FIGURE 2.

PolyUb linkage profiles of mouse tissues. A, Coomassie Blue stained gel shows total protein of five tissue homogenates (15 μg) that were fully labeled by stable isotope. B, relative abundance of polyUb linkages and the proteasome subunit Rpn2, based on the relative intensity of conserved peptide pairs from unlabeled HEK293 lysate and labeled mouse samples by LC-MS analysis. The experiments were repeated once. All data are normalized according to the abundance of each peptide in HEK293 total cell lysate and is shown as mean and S.E. The level of Lys¹¹ linkages in muscle was below the detection limit. The level of unmodified ubiquitin TLS peptide (supplemental Table S2, roughly reflecting the sum of ubiquitin in the region larger than the 80-kDa region) is 52, 59, 48, 69, and 41% in lung, cortex, muscle, liver, and kidney, respectively, compared with HEK293 cells (100%). Statistical significance (asterisk) was calculated between HEK293 cells and different tissues using Student's t test (p < 0.05).