Ubiquitin (Ub) protein is a regulatory protein found throughout the tissues of eukaryotic organisms, which are those that have complex cell structures surrounded by membranes. Animals, plants and fungi are included in this category. The principal role of ubiquitin protein is to tag other proteins for destruction. When at least four attach themselves to another protein, the cell begins to disassemble it.
Composed of a sequence of only 76 amino acids, ubiquitin protein varies little between organisms. There is only a slight difference between the human version and that found in yeast, suggesting that its structure is essential for complex cell function. The amino sequence has been conserved over evolutionary history.
Cells are continuously building proteins that perform specific functions. Disassembly of this type of protein is an effective way to ensure that the process associated with it is halted. Ubiquitin protein plays a central role in the elimination of proteins that are no longer needed. In a processed termed ubiquitination, the proteins to be recycled are given a molecular tag, the ubiquitin protein.
Regulatory proteins that have completed their function are thought to trigger a signal that attracts the ubiquitin protein. Three types of enzymes are needed to complete the attachment. E1, or Ub-activating enzymes, put the ubiquitin into a reactive state. Attachment to the protein is catalyzed by E2, or Ub-conjugating enzymes. A third type of enzyme, E3, works to identify the protein to be removed.
A single organism may contain many different versions of these enzymes. There are over a dozen variants of the E2 enzyme in yeast, for example. A combination of these variants is thought to facilitate the ubiquitin tagging of proteins associated with specific functions.
A protein identified by four or more tags is fed into the proteasome, a hollow structure that breaks down proteins into individual parts. The ubiquitin protein tag acts like a chemical key to open the proteasome. When disassembly begins, the Ub tags are released and may be used again. The 2004 Nobel Prize in Chemistry was awarded to the discoverers of this process.
The ubiquitin protein plays a central role in many different cellular processes. Dysfunction in Ub mediated processes can lead to a number of pathological conditions. Certain cancers, disorders of the immune system and degenerative nerve diseases have been tied to improper Ub function, suggesting possible treatment options or avenues for further research.