StUbEx PLUS – a modified Stable tagged Ubiquitin Exchange system for Peptide Level purification and in-depth mapping of Ubiquitination Sites. Applying StUbEx PLUS to U2OS cells treated with proteasomal inhibitors MG132 or Bortezomib resulted in the identification of 41,582 sites mapping on 7,762 proteins. In addition, 72 proteins were identified with N-terminal protein ubiquitination.