The Proteasome

What is MG132 and what does it do?

Pharmalogical inhibitor of the proteasome

Describe the process of protein ubiquitination and degradation by the proteasome

-First ubiquitin is bound to E1 and then transferred to E2/E3 complex -E2/E3 complexes (about 300 different types) recognize different degradation signals on target proteins (via E3) -E1 continues to add ubiquitin once the first coupling has taken place LOOK AT PPT

How do cells distinguish misfolded proteins?

-Large hydrophobic patches on its surface -Failed to fold correctly -Suffered an accident that partly unfolded it -Dangerous because can form aggregates with other similar proteins -Chaperones refold proteins and also therefore prevent protein aggregation

Describe the role of chaperone proteins

-Molecular chaperones guide and perfect protein folding -Molten globule= partially folded protein

Describe proteasome structure and function

-Multiprotein complexes that consist of four rings of subunits stacked one on top of another with a cap at either end of the stack -They assume a "barrel-shaped" form and are located in both the nucleus and cytosol of cells Covalent binding of ubiquitin -- Recognition of ubiquinated proteins by proteasome -- Removal of ubiquitin and unfolding of protein -- Digestion of the protein -- Release of small peptides

Describe the steps in the creation of a functional protein

After a mRNA molecule is translate into a polypeptide chain, the protein must be folded correctly into its 3D shape in order to function. Nascent polypeptide chain -- Folding and cofactor binding (noncovalent interactions) -- covalent modification by glycosylation, phosphorylation, acetylation, etc. -- binding to other protein subunits -- mature functional protein

Describe protein half-life and how that leads to destruction (recall pulse-chase too)

Every protein is thought to have a characteristic longevity which is expressed as a half life or the period of time during which it has a 50% likelihood of being destroyed The mechanisms that regulate protein turnover are not well understood However, one of the factors governing protein turnover is the specific amino acids that resides at the N-terminus of a polypeptide chain.

Describe how proteins can be folded co-transtionally

Growing polypeptide chain folds into secondary and tertiary structure as soon as is translated - N terminal domain folds first Growing polypeptide chain -- folded N-terminal domain -- folding C-terminal domain -- folding of protein completed after release from ribosome LOOK AT PPT

Describe the process of protein folding by HSP60

Hsp60 forms a barrel like structure Sequesters misfolded proteins (by hydrophobic interactions along the rim of the barrel) after released from the ribosome and keeps the protein in its "isolation chamber" to allow protein folding

Describe the process of protein folding by HSP70

Hsp=heat shock proteins (a family of molecular chaperones) Hsp70 (BIP) proteins act early and recognize a small stretch of hydrophobic amino acids on the proteins surface before the protein leaves the ribosome Aided by set of smaller HSP40 proteins (not shown) HSP binds and releases ATP each cycle of folding

Describe the process of regulated protein destruction

Proteolytic pathways -Recognize and eliminate misfolded proteins -Confer short life times on specific normal proteins whose concentration must change promptly with alterations in the state of the cell LOOK AT PPT