Ubiquitination enzymes

Recently, murine homologues of Ubc6 and Ubc7 have been identified that play a role in the initiation of retrograde transport of T cell receptor subunits (Tiwari and Weissman 2001). Considering the data obtained for yeast, it may be possible that in mammalian cells ubiquitinating enzymes with a taste for MHC class I heavy chains are recruited to the site where heavy chains destined for degradation exit the ER. 

Chromatin is composed of nucleosomes, where each comprise 147 base pairs of DNA wrapped around an octamer oftwo copies of each histone H2A, H2B, H3, and H4. Nucleosomes are folded into higher-order structures that are stabilized by linker histones. Chromatin structure can be altered by enzymes that posttranslationally modify histones (e.g., through phosphorylation, acetylation, methylation, or ubiquitination) or by ATP-driven chromatin-remodeling complexes that alter nucleosome position and/or composition. 

Small tfbiquitin-like modifier represents a family of evolutionary conserved proteins that are distantly related in amino-acid sequence to ubiquitin, but share the same structural folding with ubiquitin proteins. SUMO proteins are covalently conjugated to protein substrates by an isopeptide bond through their carboxyl termini. SUMO addition to lysine residues of target proteins, termed SUMOylation, mediates post-transla-tional modification and requires a set of enzymes that are distinct from those that act on ubiquitin. SUMOylation regulates the activity of a variety of tar get proteins including transcription factors. 

Ubiquitin tags proteins for protein degradation. The ubiquitination requires three different enzymatic activities, a ubiquitin-activating enzyme (El), a ubiquitin-conjugating enzyme (E2 or Ubc) and a ubiquitin ligase (E3). The action of all three enzymes leads to the establishment of a poly-ubiquitin chain on target proteins which are then recognized and proteolyzed by the 26S proteasome. 

Examples of such systems include the reactions of kinases, phosphatases, hydroxylases, acetylases, ubiquitin transferases, and many other enzyme classes that represent attractive targets for drug discovery. There are several mechanisms by which an enzyme can catalyze these types of reactions, and the details of the mechanism are important in determining the best approach to designing activity assays for the enzyme and for proper evaluation of inhibitors that are identified through those activity assays. 

Protein fragment complementation assays are based on an enzyme reassembly strategy whereby a protein-protein interaction promotes the efficient refolding and complementation of enzyme fragments to restore an active enzyme. The approach was initially developed using the reconstitution of ubiquitin as a sensor for protein-protein interactions (Johnsson and Varshavsky, 1994). Ubiquitin is a 76 amino acid protein that... 

The natural product panepophenanthrin (6/1-170), isolated in 2002 from the fermented broth of the mushroom strain Panus radus IFO 8994 [90], is the first example of an inhibitor of the ubiquitin-activating enzyme [91]. Retrosynthetic analysis based on a biomimetic analysis led to the conjugated diene 6/1-172 by a retro-Diels-Alder reaction via the hemiacetal 6/1-171. Further disconnections of 6/1-172 produces the vinyl stannane 6/1-173 and the vinyl bromide 6/1-174 [92]. 

Proteins which are destined for degradation by the proteosome are first modified by the enzyme-catalysed attachment of numerous molecules of the protein ubiquitin, through amino groups to the protein targeted for degradation. This marks out the protein for ATP-dependent hydrolysis by the 26S proteosome, releasing peptides and ubiquitin... 

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