitin-recognizing site(s) on the regulatory particle.
The protein is unfolded by the ATPases using the energy of ATP
The unfolded protein is translocated into the central cavity of the core particle.
Several active sites on the inner surface of the two middle "donuts" break various specific peptide bonds of the chain.
This produces a set of peptides averaging about 8 amino acids long.
These leave the core particle by an unknown route where
- they may be further broken down into individual amino acids by peptidases in the cytosol or
- in mammals, they may be incorporated in a class I histocompatibility molecule to be presented to the immune system as a potential antigen [see below].
The regulatory particle releases the ubiquitins for reuse.

Bortezomib (Velcade®)

On 13 May 2003, the U.S. Food and Drug Administration (FDA) approved a drug called bortezomib (Velcade®) (formerly known as LDP-341) to treat patients with multiple myeloma, a cancer of plasma cells.

The drug blocks the proteolytic action of the proteasome.

The failure to degrade IκB blocks the signaling action of the transcription factor NF-κB. [Discussion] Dozens of genes needed for proliferation and adhesion of myeloma cells are turned off.
The failure to degrade cyclins inhibits completion of the cell cycle and hence the mitotic proliferation of the cancerous cells.
The drug seems to work especially well when used with conventional "chemo" drugs probably by inhibiting the ability of the cancer cell to protect itself against the damage the "chemo" causes.
Inhibition of Bcl-2 leads to death of the cell by apoptosis.
Angiogenesis and metastasis are also inhibited.

The drug is given intermittently, and its action is reversible. Cancer cells are killed while normal cells are spared.

Antigen Processing by Proteasomes

In mammals, activation of the immune system

leads to the release of the cytokine interferon-gamma.
This causes three of the subunits in the core particle to be replaced by substitute subunits.
The peptides generated in this altered proteasome are picked up by TAP (= transporter associated with antigen processing) proteins and transported from the cytosol into the endoplasmic reticulum where
each enters the groove at the surface of a class I histocompatibility molecule.
This complex then moves through the Golgi apparatus and is inserted in the plasma membrane where it can be "recognized" by CD8⁺ T cells.

Link to a discussion of antigen presentation in the class I pathway.

It is probably no coincidence that the genes encoding

the three substitute core particle subunits
TAP
all the MHC molecules

are clustered together on the same chromosome (#6 in humans).

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16 May 2004