9.9 : The Proteasome

Due to errors in transcription, RNA splicing, and translation, some proteins never fold correctly and need to be degraded. 

In eukaryotes, the major pathway for selective protein degradation is the ubiquitin-proteasome pathway. 

A ubiquitin ligase can differentiate between a normal protein and a target protein by recognizing certain degradation signals on their surface. 

It then catalyses the transfer of several ubiquitin molecules to a specific amino acid on the target proteins to mark them for degradation. 

These polyubiquitinated proteins are degraded by an ATP-dependent protease complex called the proteasome.

Each proteasome consists of a central hollow cylinder or the core, and large ring-shaped protein complexes called the caps at one or both ends of the core.

The core is formed by multiple protein subunits that assemble as a stack of rings. The proteolytic active sites of the core lie in its hollow inner chamber.

The caps at the end of the proteasome core act as the gatekeepers and only allow proteins marked by ubiquitin for entry into the core. 

The cap contains a deubiquitinase that cleaves ubiquitin from the substrate protein so that the released ubiquitin can be recycled. 

The cap then uses energy from ATP hydrolysis to unfold the target protein and starts feeding the protein into the core. 

With successive rounds of ATP hydrolysis, the unfolded protein reaches the proteasome core, where it is digested by the proteases lining the inner chamber.

The proteasome converts the entire protein into short peptide chains, which are then released into the cytosol.

These peptides are then further degraded by the cytosolic peptidases into their constituent amino acids, which can eventually be reused by the cell.