A highly conserved protein involved in tagging proteins for degradation by the cell,
part of the waste-disposal machinery.
Proteins in the cell are always in a state of flux. New ones are being created as old
ones are destroyed. This keeps partially unfolded or degraded proteins from hanging around and mucking up the
works. Also, protein disposal systems allow molecules that are only important for a short period of time, such
as signalling proteins, to be rapidly flushed out of the system as they are no longer wanted. Proteins
like ubiqutin can label the trash and help guide it to the lysosome, where it will be digested.
Ubiquitin is a 76 amino acid (about 8.5 kDa) protein found in most eukaryotes.
It is highly conserved (yeast and human proteins are 53% identical!).
Ubiquitin ligates to proteins, committing them to the degradation pathway.
There are three accessory enzymes that are involved in tagging a protein with ubiquitin,
creatively named E1, E2 and E3.
The following schematic shows the chain of events involved in ubiquitination of a
E1 - the ubiquitin activating enzyme
(1) Ubiquitin-C-O + ATP ----> P-P + Ubiquitin-C~AMP
(carbonyl from the C-term (ubiquitinyl acyladenylate)
(2) Ubiquitin-C~AMP + E1-SH ----> AMP + Ubiquitin-C~S-E1
E2 - ubiquitin carrier protein
(3) E2-SH + Ubiqutin-C~S-E1 ----> E1-SH + Ubiquitin-C~S-E2
E3 - ligase
E3 + target protein ----> E3:target protein
E3:target + E2-S~C-Ubiquitin ----> E2-SH + E3 + Protein-Ubiquitin
Proteins are targeted on free amino
groups such as the N-terminus
or the sidechain
amino group on lysine
Ubiquitin itself has exposed surface lysines which can be tagged, sometimes leading to chains of ubiquitin
attached to a target protein.