Ubiquitin

Ubiquitin, also known as ubiquitous immunopoietic polypeptides, is a polypeptide that occurs in all nucleated cells and plays a major role in the proteolysis of molecules.

• Degradation of proteins is also called ubiquitination. Ubiquitin marks the proteins to be degraded. It consists of 76 amino acids, has a size of 8.5 kDa and is found in all eukaryotic cells.
• Ubiquitin is a highly conserved protein: it differs in humans and in yeast fungi only in 4% of the amino acids.
• The molecule has a globular structure, only one C-terminal glycine-glycine motif stands out.
• Ubiquitin is enzymatically coupled to other proteins by a ubiquitin-protein ligase. These change their properties through this"ubiquitination". Depending on the number and type of ubiquitin bindings, an ubiquitinated target protein can thus be promoted or hindered in its interaction with other proteins, its activity can be influenced, its localisation in the cell changed or its degradation accelerated.
• During protein quality control, several chain-linked ubiquitins mark the poly-ubiquitinated protein for degradation in the proteasome.

1. Yi J et al (2007) Emerging roles for ubiquitin and protein degradation in neuronal function. Pharmacol Rev 59: 14-39