FUNCTION Exists either covalently attached to another protein, or free (unanchored). When covalently bound, it is conjugated to target proteins via an isopeptide bond either as a monomer (monoubiquitin), a polymer linked via different Lys residues of the ubiquitin (polyubiquitin chains) or a linear polymer linked via the initiator Met of the ubiquitin (linear polyubiquitin chains). Polyubiquitin chains, when attached to a target protein, have different functions depending on the Lys residue of the ubiquitin that is linked: Lys-48-linked is involved in protein degradation via the proteasome. Linear polymer chains formed via attachment by the initiator Met lead to cell signaling. Ubiquitin is usually conjugated to Lys residues of target proteins, however, in rare cases, conjugation to Cys or Ser residues has been observed. When polyubiquitin is free (unanchored-polyubiquitin), it also has distinct roles, such as in activation of protein kinases, and in signaling (By similarity).FUNCTION Component of the 60S subunit of the ribosome.SUBUNIT Part of the 60S ribosomal subunit.SUBCELLULAR LOCATION In Drosophila ubiquitin is encoded by 3 different genes. RpL40 and RpS27A genes code for a single copy of ubiquitin fused to the ribosomal proteins eL40 and eS31, respectively. Ubi-p63E gene codes for a polyubiquitin precursor with 10 exact head to tail repeats.MISCELLANEOUS For a better understanding, features related to ubiquitin are only indicated for the first chain.SIMILARITY In the N-terminal section; belongs to the ubiquitin family.SIMILARITY In the C-terminal section; belongs to the eukaryotic ribosomal protein eL40 family.
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