Ubiquitin ligase

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  Ubiquitin ligase structure

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  Ubiquitin lysine residues

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  Phosphodegron binding by ubiquitin ligase

Ubiquitin Ligase

Ubiquitin ligases, also known as E3 ubiquitin-protein ligases, are enzymes that play a crucial role in the process of ubiquitination, a post-translational modification that targets proteins for degradation by the proteasome. Ubiquitin ligases are responsible for the transfer of ubiquitin from an E2 ubiquitin-conjugating enzyme to a substrate protein, thereby tagging it for degradation or other cellular processes.

Structure and Function

Ubiquitin ligases are part of the ubiquitin-proteasome system, which is essential for maintaining cellular protein homeostasis. They are characterized by their ability to recognize specific substrate proteins and facilitate the covalent attachment of ubiquitin molecules. This process involves three main steps:

1. Activation: Ubiquitin is activated by an E1 ubiquitin-activating enzyme in an ATP-dependent manner.
2. Conjugation: The activated ubiquitin is transferred to an E2 ubiquitin-conjugating enzyme.
3. Ligation: The E3 ubiquitin ligase mediates the transfer of ubiquitin from the E2 enzyme to the substrate protein.

Ubiquitin ligases are highly diverse and are classified into several families based on their structural domains and mechanisms of action. The two main types of ubiquitin ligases are the RING (Really Interesting New Gene) finger and the HECT (Homologous to the E6-AP Carboxyl Terminus) domain ligases.

Types of Ubiquitin Ligases

RING Finger Ligases

RING finger ubiquitin ligases are characterized by the presence of a RING domain, which facilitates the direct transfer of ubiquitin from the E2 enzyme to the substrate. These ligases function as scaffolds that bring the E2 enzyme and the substrate into close proximity, allowing for efficient ubiquitination.

HECT Domain Ligases

HECT domain ubiquitin ligases possess a HECT domain that forms a thioester bond with ubiquitin before transferring it to the substrate. This two-step mechanism involves the formation of an intermediate ubiquitin-ligase complex, which is then resolved by the transfer of ubiquitin to the substrate.

Biological Roles

Ubiquitin ligases are involved in a wide range of cellular processes, including:

• Protein Degradation: By tagging proteins with ubiquitin, ligases mark them for degradation by the proteasome, thus regulating protein levels within the cell.
• Cell Cycle Regulation: Ubiquitin ligases control the degradation of key cell cycle regulators, ensuring proper cell cycle progression.
• DNA Repair: Certain ubiquitin ligases are involved in the ubiquitination of proteins that participate in DNA repair pathways.
• Signal Transduction: Ubiquitin ligases modulate signaling pathways by regulating the stability of signaling proteins.

Clinical Significance

Dysregulation of ubiquitin ligases has been implicated in various diseases, including cancer, neurodegenerative diseases, and immune disorders. Mutations or alterations in the expression of specific ubiquitin ligases can lead to aberrant protein degradation, contributing to disease pathogenesis.

Related Pages

• Ubiquitin
• Proteasome
• Protein degradation
• Post-translational modification