Ubiquitin homologs; Includes ubiquitin and ubiquitin-like proteins. Ubiquitin-mediated proteolysis is part of the regulated turnover of proteins required for controlling cell cycle progression. Other family members are protein modifiers that perform a wide range of functions. Ubiquitination usually results in a covalent bond between the c-terminus of ubiquitin and the epsilon-amino group of a substrate lysine. The three-step mechanism requires an activating enzyme (E1) that forms a thiol ester with the c-terminal carboxy group, a conjugating enzyme (E2) that transiently carries the activated ubiquitin molecule as a thiol ester, and a ligase (E3) that transfers the activated ubiquitin from the E2 to the substrate lysine residue. In poly-ubiquitination ubiquitin itself is the substrate.