Ubiquitin ligase

A ubiquitin ligase (also called an E3 ubiquitin ligase) is a protein that in combination with an E2 ubiquitin-conjugating enzyme causes the attachment of ubiquitin to a lysine on a target protein via an isopeptide bond; the E3 ubiquitin ligase targets specific protein substrates for degradation by the proteasome. In general, the ubiquitin ligase is involved in polyubiquitination: A second ubiquitin is attached to the first, a third is attached to the second, and so forth. Polyubiquitination marks proteins for degradation by the proteasome.

However, there are some ubiquitination events that are limited to mono-ubiquitination, in which only a single ubiquitin is added by the ubiquitin ligase to a substrate molecule. Mono-ubiquitinated proteins are not targeted to the proteasome for degradation, but may instead be altered in their cellular location or function, for example, via binding other proteins that have domains capable of binding ubiquitin.

Further complicating matters, different lysines on ubiquitin can be targeted by an E3 to make chains. The most common lysine is Lys48 on the ubiquitin chain. This is the lysine used to make polyubiquitin which is recognized by the proteasome. However, Lys63 can also be used, and chains using this lysine are important for DNA repair, amongst other functions.

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