Ubiquitin

File:Ubiquitin cartoon-2-.png

Cartoon representation of ubiquitin.

File:Ubiquitin 1UBQ surface.png

Surface representation of ubiquitin.

Ubiquitin is a small regulatory protein that is found in most eukaryotic cells. It was first identified in 1975 and plays a crucial role in the process of ubiquitination, where it is attached to substrate proteins to label them for various cellular processes.

Structure

Ubiquitin is a highly conserved protein consisting of 76 amino acids and has a molecular weight of approximately 8.5 kDa. Its structure is characterized by a compact globular fold with a flexible C-terminal tail. The protein contains seven lysine residues, which are key to its function in forming polyubiquitin chains.

Function

Ubiquitin's primary role is to tag proteins for proteasomal degradation. This process is essential for maintaining cellular homeostasis by regulating the concentration of specific proteins and removing damaged or misfolded proteins. Ubiquitin can also signal for other cellular processes, such as DNA repair, cell cycle regulation, and endocytosis.

Ubiquitination

Diagram of the ubiquitination process.

Ubiquitination is the process by which ubiquitin is covalently attached to a substrate protein. This is achieved through an enzymatic cascade involving three types of enzymes: E1 (ubiquitin-activating enzyme), E2 (ubiquitin-conjugating enzyme), and E3 (ubiquitin ligase). The ubiquitin molecule is attached to the lysine residue of the substrate protein via an isopeptide bond.

Glycine-lysine isopeptide bond in ubiquitination.

Polyubiquitin Chains

Ubiquitin can form polyubiquitin chains through linkage at any of its seven lysine residues or the N-terminal methionine. The type of linkage determines the fate of the substrate protein. For example, K48-linked chains typically signal for proteasomal degradation, while K63-linked chains are involved in non-proteolytic functions such as DNA repair and signaling.

File:Diubiquitin-lysine-48.png

K48-linked diubiquitin.

File:Diubiquitin-lysine-63.png

K63-linked diubiquitin.

Biological Significance

Ubiquitin-mediated processes are critical for numerous cellular functions. Dysregulation of ubiquitination pathways is implicated in various diseases, including cancer, neurodegenerative disorders, and immune system dysfunctions. As such, the ubiquitin-proteasome system is a target for therapeutic interventions.

Related Pages

• Proteasome
• Protein degradation
• Post-translational modification

References

• Hershko, A., & Ciechanover, A. (1998). The ubiquitin system. Annual Review of Biochemistry, 67, 425-479.
• Pickart, C. M. (2001). Mechanisms underlying ubiquitination. Annual Review of Biochemistry, 70, 503-533.