A small regulatory protein that tags other proteins for degradation, localization, or functional modification. It is covalently attached to target proteins through a complex enzymatic cascade involving E1, E2, and E3 enzymes.
Named 'ubiquitin' in 1975 because it was found to be ubiquitous (present everywhere) in eukaryotic cells. Discovered by Aaron Ciechanover, Avram Hershko, and Irwin Rose, who won the 2004 Nobel Prize for uncovering this protein degradation system.
Think of ubiquitin as the cellular equivalent of a 'kick me' sign - once it's attached to a protein, that protein is marked for destruction! But plot twist: sometimes multiple ubiquitin molecules create different 'zip codes' that send proteins to different cellular locations instead.
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