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GREG WILLIAMS / SCIENCE SOURCE / SCIENCE PHOTO LIBRARY GREG WILLIAMS / SCIENCE SOURCE / SCIENCE PHOTO LIBRARY
Ubiquitin is a tiny regulatory protein involved in several processes generally associated with protein housekeeping. The best known role for ubiquitin is to tag and identify for degradation proteins that are damaged or no longer needed by the cell. Ubiquitin links to a target protein by using the C-terminal Gly residue, shown here in ball-and-stick form. The linking reaction uses a Lys side chain on the target protein to form an isopeptide bond. Multiple copies of ubiquitin are usually required to signal a particular cellar process. The role of ubiquination depends on the number and arrangement of ubiquitin molecules. Ubiquitin can use of any seven Lys residues (blue) to form iopeptide bonds to other ubiquitin units to multiple sites on the target protein.
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