Abstract: Disclosed herein are compounds, compositions, and methods for identifying substrates of E3 ubiquitin ligases. The proteins, vectors, compositions, and may be utilized in methods for orthogonal ubiquitin transfer (OUT) between variant components of the ubiquitin cascade such as a variant ubiquitin (UB), a variant ubiquitin activating enzyme (E1), a variant ubiquitin-conjugating enzyme (E2), and a variant ubiquitin ligase (E3), which may include a variant HECT type, U-box type, RBR type, and/or Ring type E3 ligase. The variant components of the ubiquitin cascade typically include one or more mutations relative to their wild-type counterparts, such as amino acid substitutions, such that the variant components interact specifically and orthogonally with each of their variant components and do not interact with the wild-type counterpart of their variant components.