Natural substrates of the Ubiquitin proteolytic pathway

Ubiquitin (Ub) is a highly conserved, 76 residue protein found only in eukaryotes. The macromolecule is unique in its ability to become reversibly cross-linked to other cellular proteins. Formation of isopeptide bonds between the carboxyl terminus of Ub and lysine amino groups on acceptor protein is associated with various biological processes ranging from insect flight-muscle organization to chromatin structure and lymphocytes homing. In these situations Ub may function as a movable binding site promoting the association of proteins that do not have complementary surfaces. Ub is most fully characterized, however, as an essential component of a major cellular proteolytic pathway(s).