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Assay Method Information

Assay Name:  Pharmaceutical Assay
Description:  To determine the HECT E3 ligase selectivity of the compounds, a panel of biochemical HECT E3 ligase autoubiquitinylation assays was employed (Smurf-1, Smurf-2, WWP1, WWP2, ITCH, Nedd4, Nedd4L and E6AP). The conjugation of ubiquitin to a protein substrate is a multistep process. In an initial ATP-requiring step, a thioester bond is formed between the carboxyl terminus of ubiquitin and an internal cystein residue of the ubiquitin-activating enzyme (E1). Activated ubiquitin is then transferred to a specific cystein residue of an ubiquitin-conjugating enzyme (E2). E2s donate ubiquitin to a HECT E3 ligase (E3) from which it is transferred to the substrate protein. HECT E3 ligases can auto-ubiquitinylate. This event is detected in the TR-FRET (Time-Resolved Fluorescence Resonance Energy Transfer) assay used in this panel. The reaction mix contains E1, E2, tagged-E3, biotin-conjugated ubiquitin, the compound and ATP in a suitable buffer and is incubated for 45 minutes to allow auto-ubiquitinylation of the E3 ligase. To measure the extent of ubiquitinylated E3 ligase by TR-FRET, the donor fluorophore Europium cryptate (Eu3+ cryptate), conjugated to streptavidin which subsequently binds to biotinylated ubiquitin, and the modified allophycocyanin XL665 (HTRF primary acceptor fluorophore) coupled to a tag-specific antibody (HA, His or GST), which recognizes the respective E3 ligase fusion proteins, are added after the reaction is complete. When these two fluorophores are brought together by a biomolecular interaction (in this case ubiquitinylation of the E3 ligase), a portion of the energy captured by the Cryptate during excitation is released through fluorescence emission at 620 nm, while the remaining energy is transferred to XL665. This energy is then released by XL665 as specific fluorescence at 665 nm. Light at 665 nm is emitted only through FRET with Europium. Because Europium Cryptate is present in the assay, light at 620 nm is detected even when the biomolecular interaction does not bring XL665 within close proximity.
Affinity data for this assay
 

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Last update November 1, 2007
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