Variation on a Theme

Science  01 Jul 2005:
Vol. 309, Issue 5731, pp. 15
DOI: 10.1126/science.309.5731.15m

Ubiquitination is a protein modification used by eukaryotic cells to regulate a broad range of cellular functions. So far, E3 ubiquitin ligases have only been known to catalyze the formation of an isopeptide bond between ubiquitin and a lysine (or the N terminus) of the substrate. Cadwell and Coscoy (p. 127) show that MIR1, a virally encoded E3 ubiquitin ligase promotes ubiquitination of its substrate by a novel and unexpected mechanism. Ubiquitination of major histocompatibility complex class I (MHC I) molecules by MIR1 does not involve an isopeptide bond but rather a thiol-ester bond at a unique cysteine residue encoded within the MHC I intracytoplasmic domain. This finding broadens the range of candidate substrates for a potentially reversible form of ubiquitination.

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