NCIt definition : The type of ubiquitin modification determines the functional consequences for the
modified protein. Monoubiquitination does not serve as a proteasome targeting signal,
but plays other distinct roles. Target proteins are monoubiquitinated on one or more
lysine residues, which serve as a signal for intracellular sorting as well as endocytosis
of cell-surface proteins. Monoubiquitination of cell-surface receptors serves as a
signal to target them to lysosome, where they are degraded by lysosomal proteases.
(From PMID and NCI);