ubiquitin Protein, Human (HA)

Ubiquitin, a versatile protein, exists in either covalently attached forms to other proteins or in a free, unanchored state. In its covalently bound state, ubiquitin forms isopeptide bonds with target proteins, presenting as monoubiquitin, polyubiquitin chains linked through different Lys residues, or linear polyubiquitin chains initiated at the Met residue of ubiquitin. The diverse functions of polyubiquitin chains are dictated by the specific Lys residue involved: Lys-6-linked ubiquitin may play a role in DNA repair, Lys-11-linked in endoplasmic reticulum-associated degradation (ERAD) and cell-cycle regulation, Lys-29-linked in proteotoxic stress response and cell cycle, Lys-33-linked in kinase modification, Lys-48-linked in protein degradation via the proteasome, and Lys-63-linked in endocytosis, DNA damage responses, and NF-kappa-B activation. Linear polyubiquitin chains, initiated at Met, are implicated in cell signaling. While ubiquitin typically conjugates to Lys residues, rare instances involve conjugation to Cys or Ser residues. In its free form, unanchored-polyubiquitin contributes to various cellular processes, including the activation of protein kinases and signaling events. Ubiquitin's multifaceted roles underline its significance in orchestrating diverse cellular functions and regulatory pathways.