Ubiquitin: A small but extremely important protein that acts the "kiss of death" to other proteins. Ubiquitin consists of only 76 amino acids. In the normal course of events, proteins are inactivated by the attachment of ubiquitin to them, a process called ubiquitination. Ubiquitin acts as a tag by which the protein-transport machinery ferries a protein to the proteasome for degradation. Antagonizing this process are enzymes that remove ubiquitin from proteins.
Ubiquitin is appropriately named since it is ubiquitous and is present in virtually all types of cells. It is also one of the most highly conserved (least changed) proteins during evolution. The amino acid sequence of ubiquitin is identical in all creatures from insects to humans. Evolution has not changed it.
The cell functions as a highly-efficient checking station where proteins are built up and broken down at a rapid rate. The degradation is not indiscriminate but takes place through a process that is controlled in detail so that the proteins to be broken down at any given moment are given a molecular label.This label has been called a "kiss of death." The labelled proteins are then fed into the cells' "waste disposers", the so called proteasomes, where they are chopped into small pieces and destroyed.
The kiss-of-death is ubiquitin. It fastens to the protein to be destroyed, accompanies it to the proteasome where it is recognised as the key in a lock, and signals that a protein is on the way for disassembly. Shortly before the protein is squeezed into the proteasome, its ubiquitin label is disconnected for re-use.
In 2004, Aaron Ciechanover and Avram Hershko of the Technion Israel Institute of Technology in Haifa, Israel and Irwin Rose of the University of California, Irvine, USA shared the Nobel Prize in Chemistry "for the discovery of ubiquitin-mediated protein degradation."