Ubiquitin 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). Ubiquitin is synthesized as a polyubiquitin precursor with exact head to tail repeats. Some ubiquitin genes contain a single copy of ubiquitin fused to a ribosomal protein. In D.discoideum there are 9 genes: ubqA: 5 copies of Ub and a final Asn; ubqB: 1 copy of Ub and ribosomal protein L40; ubqC: 1 copy of Ub and ribosomal protein S27A; ubqD: 3 copies of Ub and a final Leu; ubqF: 7 copies of Ub and a final Asn; ubqG: 5 copies of Ub and a final Leu; ubqH: 5 copies of Ub and a final Asn; ubqI: 4 copies of Ub and a final Asn; ubqJ: 4 copies of Ub and a final Asn. For the sake of clarity sequence features are annotated only for the first chain, and are not repeated for each of the following chains. Belongs to the ubiquitin family.