Figure 4: The proteasome favours threonine as the active-site nucleophile.

(a) Growth tests by serial dilution of WT and pre2 (β5) mutant yeast cultures reveal growth defects of the active-site mutants under the indicated conditions after 2 days (2 d) of incubation. (b) Purified WT and mutant proteasomes were tested for their chymotrypsin-like activity (β5) using the substrate Suc-LLVY-AMC. Relative fluorescence units were measured in triplicate after 1 h of incubation at room temperature and are given as mean values. S.d.’s are indicated by error bars. (c) Illustration of the 2F_O–F_C electron-density map (blue mesh contoured at 1σ) for the β5-T1C propeptide fragment. The prosegment is cleaved but still bound in the substrate-binding channel. Notably, His(-2) does not occupy the S1 pocket formed by Met45, similar to what was observed for the β5-T1A-K81R mutant. (d) Structural superposition of the β5-T1A-K81R and the β5-T1C mutant subunits onto the WT β5 subunit. (e) Structural superposition of the β5-T1C propeptide onto the β1-T1A active site (blue) and the WT β5 active site in complex with the proteasome inhibitor MG132 (ref. 30). The inhibitor as well as the propeptides adopt similar conformations in the substrate-binding channel. (f) Structural superposition of the WT β5 and β5-T1C mutant active sites illustrates the different orientations of the hydroxyl group of Thr1 and the thiol side chain of Cys1. The SH group is rotated by 74° compared with the OH group. (g) Structural superposition of the WT β5 and β5-T1S mutant active sites reveals different orientations of the hydroxyl groups of Thr1 and Ser1, respectively. The 2F_O–F_C electron-density map for Ser1 (blue mesh contoured at 1σ) is illustrated. (h) The methyl group of Thr1 is anchored by hydrophobic interactions with Ala46C^β and Thr3C^γ. Ser1 lacks this stabilization and is therefore rotated by 60°.