Ubiquitin

Ubiquitin is a protein, or rather, a polypeptide, that occurs in most eukaryotes. Its main function is to mark other proteins for proteolysis. It can also mark trnasmembrane? proteins (for example, receptor?s) for removal from the membrane. The marking of the protein is done by the ubiquitin binging to a lysin? residue of the target protein.

The process of marking a protein with ubiquitin consists of a series of steps.

1. Activation of ubiquitin by binding of the [carboxy group]? of the terminal glycin? of ubiquitin to the SH group of an ubiquitin-activating enzyme. This step requires ATP as an energy source, and results in a thioester? bond between ubiquitin ant the enzyme E1.
2. Transfer of ubiquitin from E1 to the ubiquitin-conjugating enzyme E2 via transacylation?.
3. The final transfer of ubiquitin to the target protein can occur either
   1. directly from E2. This is primarily used when ubiquitin is transfered to another ubiquitin already in place, creating a branched ubiquitin chain.
   2. via an E3 enzyme, which binds specifically to both E2 and the target protein. Tis is the usual way to mark specific peoteins for proteolysis.

Finally, the marked protein is digested in the 26S-proteasome into small peptides, amino acids, and ubiquitin to be reused.

Literature : Jentsch (1992); Hicke and Riezmann (1996) Anyone knows the full titles?