A ubiquitin ligase, or E3, promotes transfer of ubiquitin to a selected substrate within the ubiquitination system. Many E3s provide substrate recognition, while the detailed chemistry differs between E3 families. A ubiquitin-conjugating enzyme supplies activated ubiquitin. The resulting modification can signal protein degradation or other fates depending on substrate, linkage and context, so ubiquitination is not synonymous with immediate destruction. [ref]
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