Ubiquitin (Ub)-binding domains (UBDs) are key elements in conveying Ub-based cellular signals. UBD-containing proteins interact with ubiquitinated targets and control numerous biological processes. They themselves undergo UBD-dependent monoubiquitination, which promotes intramolecular binding of the UBD to the attached Ub and leads to their inactivation. Here, we report that, in contrast to the established ubiquitination pathway, the presence of UBDs allows the ubiquitination of host proteins independently of E3 ligases. UBDs of different types, including UBA, UIM, UBM, NFZ, and UBZ, can directly cooperate with Ub-charged E2 enzymes to promote monoubiquitination. Using FRET and siRNA technologies, we verify that Ub-loaded E2 and substrates interact in cells and that E2 enzymes are essential for their monoubiquitination in vivo. This modification is mechanistically and functionally distinct from E3-mediated and growth factor-dependent monoubiquitination.