Ubiquitin is a highly conserved 76-residue protein of apparently all eukaryotic cells. It exists in a free form or as part of a branched, covalently linked protein-protein complex in which ubiquitin is attached by isopeptide formation at its C-terminus to the epsilon-amino groups of lysine side chains of acceptor proteins. This type of protein modification, termed ubiquitination, is known to mark proteins for degradation by the 26S proteasome. Ubiquitination can also target proteins to lysosomes by the endocytic route or mediate other regulatory functions (Hochstrasser, 1996, Annu. Rev. Genet., 405-39).

Ubiquitin-like modifiers (UBLs) are ubiquitin-like proteins which function as modifiers in a manner to that of ubiquitin, i.e. they exist either in a free form or , when catalysed by specific enzymes, attached covalently to other proteins by their C-termini. Examples are ubiquitin itself, SUMOs, and RUB1/NEDD8 (Jentsch and Pyrowolakis, 2000, Trends Cell. Biol., 10:335-42).

Ubiquitin-domain proteins (UDPs) contain domains that are related to ubiquitin but otherwise are unrelated in sequence to each other; in contrast to UBLs, these proteins are not conjugated to other proteins. Examples are RAD23, BAG-1, Scythe, and Parkin (Jentsch and Pyrowolakis, 2000, Trends Cell. Biol., 10:335-42).

Usurpin is an alias for FLIP, the enzymatically inactive caspase-8 homolog, which can prevent receptor-mediated apoptosis More...

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