Table 2. Peptides unique to wt UB identified by proteomics.

Peptides with the sequence “EGIDPPDQQR” and “ESTLHLVLR” would originate from wt UB but not from xUB (Fig. 11). The two peptides were present in all data sets collected with the OUT cascades of E4B and CHIP. This suggests that wt UB was conjugated to E4B or CHIP substrates that were purified by binding to HBT-xUB.

Peptide	Screen with the OUT cascade of E4B			Screen with the OUT cascade of CHIP
	No. of PSMs (screen 1)	No. of PSMs (screen 2)	No. of PSMs (screen 3)	No. of PSMs (screen 1)	No. of PSMs (screen 2)	No. of PSMs (screen 3)
EGIDPPDQQR	11	1	7	19	20	9
ESTLHLVLR	1	2	1	2	4	2