FIGURE 2.

Lactacystin but not PI-1840 binds covalently to CT-L of the 20 S proteasome. A–C, LC-MS/MS analysis. Purified rabbit 20 S proteasome was incubated either with vehicle (A), PI-1840 (B), or lactacystin (C), and the tryptic digests were analyzed by LC-MS/MS as described under “Experimental Procedures.” The b ions (red) and y ions (blue) designate the N and C termini of the peptide, respectively. The number next to each ion represents the number of amino acids in that fragment (i.e. y4 = LAFK from C terminus of the peptide).