A highly conserved 76-amino acid peptide universally found in eukaryotic cells that functions as a marker for intracellular PROTEIN TRANSPORT and degradation. Ubiquitin becomes activated through a series of complicated steps and forms an isopeptide bond to lysine residues of specific proteins within the cell. These "ubiquitinated" proteins can be recognized and degraded by proteosomes or be transported to specific compartments within the cell.

Conserved nuclear and cytoplasmic Ubiquitins are synthesized as either a polyubiquitin precursor with head to tail repeats or as a single ubiquitin fused to an unrelated tail protein and are typically involved in selective ATP-dependent nonlysosomal degradation of cellular proteins, histone modification, maintenance of chromatin structure, regulation of gene expression, stress response, and ribosome biogenesis. Ubiquitins often appear to be covalently bound to proteins as a marker for degradation. Ubiquitin-dependent proteolysis by the 26S proteasome plays a pivotal role in cell cycle progression.

Ubiquitous intracellular protein which, when covalently attached to other proteins, marks them for degradation or targeted transport.